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Title: The Mechanical Properties of Wood

Author: Samuel J. Record

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*** START OF THE PROJECT GUTENBERG EBOOK THE
MECHANICAL PROPERTIES OF WOOD ***

THE MECHANICAL
PROPERTIES OF WOOD

Frontispiece.
Photomicrograph of a small block of
western hemlock. At the top is the
cross section showing to the right the
late wood of one season's growth, to
the left the early wood of the next
season. The other two sections are
longitudinal and show the fibrous
character of the wood. To the left is
the radial section with three rays
crossing it. To the right is the
tangential section upon which the

rays appear as vertical rows of beads.
× 35. Photo by the author.

THE MECHANICAL PROPERTIES OF WOOD
Including a Discussion
of the Factors Affecting the Mechanical Properties,
and Methods of Timber Testing

BY

SAMUEL J. RECORD, M.A., M.F.

ASSISTANT PROFESSOR OF FOREST PRODUCTS, YALE
UNIVERSITY

FIRST EDITION
FIRST THOUSAND
1914

BY THE SAME AUTHOR

Identification of the Economic Woods of the United States.
8vo, vi + 117 pages, 15 figures. Cloth, $1.25 net.

TO THE STAFF OF THE
FOREST PRODUCTS LABORATORY, AT MADISON,
WISCONSIN
IN APPRECIATION OF THE MANY OPPORTUNITIES
AFFORDED AND COURTESIES EXTENDED
THE AUTHOR

PREFACE
This book was written primarily for students of forestry to whom a
knowledge of the technical properties of wood is essential. The mechanics
involved is reduced to the simplest terms and without reference to higher
mathematics, with which the students rarely are familiar. The intention
throughout has been to avoid all unnecessarily technical language and
descriptions, thereby making the subject-matter readily available to every
one interested in wood.
Part I is devoted to a discussion of the mechanical properties of wood—the
relation of wood material to stresses and strains. Much of the subject-matter
is merely elementary mechanics of materials in general, though written with
reference to wood in particular. Numerous tables are included, showing the
various strength values of many of the more important American woods.
Part II deals with the factors affecting the mechanical properties of wood.
This is a subject of interest to all who are concerned in the rational use of
wood, and to the forester it also, by retrospection, suggests ways and means
of regulating his forest product through control of the conditions of
production. Attempt has been made, in the light of all data at hand, to
answer many moot questions, such as the effect on the quality of wood of
rate of growth, season of cutting, heartwood and sapwood, locality of
growth, weight, water content, steaming, and defects.
Part III describes methods of timber testing. They are for the most part
those followed by the U.S. Forest Service. In schools equipped with the
necessary machinery the instructions will serve to direct the tests; in others
a study of the text with reference to the illustrations should give an adequate
conception of the methods employed in this most important line of research.
The appendix contains a copy of the working plan followed by the U.S.
Forest Service in the extensive investigations covering the mechanical

properties of the woods grown in the United States. It contains many
valuable suggestions for the independent investigator. In addition four
tables of strength values for structural timbers, both green and air-seasoned,
are included. The relation of the stresses developed in different structural
forms to those developed in the small clear specimens is given.
In the bibliography attempt was made to list all of the important
publications and articles on the mechanical properties of wood, and timber
testing. While admittedly incomplete, it should prove of assistance to the
student who desires a fuller knowledge of the subject than is presented here.
The writer is indebted to the U.S. Forest Service for nearly all of his tables
and photographs as well as many of the data upon which the book is based,
since only the Government is able to conduct the extensive investigations
essential to a thorough understanding of the subject. More than eighty
thousand tests have been made at the Madison laboratory alone, and the
work is far from completion.
The writer also acknowledges his indebtedness to Mr. Emanuel Fritz, M.E.,
M.F., for many helpful suggestions in the preparation of Part I; and
especially to Mr. Harry Donald Tiemann, M.E., M.F., engineer in charge of
Timber Physics at the Government Forest Products Laboratory, Madison,
Wisconsin, for careful revision of the entire manuscript.
SAMUEL J. RECORD.
YALE FOREST SCHOOL, July 1, 1914.

CONTENTS

PREFACE

PART I
THE MECHANICAL
PROPERTIES OF WOOD

Introduction
Fundamental considerations and
definitions
Tensile strength
Compressive or crushing strength
Shearing strength
Transverse or bending strength:
Beams
Toughness: Torsion
Hardness
Cleavability

PART II
FACTORS AFFECTING THE
MECHANICAL
PROPERTIES OF WOOD

Introduction
Rate of growth

Heartwood and sapwood
Weight, density, and specific
gravity
Color
Cross grain
Knots
Frost splits
Shakes, galls, pitch pockets
Insect injuries
Marine wood-borer injuries
Fungous injuries
Parasitic plant injuries
Locality of growth
Season of cutting
Water content
Temperature
Preservatives

PART III
TIMBER TESTING

Working plan
Forms of material tested
Size of test specimens
Moisture determination
Machine for static tests
Speed of testing machine
Bending large beams
Bending small beams
Endwise compression
Compression across the grain
Shear along the grain
Impact test
Hardness test: Abrasion and
indentation

Cleavage test
Tension test parallel to the grain
Tension test at right angles to the
grain
Torsion test
Special tests
Spike pulling test
Packing boxes
Vehicle and implement woods
Cross-arms
Other tests

APPENDIX

Sample working plan of United
States Forest Service
Strength values for structural
timbers

BIBLIOGRAPHY

Part I: Some general works on
mechanics, materials of
construction, and testing of
materials
Part II: Publications and articles on
the mechanical properties of wood,
and timber testing
Part III: Publications of the United
States Government on the
mechanical properties of wood,
and timber testing

ILLUSTRATIONS

Frontispiece. Photomicrograph of a
small block of western hemlock
1. Stress-strain diagrams of two
longleaf pine beams
2. Compression across the grain
3. Side view of failures in
compression across the grain
4. End view of failures in
compression across the grain
5. Testing a buggy-spoke in
endwise compression
6. Unequal distribution of stress in
a long column due to lateral
bending
7. Endwise compression of a short
column
8. Failures of a short column of
green spruce
9. Failures of short columns of dry
chestnut
10. Example of shear along the
grain
11. Failures of test specimens in
shear along the grain
12. Horizontal shear in a beam
13. Oblique shear in a short
column
14. Failure of a short column by
oblique shear
15. Diagram of a simple beam
16. Three common forms of beams
—(1) simple, (2) cantilever, (3)
continuous

17. Characteristic failures of
simple beams
18. Failure of a large beam by
horizontal shear
19. Torsion of a shaft
20. Effect of torsion on different
grades of hickory
21. Cleavage of highly elastic
wood
22. Cross-sections of white ash, red
gum, and eastern hemlock
23. Cross-section of longleaf pine
24. Relation of the moisture
content to the various strength
values of spruce
25. Cross-section of the wood of
western larch showing fissures in
the thick-walled cells of the late
wood
26. Progress of drying throughout
the length of a chestnut beam
27. Excessive season checking
28. Control of season checking by
the use of S-irons
29. Static bending test on a large
beam
30. Two methods of loading a
beam
31. Static bending test on a small
beam
32. Sample log sheet, giving full
details of a transverse bending test
on a small pine beam
33. Endwise compression test
34. Sample log sheet of an endwise
compression test on a short pine
column

35. Compression across the grain
36. Vertical section of shearing tool
37. Front view of shearing tool
38. Two forms of shear test
specimens
39. Making a shearing test
40. Impact testing machine
41. Drum record of impact bending
test
42. Abrasion machine for testing
the wearing qualities of woods
43. Design of tool for testing the
hardness of woods by indentation
44. Design of tool for cleavage test
45. Design of cleavage test
specimen
46. Designs of tension test
specimens used in United States
47. Design of tension test specimen
used in New South Wales
48. Design of tool and specimen
for testing tension at right angles to
the grain
49. Making a torsion test on
hickory
50. Method of cutting and marking
test specimens
51. Diagram of specific gravity
apparatus

TABLES

I. Comparative strength of iron,
steel, and wood

II. Ratio of strength of wood in
tension and in compression
III. Right-angled tensile strength of
small clear pieces of 25 woods in
green condition
IV. Results of compression tests
across the grain on 51 woods in
green condition, and comparison
with white oak
V. Relation of fibre stress at elastic
limit in bending to the crushing
strength of blocks cut therefrom in
pounds per square inch
VI. Results of endwise
compression tests on small clear
pieces of 40 woods in green
condition
VII. Shearing strength along the
grain of small clear pieces of 41
woods in green condition
VIII. Shearing strength across the
grain of various American woods
IX. Results of static bending tests
on small clear beams of 49 woods
in green condition
X. Results of impact bending tests
on small clear beams of 34 woods
in green condition
XI. Manner of first failure of large
beams
XII. Hardness of 32 woods in
green condition, as indicated by the
load required to imbed a 0.444-
inch steel ball to one-half its
diameter
XIII. Cleavage strength of small
clear pieces of 32 woods in green

condition
XIV. Specific gravity, and
shrinkage of 51 American woods
XV. Effect of drying on the
mechanical properties of wood,
shown in ratio of increase due to
reducing moisture content from the
green condition to kiln-dry
XVI. Effect of steaming on the
strength of green loblolly pine
XVII. Speed-strength moduli, and
relative increase in strength at rates
of fibre strain increasing in
geometric ratio
XVIII. Results of bending tests on
green structural timbers
XIX. Results of compression and
shear tests on green structural
timbers
XX. Results of bending tests on
air-seasoned structural timbers
XXI. Results of compression and
shear tests on air-seasoned
structural timbers
XXII. Working unit stresses for
structural timber expressed in
pounds per square inch

INDEX

FOOTNOTES

PART I
THE MECHANICAL PROPERTIES OF WOOD

INTRODUCTION

The mechanical properties of wood are its fitness and ability to resist
applied or external forces. By external force is meant any force outside of a
given piece of material which tends to deform it in any manner. It is largely
such properties that determine the use of wood for structural and building
purposes and innumerable other uses of which furniture, vehicles,
implements, and tool handles are a few common examples.
Knowledge of these properties is obtained through experimentation either in
the employment of the wood in practice or by means of special testing
apparatus in the laboratory. Owing to the wide range of variation in wood it
is necessary that a great number of tests be made and that so far as possible
all disturbing factors be eliminated. For comparison of different kinds or
sizes a standard method of testing is necessary and the values must be
expressed in some defined units. For these reasons laboratory experiments
if properly conducted have many advantages over any other method.
One object of such investigation is to find unit values for strength and
stiffness, etc. These, because of the complex structure of wood, cannot have
a constant value which will be exactly repeated in each test, even though no
error be made. The most that can be accomplished is to find average values,
the amount of variation above and below, and the laws which govern the
variation. On account of the great variability in strength of different
specimens of wood even from the same stick and appearing to be alike, it is
important to eliminate as far as possible all extraneous factors liable to
influence the results of the tests.

The mechanical properties of wood considered in this book are: (1) stiffness
and elasticity, (2) tensile strength, (3) compressive or crushing strength, (4)
shearing strength, (5) transverse or bending strength, (6) toughness, (7)
hardness, (8) cleavability, (9) resilience. In connection with these,
associated properties of importance are briefly treated.
In making use of figures indicating the strength or other mechanical
properties of wood for the purpose of comparing the relative merits of
different species, the fact should be borne in mind that there is a
considerable range in variability of each individual material and that small
differences, such as a few hundred pounds in values of 10,000 pounds,
cannot be considered as a criterion of the quality of the timber. In testing
material of the same kind and grade, differences of 25 per cent between
individual specimens may be expected in conifers and 50 per cent or even
more in hardwoods. The figures given in the tables should be taken as
indications rather than fixed values, and as applicable to a large number
collectively and not to individual pieces.

FUNDAMENTAL CONSIDERATIONS AND DEFINITIONS

Study of the mechanical properties of a material is concerned mostly with
its behavior in relation to stresses and strains, and the factors affecting this
behavior. A stress is a distributed force and may be defined as the mutual
action (1) of one body upon another, or (2) of one part of a body upon
another part. In the first case the stress is external; in the other internal. The
same stress may be internal from one point of view and external from
another. An external force is always balanced by the internal stresses when
the body is in equilibrium.
If no external forces act upon a body its particles assume certain relative
positions, and it has what is called its natural shape and size. If sufficient
external force is applied the natural shape and size will be changed. This
distortion or deformation of the material is known as the strain. Every
stress produces a corresponding strain, and within a certain limit (see elastic
limit, page 5) the strain is directly proportional to the stress producing it.1
The same intensity of stress, however, does not produce the same strain in

different materials or in different qualities of the same material. No strain
would be produced in a perfectly rigid body, but such is not known to exist.
Stress is measured in pounds (or other unit of weight or force). A unit
stress is the stress on a unit of the sectional area.

P
( Unit stress = --- )
A

For instance, if a load (P) of one hundred pounds is uniformly supported by
a vertical post with a cross-sectional area (A) of ten square inches, the unit
compressive stress is ten pounds per square inch.
Strain is measured in inches (or other linear unit). A unit strain is the strain
per unit of length. Thus if a post 10 inches long before compression is 9.9
inches long under the compressive stress, the total strain is 0.1 inch, and the
unit strain is

l 0.1
--- = ----- = 0.01 inch per inch of length.
L 10

As the stress increases there is a corresponding increase in the strain. This
ratio may be graphically shown by means of a diagram or curve plotted
with the increments of load or stress as ordinates and the increments of
strain as abscissæ. This is known as the stress-strain diagram. Within the
limit mentioned above the diagram is a straight line. (See Fig. 1.) If the
results of similar experiments on different specimens are plotted to the same
scales, the diagrams furnish a ready means for comparison. The greater the
resistance a material offers to deformation the steeper or nearer the vertical
axis will be the line.

Figure 1
Stress-strain diagrams of two
longleaf pine beams. E.L. = elastic
limit. The areas of the triangles
0(EL)A and 0(EL)B represent the
elastic resilience of the dry and green
beams, respectively.

There are three kinds of internal stresses, namely, (1) tensile, (2)
compressive, and (3) shearing. When external forces act upon a bar in a
direction away from its ends or a direct pull, the stress is a tensile stress;
when toward the ends or a direct push, compressive stress. In the first
instance the strain is an elongation; in the second a shortening. Whenever
the forces tend to cause one portion of the material to slide upon another
adjacent to it the action is called a shear. The action is that of an ordinary
pair of shears. When riveted plates slide on each other the rivets are sheared
off.

These three simple stresses may act together, producing compound stresses,
as in flexure. When a bow is bent there is a compression of the fibres on the
inner or concave side and an elongation of the fibres on the outer or convex
side. There is also a tendency of the various fibres to slide past one another
in a longitudinal direction. If the bow were made of two or more separate
pieces of equal length it would be noted on bending that slipping occurred
along the surfaces of contact, and that the ends would no longer be even. If
these pieces were securely glued together they would no longer slip, but the
tendency to do so would exist just the same. Moreover, it would be found in
the latter case that the bow would be much harder to bend than where the
pieces were not glued together—in other words, the stiffness of the bow
would be materially increased.
Stiffness is the property by means of which a body acted upon by external
forces tends to retain its natural size and shape, or resists deformation. Thus
a material that is difficult to bend or otherwise deform is stiff; one that is
easily bent or otherwise deformed is flexible. Flexibility is not the exact
counterpart of stiffness, as it also involves toughness and pliability.
If successively larger loads are applied to a body and then removed it will
be found that at first the body completely regains its original form upon
release from the stress—in other words, the body is elastic. No substance
known is perfectly elastic, though many are practically so under small
loads. Eventually a point will be reached where the recovery of the
specimen is incomplete. This point is known as the elastic limit, which may
be defined as the limit beyond which it is impossible to carry the distortion
of a body without producing a permanent alteration in shape. After this limit
has been exceeded, the size and shape of the specimen after removal of the
load will not be the same as before, and the difference or amount of change
is known as the permanent set.
Elastic limit as measured in tests and used in design may be defined as that
unit stress at which the deformation begins to increase in a faster ratio than
the applied load. In practice the elastic limit of a material under test is
determined from the stress-strain diagram. It is that point in the line where
the diagram begins perceptibly to curve.2 (See Fig. 1.)

Resilience is the amount of work done upon a body in deforming it. Within
the elastic limit it is also a measure of the potential energy stored in the
material and represents the amount of work the material would do upon
being released from a state of stress. This may be graphically represented
by a diagram in which the abscissæ represent the amount of deflection and
the ordinates the force acting. The area included between the stress-strain
curve and the initial line (which is zero) represents the work done. (See Fig.
1.) If the unit of space is in inches and the unit of force is in pounds the
result is inch-pounds. If the elastic limit is taken as the apex of the triangle
the area of the triangle will represent the elastic resilience of the specimen.
This amount of work can be applied repeatedly and is perhaps the best
measure of the toughness of the wood as a working quality, though it is not
synonymous with toughness.
Permanent set is due to the plasticity of the material. A perfectly plastic
substance would have no elasticity and the smallest forces would cause a
set. Lead and moist clay are nearly plastic and wood possesses this property
to a greater or less extent. The plasticity of wood is increased by wetting,
heating, and especially by steaming and boiling. Were it not for this
property it would be impossible to dry wood without destroying completely
its cohesion, due to the irregularity of shrinkage.
A substance that can undergo little change in shape without breaking or
rupturing is brittle. Chalk and glass are common examples of brittle
materials. Sometimes the word brash is used to describe this condition in
wood. A brittle wood breaks suddenly with a clean instead of a splintery
fracture and without warning. Such woods are unfitted to resist shock or
sudden application of load.
The measure of the stiffness of wood is termed the modulus of elasticity
(or coefficient of elasticity). It is the ratio of stress per unit of area to the
deformation per unit of length.

unit stress
( E = ------------- )
unit strain

It is a number indicative of stiffness, not of strength, and only applies to
conditions within the elastic limit. It is nearly the same whether derived
from compression tests or from tension tests.
A large modulus indicates a stiff material. Thus in green wood tested in
static bending it varies from 643,000 pounds per square inch for arborvitæ
to 1,662,000 pounds for longleaf pine, and 1,769,000 pounds for pignut
hickory. (See Table IX.) The values derived from tests of small beams of
dry material are much greater, approaching 3,000,000 for some of our
woods. These values are small when compared with steel which has a
modulus of elasticity of about 30,000,000 pounds per square inch. (See
Table I.)

TABLE I
COMPARATIVE STRENGTH OF IRON, STEEL, AND
WOOD
Modulus
of Modulus
Tensile Crushing
elasticity of
Sp. strength strength
MATERIAL in rupture
gr.,dry bending
Lbs. per Lbs. per Lbs. per Lbs. per
sq. in. sq. in. sq. in. sq. in.
Cast iron,
cold blast 7.1 17,270,000 16,700 106,000 38,500
(Hodgkinson)
Bessenger
steel, high
7.8 29,215,000 88,400 225,600
grade
(Fairbain).
Longleaf
pine, 3.5%
.63 2,800,000 13,000 21,000
moisture
(U.S.)

Redspruce,
3.5%
.41 1,800,000 8,800 14,500
moisture
(U.S.)
Pignut
hickory,
3.5% .86 2,370,000 11,130 24,000
moisture
(U.S.)
NOTE.—Great variation may be found in different samples
of metals as well as of wood. The examples given represent
reasonable values.

TENSILE STRENGTH

Tension results when a pulling force is applied to opposite ends of a body.
This external pull is communicated to the interior, so that any portion of the
material exerts a pull or tensile force upon the remainder, the ability to do
so depending upon the property of cohesion. The result is an elongation or
stretching of the material in the direction of the applied force. The action is
the opposite of compression.
Wood exhibits its greatest strength in tension parallel to the grain, and it is
very uncommon in practice for a specimen to be pulled in two lengthwise.
This is due to the difficulty of making the end fastenings secure enough for
the full tensile strength to be brought into play before the fastenings shear
off longitudinally. This is not the case with metals, and as a result they are
used in almost all places where tensile strength is particularly needed, even
though the remainder of the structure, such as sills, beams, joists, posts, and
flooring, may be of wood. Thus in a wooden truss bridge the tension
members are steel rods.
The tensile strength of wood parallel to the grain depends upon the strength
of the fibres and is affected not only by the nature and dimensions of the
wood elements but also by their arrangement. It is greatest in straight-
grained specimens with thick-walled fibres. Cross grain of any kind

materially reduces the tensile strength of wood, since the tensile strength at
right angles to the grain is only a small fraction of that parallel to the grain.

TABLE II
RATIO OF STRENGTH OF WOOD IN
TENSION AND IN COMPRESSION
(Bul. 10, U. S. Div. of Forestry, p. 44)
A stick 1
Ratio: square inch in
KIND R= cross section.
OF Tensile strength Weight
WOOD --------------------- required to—
compressive strength Pull Crush
apart endwise
Hickory 3.7 32,000 8,500
Elm 3.8 29,000 7,500
Larch 2.3 19,400 8,600
Longleaf
2.2 17,300 7,400
Pine
NOTE.—Moisture condition not given.

Failure of wood in tension parallel to the grain occurs sometimes in flexure,
especially with dry material. The tension portion of the fracture is nearly the
same as though the piece were pulled in two lengthwise. The fibre walls are
torn across obliquely and usually in a spiral direction. There is practically
no pulling apart of the fibres, that is, no separation of the fibres along their
walls, regardless of their thickness. The nature of tension failure is
apparently not affected by the moisture condition of the specimen, at least
not so much so as the other strength values.3
Tension at right angles to the grain is closely related to cleavability. When
wood fails in this manner the thin fibre walls are torn in two lengthwise
while the thick-walled fibres are usually pulled apart along the primary
wall.

TABLE III
TENSILE STRENGTH AT
RIGHT ANGLES TO THE
GRAIN OF SMALL CLEAR
PIECES OF 25 WOODS IN
GREEN CONDITION
(Forest Service Cir. 213)
When
surface When
of surface of
COMMON failure failure is
NAME OF is tangential
SPECIES radial
Lbs.
Lbs. per
per sq.
sq. inch
inch
Hardwoods
Ash, white 645 671
Basswood 226 303
Beech 633 969
Birch,
446 526
yellow
Elm,
765 832
slippery
Hackberry 661 786
Locust,
1,133 1,445
honey
Maple,
610 864
sugar
Oak, post 714 924
red 639 874
swamp
757 909
white
white 622 749
yellow 728 929

Sycamore 540 781
Tupelo 472 796
Conifers
Arborvitæ 241 235
Cypress,
242 251
bald
Fir, white 213 304
Hemlock 271 323
Pine,
240 298
longleaf
red 179 205
sugar 239 304
western
230 252
yellow
white 225 285
Tamarack 236 274

COMPRESSIVE OR CRUSHING STRENGTH

Compression across the grain is very closely related to hardness and
transverse shear. There are two ways in which wood is subjected to stress of
this kind, namely, (1) with the load acting over the entire area of the
specimen, and (2) with a load concentrated over a portion of the area. (See
Fig. 2.) The latter is the condition more commonly met with in practice, as,
for example, where a post rests on a horizontal sill, or a rail rests on a cross-
tie. The former condition, however, gives the true resistance of the grain to
simple crushing.]

Figure 2
Compression across the grain.

The first effect of compression across the grain is to compact the fibres, the
load gradually but irregularly increasing as the density of the material is
increased. If the specimen lies on a flat surface and the load is applied to
only a portion of the upper area, the bearing plate indents the wood,
crushing the upper fibres without affecting the lower part. (See Fig. 3.) As
the load increases the projecting ends sometimes split horizontally. (See
Fig. 4.) The irregularities in the load are due to the fact that the fibres
collapse a few at a time, beginning with those with the thinnest walls. The
projection of the ends increases the strength of the material directly beneath
the compressing weight by introducing a beam action which helps support
the load. This influence is exerted for a short distance only.

Figure 3
Side view of failures in compression
across the grain, showing crushing of
blocks under bearing plate. Specimen
at right shows splitting at ends.

Figure 4
End view of failures in compression
across the grain, showing splitting of
the ends of the test specimens.

TABLE IV
RESULTS OF COMPRESSION
TESTS ACROSS THE GRAIN
ON 51 WOODS IN GREEN
CONDITION, AND

COMPARISON WITH WHITE
OAK
(U. S. Forest Service)
Fiber
stress
in per
Fibre stress at cent of
elastic limit white
COMMON
perpendicula oak, or
NAME OF
r to grain 853
SPECIES
pounds
per sq.
in.
Lbs. per sq. Per
inch cent
Osage
2,260 265.0
orange
Honey
1,684 197.5
locust
Black
1,426 167.2
locust
Post oak 1,148 134.6
Pignut
1,142 133.9
hickory
Water
1,088 127.5
hickory
Shagbark
1,070 125.5
hickory
Mockernut
1,012 118.6
hickory
Big
shellbark 997 116.9
hickory
Bitternut
986 115.7
hickory

Nutmeg
938 110.0
hickory
Yellow oak 857 100.5
White oak 853 100.0
Bur oak 836 98.0
White ash 828 97.1
Red oak 778 91.2
Sugar
742 87.0
maple
Rock elm 696 81.6
Beech 607 71.2
Slippery
599 70.2
elm
Redwood 578 67.8
Bald
548 64.3
cypress
Red maple 531 62.3
Hackberry 525 61.6
Incense
518 60.8
cedar
Hemlock 497 58.3
Longleaf
491 57.6
pine
Tamarack 480 56.3
Silver
456 53.5
maple
Yellow
454 53.2
birch
Tupelo 451 52.9
Black
444 52.1
cherry
Sycamore 433 50.8
Douglas fir 427 50.1
Cucumber
408 47.8
tree

Shortleaf
400 46.9
pine
Red pine 358 42.0
Sugar pine 353 41.1
White elm 351 41.2
Western
348 40.8
yellow pine
Lodgepole
348 40.8
pine
Red spruce 345 40.5
White pine 314 36.8
Engelman
290 34.0
spruce
Arborvitæ 288 33.8
Largetooth
269 31.5
aspen
White
262 30.7
spruce
Butternut 258 30.3
Buckeye
210 24.6
(yellow)
Basswood 209 24.5
Black
193 22.6
willow
When wood is used for columns, props, posts, and spokes, the weight of the
load tends to shorten the material endwise. This is endwise compression,
or compression parallel to the grain. In the case of long columns, that is,
pieces in which the length is very great compared with their diameter, the
failure is by sidewise bending or flexure, instead of by crushing or splitting.
(See Fig. 5.) A familiar instance of this action is afforded by a flexible
walking-stick. If downward pressure is exerted with the hand on the upper
end of the stick placed vertically on the floor, it will be noted that a definite
amount of force must be applied in each instance before decided flexure
takes place. After this point is reached a very slight increase of pressure

very largely increases the deflection, thus obtaining so great a leverage
about the middle section as to cause rupture.

Figure 5
Testing a buggy spoke in endwise
compression, illustrating the failure
by sidewise bending of a long
column fixed only at the lower end.
Photo by U. S. Forest Service

The lateral bending of a column produces a combination of bending with
compressive stress over the section, the compressive stress being maximum
at the section of greatest deflection on the concave side. The convex surface
is under tension, as in an ordinary beam test. (See Fig. 6.) If the same stick
is braced in such a way that flexure is prevented, its supporting strength is
increased enormously, since the compressive stress acts uniformly over the
section, and failure is by crushing or splitting, as in small blocks. In all
columns free to bend in any direction the deflection will be seen in the
direction in which the column is least stiff. This sidewise bending can be

overcome by making pillars and columns thicker in the middle than at the
ends, and by bracing studding, props, and compression members of trusses.
The strength of a column also depends to a considerable extent upon
whether the ends are free to turn or are fixed.

Figure 6
Unequal distribution of stress in a
long column due to lateral bending.

The complexity of the computations depends upon the way in which the
stress is applied and the manner in which the stick bends. Ordinarily where
the length of the test specimen is not greater than four diameters and the
ends are squarely faced (See Fig. 7.), the force acts uniformly over each
square inch of area and the crushing strength is equal to the maximum load
(P) divided by the area of the cross-section (A).

( C = ---P )

A

Figure 7
Endwise compression of a short
column.

It has been demonstrated4 that the ultimate strength in compression parallel
to the grain is very nearly the same as the extreme fibre stress at the elastic
limit in bending. (See Table 5.) In other words, the transverse strength of
beams at elastic limit is practically equal to the compressive strength of the
same material in short columns. It is accordingly possible to calculate the
approximate breaking strength of beams from the compressive strength of
short columns except when the wood is brittle. Since tests on endwise
compression are simpler, easier to make, and less expensive than transverse
bending tests, the importance of this relation is obvious, though it does not
do away with the necessity of making beam tests.

TABLE V
RELATION OF FIBRE STRESS AT ELASTIC
LIMIT (r) IN BENDING TO THE CRUSHING
STRENGTH (C) OF BLOCKS CUT THEREFROM,
IN POUNDS PER SQUARE INCH
(Forest Service Bul. 70, p. 90)
LONGLEAF PINE
Kiln-
Soaked Green 14 11.5 9.5 dry
MOISTURE
50 per 23 per per per per 6.2
CONDITION
cent cent cent cent cent per
cent
Number of
5 5 5 5 4 5
tests averaged
r in bending 4,920 5,944 6,924 7,8529,280 11,550
C in
4,668 5,100 6,466 7,4668,98510,910
compression
Per cent r is in
5.5 16.5 7.1 5.2 3.3 5.9
excess of C
SPRUCE
Kiln-
Green
Soaked 10 8.1 dry
MOISTURE 30
30 per per per 3.9
CONDITION per
cent cent cent per
cent
cent
Number of tests
5 4 5 3 4
averaged
r in bending 3,002 3,362 6,4588,40010,170
C in compression 2,680 3,025 6,1207,610 9,335
Per cent r is in excess
12.0 11.1 5.5 10.4 9.0
of C

When a short column is compressed until it breaks, the manner of failure
depends partly upon the anatomical structure and partly upon the degree of
humidity of the wood. The fibres (tracheids in conifers) act as hollow tubes

bound closely together, and in giving way they either (1) buckle, or (2)
bend.5
The first is typical of any dry thin-walled cells, as is usually the case in
seasoned white pine and spruce, and in the early wood of hard pines,
hemlock, and other species with decided contrast between the two portions
of the growth ring. As a rule buckling of a tracheid begins at the bordered
pits which form places of least resistance in the walls. In hardwoods such as
oak, chestnut, ash, etc., buckling occurs only in the thinnest-walled
elements, such as the vessels, and not in the true fibres.

According to Jaccard6 the folding of the cells is accompanied by
characteristic alterations of their walls which seem to split them into
extremely thin layers. When greatly magnified, these layers appear in
longitudinal sections as delicate threads without any definite arrangements,
while on cross section they appear as numerous concentric strata. This may
be explained on the ground that the growth of a fibre is by successive layers
which, under the influence of compression, are sheared apart. This is
particularly the case with thick-walled cells such as are found in late wood.

TABLE VI
RESULTS OF ENDWISE
COMPRESSION TESTS ON
SMALL CLEAR PIECES OF 40
WOODS IN GREEN CONDITION
(Forest Service Cir. 213)
Fibre
stress Modulus
Crushing
at of
strength
COMMON elastic elasticity
NAME OF limit
SPECIES Lbs.
per Lbs. per Lbs. per
sq. sq. inch sq. inch
inch
Hardwoods

Ash, white 3,510 4,220 1,531,000
Basswood 780 1,820 1,016,000
Beech 2,770 3,480 1,412,000
Birch,
2,570 3,400 1,915,000
yellow
Elm,
3,410 3,990 1,453,000
slippery
Hackberry 2,730 3,310 1,068,000
Hickory,
big 3,570 4,520 1,658,000
shellbark
bitternut 4,330 4,570 1,616,000
3,990 4,320 1,359,000
mockernut
nutmeg 3,620 3,980 1,411,000
pignut 3,520 4,820 1,980,000
shagbark 3,730 4,600 1,943,000
water 3,240 4,660 1,926,000
Locust,
4,300 4,970 1,536,000
honey
Maple,
3,040 3,670 1,463,000
sugar
Oak, post 2,780 3,330 1,062,000
red 2,290 3,210 1,295,000
swamp
3,470 4,360 1,489,000
white
white 2,400 3,520 946,000
yellow 2,870 3,700 1,465,000
Osage
3,980 5,810 1,331,000
orange
Sycamore 2,320 2,790 1,073,000
Tupelo 2,280 3,550 1,280,000
Conifers
Arborvitæ 1,420 1,990 754,000

Cedar,
2,710 3,030 868,000
incense
Cypress,
3,560 3,960 1,738,000
bald
Fir, alpine 1,660 2,060 882,000
amabilis 2,763 3,040 1,579,000
Douglas 2,390 2,920 1,440,000
white 2,610 2,800 1,332,000
Hemlock 2,110 2,750 1,054,000
Pine,
2,290 2,530 1,219,000
lodgepole
longleaf 3,420 4,280 1,890,000
red 2,470 3,080 1,646,000
sugar 2,340 2,600 1,029,000
western
2,100 2,420 1,271,000
yellow
white 2,370 2,720 1,318,000
Redwood 3,420 3,820 1,175,000
Spruce,
1,880 2,170 1,021,000
Engelmann
Tamarack 3,010 3,480 1,596,000

The second case, where the fibres bend with more or less regular curves
instead of buckling, is characteristic of any green or wet wood, and in dry
woods where the fibres are thick-walled. In woods in which the fibre walls
show all gradations of thickness—in other words, where the transition from
the thin-walled cells of the early wood to the thick-walled cells of the late
wood is gradual—the two kinds of failure, namely, buckling and bending,
grade into each other. In woods with very decided contrast between early
and late wood the two forms are usually distinct. Except in the case of
complete failure the cavity of the deformed cells remains open, and in
hardwoods this is true not only of the wood fibres but also of the tube-like
vessels. In many cases longitudinal splits occur which isolate bundles of
elements by greater or less intervals. The splitting occurs by a tearing of the
fibres or rays and not by the separation of the rays from the adjacent
elements.

Figure 8
Failures of short columns of green
spruce.

Figure 9
Failures of short columns of dry
chestnut.

Moisture in wood decreases the stiffness of the fibre walls and enlarges the
region of failure. The curve which the fibre walls make in the region of

failure is more gradual and also more irregular than in dry wood, and the
fibres are more likely to be separated.
In examining the lines of rupture in compression parallel to the grain it
appears that there does not exist any specific type, that is, one that is
characteristic of all woods. Test blocks taken from different parts of the
same log may show very decided differences in the manner of failure, while
blocks that are much alike in the size, number, and distribution of the
elements of unequal resistance may behave very similarly. The direction of
rupture is, according to Jaccard, not influenced by the distribution of the
medullary rays.7 These are curved with the bundles of fibres to which they
are attached. In any case the failure starts at the weakest points and follows
the lines of least resistance. The plane of failure, as visible on radial
surfaces, is horizontal, and on the tangential surface it is diagonal.

SHEARING STRENGTH

Whenever forces act upon a body in such a way that one portion tends to
slide upon another adjacent to it the action is called a shear.8 In wood this
shearing action may be (1) along the grain, or (2) across the grain. A
tenon breaking out its mortise is a familiar example of shear along the
grain, while the shoving off of the tenon itself would be shear across the
grain. The use of wood for pins or tree-nails involves resistance to shear
across the grain. Another common instance of the latter is where the steel
edge of the eye of an axe or hammer tends to cut off the handle. In Fig. 10
the action of the wooden strut tends to shear off along the grain the portion
AB of the wooden tie rod, and it is essential that the length of this portion be
great enough to guard against it. Fig. 11 shows characteristic failures in
shear along the grain.

Figure 10
Example of shear along the grain.

Figure 11
Failures of test specimens in shear
along the grain. In the block at the
left the surface of failure is radial; in
the one at the right, tangential.

TABLE VII
SHEARING STRENGTH
ALONG THE GRAIN OF
SMALL CLEAR PIECES
OF 41 WOODS IN GREEN
CONDITION
(Forest Service Cir. 213)
COMMON When When
NAME OF surface surface of
SPECIES of failure is
failure tangential

is
radial
Lbs.
Lbs. per
per sq.
sq. inch
inch
Hardwoods
Ash, black 876 832
white 1,360 1,312
Basswood 560 617
Beech 1,154 1,375
Birch,
1,103 1,188
yellow
Elm,
1,197 1,174
slippery
white 778 872
Hackberry 1,095 1,161
Hickory,
big 1,134 1,191
shellbark
bitternut 1,134 1,348
1,251 1,313
mockernut
nutmeg 1,010 1,053
pignut 1,334 1,457
shagbark 1,230 1,297
water 1,390 1,490
Locust,
1,885 2,096
honey
Maple, red 1,130 1,330
sugar 1,193 1,455
Oak, post 1,196 1,402
red 1,132 1,195
swamp
1,198 1,394
white
white 1,096 1,292

yellow 1,162 1,196
Sycamore 900 1,102
Tupelo 978 1,084
Conifers
Arborvitæ 617 614
Cedar,
613 662
incense
Cypress,
836 800
bald
Fir, alpine 573 654
amabilis 517 639
Douglas 853 858
white 742 723
Hemlock 790 813
Pine,
672 747
lodgepole
longleaf 1,060 953
red 812 741
sugar 702 714
western
686 706
yellow
white 649 639
Spruce,
607 624
Engelmann
Tamarack 883 843
Both shearing stresses may act at the same time. Thus the weight carried by
a beam tends to shear it off at right angles to the axis; this stress is equal to
the resultant force acting perpendicularly at any point, and in a beam
uniformly loaded and supported at either end is maximum at the points of
support and zero at the centre. In addition there is a shearing force tending
to move the fibres of the beam past each other in a longitudinal direction.
(See Fig. 12.) This longitudinal shear is maximum at the neutral plane and
decreases toward the upper and lower surfaces.

Figure 12
Horizontal shear in a beam.

Shearing across the grain is so closely related to compression at right angles
to the grain and to hardness that there is little to be gained by making
separate tests upon it. Knowledge of shear parallel to the grain is important,
since wood frequently fails in that way. The value of shearing stress parallel
to the grain is found by dividing the maximum load in pounds (P) by the
area of the cross section in inches (A).

P
( Shear = --- )
A

Oblique shearing stresses are developed in a bar when it is subjected to
direct tension or compression. The maximum shearing stress occurs along a
plane when it makes an angle of 45 degrees with the axis of the specimen.
In this case,

P
shear = -----.
2A

When the value of the angle θ is less than 45 degrees,

P

the shear along the plane = --- sin θ cos θ.
A

(See Fig. 13.) The effect of oblique shear is often visible in the failures of
short columns. (See Fig. 14.)

Figure 13
Oblique shear in a short column.

Figure 14
Failure of short column by oblique
shear.

TABLE VIII
SHEARING STRENGTH ACROSS THE
GRAIN OF VARIOUS AMERICAN
WOODS
(J.C. Trautwine. Jour. Franklin Institute. Vol.
109, 1880, pp. 105-106)
Lbs. Lbs.
KIND OF KIND OF
per sq. per sq.
WOOD WOOD
inch inch
Ash 6,280 Hickory 7,285
Beech 5,223 Locust 7,176
Birch 5,595 Maple 6,355

Cedar
1,372 Oak 4,425
(white)
Cedar
1,519 Oak (live) 8,480
(white)
Cedar
(Central 3,410 Pine (white ) 2,480
Amer.)
Pine (northern
Cherry 2,945 4,340
yellow)
Pine
Chestnut 1,536 5,735
(southernyellow)
Pine (very
Dogwood 6,510 5,053
resinous yellow)
Ebony 7,750 Poplar 4,418
Gum 5,890 Spruce 3,255
Hemlock 2,750 Walnut (black) 4,728
Hickory 6,045 Walnut (common) 2,830
NOTE.—Two specimens of each were
tested. All were fairly seasoned and without
defects. The piece sheared off was 5/8 in.
The single circular area of each pin was
0.322 sq. in.

TRANSVERSE OR BENDING STRENGTH: BEAMS

When external forces acting in the same plane are applied at right angles to
the axis of a bar so as to cause it to bend, they occasion a shortening of the
longitudinal fibres on the concave side and an elongation of those on the
convex side. Within the elastic limit the relative stretching and contraction
of the fibres is directly9] proportional to their distances from a plane
intermediate between them—the neutral plane. (N1P in Fig. 15.) Thus the
fibres half-way between the neutral plane and the outer surface experience
only half as much shortening or elongation as the outermost or extreme
fibres. Similarly for other distances. The elements along the neutral plane
experience no tension or compression in an axial direction. The line of

intersection of this plane and the plane of section is known as the neutral
axis (N A in Fig. 15.) of the section.

Figure 15
Diagram of a simple beam. N1P =
neutral plane, N A = neutral axis of
section R S.

If the bar is symmetrical and homogeneous the neutral plane is located half-
way between the upper and lower surfaces, so long as the deflection does
not exceed the elastic limit of the material. Owing to the fact that the tensile
strength of wood is from two to nearly four times the compressive strength,
it follows that at rupture the neutral plane is much nearer the convex than
the concave side of the bar or beam, since the sum of all the compressive
stresses on the concave portion must always equal the sum of the tensile
stresses on the convex portion. The neutral plane begins to change from its
central position as soon as the elastic limit has been passed. Its location at
any time is very uncertain.
The external forces acting to bend the bar also tend to rupture it at right
angles to the neutral plane by causing one transverse section to slip past
another. This stress at any point is equal to the resultant perpendicular to the
axis of the forces acting at this point, and is termed the transverse shear
(or in the case of beams, vertical shear).
In addition to this there is a shearing stress, tending to move the fibres past
one another in an axial direction, which is called longitudinal shear (or in
the case of beams, horizontal shear). This stress must be taken into

consideration in the design of timber structures. It is maximum at the
neutral plane and decreases to zero at the outer elements of the section. The
shorter the span of a beam in proportion to its height, the greater is the
liability of failure in horizontal shear before the ultimate strength of the
beam is reached.

Beams

There are three common forms of beams, as follows:
(1) Simple beam—a bar resting upon two supports, one near each end. (See
Fig. 16, No. 1.)
(2) Cantilever beam—a bar resting upon one support or fulcrum, or that
portion of any beam projecting out of a wall or beyond a support. (See Fig.
16, No. 2.)
(3) Continuous beam—a bar resting upon more than two supports. (See
Fig. 16, No. 3.)

Figure 16
Three common forms of beams. 1.
Simple. 2. Cantilever. 3. Continuous.

Stiffness of Beams

The two main requirements of a beam are stiffness and strength. The
formulæ for the modulus of elasticity (E) or measure of stiffness of a
rectangular prismatic simple beam loaded at the centre and resting freely on
supports at either end is:10

P' l3
E= -----------
4 D b h3

b = breadth or width of beam, inches.
h = height or depth of beam, inches.
l = span (length between points of supports) of beam, inches.
D = deflection produced by load P', inches.
P' = load at or below elastic limit, pounds.

From this formulæ it is evident that for rectangular beams of the same
material, mode of support, and loading, the deflection is affected as follows:
(1) It is inversely proportional to the width for beams of the same length
and depth. If the width is tripled the deflection is one-third as great.
(2) It is inversely proportional to the cube of the depth for beams of the
same length and breadth. If the depth is tripled the deflection is one twenty-
seventh as great.
(3) It is directly proportional to the cube of the span for beams of the same
breadth and depth. Tripling the span gives twenty-seven times the
deflection.

The number of pounds which concentrated at the centre will deflect a
rectangular prismatic simple beam one inch may be found from the
preceding formulæ by substituting D = 1" and solving for P'. The formulæ
then becomes:

4 E b h3
Necessary weight (P') = ----------
l3

In this case the values for E are read from tables prepared from data
obtained by experimentation on the given material.

Strength of Beams

The measure of the breaking strength of a beam is expressed in terms of
unit stress by a modulus of rupture, which is a purely hypothetical
expression for points beyond the elastic limit. The formulæ used in
computing this modulus is as follows:

1.5 P l
R = ---------
b h2

b, h, l = breadth, height, and span, respectively, as in preceding formulæ.
R = modulus of rupture, pounds per square inch.
P = maximum load, pounds.

In calculating the fibre stress at the elastic limit the same formulæ is used
except that the load at elastic limit (P1) is substituted for the maximum load
(P).

From this formulæ it is evident that for rectangular prismatic beams of the
same material, mode of support, and loading, the load which a given beam
can support varies as follows:
(1) It is directly proportional to the breadth for beams of the same length
and depth, as is the case with stiffness.
(2) It is directly proportional to the square of the height for beams of the
same length and breadth, instead of as the cube of this dimension as in
stiffness.
(3) It is inversely proportional to the span for beams of the same breadth
and depth and not to the cube of this dimension as in stiffness.
The fact that the strength varies as the square of the height and the stiffness
as the cube explains the relationship of bending to thickness. Were the law
the same for strength and stiffness a thin piece of material such as a sheet of
paper could not be bent any further without breaking than a thick piece, say
an inch board.

TABLE IX
RESULTS OF STATIC BENDING TESTS ON SMALL
CLEAR BEAMS OF 49 WOODS IN GREEN
CONDITION
(Forest Service Cir. 213)
Fibre Work in Bending
stress Modulus Modulus
at of of To To
elastic rupture elasticity elastic maximum Total
COMMON limit load
limit
NAME OF
In.- In.-
SPECIES
Lbs. lbs. In.-lbs. lbs.
Lbs. per Lbs. per
per per per cu. per
sq. in. sq. in.
sq. in. cu. inch cu.
inch inch
Hardwoods
Ash, black 2,580 6,000 960,000 0.41 13.1 38.9

white 5,180 9,920 1,416,000 1.10 20.0 43.7
Basswood 2,480 4,450 842,000 .45 5.8 8.9
Beech 4,490 8,610 1,353,000 .96 14.1 31.4
Birch,
4,190 8,390 1,597,000 .62 14.2 31.5
yellow
Elm, rock 4,290 9,430 1,222,000 .90 19.4 47.4
slippery 5,560 9,510 1,314,000 1.32 11.7 44.2
white 2,850 6,940 1,052,000 .44 11.8 27.4
Gum, red 3,460 6,450 1,138,000
Hackberry 3,320 7,800 1,170,000 .56 19.6 52.9
Hickory,
big 6,370 11,110 1,562,000 1.47 24.3 78.0
shellbark
bitternut 5,470 10,280 1,399,000 1.22 20.0 75.5
6,550 11,110 1,508,000 1.50 31.7 84.4
mockernut
nutmeg 4,860 9,060 1,289,000 1.06 22.8 58.2
pignut 5,860 11,810 1,769,000 1.12 30.6 86.7
shagbark 6,120 11,000 1,752,000 1.22 18.3 72.3
water 5,980 10,740 1,563,000 1.29 18.8 52.9
Locust,
6,020 12,360 1,732,000 1.28 17.3 64.4
honey
Maple, red 4,450 8,310 1,445,000 .78 9.8 17.1
sugar 4,630 8,860 1,462,000 .88 12.7 32.0
Oak, post 4,720 7,380 913,000 1.39 9.1 17.4
red 3,490 7,780 1,268,000 .60 11.4 26.0
swamp
5,380 9,860 1,593,000 1.05 14.5 37.6
white
tanbark 6,580 10,710 1,678,000 1.49
white 4,320 8,090 1,137,000 .95 12.1 36.7
yellow 5,060 8,570 1,219,000 1.20 11.7 30.7
Osage
7,760 13,660 1,329,000 2.53 37.9 101.7
orange
Sycamore 2,820 6,300 961,000 .51 7.1 13.6
Tupelo 4,300 7,380 1,045,000 1.00 7.8 20.9

Conifers
Arborvitæ 2,600 4,250 643,000 .60 5.7 9.5
Cedar,
3,950 6,040 754,000
incense
Cypress,
4,430 7,110 1,378,000 .96 5.1 15.4
bald
Fir, alpine 2,366 4,450 861,000 .66 4.4 7.4
amabilis 4,060 6,570 1,323,000
Douglas 3,570 6,340 1,242,000 .59 6.6 13.6
white 3,880 5,970 1,131,000 .77 5.2 14.9
Hemlock 3,410 5,770 917,000 .73 6.6 12.9
Pine,
3,080 5,130 1,015,000 .54 5.1 7.4
lodgepole
longleaf 5,090 8,630 1,662,000 .88 8.1 34.8
red 3,740 6,430 1,384,000 .59 5.8 28.0
shortleaf 4,360 7,710 1,395,000
sugar 3,330 5,270 966,000 .66 5.0 11.6
west,
3,180 5,180 1,111,000 .52 4.3 15.6
yellow
White 3,410 5,310 1,073,000 .62 5.9 13.3
Redwood 4,530 6,560 1,024,000
Spruce,
2,740 4,550 866,000 .50 4.8 6.1
Engelmann
red 3,440 5,820 1,143,000 .62 6.0
white 3,160 5,200 968,000 .58 6.6
Tamarack 4,200 7,170 1,236,000 .84 7.2 30.0

Kinds of Loads

There are various ways in which beams are loaded, of which the following
are the most important:
(1) Uniform load occurs where the load is spread evenly over the beam.
(2) Concentrated load occurs where the load is applied at single point or
points.

(3) Live or immediate load is one of momentary or short duration at any
one point, such as occurs in crossing a bridge.
(4) Dead or permanent load is one of constant and indeterminate duration,
as books on a shelf. In the case of a bridge the weight of the structure itself
is the dead load. All large beams support a uniform dead load consisting of
their own weight.
The effect of dead load on a wooden beam may be two or more times that
produced by an immediate load of the same weight. Loads greater than the
elastic limit are unsafe and will generally result in rupture if continued long
enough. A beam may be considered safe under permanent load when the
deflections diminish during equal successive periods of time. A continual
increase in deflection indicates an unsafe load which is almost certain to
rupture the beam eventually.
Variations in the humidity of the surrounding air influence the deflection of
dry wood under dead load, and increased deflections during damp weather
are cumulative and not recovered by subsequent drying. In the case of
longleaf pine, dry beams may with safety be loaded permanently to within
three-fourths of their elastic limit as determined from ordinary static tests.
Increased moisture content, due to greater humidity of the air, lowers the
elastic limit of wood so that what was a safe load for the dry material may
become unsafe.
When a dead load not great enough to rupture a beam has been removed,
the beam tends gradually to recover its former shape, but the recovery is not
always complete. If specimens from such a beam are tested in the ordinary
testing machine it will be found that the application of the dead load did not
affect the stiffness, ultimate strength, or elastic limit of the material. In
other words, the deflections and recoveries produced by live loads are the
same as would have been produced had not the beam previously been
subjected to a dead load.11
Maximum load is the greatest load a material will support and is usually
greater than the load at rupture.
Safe load is the load considered safe for a material to support in actual
practice. It is always less than the load at elastic limit and is usually taken

as a certain proportion of the ultimate or breaking load.
The ratio of the breaking to the safe load is called the factor of safety.

ultimate strength
( Factor of safety = ------------------- )
safe load

In order to make due allowance for the natural variations and imperfections
in wood and in the aggregate structure, as well as for variations in the load,
the factor of safety is usually as high as 6 or 10, especially if the safety of
human life depends upon the structure. This means that only from one-sixth
to one-tenth of the computed strength values is considered safe to use. If the
depth of timbers exceeds four times their thickness there is a great tendency
for the material to twist when loaded. It is to overcome this tendency that
floor joists are braced at frequent intervals. Short deep pieces shear out or
split before their strength in bending can fully come into play.

Application of Loads

There are three12 general methods in which loads may be applied to beams,
namely:
(1) Static loading or the gradual imposition of load so that the moving parts
acquire no appreciable momentum. Loads are so applied in the ordinary
testing machine.
(2) Sudden imposition of load without initial velocity. "Thus in the case
of placing a load on a beam, if the load be brought into contact with the
beam, but its weight sustained by external means, as by a cord, and then this
external support be suddenly (instantaneously) removed, as by quickly
cutting the cord, then, although the load is already touching the beam (and
hence there is no real impact), yet the beam is at first offering no resistance,
as it has yet suffered no deformation. Furthermore, as the beam deflects the
resistance increases, but does not come to be equal to the load until it has
attained its normal deflection. In the meantime there has been an

unbalanced force of gravity acting, of a constantly diminishing amount,
equal at first to the entire load, at the normal deflection. But at this instant
the load and the beam are in motion, the hitherto unbalanced force having
produced an accelerated velocity, and this velocity of the weight and beam
gives to them an energy, or vis viva, which must now spend itself in
overcoming an excess of resistance over and above the imposed load, and
the whole mass will not stop until the deflection (as well as the resistance)
has come to be equal to twice that corresponding to the static load imposed.
Hence we say the effect of a suddenly imposed load is to produce twice the
deflection and stress of the same load statically applied. It must be evident,
however, that this case has nothing in common with either the ordinary
'static' tests of structural materials in testing-machines, or with impact
tests."13

(3) Impact, shock, or blow.14 There are various common uses of wood
where the material is subjected to sudden shocks and jars or impact. Such is
the action on the felloes and spokes of a wagon wheel passing over a rough
road; on a hammer handle when a blow is struck; on a maul when it strikes
a wedge.
Resistance to impact is resistance to energy which is measured by the
product of the force into the space through which it moves, or by the
product of one-half the moving mass which causes the shock into the square
of its velocity. The work done upon the piece at the instant the velocity is
entirely removed from the striking body is equal to the total energy of that
body. It is impossible, however, to get all of the energy of the striking body
stored in the specimen, though the greater the mass and the shorter the
space through which it moves, or, in other words, the greater the proportion
of weight and the smaller the proportion of velocity making up the energy
of the striking body, the more energy the specimen will absorb. The rest is
lost in friction, vibrations, heat, and motion of the anvil.
In impact the stresses produced become very complex and difficult to
measure, especially if the velocity is high, or the mass of the beam itself is
large compared to that of the weight.
The difficulties attending the measurement of the stresses beyond the elastic
limit are so great that commonly they are not reckoned. Within the elastic

limit the formulæ for calculating the stresses are based on the assumption
that the deflection is proportional to the stress in this case as in static tests.
A common method of making tests upon the resistance of wood to shock is
to support a small beam at the ends and drop a heavy weight upon it in the
middle. (See Fig. 40.) The height of the weight is increased after each drop
and records of the deflection taken until failure. The total work done upon
the specimen is equal to the area of the stress-strain diagram plus the effect
of local inertia of the molecules at point of contact.
The stresses involved in impact are complicated by the fact that there are
various ways in which the energy of the striking body may be spent:
(a) It produces a local deformation of both bodies at the surface of contact,
within or beyond the elastic limit. In testing wood the compression of the
substance of the steel striking-weight may be neglected, since the steel is
very hard in comparison with the wood. In addition to the compression of
the fibres at the surface of contact resistance is also offered by the inertia of
the particles there, the combined effect of which is a stress at the surface of
contact often entirely out of proportion to the compression which would
result from the action of a static force of the same magnitude. It frequently
exceeds the crushing strength at the extreme surface of contact, as in the
case of the swaging action of a hammer on the head of an iron spike, or of a
locomotive wheel on the steel rail. This is also the case when a bullet is shot
through a board or a pane of glass without breaking it as a whole.
(b) It may move the struck body as a whole with an accelerated velocity, the
resistance consisting of the inertia of the body. This effect is seen when a
croquet ball is struck with a mallet.
(c) It may deform a fixed body against its external supports and resistances.
In making impact tests in the laboratory the test specimen is in reality in the
nature of a cushion between two impacting bodies, namely, the striking
weight and the base of the machine. It is important that the mass of this
base be sufficiently great that its relative velocity to that of the common
centre of gravity of itself and the striking weight may be disregarded.
(d) It may deform the struck body as a whole against the resisting stresses
developed by its own inertia, as, for example, when a baseball bat is broken

by striking the ball.

TABLE X
RESULTS OF IMPACT BENDING
TESTS ON SMALL CLEAR
BEAMS OF 34 WOODS IN
GREEN CONDITION
(Forest Service Cir. 213)
Work
Fibre
in
stress Modulus
bending
at of
COMMON to
elastic elasticity
NAME OF elastic
limit
SPECIES limit
Lbs. In.-lbs.
Lbs. per
per sq. per cu.
sq. in.
in. inch
Hardwoods
Ash, black 7,840 955,000 3.69
white 11,710 1,564,000 4.93
Basswood 5,480 917,000 1.84
Beech 11,760 1,501,000 5.10
Birch,
11,080 1,812,000 3.79
yellow
Elm, rock 12,0901,367,000 6.52
slippery 11,700 1,569,000 4.86
white 9,910 1,138,000 4.82
Hackberry 10,4201,398,000 4.48
Locust,
13,460 2,114,000 4.76
honey
Maple, red 11,670 1,411,000 5.45
sugar 11,680 1,680,000 4.55
Oak, post 11,260 1,596,000 4.41
red 10,5801,506,000 4.16

swamp
13,2802,048,000 4.79
white
white 9,860 1,414,000 3.84
yellow 10,8401,479,000 4.44
Osage
15,5201,498,000 8.92
orange
Sycamore 8,180 1,165,000 3.22
Tupelo 7,650 1,310,000 2.49
Conifers
Arborvitæ 5,290 778,000 2.04
Cypress,
8,290 1,431,000 2.71
bald
Fir, alpine 5,280 980,000 1.59
Douglas 8,870 1,579,000 2.79
white 7,230 1,326,000 2.21
Hemlock 6,330 1,025,000 2.19
Pine,
6,870 1,142,000 2.31
lodgepole
longleaf 9,680 1,739,000 3.02
red 7,480 1,438,000 2.18
sugar 6,740 1,083,000 2.34
western
7,070 1,115,000 2.51
yellow
white 6,490 1,156,000 2.06
Spruce,
6,300 1,076,000 2.09
Engelmann
Tamarack 7,750 1,263,000 2.67

Impact testing is difficult to conduct satisfactorily and the data obtained are
of chief value in a relative sense, that is, for comparing the shock-resisting
ability of woods of which like specimens have been subjected to exactly
identical treatment. Yet this test is one of the most important made on wood,
as it brings out properties not evident from other tests. Defects and
brittleness are revealed by impact better than by any other kind of test. In
common practice nearly all external stresses are of the nature of impact. In
fact, no two moving bodies can come together without impact stress. Impact

is therefore the commonest form of applied stress, although the most
difficult to measure.

Failures in Timber Beams

If a beam is loaded too heavily it will break or fail in some characteristic
manner. These failures may be classified according to the way in which
they develop, as tension, compression, and horizontal shear; and according
to the appearance of the broken surface, as brash, and fibrous. A number of
forms may develop if the beam is completely ruptured.
Since the tensile strength of wood is on the average about three times as
great as the compressive strength, a beam should, therefore, be expected to
fail by the formation in the first place of a fold on the compression side due
to the crushing action, followed by failure on the tension side. This is
usually the case in green or moist wood. In dry material the first visible
failure is not infrequently on the lower or tension side, and various attempts
have been made to explain why such is the case.15
Within the elastic limit the elongations and shortenings are equal, and the
neutral plane lies in the middle of the beam. (See page 23.) Later the top
layer of fibres on the upper or compression side fail, and on the load
increasing, the next layer of fibres fail, and so on, even though this failure
may not be visible. As a result the shortenings on the upper side of the beam
become considerably greater than the elongations on the lower side. The
neutral plane must be presumed to sink gradually toward the tension side,
and when the stresses on the outer fibres at the bottom have become
sufficiently great, the fibres are pulled in two, the tension area being much
smaller than the compression area. The rupture is often irregular, as in
direct tension tests. Failure may occur partially in single bundles of fibres
some time before the final failure takes place. One reason why the failure of
a dry beam is different from one that is moist, is that drying increases the
stiffness of the fibres so that they offer more resistance to crushing, while it
has much less effect upon the tensile strength.
There is considerable variation in tension failures depending upon the
toughness or the brittleness of the wood, the arrangement of the grain,

defects, etc., making further classification desirable. The four most common
forms are:
(1) Simple tension, in which there is a direct pulling in two of the wood on
the under side of the beam due to a tensile stress parallel to the grain, (See
Fig. 17, No. 1.) This is common in straight-grained beams, particularly
when the wood is seasoned.
(2) Cross-grained tension, in which the fracture is caused by a tensile
force acting oblique to the grain. (See Fig. 17, No. 2.) This is a common
form of failure where the beam has diagonal, spiral or other form of cross
grain on its lower side. Since the tensile strength of wood across the grain is
only a small fraction of that with the grain it is easy to see why a cross-
grained timber would fail in this manner.
(3) Splintering tension, in which the failure consists of a considerable
number of slight tension failures, producing a ragged or splintery break on
the under surface of the beam. (See Fig. 17, No. 3.) This is common in
tough woods. In this case the surface of fracture is fibrous.
(4) Brittle tension, in which the beam fails by a clean break extending
entirely through it. (See Fig. 17, No. 4.) It is characteristic of a brittle wood
which gives way suddenly without warning, like a piece of chalk. In this
case the surface of fracture is described as brash.
Compression failure (see Fig. 17, No. 5) has few variations except that it
appears at various distances from the neutral plane of the beam. It is very
common in green timbers. The compressive stress parallel to the fibres
causes them to buckle or bend as in an endwise compressive test. This
action usually begins on the top side shortly after the elastic limit is reached
and extends downward, sometimes almost reaching the neutral plane before
complete failure occurs. Frequently two or more failures develop at about
the same time.

Figure 17
Characteristic failures of simple
beams.

Horizontal shear failure, in which the upper and lower portions of the
beam slide along each other for a portion of their length either at one or at
both ends (see Fig. 17, No. 6), is fairly common in air-dry material and in
green material when the ratio of the height of the beam to the span is
relatively large. It is not common in small clear specimens. It is often due to
shake or season checks, common in large timbers, which reduce the actual
area resisting the shearing action considerably below the calculated area
used in the formulæ for horizontal shear. (See page 98 for this formulæ.)
For this reason it is unsafe, in designing large timber beams, to use shearing
stresses higher than those calculated for beams that failed in horizontal
shear. The effect of a failure in horizontal shear is to divide the beam into
two or more beams the combined strength of which is much less than that
of the original beam. Fig. 18 shows a large beam in which two failures in
horizontal shear occurred at the same end. That the parts behave

independently is shown by the compression failure below the original
location of the neutral plane.

Figure 18
Failure of a large beam by horizontal
shear. Photo by U. S, Forest Service.

Table XI gives an analysis of the causes of first failure in 840 large timber
beams of nine different species of conifers. Of the total number tested 165
were air-seasoned, the remainder green. The failure occurring first signifies
the point of greatest weakness in the specimen under the particular
conditions of loading employed (in this case, third-point static loading).

TABLE XI
MANNER OF FIRST FAILURE OF LARGE
BEAMS
(Forest Service Bul. 108, p. 56)

Total
COMMON Per cent of total failing by
numbe
NAME OF
r of
SPECIES Tension Compression Shear
tests
Longleaf
pine:
green 17 18 24 58
dry 9 22 22 56
Douglas
fir:
green 191 27 72 1
dry 91 19 76 5
Shortleaf
pine:
green 48 27 56 17
dry 13 54 46
Western
larch:
green 62 23 71 6
dry 52 54 19 27
Loblolly
pine:
green 111 40 53 7
dry 25 60 12 28
Tamarack:
green 30 37 53 10
dry 9 45 22 33
Western
hemlock:
green 39 21 74 5
dry 44 11 66 23
Redwood:
green 28 43 50 7
dry 12 83 17

Norway
pine:
green 49 18 76 6
dry 10 30 60 10
NOTE.—These tests were made on timbers
ranging in cross section from 4" × 10" to 8" ×
16", and with a span of 15 feet.

TOUGHNESS: TORSION

Toughness is a term applied to more than one property of wood. Thus wood that is
difficult to split is said to be tough. Again, a tough wood is one that will not rupture
until it has deformed considerably under loads at or near its maximum strength, or
one which still hangs together after it has been ruptured and may be bent back and
forth without breaking apart. Toughness includes flexibility and is the reverse of
brittleness, in that tough woods break gradually and give warning of failure. Tough
woods offer great resistance to impact and will permit rougher treatment in
manipulations attending manufacture and use. Toughness is dependent upon the
strength, cohesion, quality, length, and arrangement of fibre, and the pliability of the
wood. Coniferous woods as a rule are not as tough as hardwoods, of which hickory
and elm are the best examples.

Figure 19
Torsion of a shaft.

The torsion or twisting test is useful in determining the toughness of wood. If the
ends of a shaft are turned in opposite directions, or one end is turned and the other is
fixed, all of the fibres except those at the axis tend to assume the form of helices.
(See Fig. 19.) The strain produced by torsion or twisting is essentially shear
transverse and parallel to the fibres, combined with longitudinal tension and
transverse compression. Within the elastic limit the strains increase directly as the
distance from the axis of the specimen. The outer elements are subjected to tensile
stresses, and as they become twisted tend to compress those near the axis. The
elongated elements also contract laterally. Cross sections which were originally
plane become warped. With increasing strain the lateral adhesion of the outer fibres
is destroyed, allowing them to slide past each other, and reducing greatly their power

of resistance. In this way the strains on the fibres nearer the axis are progressively
increased until finally all of the elements are sheared apart. It is only in the toughest
materials that the full effect of this action can be observed. (See Fig. 20.) Brittle
woods snap off suddenly with only a small amount of torsion, and their fracture is
irregular and oblique to the axis of the piece instead of frayed out and more nearly
perpendicular to the axis as is the case with tough woods.

Figure 20
Effect of torsion on different grades of
hickory. Photo by U. S. Forest Service.

HARDNESS

The term hardness is used in two senses, namely: (1) resistance to indentation, and
(2) resistance to abrasion or scratching. In the latter sense hardness combined with
toughness is a measure of the wearing ability of wood and is an important
consideration in the use of wood for floors, paving blocks, bearings, and rollers.
While resistance to indentation is dependent mostly upon the density of the wood,
the wearing qualities may be governed by other factors such as toughness, and the
size, cohesion, and arrangement of the fibres. In use for floors, some woods tend to
compact and wear smooth, while others become splintery and rough. This feature is

affected to some extent by the manner in which the wood is sawed; thus edge-grain
pine flooring is much better than flat-sawn for uniformity of wear.

TABLE XII
HARDNESS OF 32 WOODS IN GREEN
CONDITION, AS INDICATED BY THE
LOAD REQUIRED TO IMBED A 0.444-INCH
STEEL BALL TO ONE-HALF ITS
DIAMETER
(Forest Service Cir. 213)
COMMON End Radial Tangential
Average
NAME OF surface surface surface
SPECIES Pounds Pounds Pounds Pounds
Hardwoods
1 Osage
1,971 1,838 2,312 1,762
orange
2 Honey
1,851 1,862 1,860 1,832
locust
3 Swamp
1,174 1,205 1,217 1,099
white oak
4 White oak 1,164 1,183 1,163 1,147
5 Post oak 1,099 1,139 1,068 1,081
6 Black oak 1,069 1,093 1,083 1,031
7 Red oak 1,043 1,107 1,020 1,002
8 White ash 1,046 1,121 1,000 1,017
9 Beech 942 1,012 897 918
10 Sugar
937 992 918 901
maple
11 Rock elm 910 954 883 893
12
799 829 795 773
Hackberry
13 Slippery
788 919 757 687
elm
14 Yellow
778 827 768 739
birch
15 Tupelo 738 814 666 733
16 Red
671 766 621 626
maple

17
608 664 560 599
Sycamore
18 Black ash 551 565 542 546
19 White
496 536 456 497
elm
20
239 273 226 217
Basswood
Conifers
1 Longleaf
532 574 502 521
pine
2 Douglas
410 415 399 416
fir
3 Bald
390 460 355 354
cypress
4 Hemlock 384 463 354 334
5 Tamarack 384 401 380 370
6 Red pine 347 355 345 340
7 White fir 346 381 322 334
8 Western
328 334 307 342
yellow pine
9 Lodgepole
318 316 318 319
pine
10 White
299 304 294 299
pine
11
Engelmann 266 272 253 274
pine
12 Alpine fir 241 284 203 235
NOTE.—Black locust and hickory are not
included in this table, but their position would be
near the head of the list.

Tests for either form of hardness are of comparative value only. Tests for indentation
are commonly made by penetrations of the material with a steel punch or ball.16 Tests
for abrasion are made by wearing down wood with sandpaper or by means of a sand
blast.

CLEAVABILITY

Cleavability is the term used to denote the facility with which wood is split. A
splitting stress is one in which the forces act normally like a wedge. (See Fig. 21.)
The plane of cleavage is parallel to the grain, either radially or tangentially.

Figure 21
Cleavage of highly elastic wood. The cleft
runs far ahead of the wedge.

This property of wood is very important in certain uses such as firewood, fence rails,
billets, and squares. Resistance to splitting or low cleavability is desirable where
wood must hold nails or screws, as in box-making. Wood usually splits more readily
along the radius than parallel to the growth rings though exceptions occur, as in the
case of cross grain.
Splitting involves transverse tension, but only a portion of the fibres are under stress
at a time. A wood of little stiffness and strong cohesion across the grain is difficult to
split, while one with great stiffness, such as longleaf pine, is easily split. The form of
the grain and the presence of knots greatly affect this quality.

TABLE XIII
CLEAVAGE STRENGTH OF SMALL
CLEAR PIECES OF 32 WOODS IN GREEN

CONDITION
(Forest Service Cir. 213)
When
When surface of
surface of
COMMON failure is
failure is
NAME OF tangential
radial
SPECIES
Lbs. per sq.
Lbs. per sq. inch
inch
Hardwoods
Ash, black 275 260
white 333 346
Bashwood 130 168
Beech 339 527
Birch, yellow 294 287
Elm, slippery 401 424
white 210 270
Hackberr 422 436
Locust, honey 552 610
Maple, red 297 330
sugar 376 513
Oak, post 354 487
red 380 470
swamp white 428 536
white 382 457
yellow 379 470
Sycamore 265 425
Tupelo 277 380
Conifers
Arborvitæ 148 139
Cypress, bald 167 154
Fir, alpine 130 133
Douglas 139 127
white 145 187
Hemlock 168 151
Pine, lodgepole 142 140
longleaf 187 180
red 161 154
sugar 168 189

western yellow 162 187
white 144 160
Spruce,
110 135
Engelmann
Tamarack 167 159

PART II
FACTORS AFFECTING THE MECHANICAL
PROPERTIES OF WOOD

INTRODUCTION

Wood is an organic product—a structure of infinite variation of detail and design.17 It
is on this account that no two woods are alike—in reality no two specimens from the
same log are identical. There are certain properties that characterize each species, but
they are subject to considerable variation. Oak, for example, is considered hard,
heavy, and strong, but some pieces, even of the same species of oak, are much
harder, heavier, and stronger than others. With hickory are associated the properties
of great strength, toughness, and resilience, but some pieces are comparatively weak
and brash and ill-suited for the exacting demands for which good hickory is
peculiarly adapted.
It follows that no definite value can be assigned to the properties of any wood and
that tables giving average results of tests may not be directly applicable to any
individual stick. With sufficient knowledge of the intrinsic factors affecting the
results it becomes possible to infer from the appearance of material its probable
variation from the average. As yet too little is known of the relation of structure and
chemical composition to the mechanical and physical properties to permit more than
general conclusions.

RATE OF GROWTH

To understand the effect of variations in the rate of growth it is first necessary to
know how wood is formed. A tree increases in diameter by the formation, between
the old wood and the inner bark, of new woody layers which envelop the entire stem,
living branches, and roots. Under ordinary conditions one layer is formed each year
and in cross section as on the end of a log they appear as rings—often spoken of as
annual rings. These growth layers are made up of wood cells of various kinds, but
for the most part fibrous. In timbers like pine, spruce, hemlock, and other coniferous

or softwood species the wood cells are mostly of one kind, and as a result the
material is much more uniform in structure than that of most hardwoods. (See
Frontispiece.) There are no vessels or pores in coniferous wood such as one sees so
prominently in oak and ash, for example. (See Fig. 22.)

Figure 22
Cross sections of a ring-porous hardwood
(white ash), a diffuse-porous hardwood
(red gum), and a non-porous or coniferous
wood (eastern hemlock). × 30.
Photomicrographs by the author.

The structure of the hardwoods is more complex. They are more or less filled with
vessels, in some cases (oak, chestnut, ash) quite large and distinct, in others
(buckeye, poplar, gum) too small to be seen plainly without a small hand lens. In
discussing such woods it is customary to divide them into two large classes—ring-
porous and diffuse-porous. (See Fig. 22.) In ring-porous species, such as oak,
chestnut, ash, black locust, catalpa, mulberry, hickory, and elm, the larger vessels or
pores (as cross sections of vessels are called) become localized in one part of the
growth ring, thus forming a region of more or less open and porous tissue. The rest
of the ring is made up of smaller vessels and a much greater proportion of wood
fibres. These fibres are the elements which give strength and toughness to wood,
while the vessels are a source of weakness.

In diffuse-porous woods the pores are scattered throughout the growth ring instead
of being collected in a band or row. Examples of this kind of wood are gum, yellow
poplar, birch, maple, cottonwood, basswood, buckeye, and willow. Some species,
such as walnut and cherry, are on the border between the two classes, forming a sort
of intermediate group.
If one examines the smoothly cut end of a stick of almost any kind of wood, he will
note that each growth ring is made up of two more or less well-defined parts. That
originally nearest the centre of the tree is more open textured and almost invariably
lighter in color than that near the outer portion of the ring. The inner portion was
formed early in the season, when growth was comparatively rapid and is known as
early wood (also spring wood); the outer portion is the late wood, being produced in
the summer or early fall. In soft pines there is not much contrast in the different parts
of the ring, and as a result the wood is very uniform in texture and is easy to work. In
hard pine, on the other hand, the late wood is very dense and is deep-colored,
presenting a very decided contrast to the soft, straw-colored early wood. (See Fig.
23.) In ring-porous woods each season's growth is always well defined, because the
large pores of the spring abut on the denser tissue of the fall before. In the diffuse-
porous, the demarcation between rings is not always so clear and in not a few cases
is almost, if not entirely, invisible to the unaided eye. (See Fig. 22.)

Figure 23
Cross section of longleaf pine showing
several growth rings with variations in the

width of the dark-colored late wood.
Seven resin ducts are visible. × 33.
Photomicrograph by U.S. Forest Service.

If one compares a heavy piece of pine with a light specimen it will be seen at once
that the heavier one contains a larger proportion of late wood than the other, and is
therefore considerably darker. The late wood of all species is denser than that formed
early in the season, hence the greater the proportion of late wood the greater the
density and strength. When examined under a microscope the cells of the late wood
are seen to be very thick-walled and with very small cavities, while those formed
first in the season have thin walls and large cavities. The strength is in the walls, not
the cavities. In choosing a piece of pine where strength or stiffness is the important
consideration, the principal thing to observe is the comparative amounts of early and
late wood. The width of ring, that is, the number per inch, is not nearly so important
as the proportion of the late wood in the ring.
It is not only the proportion of late wood, but also its quality, that counts. In
specimens that show a very large proportion of late wood it may be noticeably more
porous and weigh considerably less than the late wood in pieces that contain but
little. One can judge comparative density, and therefore to some extent weight and
strength, by visual inspection.
The conclusions of the U.S. Forest Service regarding the effect of rate of growth on
the properties of Douglas fir are summarized as follows:
"1. In general, rapidly grown wood (less than eight rings per inch) is relatively weak.
A study of the individual tests upon which the average points are based shows,
however, that when it is not associated with light weight and a small proportion of
summer wood, rapid growth is not indicative of weak wood.
"2. An average rate of growth, indicated by from 12 to 16 rings per inch, seems to
produce the best material.
"3. In rates of growths lower than 16 rings per inch, the average strength of the
material decreases, apparently approaching a uniform condition above 24 rings per
inch. In such slow rates of growth the texture of the wood is very uniform, and
naturally there is little variation in weight or strength.
"An analysis of tests on large beams was made to ascertain if average rate of growth
has any relation to the mechanical properties of the beams. The analysis indicated
conclusively that there was no such relation. Average rate of growth [without
consideration also of density], therefore, has little significance in grading structural

timber."18 This is because of the wide variation in the percentage of late wood in
different parts of the cross section.
Experiments seem to indicate that for most species there is a rate of growth which, in
general, is associated with the greatest strength, especially in small specimens. For
eight conifers it is as follows:19

Rings per inch
Douglas fir 24
Shortleaf pine 12
Loblolly pine 6
Western larch 18
Western hemlock 14
Tamarack 20
Norway pine 18
Redwood 30

No satisfactory explanation can as yet be given for the real causes underlying the
formation of early and late wood. Several factors may be involved. In conifers, at
least, rate of growth alone does not determine the proportion of the two portions of
the ring, for in some cases the wood of slow growth is very hard and heavy, while in
others the opposite is true. The quality of the site where the tree grows undoubtedly
affects the character of the wood formed, though it is not possible to formulate a rule
governing it. In general, however, it may be said that where strength or ease of
working is essential, woods of moderate to slow growth should be chosen. But in
choosing a particular specimen it is not the width of ring, but the proportion and
character of the late wood which should govern.
In the case of the ring-porous hardwoods there seems to exist a pretty definite
relation between the rate of growth of timber and its properties. This may be briefly
summed up in the general statement that the more rapid the growth or the wider the
rings of growth, the heavier, harder, stronger, and stiffer the wood. This, it must be
remembered, applies only to ring-porous woods such as oak, ash, hickory, and others
of the same group, and is, of course, subject to some exceptions and limitations.
In ring-porous woods of good growth it is usually the middle portion of the ring in
which the thick-walled, strength-giving fibres are most abundant. As the breadth of
ring diminishes, this middle portion is reduced so that very slow growth produces
comparatively light, porous wood composed of thin-walled vessels and wood

parenchyma. In good oak these large vessels of the early wood occupy from 6 to 10
per cent of the volume of the log, while in inferior material they may make up 25 per
cent or more. The late wood of good oak, except for radial grayish patches of small
pores, is dark colored and firm, and consists of thick-walled fibres which form one-
half or more of the wood. In inferior oak, such fibre areas are much reduced both in
quantity and quality. Such variation is very largely the result of rate of growth.
Wide-ringed wood is often called "second-growth," because the growth of the young
timber in open stands after the old trees have been removed is more rapid than in
trees in the forest, and in the manufacture of articles where strength is an important
consideration such "second-growth" hardwood material is preferred. This is
particularly the case in the choice of hickory for handles and spokes. Here not only
strength, but toughness and resilience are important. The results of a series of tests
on hickory by the U.S. Forest Service show that "the work or shock-resisting ability
is greatest in wide-ringed wood that has from 5 to 14 rings per inch, is fairly constant
from 14 to 38 rings, and decreases rapidly from 38 to 47 rings. The strength at
maximum load is not so great with the most rapid-growing wood; it is maximum
with from 14 to 20 rings per inch, and again becomes less as the wood becomes
more closely ringed. The natural deduction is that wood of first-class mechanical
value shows from 5 to 20 rings per inch and that slower growth yields poorer stock.
Thus the inspector or buyer of hickory should discriminate against timber that has
more than 20 rings per inch. Exceptions exist, however, in the case of normal growth
upon dry situations, in which the slow-growing material may be strong and tough."20
The effect of rate of growth on the qualities of chestnut wood is summarized by the
same authority as follows: "When the rings are wide, the transition from spring wood
to summer wood is gradual, while in the narrow rings the spring wood passes into
summer wood abruptly. The width of the spring wood changes but little with the
width of the annual ring, so that the narrowing or broadening of the annual ring is
always at the expense of the summer wood. The narrow vessels of the summer wood
make it richer in wood substance than the spring wood composed of wide vessels.
Therefore, rapid-growing specimens with wide rings have more wood substance than
slow-growing trees with narrow rings. Since the more the wood substance the greater
the weight, and the greater the weight the stronger the wood, chestnuts with wide
rings must have stronger wood than chestnuts with narrow rings. This agrees with
the accepted view that sprouts (which always have wide rings) yield better and
stronger wood than seedling chestnuts, which grow more slowly in diameter."21
In diffuse-porous woods, as has been stated, the vessels or pores are scattered
throughout the ring instead of collected in the early wood. The effect of rate of
growth is, therefore, not the same as in the ring-porous woods, approaching more
nearly the conditions in the conifers. In general it may be stated that such woods of

medium growth afford stronger material than when very rapidly or very slowly
grown. In many uses of wood, strength is not the main consideration. If ease of
working is prized, wood should be chosen with regard to its uniformity of texture
and straightness of grain, which will in most cases occur when there is little contrast
between the late wood of one season's growth and the early wood of the next.

HEARTWOOD AND SAPWOOD

Examination of the end of a log of many species reveals a darker-colored inner
portion—the heartwood, surrounded by a lighter-colored zone—the sapwood. In
some instances this distinction in color is very marked; in others, the contrast is
slight, so that it is not always easy to tell where one leaves off and the other begins.
The color of fresh sapwood is always light, sometimes pure white, but more often
with a decided tinge of green or brown.
Sapwood is comparatively new wood. There is a time in the early history of every
tree when its wood is all sapwood. Its principal functions are to conduct water from
the roots to the leaves and to store up and give back according to the season the food
prepared in the leaves. The more leaves a tree bears and the more thrifty its growth,
the larger the volume of sapwood required, hence trees making rapid growth in the
open have thicker sapwood for their size than trees of the same species growing in
dense forests. Sometimes trees grown in the open may become of considerable size,
a foot or more in diameter, before any heartwood begins to form, for example, in
second-growth hickory, or field-grown white and loblolly pines.
As a tree increases in age and diameter an inner portion of the sapwood becomes
inactive and finally ceases to function. This inert or dead portion is called heartwood,
deriving its name solely from its position and not from any vital importance to the
tree, as is shown by the fact that a tree can thrive with its heart completely decayed.
Some, species begin to form heartwood very early in life, while in others the change
comes slowly. Thin sapwood is characteristic of such trees as chestnut, black locust,
mulberry, Osage orange, and sassafras, while in maple, ash, gum, hickory, hackberry,
beech, and loblolly pine, thick sapwood is the rule.
There is no definite relation between the annual rings of growth and the amount of
sapwood. Within the same species the cross-sectional area of the sapwood is roughly
proportional to the size of the crown of the tree. If the rings are narrow, more of them
are required than where they are wide. As the tree gets larger, the sapwood must
necessarily become thinner or increase materially in volume. Sapwood is thicker in
the upper portion of the trunk of a tree than near the base, because the age and the
diameter of the upper sections are less.

When a tree is very young it is covered with limbs almost, if not entirely, to the
ground, but as it grows older some or all of them will eventually die and be broken
off. Subsequent growth of wood may completely conceal the stubs which, however,
will remain as knots. No matter how smooth and clear a log is on the outside, it is
more or less knotty near the middle. Consequently the sapwood of an old tree, and
particularly of a forest-grown tree, will be freer from knots than the heartwood.
Since in most uses of wood, knots are defects that weaken the timber and interfere
with its ease of working and other properties, it follows that sapwood, because of its
position in the tree, may have certain advantages over heartwood.
It is really remarkable that the inner heartwood of old trees remains as sound as it
usually does, since in many cases it is hundreds of years, and in a few instances
thousands of years, old. Every broken limb or root, or deep wound from fire, insects,
or falling timber, may afford an entrance for decay, which, once started, may
penetrate to all parts of the trunk. The larvæ of many insects bore into the trees and
their tunnels remain indefinitely as sources of weakness. Whatever advantages,
however, that sapwood may have in this connection are due solely to its relative age
and position.
If a tree grows all its life in the open and the conditions of soil and site remain
unchanged, it will make its most rapid growth in youth, and gradually decline. The
annual rings of growth are for many years quite wide, but later they become
narrower and narrower. Since each succeeding ring is laid down on the outside of the
wood previously formed, it follows that unless a tree materially increases its
production of wood from year to year, the rings must necessarily become thinner. As
a tree reaches maturity its crown becomes more open and the annual wood
production is lessened, thereby reducing still more the width of the growth rings. In
the case of forest-grown trees so much depends upon the competition of the trees in
their struggle for light and nourishment that periods of rapid and slow growth may
alternate. Some trees, such as southern oaks, maintain the same width of ring for
hundreds of years. Upon the whole, however, as a tree gets larger in diameter the
width of the growth rings decreases.
It is evident that there may be decided differences in the grain of heartwood and
sapwood cut from a large tree, particularly one that is overmature. The relationship
between width of growth rings and the mechanical properties of wood is discussed
under Rate of Growth. In this connection, however, it may be stated that as a general
rule the wood laid on late in the life of a tree is softer, lighter, weaker, and more
even-textured than that produced earlier. It follows that in a large log the sapwood,
because of the time in the life of the tree when it was grown, may be inferior in
hardness, strength, and toughness to equally sound heartwood from the same log.

After exhaustive tests on a number of different woods the U.S. Forest Service
concludes as follows: "Sapwood, except that from old, overmature trees, is as strong
as heartwood, other things being equal, and so far as the mechanical properties go
should not be regarded as a defect."22 Careful inspection of the individual tests made
in the investigation fails to reveal any relation between the proportion of sapwood
and the breaking strength of timber.
In the study of the hickories the conclusion was: "There is an unfounded prejudice
against the heartwood. Specifications place white hickory, or sapwood, in a higher
grade than red hickory, or heartwood, though there is no inherent difference in
strength. In fact, in the case of large and old hickory trees, the sapwood nearest the
bark is comparatively weak, and the best wood is in the heart, though in young trees
of thrifty growth the best wood is in the sap."23 The results of tests from selected
pieces lying side by side in the same tree, and also the average values for heartwood
and sapwood in shipments of the commercial hickories without selection, show
conclusively that "the transformation of sapwood into heartwood does not affect
either the strength or toughness of the wood.... It is true, however, that sapwood is
usually more free from latent defects than heartwood."24
Specifications for paving blocks often require that longleaf pine be 90 per cent heart.
This is on the belief that sapwood is not only more subject to decay, but is also
weaker than heartwood. In reality there is no sound basis for discrimination against
sapwood on account of strength, provided other conditions are equal. It is true that
sapwood will not resist decay as long as heartwood, if both are untreated with
preservatives. It is especially so of woods with deep-colored heartwood, and is due
to infiltrations of tannins, oils, and resins, which make the wood more or less
obnoxious to decay-producing fungi. If, however, the timbers are to be treated,
sapwood is not a defect; in fact, because of the relative ease with which it can be
impregnated with preservatives it may be made more desirable than heartwood.25
In specifications for structural timbers reference is sometimes made to "boxheart,"
meaning the inclusion of the pith or centre of the tree within a cross section of the
timber. From numerous experiments it appears that the position of the pith does not
bear any relation to the strength of the material. Since most season checks, however,
are radial, the position of the pith may influence the resistance of a seasoned beam to
horizontal shear, being greatest when the pith is located in the middle half of the
section.26

WEIGHT, DENSITY, AND SPECIFIC GRAVITY

From data obtained from a large number of tests on the strength of different woods it
appears that, other things being equal, the crushing strength parallel to the grain,
fibre stress at elastic limit in bending, and shearing strength along the grain of wood
vary in direct proportion to the weight of dry wood per unit of volume when green.
Other strength values follow different laws. The hardness varies in a slightly greater
ratio than the square of the density. The work to the breaking point increases even
more rapidly than the cube of density. The modulus of rupture in bending lies
between the first power and the square of the density. This, of course, is true only in
case the greater weight is due to increase in the amount of wood substance. A wood
heavy with resin or other infiltrated substance is not necessarily stronger than a
similar specimen free from such materials. If differences in weight are due to degree
of seasoning, in other words, to the relative amounts of water contained, the rules
given above will of course not hold, since strength increases with dryness. But of
given specimens of pine or of oak, for example, in the green condition, the
comparative strength may be inferred from the weight. It is not permissible,
however, to compare such widely different woods as oak and pine on a basis of their
weights.27
The weight of wood substance, that is, the material which composes the walls of the
fibres and other cells, is practically the same in all species, whether pine, hickory, or
cottonwood, being a little greater than half again as heavy as water. It varies slightly
from beech sapwood, 1.50, to Douglas fir heartwood, 1.57, averaging about 1.55 at
30° to 35° C., in terms of water at its greatest density 4° C. The reason any wood
floats is that the air imprisoned in its cavities buoys it up. When this is displaced by
water the wood becomes water-logged and sinks. Leaving out of consideration
infiltrated substances, the reason a cubic foot of one kind of dry wood is heavier than
that of another is because it contains a greater amount of wood substance. Density is
merely the weight of a unit of volume, as 35 pounds per cubic foot, or 0.56 grams
per cubic centimetre. Specific gravity or relative density is the ratio of the density of
any material to the density of distilled water at 4° C. (39.2° F.). A cubic foot of
distilled water at 4° C. weighs 62.43 pounds. Hence the specific gravity of a piece of
wood with a density of 35 pounds is

35
------- = 0.561 .
62.43

To find the weight per cubic foot when the specific gravity is given, simply multiply
by 62.43. Thus, 0.561 × 62.43 = 35. In the metric system, since the weight of a cubic

centimetre of pure water is one gram, the density in grams per cubic centimetre has
the same numerical value as the specific gravity.
Since the amount of water in wood is extremely variable it usually is not satisfactory
to refer to the density of green wood. For scientific purposes the density of "oven-
dry" wood is used; that is, the wood is dried in an oven at a temperature of 100°C.
(212°F.) until a constant weight is attained. For commercial purposes the weight or
density of air-dry or "shipping-dry" wood is used. This is usually expressed in
pounds per thousand board feet, a board foot being considered as one-twelfth of a
cubic foot.
Wood shrinks greatly in drying from the green to the oven-dry condition. (See Table
XIV.) Consequently a block of wood measuring a cubic foot when green will
measure considerably less when oven-dry. It follows that the density of oven-dry
wood does not represent the weight of the dry wood substance in a cubic foot of
green wood. In other words, it is not the weight of a cubic foot of green wood minus
the weight of the water which it contains. Since the latter is often a more convenient
figure to use and much easier to obtain than the weight of oven-dry wood, it is
commonly expressed in tables of "specific gravity or density of dry wood."

TABLE XIV
SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN
WOODS
(Forest Service Cir. 213)
Specific
Shrinkage from green to
gravity oven-
oven-dry condition
dry, based on
Moisture
Volume
COMMON NAME content Volume
when In
OF SPECIES when Radial Tangential
oven- volume
green
dry
Per Per
Per cent Per cent
cent cent
Hardwoods
Ash, black 77 0.466
white 38 .550 0.640 12.6 4.3 6.4
" 47 .516 .590 11.7
Basswood 110 .315 .374 14.5 6.2 8.4
Beech 61 .556 .669 16.5 4.6 10.5
Birch, yellow 72 .545 .661 17.0 7.9 9.0

Elm, rock 46 .578
slippery 57 .541 .639 15.5 5.1 9.9
white 66 .430
Gum, red 71 .434
Hackberry 50 .504 .576 14.0 4.2 8.9
Hickory, big
64 .601 17.6 7.4 11.2
shellbark
" 55 .666 20.9 7.9 14.2
bitternut 65 .624
mockernut 64 .606 16.5 6.9 10.4
" 57 .662 18.9 8.4 11.4
" 48 .666
nutmeg 76 .558
pignut 59 .627 15.0 5.6 9.8
" 54 .667 15.3 6.3 9.5
" 55 .667 16.9 6.8 10.9
" 52 .667 21.2 8.5 13.8
shagbark 65 .608 16.0 6.5 10.2
" 58 .646 18.4 7.9 11.4
" 64 .617
" 60 .653 15.5 6.5 9.7
water 74 .630
Locust, honey 53 .695 .759 8.6
Maple, red 69 .512
sugar 57 .546 .643 14.3 4.9 9.1
" 56 .577
Oak, post 64 .590 .732 16.0 5.7 10.6
red 80 .568 .660 13.1 3.7 8.3
swamp white 74 .637 .792 17.7 5.5 10.6
tanbark 88 .585
white 58 .594 .704 15.8 6.2 8.3
" 62 .603 .696 14.3 4.9 9.0
" 78 .600 .708 16.0 4.8 9.2
yellow 77 .573 .669 14.2 4.5 9.7
" 80 .550
Osage orange 31 .761 .838 8.9
Sycamore 81 .454 .526 13.5 5.0 7.3
Tupelo 121 .475 .545 12.4 4.4 7.9

TABLE XIV (CONT.)
SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN
WOODS
(Forest Service Cir. 213)
Specific
Shrinkage from green to
gravity oven-
oven-dry condition
dry, based on
Moisture
Volume
COMMON NAME content Volume
when In
OF SPECIES when Radial Tangential
oven- volume
green
dry
Per Per
Per cent Per cent
cent cent
Conifers
Arborvitæ 55 .293 .315 7.0 2.1 4.9
Cedar, incense 80 .363
Cypress, bald 79 .452 .513 11.5 3.8 6.0
Fir, alpine 47 .306 .321 9.0 2.5 7.1
amabilis 117 .383
Douglas 32 .418 .458 10.9 3.7 6.6
white 156 .350 .437 10.2 3.4 7.0
Hemlock (east.) 129 .340 .394 9.2 2.3 5.0
Pine, lodgepole 44 .370 .415 11.3 4.2 7.1
lodgepole 58 .371 .407 10.1 3.6 5.9
longleaf 63 .528 .599 12.8 6.0 7.6
red or Nor 54 .440 .507 11.5 4.5 7.2
shortleaf 52 .447
sugar 123 .360 .386 8.4 2.9 5.6
west yellow 98 .353 .395 9.2 4.1 6.4
" 125 .377 .433 11.5 4.3 7.3
" 93 .391 .435 9.9 3.8 5.8
white 74 .363 .391 7.8 2.2 5.9
Redwood 81 .334
" 69 .366
Spruce, Engelmann 45 .325 .359 10.5 3.7 6.9
" 156 .299 .335 10.3 3.0 6.2
red 31 .396
white 41 .318
Tamarack 52 .491 .558 13.6 3.7 7.4

This weight divided by 62.43 gives the specific gravity per green volume. It is purely
a fictitious quantity. To convert this figure into actual density or specific gravity of
the dry wood, it is necessary to know the amount of shrinkage in volume. If S is the
percentage of shrinkage from the green to the oven-dry condition, based on the green
volume; D, the density of the dry wood per cubic foot while green; and d the actual
density of oven-dry wood, then

D
---------- = d.
1 - .0 S

This relation becomes clearer from the following analysis: Taking V and W as the
volume and weight, respectively, when green, and v and w as the corresponding
volume and weight when oven-dry, then,

w W V-v V-v
d = --- ; D = --- ; S = ------- × 100 , and s = ------- × 100 ,
v V V v

in which S is the percentage of shrinkage from the green to the oven-dry condition,
based on the green volume, and s the same based on the oven-dry volume.
In tables of specific gravity or density of wood it should always be stated whether
the dry weight per unit of volume when green or the dry weight per unit of volume
when dry is intended, since the shrinkage in volume may vary from 6 to 50 per cent,
though in conifers it is usually about 10 per cent, and in hardwoods nearer 15 per
cent. (See Table XIV.)

COLOR

In species which show a distinct difference between heartwood and sapwood the
natural color of heartwood is invariably darker than that of the sapwood, and very
frequently the contrast is conspicuous. This is produced by deposits in the heartwood
of various materials resulting from the process of growth, increased possibly by
oxidation and other chemical changes, which usually have little or no appreciable
effect on the mechanical properties of the wood. (See Heartwood and Sapwood.)

Some experiments28 on very resinous longleaf pine specimens, however, indicate an
increase in strength. This is due to the resin which increases the strength when dry.
Spruce impregnated with crude resin and dried is greatly increased in strength
thereby.
Since the late wood of a growth ring is usually darker in color than the early wood,
this fact may be used in judging the density, and therefore the hardness and strength
of the material. This is particularly the case with coniferous woods. In ring-porous
woods the vessels of the early wood not infrequently appear on a finished surface as
darker than the denser late wood, though on cross sections of heartwood the reverse
is commonly true. Except in the manner just stated the color of wood is no indication
of strength.
Abnormal discoloration of wood often denotes a diseased condition, indicating
unsoundness. The black check in western hemlock is the result of insect attacks.29
The reddish-brown streaks so common in hickory and certain other woods are
mostly the result of injury by birds.30 The discoloration is merely an indication of an
injury, and in all probability does not of itself affect the properties of the wood.
Certain rot-producing fungi impart to wood characteristic colors which thus become
criterions of weakness. Ordinary sap-staining is due to fungous growth, but does not
necessarily produce a weakening effect.31

CROSS GRAIN

Cross grain is a very common defect in timber. One form of it is produced in lumber
by the method of sawing and has no reference to the natural arrangement of the
wood elements. Thus if the plane of the saw is not approximately parallel to the axis
of the log the grain of the lumber cut is not parallel to the edges and is termed
diagonal. This is likely to occur where the logs have considerable taper, and in this
case may be produced if sawed parallel to the axis of growth instead of parallel to
the growth rings.
Lumber and timber with diagonal grain is always weaker than straight-grained
material, the extent of the defect varying with the degree of the angle the fibres make
with the axis of the stick. In the vicinity of large knots the grain is likely to be cross.
The defect is most serious where wood is subjected to flexure, as in beams.
Spiral grain is a very common defect in a tree, and when excessive renders the
timber valueless for use except in the round. It is produced by the arrangement of the
wood fibres in a spiral direction about the axis instead of exactly vertical. Timber
with spiral grain is also known as "torse wood." Spiral grain usually cannot be

detected by casual inspection of a stick, since it does not show in the so-called
visible grain of the wood, by which is commonly meant a sectional view of the
annual rings of growth cut longitudinally. It is accordingly very easy to allow spiral-
grained material to pass inspection, thereby introducing an element of weakness in a
structure.
There are methods for readily detecting spiral grain. The simplest is that of splitting
a small piece radially. It is necessary, of course, that the split be radial, that is, in a
plane passing through the axis of the log, and not tangentially. In the latter case it is
quite probable that the wood would split straight, the line of cleavage being between
the growth rings.
In inspection, the elements to examine are the rays. In the case of oak and certain
other hardwoods these rays are so large that they are readily seen not only on a radial
surface, but on the tangential as well. On the former they appear as flakes, on the
latter as short lines. Since these rays are between the fibres it naturally follows that
they will be vertical or inclined according as the tree is straight-grained or spiral-
grained. While they are not conspicuous in the softwoods, they can be seen upon
close scrutiny, and particularly so if a small hand magnifier is used.
When wood has begun to dry and check it is very easy to see whether or not it is
straight- or spiral-grained, since the checks will for the most part follow along the
rays. If one examines a row of telephone poles, for example, he will probably find
that most of them have checks running spirally around them. If boards were sawed
from such a pole after it was badly checked they would fall to pieces of their own
weight. The only way to get straight material would be to split it out.
It is for this reason that split billets and squares are stronger than most sawed
material. The presence of the spiral grain has little, if any, effect on the timber when
it is used in the round, but in sawed material the greater the pitch of the spiral the
greater is the defect.

KNOTS

Knots are portions of branches included in the wood of the stem or larger branch.
Branches originate as a rule from the central axis of a stem, and while living increase
in size by the addition of annual woody layers which are a continuation of those of
the stem. The included portion is irregularly conical in shape with the tip at the pith.
The direction of the fibre is at right angles or oblique to the grain of the stem, thus
producing local cross grain.

During the development of a tree most of the limbs, especially the lower ones, die,
but persist for a time—often for years. Subsequent layers of growth of the stem are
no longer intimately joined with the dead limb, but are laid around it. Hence dead
branches produce knots which are nothing more than pegs in a hole, and likely to
drop out after the tree has been sawed into lumber. In grading lumber and structural
timber, knots are classified according to their form, size, soundness, and the firmness
with which they are held in place.32
Knots materially affect checking and warping, ease in working, and cleavability of
timber. They are defects which weaken timber and depreciate its value for structural
purposes where strength is an important consideration. The weakening effect is much
more serious where timber is subjected to bending and tension than where under
compression. The extent to which knots affect the strength of a beam depends upon
their position, size, number, direction of fibre, and condition. A knot on the upper
side is compressed, while one on the lower side is subjected to tension. The knot,
especially (as is often the case) if there is a season check in it, offers little resistance
to this tensile stress. Small, knots, however, may be so located in a beam along the
neutral plane as actually to increase the strength by tending to prevent longitudinal
shearing. Knots in a board or plank are least injurious when they extend through it at
right angles to its broadest surface. Knots which occur near the ends of a beam do
not weaken it. Sound knots which occur in the central portion one-fourth the height
of the beam from either edge are not serious defects.

Extensive experiments by the U.S. Forest Service33 indicate the following effects of
knots on structural timbers:
(1) Knots do not materially influence the stiffness of structural timber.
(2) Only defects of the most serious character affect the elastic limit of beams.
Stiffness and elastic strength are more dependent upon the quality of the wood fibre
than upon defects in the beam.
(3) The effect of knots is to reduce the difference between the fibre stress at elastic
limit and the modulus of rupture of beams. The breaking strength is very susceptible
to defects.
(4) Sound knots do not weaken wood when subject to compression parallel to the
grain.34

FROST SPLITS

A common defect in standing timber results from radial splits which extend inward
from the periphery of the tree, and almost, if not always, near the base. It is most
common in trees which split readily, and those with large rays and thin bark. The
primary cause of the splitting is frost, and various theories have been advanced to
explain the action.

R. Hartig35 believes that freezing forces out a part of the imbibition water of the cell
walls, thereby causing the wood to shrink, and if the interior layers have not yet been
cooled, tangential strains arise which finally produce radial clefts.
Another theory holds that the water is not driven out of the cell walls, but that
difference in temperature conditions of inner and outer layers is itself sufficient to set
up the strains, resulting in splitting. An air temperature of 14°F. or less is considered
necessary to produce frost splits.

A still more recent theory is that of Busse36 who considers the mechanical action of
the wind a very important factor. He observed: (a) Frost splits sometimes occur at
higher temperatures than 14°F. (b) Most splits take place shortly before sunrise, i.e.,
at the time of lowest air and soil temperature; they are never heard to take place at
noon, afternoon, or evening. (c) They always occur between two roots or between
the collars of two roots, (d) They are most frequent in old, stout-rooted, broad-
crowned trees; in younger stands it is always the stoutest members that are found
with frost splits, while in quite young stands they are altogether absent, (e) Trees on
wet sites are most liable to splits, due to difference in wood structure, just as
difference in wood structure makes different species vary in this regard. (f) Frost
splits are most numerous less than three feet above the ground.
When a tree is swayed by the wind the roots are counteracting forces, and the wood
fibres are tested in tension and compression by the opposing forces; where the roots
exercise tension stresses most effectively the effect of compression stresses is at a
minimum; only where the pressure is in excess of the tension, i.e., between the roots,
can a separation of the fibre result. Hence, when by frost a tension on the entire
periphery is established, and the wind localizes additional strains, failure occurs. The
stronger the compression and tension, the severer the strains and the oftener failures
occur. The occurrence of reports of frost splits on wind-still days is believed by
Busse to be due to the opening of old frost splits where the tension produced by the
frost alone is sufficient.
Frost splits may heal over temporarily, but usually open up again during the
following winter. The presence of old splits is often indicated by a ridge of callous,
the result of the cambium's effort to occlude the wound. Frost splits not only affect
the value of lumber, but also afford an entrance into the living tree for disease and
decay.

SHAKES, GALLS, PITCH POCKETS

Heart shake occurs in nearly all overmature timber, being more frequent in
hardwoods (especially oak) than in conifers. In typical heart shake the centre of the
hole shows indications of becoming hollow and radial clefts of varying size extend
outward from the pith, being widest inward. It frequently affects only the butt log,
but may extend to the entire hole and even the larger branches. It usually results
from a shrinkage of the heartwood due probably to chemical changes in the wood.
When it consists of a single cleft extending across the pith it is termed simple heart
shake. Shake of this character in straight-grained trees affects only one or two central
boards when cut into lumber, but in spiral-grained timber the damage is much
greater. When shake consists of several radial clefts it is termed star shake. In some
instances one or more of these clefts may extend nearly to the bark. In felled or
converted timber clefts due to heart shake may be distinguished from seasoning
cracks by the darker color of the exposed surfaces. Such clefts, however, tend to
open up more and more as the timber seasons.
Cup or ring shake results from the pulling apart of two or more growth rings. It is
one of the most serious defects to which sound timber is subject, as it seriously
reduces the technical properties of wood. It is very common in sycamore and in
western larch, particularly in the butt portion. Its occurrence is most frequent at the
junction of two growth layers of very unequal thickness. Consequently it is likely to
occur in trees which have grown slowly for a time, then abruptly increased, due to
improved conditions of light and food, as in thinning. Old timber is more subject to it
than young trees. The damage is largely confined to the butt log. Cup shake is often
associated with other forms of shake, and not infrequently shows traces of decay.
The causes of cup shake are uncertain. The swaying action of the wind may result in
shearing apart the growth layers, especially in trees growing in exposed places. Frost
may in some instances be responsible for cup shake or at least a contributing factor,
although trees growing in regions free from frost often have ring shake. Shrinkage of
the heartwood may be concentric as well as radial in its action, thus producing cup
shake instead of, or in connection with, heart shake.
A local defect somewhat similar in effect to cup shake is known as rind gall. If the
cambium layer is exposed by the removal of the entire bark or rind it will die.
Subsequent growth over the damaged portion does not cohere with the wood
previously formed by the old cambium. The defect resulting is termed rind gall. The
most common causes of it are fire, gnawing, blazing, chipping, sun scald, lightning,
and abrasions.

Heart break is a term applied to areas of compression failure along the grain found
in occasional logs. Sometimes these breaks are invisible until the wood is
manufactured into the finished article. The occurrence of this defect is mostly limited
to the dense hardwoods, such as hickory and to heavy tropical species. It is the
source of considerable loss in the fancy veneer industry, as the veneer from valuable
logs so affected drops to pieces.
The cause of heart break is not positively known. It is highly probable, however, that
when the tree is felled the trunk strikes across a rock or another log, and the impact
causes actual failure in the log as in a beam.
Resin or pitch pockets are of common occurrence in the wood of larch, spruce, fir,
and especially of longleaf and other hard pines. They are due to accumulations of
resin in openings between adjacent layers of growth. They are more frequent in trees
growing alone than in those of dense stands. The pockets are usually a few inches in
greatest dimension and affect only one or two growth layers. They are hidden until
exposed by the saw, rendering it impossible to cut lumber with reference to their
position. Often several boards are damaged by a single pocket. In grading lumber,
pitch pockets are classified as small, standard, and large, depending upon their width
and length.

INSECT INJURIES37

The larvæ of many insects are destructive to wood. Some attack the wood of living
trees, others only that of felled or converted material. Every hole breaks the
continuity of the fibres and impairs the strength, and if there are very many of them
the material may be ruined for all purposes where strength is required.
Some of the most common insects attacking the wood of living trees are the oak
timber worm, the chestnut timber worm, carpenter worms, ambrosia beetles, the
locust borer, turpentine beetles and turpentine borers, and the white pine weevil.
The insect injuries to forest products may be classed according to the stage of
manufacture of the material. Thus round timber with the bark on, such as poles,
posts, mine props, and sawlogs, is subject to serious damage by the same class of
insects as those mentioned above, particularly by the round-headed borers, timber
worms, and ambrosia beetles. Manufactured unseasoned products are subject to
damage from ambrosia beetles and other wood borers. Seasoned hardwood lumber
of all kinds, rough handles, wagon stock, etc., made partially or entirely of sapwood,
are often reduced in value from 10 to 90 per cent by a class of insects known as
powder-post beetles. Finished hardwood products such as handles, wagon, carriage

and machinery stock, especially if ash or hickory, are often destroyed by the powder-
post beetles. Construction timbers in buildings, bridges and trestles, cross-ties, poles,
mine props, fence posts, etc., are sometimes seriously injured by wood-boring larvæ,
termites, black ants, carpenter bees, and powder-post beetles, and sometimes reduced
in value from 10 to 100 per cent. In tropical countries termites are a very serious pest
in this respect.

MARINE WOOD-BORER INJURIES

Vast amounts of timber used for piles in wharves and other marine structures are
constantly being destroyed or seriously injured by marine borers. Almost invariably
they are confined to salt water, and all the woods commonly used for piling are
subject to their attacks. There are two genera of mollusks, Xylotrya and Teredo, and
three of crustaceans, Limnoria, Chelura, and Sphoeroma, that do serious damage in
many places along both the Atlantic and Pacific coasts.
These mollusks, which are popularly known as "shipworms," are much alike in
structure and mode of life. They attack the exposed surface of the wood and
immediately begin to bore. The tunnels, often as large as a lead pencil, extend
usually in a longitudinal direction and follow a very irregular, tangled course. Hard
woods are apparently penetrated as readily as soft woods, though in the same timber
the softer parts are preferred. The food consists of infusoria and is not obtained from
the wood substance. The sole object of boring into the wood is to obtain shelter.
Although shipworms can live in cold water they thrive best and are most destructive
in warm water. The length of time required to destroy an average barked,
unprotected pine pile on the Atlantic coast south from Chesapeake Bay and along the
entire Pacific coast varies from but one to three years.
Of the crustacean borers, Limnoria, or the "wood louse," is the only one of great
importance, although Sphoeroma is reported destructive in places. Limnoria is about
the size of a grain of rice and tunnels into the wood for both food and shelter. The
galleries extend inward radially, side by side, in countless numbers, to the depth of
about one-half inch. The thin wood partitions remaining are destroyed by wave
action, so that a fresh surface is exposed to attack. Both hard and soft woods are
damaged, but the rate is faster in the soft woods or softer portions of a wood.
Timbers seriously attacked by marine borers are badly weakened or completely
destroyed. If the original strength of the material is to be preserved it is necessary to
protect the wood from the borers. This is sometimes accomplished by proper
injection of creosote oil, and more or less successfully by the use of various kinds of
external coatings.38 No treatment, however, has proved entirely satisfactory.

FUNGOUS INJURIES39

Fungi are responsible for almost all decay of wood. So far as known, all decay is
produced by living organisms, either fungi or bacteria. Some species attack living
trees, sometimes killing them, or making them hollow, or in the case of pecky
cypress and incense cedar filling the wood with galleries like those of boring insects.
A much larger variety work only in felled or dead wood, even after it is placed in
buildings or manufactured articles. In any case the process of destruction is the same.
The mycelial threads penetrate the walls of the cells in search of food, which they
find either in the cell contents (starches, sugars, etc.), or in the cell wall itself. The
breaking down of the cell walls through the chemical action of so-called "enzymes"
secreted by the fungi follows, and the eventual product is a rotten, moist substance
crumbling readily under the slightest pressure. Some species remove the ligneous
matter and leave almost pure cellulose, which is white, like cotton; others dissolve
the cellulose, leaving a brittle, dark brown mass of ligno-cellulose. Fungi (such as
the bluing fungus) which merely stain wood usually do not affect its mechanical
properties unless the attacks are excessive.
It is evident, then, that the action of rot-causing fungi is to decrease the strength of
wood, rendering it unsound, brittle, and dangerous to use. The most dangerous kinds
are the so-called "dry-rot" fungi which work in many kinds of lumber after it is
placed in the buildings. They are particularly to be dreaded because unseen, working
as they do within the walls or inside of casings. Several serious wrecks of large
buildings have been attributed to this cause. It is stated40 that in the three years
(1911-1913) more than $100,000 was required to repair damage due to dry rot.

Dry rot develops best at 75°F. and is said to be killed by a temperature of 110°F.41
Fully 70 per cent humidity is necessary in the air in which a timber is surrounded for
the growth of this fungus, and probably the wood must be quite near its fibre
saturation condition. Nevertheless Merulius lacrymans (one of the most important
species) has been found to live four years and eight months in a dry condition.42
Thorough kiln-drying will kill this fungus, but will not prevent its redevelopment.
Antiseptic treatment, such as creosoting, is the best prevention.

All fungi require moisture and air43 for their growth. Deprived of either of these the
fungus dies or ceases to develop. Just what degree of moisture in wood is necessary
for the "dry-rot" fungus has not been determined, but it is evidently considerably
above that of thoroughly air-dry timber, probably more than 15 per cent moisture.
Hence the importance of free circulation of air about all timbers in a building.
Warmth is also conducive to the growth of fungi, the most favorable temperature
being about 90°F. They cannot grow in extreme cold, although no degree of cold

such as occurs naturally will kill them. On the other hand, high temperature will kill
them, but the spores may survive even the boiling temperature. Mould fungus has
been observed to develop rapidly at 130°F. in a dry kiln in moist air, a condition
under which an animal cannot live more than a few minutes. This fungus was killed,
however, at about 140° or 145°F.44
The fungus (Endothia parasitica And.) which causes the chestnut blight kills the
trees by girdling them and has no direct effect upon the wood save possibly the four
or five growth rings of the sapwood.45

PARASITIC PLANT INJURIES.46

The most common of the higher parasitic plants damaging timber trees are
mistletoes. Many species of deciduous trees are attacked by the common mistletoe
(Phoradendron flavescens). It is very prevalent in the South and Southwest and
when present in sufficient quantity does considerable damage. There is also a
considerable number of smaller mistletoes belonging to the genus Razoumofskya
(Arceuthobium) which are widely distributed throughout the country, and several of
them are common on coniferous trees in the Rocky Mountains and along the Pacific
coast.
One effect of the common mistletoe is the formation of large swellings or tumors.
Often the entire tree may become stunted or distorted. The western mistletoe is most
common on the branches, where it produces "witches' broom." It frequently attacks
the trunk as well, and boards cut from such trees are filled with long, radial holes
which seriously damage or destroy the value of the timber affected.
v

LOCALITY OF GROWTH

The data available regarding the effect of the locality of growth upon the properties
of wood are not sufficient to warrant definite conclusions. The subject has, however,
been kept in mind in many of the U.S. Forest Service timber tests and the following
quotations are assembled from various reports:
"In both the Cuban and longleaf pine the locality where grown appears to have but
little influence on weight or strength, and there is no reason to believe that the
longleaf pine from one State is better than that from any other, since such variations
as are claimed can be found on any 40-acre lot of timber in any State. But with

loblolly and still more with shortleaf this seems not to be the case. Being widely
distributed over many localities different in soil and climate, the growth of the
shortleaf pine seems materially influenced by location. The wood from the southern
coast and gulf region and even Arkansas is generally heavier than the wood from
localities farther north. Very light and fine-grained wood is seldom met near the
southern limit of the range, while it is almost the rule in Missouri, where forms
resembling the Norway pine are by no means rare. The loblolly, occupying both wet
and dry soils, varies accordingly." Cir. No. 12, p. 6.
" ... It is clear that as all localities have their heavy and their light timber, so they all
share in strong and weak, hard and soft material, and the difference in quality of
material is evidently far more a matter of individual variation than of soil or
climate." Ibid., p.22
"A representative committee of the Carriage Builders' Association had publicly
declared that this important industry could not depend upon the supplies of southern
timber, as the oak grown in the South lacked the necessary qualities demanded in
carriage construction. Without experiment this statement could be little better than a
guess, and was doubly unwarranted, since it condemned an enormous amount of
material, and one produced under a great variety of conditions and by at least a
dozen species of trees, involving, therefore, a complexity of problems difficult
enough for the careful investigator, and entirely beyond the few unsystematic
observations of the members of a committee on a flying trip through one of the
greatest timber regions of the world.
"A number of samples were at once collected (part of them supplied by the carriage
builders' committee), and the fallacy of the broad statement mentioned was fully
demonstrated by a short series of tests and a more extensive study into structure and
weight of these materials. From these tests it appears that pieces of white oak from
Arkansas excelled well-selected pieces from Connecticut, both in stiffness and
endwise compression (the two most important forms of resistance)." Report upon the
forestry investigations of the U.S.D.A. 1877-1898, p. 331. See also Rep. of Div. of
For., 1890, p. 209.
"In some regions there are many small, stunted hickories, which most users will not
touch. They have narrow sap, are likely to be birdpecked, and show very slow
growth. Yet five of these trees from a steep, dry south slope in West Virginia had an
average strength fully equal to that of the pignut from the better situation, and were
superior in toughness, the work to maximum load being 36.8 as against 31.2 for
pignut. The trees had about twice as many rings per inch as others from better
situations.

"This, however, is not very significant, as trees of the same species, age, and size,
growing side by side under the same conditions of soil and situation, show great
variation in their technical value. It is hard to account for this difference, but it seems
that trees growing in wet or moist situations are rather inferior to those growing on
fresher soil; also, it is claimed by many hickory users that the wood from limestone
soils is superior to that from sandy soils.
"One of the moot questions among hickory men is the relative value of northern and
southern hickory. The impression prevails that southern hickory is more porous and
brash than hickory from the north. The tests ... indicate that southern hickory is as
tough and strong as northern hickory of the same age. But the southern hickories
have a greater tendency to be shaky, and this results in much waste. In trees from
southern river bottoms the loss through shakes and grub-holes in many cases
amounts to as much as 50 per cent.
"It is clear, therefore, that the difference in northern and southern hickory is not due
to geographic location, but rather to the character of timber that is being cut. Nearly
all of that from southern river bottoms and from the Cumberland Mountains is from
large, old-growth trees; that from the north is from younger trees which are grown
under more favorable conditions, and it is due simply to the greater age of the
southern trees that hickory from that region is lighter and more brash than that from
the north." Bul. 80, pp. 52-55.

SEASON OF CUTTING

It is generally believed that winter-felled timber has decided advantages over that cut
at other seasons of the year, and to that cause alone are frequently ascribed much
greater durability, less liability to check and split, better color, and even increased
strength and toughness. The conclusion from the various experiments made on the
subject is that while the time of felling may, and often does, affect the properties of
wood, such result is due to the weather conditions rather than to the condition of the
wood.
There are two phases of this question. One is concerned with the physiological
changes which might take place during the year in the wood of a living tree. The
other deals with the purely physical results due to the weather, as differences in
temperature, humidity, moisture, and other features to be mentioned later.
Those who adhere to the first view maintain that wood cut in summer is quite
different in composition from that cut in winter. One opinion is that in summer the
"sap is up," while in winter it is "down," consequently winter-felled timber is drier. A
variation of this belief is that in summer the sap contains certain chemicals which

affect the properties of wood and does not contain them in winter. Again it is
sometimes asserted that wood is actually denser in winter than in summer, as part of
the wood substance is dissolved out in the spring and used for plant food, being
restored in the fall.
It is obvious that such views could apply only to sapwood, since it alone is in living
condition at the time of cutting. Heartwood is dead wood and has almost no function
in the existence of the tree other than the purely mechanical one of support.
Heartwood does undergo changes, but they are gradual and almost entirely
independent of the seasons.
Sapwood might reasonably be expected to respond to seasonal changes, and to some
extent it does. Just beneath the bark there is a thin layer of cells which during the
growing season have not attained their greatest density. With the exception of this
one annual ring, or portion of one, the density of the wood substance of the sapwood
is nearly the same the year round. Slight variations may occur due to impregnation
with sugar and starch in the winter and its dissolution in the growing season. The
time of cutting can have no material effect on the inherent strength and other
mechanical properties of wood except in the outermost annual ring of growth.
The popular belief that sap is up in the spring and summer and is down in the winter
has not been substantiated by experiment. There are seasonal differences in the
composition of sap, but so far as the amount of sap in a tree is concerned there is
fully as much, if not more, during the winter than in summer. Winter-cut wood is not
drier, to begin with, than summer-felled—in reality, it is likely to be wetter.47
The important consideration in regard to this question is the series of circumstances
attending the handling of the timber after it is felled. Wood dries more rapidly in
summer than in winter, not because there is less moisture at one time than another,
but because of the higher temperature in summer. This greater heat is often
accompanied by low humidity, and conditions are favorable for the rapid removal of
moisture from the exposed portions of wood. Wood dries by evaporation, and other
things being equal, this will proceed much faster in hot weather than in cold.
It is a matter of common observation that when wood dries it shrinks, and if
shrinkage is not uniform in all directions the material pulls apart, causing season
checks. (See Fig. 27.) If evaporation proceeds more rapidly on the outside than
inside, the greater shrinkage of the outer portions is bound to result in many checks,
the number and size increasing with the degree of inequality of drying.
In cold weather, drying proceeds slowly but uniformly, thus allowing the wood
elements to adjust themselves with the least amount of rupturing. In summer, drying

proceeds rapidly and irregularly, so that material seasoned at that time is more likely
to split and check.
There is less danger of sap rot when trees are felled in winter because the fungus
does not grow in the very cold weather, and the lumber has a chance to season to
below the danger point before the fungus gets a chance to attack it. If the logs in
each case could be cut into lumber immediately after felling and given exactly the
same treatment, for example, kiln-dried, no difference due to the season of cutting
would be noted.

WATER CONTENT48

Water occurs in living wood in three conditions, namely: (1) in the cell walls, (2) in
the protoplasmic contents of the cells, and (3) as free water in the cell cavities and
spaces. In heartwood it occurs only in the first and last forms. Wood that is
thoroughly air-dried retains from 8 to 16 per cent of water in the cell walls, and
none, or practically none, in the other forms. Even oven-dried wood retains a small
percentage of moisture, but for all except chemical purposes, may be considered
absolutely dry.
The general effect of the water content upon the wood substance is to render it softer
and more pliable. A similar effect of common observation is in the softening action
of water on rawhide, paper, or cloth. Within certain limits the greater the water
content the greater its softening effect.
Drying produces a decided increase in the strength of wood, particularly in small
specimens. An extreme example is the case of a completely dry spruce block two
inches in section, which will sustain a permanent load four times as great as that
which a green block of the same size will support.
The greatest increase due to drying is in the ultimate crushing strength, and strength
at elastic limit in endwise compression; these are followed by the modulus of
rupture, and stress at elastic limit in cross-bending, while the modulus of elasticity is
least affected. These ratios are shown in Table XV, but it is to be noted that they
apply only to wood in a much drier condition than is used in practice. For air-dry
wood the ratios are considerably lower, particularly in the case of the ultimate
strength and the elastic limit. Stiffness (within the elastic limit), while following a
similar law, is less affected. In the case of shear parallel to the grain, the general
effect of drying is to increase the strength, but this is often offset by small splits and
checks caused by shrinkage.

TABLE XV
EFFECT OF DRYING ON THE MECHANICAL
PROPERTIES OF WOOD, SHOWN IN RATIO OF
INCREASE DUE TO REDUCING MOISTURE CONTENT
FROM THE GREEN CONDITION TO KILN-DRY (3.5 PER
CENT)
(Forest Service Bul. 70, p. 89)
Longleaf
Spruce Chestnut
KIND OF STRENGTH pine
(1) (2) (1) (2) (1) (2)
Crushing strength parallel to
2.89 2.60 3.71 3.41 2.83 2.55
grain
Elastic limit in compression
2.60 2.34 3.80 3.49 2.40 2.26
parallel to grain
Modulus of rupture in
2.50 2.20 2.81 2.50 2.09 1.82
bending
Stress at elastic limit in
2.90 2.55 2.90 2.58 2.30 2.00
bending
Crushing strength at right
2.58 2.48
angles to grain
Shearing strength parallel to
2.01 1.91 2.03 1.95 1.55 1.47
grain
Modulus of elasticity in
1.63 1.47 2.26 2.08 1.43 1.29
compression parallel to grain
Modulus of elasticity in
1.59 1.35 1.43 1.23 1.44 1.21
bending
NOTE.—The figures in the first column show the relative increase
in strength between a green specimen and a kiln-dry specimen of
equal size. The figures in the second column show the relative
increase of strength of the same block after being dried from a
green condition to 3.5 per cent moisture, correction having been
made for shrinkage. That is, in the first column the strength values
per actual unit of area are used; in the second the values per unit of
area of green wood which shrinks to smaller size when dried. See
also Cir. 108, Fig. 1, p. 8.

The moisture content has a decided bearing also upon the manner in which wood
fails. In compression tests on very dry specimens the entire piece splits suddenly into
pieces before any buckling takes place (see Fig. 9.), while with wet material the

block gives way gradually, due to the buckling or bending of the walls of the fibres
along one or more shearing planes. (See Fig. 14.) In bending tests on wet beams, first
failure occurs by compression on top of the beam, gradually extending downward
toward the neutral axis. Finally the beam ruptures at the bottom. In the case of very
dry beams the failure is usually by splitting or tension on the under side (see Fig.
17.), without compression on the upper, and is often sudden and without warning,
and even while the load is still increasing. The effect varies somewhat with different
species, chestnut, for example, becoming more brittle upon drying than do ash,
hemlock, and longleaf pine. The tensile strength of wood is least affected by drying,
as a rule.
In drying wood no increase in strength results until the free water is evaporated and
the cell walls begin to dry49. This critical point has been called the fibre-saturation
point. (See Fig. 24.) Conversely, after the cell walls are saturated with water, any
increase in the amount of water absorbed merely fills the cavities and intercellular
spaces, and has no effect on the mechanical properties. Hence, soaking green wood
does not lessen its strength unless the water is heated, whereupon a decided
weakening results.

Figure 24
Relation of the moisture content to the
various strength values of spruce. FSP =
fibre-saturation point.

The strengthening effects of drying, while very marked in the case of small pieces,
may be fully offset in structural timbers by inherent weakening effects due to the
splitting apart of the wood elements as a result of irregular shrinkage, and in some
cases also to the slitting of the cell walls (see Fig. 25). Consequently with large
timbers in commercial use it is unsafe to count upon any greater strength, even after
seasoning, than that of the green or fresh condition.

Figure 25
Cross section of the wood of western
larch showing fissures in the thick-walled
cells of the late wood. Highly magnified.
Photo by U. S. Forest Service.

In green wood the cells are all intimately joined together and are at their natural or
normal size when saturated with water. The cell walls may be considered as made up
of little particles with water between them. When wood is dried the films of water
between the particles become thinner and thinner until almost entirely gone. As a
result the cell walls grow thinner with loss of moisture,—in other words, the cell
shrinks.
It is at once evident that if drying does not take place uniformly throughout an entire
piece of timber, the shrinkage as a whole cannot be uniform. The process of drying is
from the outside inward, and if the loss of moisture at the surface is met by a steady

capillary current of water from the inside, the shrinkage, so far as the degree of
moisture affected it, would be uniform. In the best type of dry kilns this condition is
approximated by first heating the wood thoroughly in a moist atmosphere before
allowing drying to begin.
In air-seasoning and in ordinary dry kilns this condition too often is not attained, and
the result is that a dry shell is formed which encloses a moist interior. (See Fig. 26.)
Subsequent drying out of the inner portion is rendered more difficult by this "case-
hardened" condition. As the outer part dries it is prevented from shrinking by the wet
interior, which is still at its greatest volume. This outer portion must either check
open or the fibres become strained in tension. If this outer shell dries while the fibres
are thus strained they become "set" in this condition, and are no longer in tension.
Later when the inner part dries, it tends to shrink away from the hardened outer shell,
so that the inner fibres are now strained in tension and the outer fibres are in
compression. If the stress exceeds the cohesion, numerous cracks open up, producing
a "honey-combed" condition, or "hollow-horning," as it is called. If such a case-
hardened stick of wood be resawed, the two halves will cup from the internal tension
and external compression, with the concave surface inward.

Figure 26
Progress of drying throughout the length
of a chestnut beam, the black spots
indicating the presence of free water in
the wood. The first section at the left was
cut one-fourth inch from the end, the next
one-half inch, the next one inch, and all
the others one inch apart. The illustration
shows case-hardening very clearly. Photo
by U. S. Forest Service.

For a given surface area the loss of water from wood is always greater from the ends
than from the sides, due to the fact that the vessels and other water-carriers are cut
across, allowing ready entrance of drying air and outlet for the water vapor. Water
does not flow out of boards and timbers of its own accord, but must be evaporated,

though it may be forced out of very sappy specimens by heat. In drying a log or pole
with the bark on, most of the water must be evaporated through the ends, but in the
case of peeled timbers and sawn boards the loss is greatest from the surface because
the area exposed is so much greater.
The more rapid drying of the ends causes local shrinkage, and were the material
sufficiently plastic the ends would become bluntly tapering. The rigidity of the wood
substance prevents this and the fibres are split apart. Later, as the remainder of the
stick dries many of the checks will come together, though some of the largest will
remain and even increase in size as the drying proceeds. (See Fig. 27.)

Figure 27
Excessive season checking. Photo by U.S.
Forest Service.

A wood cell shrinks very little lengthwise. A dry wood cell is, therefore, practically
of the same length as it was in a green or saturated condition, but is smaller in cross
section, has thinner walls, and a larger cavity. It is at once evident that this fact
makes shrinkage more irregular, for wherever cells cross each other at a decided
angle they will tend to pull apart upon drying. This occurs wherever pith rays and
wood fibres meet. A considerable portion of every wood is made up of these rays,
which for the most part have their cells lying in a radial direction instead of
longitudinally. (See Frontispiece.) In pine, over 15,000 of these occur on a square

inch of a tangential section, and even in oak the very large rays which are readily
visible to the eye as flakes on quarter-sawed material represent scarcely one per cent
of the number which the microscope reveals.
A pith ray shrinks in height and width, that is, vertically and tangentially as applied
to the position in a standing tree, but very little in length or radially. The other
elements of the wood shrink radially and tangentially, but almost none lengthwise or
vertically as applied to the tree. Here, then, we find the shrinkage of the rays tending
to shorten a stick of wood, while the other cells resist it, and the tendency of a stick
to get smaller in circumference is resisted by the endwise reaction or thrust of the
rays. Only in a tangential direction, or around the stick in direction of the annual
rings of growth, do the two forces coincide. Another factor to the same end is that
the denser bands of late wood are continuous in a tangential direction, while radially
they are separated by alternate zones of less dense early wood. Consequently the
shrinkage along the rings (tangential) is fully twice as much as toward the centre
(radial). (See Table XIV.) This explains why some cracks open more and more as
drying advances. (See Fig. 27.)
Although actual shrinkage in length is small, nevertheless the tendency of the rays to
shorten a stick produces strains which are responsible for some of the splitting open
of ties, posts, and sawed timbers with box heart. At the very centre of a tree the wood
is light and weak, while farther out it becomes denser and stronger. Longitudinal
shrinkage is accordingly least at the centre and greater toward the outside, tending to
become greatest in the sapwood. When a round or a box-heart timber dries fast it
splits radially, and as drying continues the cleft widens partly on account of the
greater tangential shrinkage and also because the greater contraction of the outer
fibres warps the sections apart. If a small hardwood stem is split while green for a
short distance at the end and placed where it can dry out rapidly, the sections will
become bow-shaped with the concave sides out. These various facts, taken together,
explain why, for example, an oak tie, pole, or log may split open its entire length if
drying proceeds rapidly and far enough. Initial stresses in the living trees produce a
similar effect when the log is sawn into boards. This is especially so in Eucalyptus
globulus and to a less extent with any rapidly grown wood.
The use of S-shaped thin steel clamps to prevent large checks and splits is now a
common practice in this country with crossties and poles as it has been for a long
time in European countries. These devices are driven into the butts of the timbers so
as to cross incipient checks and prevent their widening. In place of the regular S-
hook another of crimped iron has been devised. (See Fig. 28.) Thin straps of iron
with one tapered edge are run between intermeshing cogs and crimped, after which
they may be cut off any length desired. The time for driving S-irons of either form is
when the cracks first appear.

Figure 28
Control of season checking by the use of
S-irons. Photo by U. S. Forest Service.

The tendency of logs to split emphasizes the importance of converting them into
planks or timbers while in a green condition. Otherwise the presence of large checks
may render much lumber worthless which might have been cut out in good
condition. The loss would not be so great if logs were perfectly straight-grained, but
this is seldom the case, most trees growing more or less spirally or irregularly. Large
pieces crack more than smaller ones, quartered lumber less than that sawed through
and through, thin pieces, especially veneers, less than thicker boards.
In order to prevent cracks at the ends of boards, small straps of wood may be nailed
on them or they may be painted. This method is usually considered too expensive,
except in the case of valuable material. Squares used for shuttles, furniture, gun-
stocks, and tool handles should always be protected at the ends. One of the best
means is to dip them into melted paraffine, which seals the ends and prevents loss of
moisture there. Another method is to glue paper on the ends. In some cases abroad
paper is glued on to all the surfaces of valuable exotic balks. Other substances
sometimes employed for the purpose of sealing the wood are grease, carbolineum,
wax, clay, petroleum, linseed oil, tar, and soluble glass. In place of solid beams,
built-up material is often preferable, as the disastrous results of season checks are
thereby largely overcome or minimized.

TEMPERATURE

The effect of temperature on wood depends very largely upon the moisture content
of the wood and the surrounding medium. If absolutely dry wood is heated in
absolutely dry air the wood expands. The extent of this expansion is denoted by a
coefficient corresponding to the increase in length or other dimensions for each
degree rise in temperature divided by the original length or other dimension of the
specimen. The coefficient of linear expansion of oak has been found to be
.00000492; radial expansion, .0000544, or about eleven times the longitudinal.
Spruce expands less than oak, the ratio of radial to longitudinal expansion being
about six to one. Metals and glass expand equally in all directions, since they are
homogeneous substances, while wood is a complicated structure. The coefficient of
expansion of iron is .0000285, or nearly six times the coefficient of linear expansion
of oak and seven times that of spruce50.
Under ordinary conditions wood contains more or less moisture, so that the
application of heat has a drying effect which is accompanied by shrinkage. This
shrinkage completely obscures the expansion due to the heating.
Experiments made at the Yale Forest School revealed the effect of temperature on
the crushing strength of wet wood. In the case of wet chestnut wood the strength
decreases 0.42 per cent for each degree the water is heated above 60° F.; in the case
of spruce the decrease is 0.32 per cent.
The effects of high temperature on wet wood are very marked. Boiling produces a
condition of great pliability, especially in the case of hardwoods. If wood in this
condition is bent and allowed to dry, it rigidly retains the shape of the bend, though
its strength may be somewhat reduced. Except in the case of very dry wood the
effect of cold is to increase the strength and stiffness of wood. The freezing of any
free water in the pores of the wood will augment these conditions.
The effect of steaming upon the strength of cross-ties was investigated by the U.S.
Forest Service in 1904. The conclusions were summarized as follows:
"(1) The steam at pressure up to 40 pounds applied for 4 hours, or at a pressure of 20
pounds up to 20 hours, increases the weight of ties. At 40 pounds' pressure applied
for 4 hours and at 20 pounds for 5 hours the wood began to be scorched.
"(2) The steamed and saturated wood, when tested immediately after treatment,
exhibited weaknesses in proportion to the pressure and duration of steaming. (See
Table XVI.) If allowed to air-dry subsequently the specimens regained the greater
part of their strength, provided the pressure and duration had not exceeded those
cited under (1). Subsequent immersion in water of the steamed wood and dried

specimens showed that they were weaker than natural wood similarly dried and
resoaked."51

TABLE XVI
EFFECT OF STEAMING ON THE STRENGTH OF GREEN LOBLOLLY
PINE
(Forest Service, Cir. 39)
Cylinder conditions Strength
Steaming Static Impact
Height
Bending Average
Compression of drop
modulus of the
Treatment Period Pressure Temperature parallel to causing
of three
grain complete
rupture strengths
failure
Lbs. per Per cent Per cent Per cent Per cent
Hrs. °F.
sq. inch Untreated wood = 100%
4 230[a] 91.3 79.1 96.4 88.9
4 10 238 78.2 93.7 93.3 88.4
Steam, at 4 20 253 83.3 84.2 91.4 80.8
pressures 4 30 269 80.4 78.4 89.8 82.9
various 4 40 283 78.1 74.4 74.0 75.5
4 50 292 75.8 71.5 63.9 70.4
4 100 337 41.4 65.0 55.2 53.9
1 20 257 100.6 98.6 86.7 95.3
2 20 267 88.4 93.0 107.0 96.1
3 20 260 90.0 93.6 84.1 89.2
Steam, for
4 20 253 83.3 84.2 91.4 86.3
various
periods 5 20 253 85.0 78.1 84.2 82.4
6 20 242 95.2 89.8 76.0 87.0
10 20 255 73.7 82.0 76.0 77.2
20 20 258 67.5 65.0 99.0 77.2
[Footnote a: It will be noted that the temperature was 230°. This is the maximum
temperature by the maximum-temperature recording thermometer, and is due to the
handling of the exhaust valve. The average temperature was that of exhaust steam.]

"(3) A high degree of steaming is injurious to wood in strength and spike-holding
power. The degree of steaming at which pronounced harm results will depend upon
the quality of the wood and its degree of seasoning, and upon the pressure

(temperature) of steam and the duration of its application. For loblolly pine the limit
of safety is certainly 30 pounds for 4 hours, or 20 pounds for 6 hours."52
Experiments made at the Yale Forest School showed that steaming above 30 pounds'
gauge pressure reduces the strength of wood permanently while wet from 25 to 75
per cent.

PRESERVATIVES

The exact effects of chemical impregnation upon the mechanical properties of wood
have not been fully determined, though they have been the subject of considerable
investigation.53 More depends upon the method of treatment than upon the
preservatives used. Thus preliminary steaming at too high pressure or for too long a
period will materially weaken the wood, (See Tempurature, supra.)
The presence of zinc chloride does not weaken wood under static loading, although
the indications are that the wood becomes brittle under impact. If the solution is too
strong it will decompose the wood.
Soaking in creosote oil causes wood to swell, and accordingly decreases the strength
to some extent, but not nearly so much so as soaking in water.54
Soaking in kerosene seems to have no significant weakening effect.55

PART III
TIMBER TESTING56

WORKING PLAN

Preliminary to making a series of timber tests it is very important that a working plan
be prepared as a guide to the investigation. This should embrace: (1) the purpose of
the tests; (2) kind, size, condition, and amount of material needed; (3) full
description of the system of marking the pieces; (4) details of any special apparatus
and methods employed; (5) proposed method of analyzing the data obtained and the
nature of the final report. Great care should be taken in the preparation of this plan in
order that all problems arising may be anticipated so far as possible and delays and
unnecessary work avoided. A comprehensive study of previous investigations along
the same or related lines should prove very helpful in outlining the work and
preparing the report. (For sample working plan see Appendix.)

FORMS OF MATERIAL TESTED

In general, four forms of material are tested, namely: (1) large timbers, such as
bridge stringers, car sills, large beams, and other pieces five feet or more in length,
of actual sizes and grades in common use; (2) built-up structural forms and
fastenings, such as built-up beams, trusses, and various kind of joints; (3) small clear
pieces, such as are used in compression, shear, cleavage, and small cross-breaking
tests; (4) manufactured articles, such as axles, spokes, shafts, wagon-tongues, cross-
arms, insulator pins, barrels, and packing boxes.
As the moisture content is of fundamental importance (see Water Content, pages 75-
84.), all standard tests are usually made in the green condition. Another series is also
usually run in an air-dry condition of about 12 per cent moisture. In all cases the
moisture is very carefully determined and stated with the results in the tables.

SIZE OF TEST SPECIMENS

The size of the test specimen must be governed largely by the purpose for which the
test is made. If the effect of a single factor, such as moisture, is the object of
experiment, it is necessary to use small pieces of wood in order to eliminate so far as
possible all disturbing factors. If the specimens are too large, it is impossible to
secure enough perfect pieces from one tree to form a series for various tests.
Moreover, the drying process with large timbers is very difficult and irregular, and
requires a long period of time, besides causing checks and internal stresses which
may obscure the results obtained.
On the other hand, the smaller the dimensions of the test specimen the greater
becomes the relative effect of the inherent factors affecting the mechanical
properties. For example, the effect of a knot of given size is more serious in a small
stick than in a large one. Moreover, the smaller the specimen the fewer growth rings
it contains, hence there is greater opportunity for variation due to irregularities of
grain.
Tests on large timbers are considered necessary to furnish designers data on the
probable strength of the different sizes and grades of timber on the market; their
coefficients of elasticity under bending (since the stiffness rather than the strength
often determines the size of a beam); and the manner of failure, whether in bending
fibre stress or horizontal shear. It is believed that this information can only be
obtained by direct tests on the different grades of car sills, stringers, and other
material in common use.
When small pieces are selected for test they very often are clear and straight-grained,
and thus of so much better grade than the large sticks that tests upon them may not
yield unit values applicable to the larger sizes. Extensive experiments show,
however, (1) that the modulus of elasticity is approximately the same for large
timbers as for small clear specimens cut from them, and (2) that the fibre stress at
elastic limit for large beams is, except in the weakest timbers, practically equal to the
crushing strength of small clear pieces of the same material.57

MOISTURE DETERMINATION

In order for tests to be comparable, it is necessary to know the moisture content of
the specimens at the zone of failure. This is determined from disks an inch thick cut
from the timber immediately after testing.
In cases, as in large beams, where it is desirable to know not only the average
moisture content but also its distribution through the timber, the disks are cut up so
as to obtain an outside, a middle, and an inner portion, of approximately equal areas.
Thus in a section 10" × 12" the outer strip would be one inch wide, and the second

one a little more than an inch and a quarter. Moisture determinations are made for
each of the three portions separately.
The procedure is as follows:
(1) Immediately after sawing, loose splinters are removed and each section is
weighed.
(2) The material is put into a drying oven at 100° C. (212° F.) and dried until the
variation in weight for a period of twenty-four hours is less than 0.5 per cent.
(3) The disk is again carefully weighed.
(4) The loss in weight expressed in per cent of the dry weight indicates the moisture
content of the specimen from which the specimen was cut.

MACHINE FOR STATIC TESTS

The standard screw machines used for metal tests are also used for wood, but in the
case of wood tests the readings must be taken "on the fly," and the machine operated
at a uniform speed without interruption from beginning to end of the test. This is on
account of the time factor in the strength of wood. (See Speed of Testing Machine,
page 92.)
The standard machines for static tests can be used for transverse bending,
compression, tension, shear, and cleavage. A common form consists of three main
parts, namely: (1) the straining mechanism, (2) the weighing apparatus, and (3) the
machinery for communicating motion to the screws.
The straining mechanism consists of two parts, one of which is a movable crosshead
operated by four (sometimes two or three) upright steel straining screws which pass
through openings in the platform and bear upward on the bed of the machine upon
which the weighing platform rests as a fulcrum. At the lower ends of these screws
are geared nuts all rotated simultaneously by a system of gears which cause the
movable crosshead to rise and fall as desired.
The stationary part of the straining mechanism, which is used only for tension and
cleavage tests, consists of a steel cage above the movable crosshead and rests
directly upon the weighing platform. The top of the cage contains a square hole into
which one end of the test specimen may be clamped, the crosshead containing a
similar clamp for the other end, in making tension tests.

For testing long beams a special form of machine with an extended platform is used.
(See Fig. 29.)
The weighing platform rests upon knife edges carried by primary levers of the
weighing apparatus, the fulcrum being on the bed of the machine, and any pressure
upon it is directly transmitted through a series of levers to the weighing beam. This
beam is adjusted by means of a poise running on a screw. In operation the beam is
kept floating by means of another poise moved back and forth by a screw which is
operated by a hand wheel or automatically. The larger units of stress are read from
the graduations along the side of the beam, while the intermediate smaller weights
are observed on the dial on the rear end of the beam.
The machine is driven by power from a shaft or a motor and is so geared that various
speeds are obtainable. One man can operate it.
In making tests the operation of the straining screws is always downward so as to
bring pressure to bear upon the weighing platform. For tests in tension and cleavage
the specimen is placed between the top of the stationary cage and the movable head
and subjected to a pull. For tests in transverse bending, compression, and cleavage
the specimen is placed between the movable head and the platform, and a direct
compression force applied.
Testing machines are usually calibrated to a portion of their capacity before leaving
the factory. The delicacy of the weighing levers is verified by determining the
number of pounds necessary to move the beam between the stops while a load of
1,000 pounds rests on the platform. The usual requirement is that ten pounds should
accomplish this movement.
The size of machine suitable for compression tests on 2" × 2" sticks or for 2" × 2"
beams with 26 to 36-inch span has a capacity of 30,000 pounds.

SPEED OF TESTING MACHINE

In instructions for making static tests the rate of application of the stress, i.e., the
speed of the machine, is given because the strength of wood varies with the speed at
which the fibres are strained. The speed of the crosshead of the testing machine is
practically never constant, due to mechanical defects of the apparatus and variations
in the speed of the motor, but so long as it does not exceed 25 per cent the results
will not be appreciably affected. In fact, a change in speed of 50 per cent will not
cause the strength of the wood to vary more than 2 per cent.58

Following are the formulæ used in determining the speed of the movable head of the
machine in inches per minute (n):

(1) For endwise compression n= Zl
Z l2
(2) For beams (centre loading) n= ------
6h
Z l2
(3) For beams (third-point loading) n = ------
5.4 h

Z = rate of fibre strain per inch of fibre length.
l = span of beam or length of compression specimen.
h = height of beam.

The values commonly used for Z are as follows:
Bending large beams Z = 0.0007
Bending small beams Z = 0.0015
Endwise compression-large specimens Z = 0.0015
Endwise compression-small specimens Z = 0.003
Right-angled compression-large specimens Z = 0.007
Right-angled compression-small specimens Z = 0.015
Shearing parallel to the grain Z = 0.015

Example: At what speed should the crosshead move to give the required rate of fibre strain in testing a small beam
2" × 2" × 30". (Span = 28".) Substituting these values in equation (2) above:

(0.0015 × 282)
n = ----------------- = 0.1 inch per minute.
(6 × 2)

In order that tests may be intelligently compared, it is important that account be taken of the speed at which the
stress was applied. In determining the basis for a ratio between time and strength the rate of strain, which is
controllable, and not the ratio of stress, which is circumstantial, should be used. In other words, the rate at which
the movable head of the testing machine descends and not the rate of increase in the load is to be regulated. This
ratio, to which the name speed-strength modulus has been given, may be expressed as a coefficient which, if
multiplied into any proportional change in speed, will give the proportional change in strength. This ratio is
derived from empirical curves. (See Table XVII.)

TABLE XVII
SPEED-STRENGTH MODULI AND RELATIVE INCREASE IN STRENGTH AT RATES OF FIBRE STR
loc. cit.)
(Values in parentheses are approximate)
Rate of fibre strain.
2/3 2 6 18
Ten-thousandths inch per minute per inch
COMPRESSION Speed of crosshead.
0.000383 0.00115 0.00345 0.01
Inches per minute
Specimens Wet Dry All Wet Dry All Wet Dry All Wet Dr

Relative crushing strength 100.0 100.0 100.0 103.4100.8101.5 107.5102
Speed-strength modulus,
0.017 (0.006) (0.009) 0.0330.0120.016 0.0470.02
T
Speed of crosshead.
0.0072 0.0216 0.0648 0.19
Inches per minute
Specimens Wet Dry All Wet Dry All Wet Dry All Wet Dr
BENDING
Relative crushing strength 97.4 99.0 98.2 100.0 100.0 100.0 105.1102.1103.7 111.3 105
Speed-strength modulus,
(0.014) (0.005) 0.0120.033 0.014 0.026 0.0490.0260.037 0.0530.0
T
NOTE.—The usual speeds of testing at the U.S. Forest Service laboratory are at rates of fibre strain of 15 and 10 ten
respectively.

BENDING LARGE BEAMS

Apparatus: A static bending machine (described above), with a special crosshead for third-point loading and a long
platform bearing knife-edge supports, is required. (See Fig. 29.)

Figure 29
Static bending test on large beam. Note arrangement of
wire and scale for measuring deflection; also method of
applying load at "third-points."

Preparing the material: Standard sizes and grades of beams and timbers in common use are employed. The ends
are roughly squared and the specimen weighed and measured, taking the cross-sectional dimensions midway of the
length. Weights should be to the nearest pound, lengths to the nearest 0.1 inch, and cross-sectional dimensions to
the nearest 0.01 inch.
Marking and sketching: The butt end of the beam is marked A and the top end B. While facing A, the top side is
marked a, the right hand b, the bottom c, the left hand d. Sketches are made of each side and end, showing (1) size,
location, and condition of knots, checks, splits, and other defects; (2) irregularities of grain; (3) distribution of
heartwood and sapwood; and on the ends: (4) the location of the pith and the arrangement of the growth rings, (5)
number of rings per inch, and (6) the proportion of late wood.
The number of rings per inch and the proportion of late wood should always be determined along a radius or a line
normal to the rings. The average number of rings per inch is the total number of rings divided by the length of the
line crossing them. The proportion of late wood is equal to the sum of the widths of the late wood crossed by the

line, divided by the length of the line. Rings per inch should be to the nearest 0.1; late wood to the nearest 0.1 per
cent.
Since in large beams a great variation in rate of growth and relative amount of late wood is likely in different parts
of the section, it is advisable to consider the cross section in three volumes, namely, the upper and lower quarters
and the middle half. The determination should be made upon each volume separately, and the average for the entire
cross section obtained from these results.
At the conclusion of the test the failure, as it appears on each surface, is traced on the sketches, with the failures
numbered in the order of their occurrence. If the beam is subsequently cut up and used for other tests an additional
sketch may be desirable to show the location of each piece.
Adjusting specimen in machine: The beam is placed in the machine with the side marked a on top, and with the
ends projecting equally beyond the supports. In order to prevent crushing of the fibre at the points where the stress
is applied it is necessary to use bearing blocks of maple or other hard wood with a convex surface in contact with
the beam. Roller bearings should be placed between the bearing blocks and the knife edges of the crosshead to
allow for the shortening due to flexure. (See Fig. 29.) Third-point loading is used, that is, the load is applied at two
points one-third the span of the beam apart. (See Fig. 30.) This affords a uniform bending moment throughout the
central third of the beam.

Figure 30
Two methods of loading a beam, namely, third-point
loading (upper), and centre loading (lower).

Measuring the deflection: The method of measuring the deflection should be such that any compression at the
points of support or at the application of the load will not affect the reading. This may be accomplished by driving
a small nail near each end of the beam, the exact location being on the neutral plane and vertically above each
knife-edge support. Between these nails a fine wire is stretched free of the beam and kept taut by means of a
rubber band or coiled spring on one end. Behind the wire at a point on the beam midway between the supports a
steel scale graduated to hundredths of an inch is fastened vertically by means of thumb-tacks or small screws
passing through holes in it. Attachment should be made on the neutral plane.
The first reading is made when the scale beam is balanced at zero load, and afterward at regular increments of the
load which is applied continuously and at a uniform speed. (See Speed of Testing Machine, page 92.) If desired,
however, the load may be read at regular increments of deflection. The deflection readings should be to the nearest
0.01 inch. To avoid error due to parallax, the readings may be taken by means of a reading telescope about ten feet
distant and approximately on a level with the wire. A mirror fastened to the scale will increase the accuracy of the
readings if the telescope is not used. As in all tests on timber, the strain must be continuous to rupture, not

intermittent, and readings must be taken "on the fly." The weighing beam is kept balanced after the yield point is
reached and the maximum load, and at least one point beyond it, noted.
Log of the test: The proper log sheet for this test consists of a piece of cross-section paper with space at the margin
for notes. (See Fig. 32.) The load in some convenient unit (1,000 to 10,000 pounds, depending upon the
dimensions of the specimen) is entered on the ordinates, the deflection in tenths of an inch on the abscissæ. The
increments of load should be chosen so as to furnish about ten points on the stress-strain diagram below the elastic
limit.
As the readings of the wire on the scale are made they are entered directly in their proper place on the cross-section
paper. In many cases a test should be continued until complete failure results. The points where the various failures
occur are indicated on the stress-strain diagram. A brief description of the failure is made on the margin of the log
sheet, and the form traced on the sketches.
Disposal of the specimen: Two one-inch sections are cut from the region of failure to be used in determining the
moisture content. (See Moisture Determination, page 90.) A two-inch section may be cut for subsequent reference
and identification, and possible microscopic study. The remainder of the beam may be cut into small beams and
compression pieces.
Calculating the results: The formulæ used in calculating the results of tests on large rectangular simple beams
loaded at third points of the span are as follows:

0.75 P
(1) J = --------
bh

l (P1 + 0.75 W)
(2) r = --------------------
b h2

l (P + 0.75 W)
(3) R = ----------------
b h2

P1l3
(4) E = ---------------
4.7 D b h 3

0.87 P 1 D
(5) S = --------------
2V

b, h, l = breadth, height, and span of specimen, inches.
D = total deflection at elastic limit, inches.
P = maximum load, pounds.
P1 = load at elastic limit, pounds.
E = modulus of elasticity, pounds per square inch.
r = fibre stress at elastic limit, pounds per sq. inch.
R = modulus of rupture, pounds per square inch.
S = elastic resilience or work to elastic limit, inch-pounds per cu. in.
J = greatest calculated longitudinal shear, pounds per square inch.

V = volume of beam, cubic inches.
W = weight of the beam.

In large beams the weight should be taken into account in calculating the fibre stress. In (2) and (3) three-fourths of
the weight of the beam is added to the load for this reason.

BENDING SMALL BEAMS

Apparatus: An ordinary static bending machine, a steel I-beam bearing two adjustable knife-edge supports to rest
on the platform, and a special deflectometer, are required. (See Fig. 31.)

Figure 31
Static bending test on small beam. Note the use of the
deflectometer with indicator and dial for measuring the
deflection; also roller bearings between beam and
supports.

Preparing the material: The specimens may be of any convenient size, though beams 2" × 2" × 30" tested over a
28-inch span, are considered best. The beams are surfaced on all four sides, care being taken that they are not
damaged by the rollers of the surfacing machine. Material for these tests is sometimes cut from large beams after
failure. The specimens are carefully weighed in grams, and all dimensions measured to the nearest 0.01 inch. If to
be tested in a green or fresh condition the specimens should be kept in a damp box or covered with moist sawdust
until needed. No defects should be allowed in these specimens.
Marking and sketching: Sketches are made of each end of the specimen to show the character of the growth, and
after testing, the manner of failure is shown for all four sides. In obtaining data regarding the rate of growth and
the proportion of late wood the same procedure is followed as with large beams.
Adjusting specimen in machine: The beam should be correctly centred in the machine and each end should have a
plate with roller bearings between it and the support. Centre loading is used. Between the movable head of the
machine and the specimen is placed a bearing block of maple or other hard wood, the lower surface of which is
curved in a direction along the beam, the curvature of which should be slightly less than that of the beam at
rupture, in order to prevent the edges from crushing into the fibres of the test piece.
Measuring the deflection: The method of measuring deflection of large beams can be used for small sizes, but
because of the shortness of the span and consequent slight deformation in the latter, it is hardly accurate enough for
good work. The special deflectometer shown in Fig. 31 allows closer reading, as it magnifies the deflection ten

times. It rests on two small nails driven in the beam on the neutral plane and vertically above the supports. The fine
wire on the wheel at the base of the indicator is attached to another small nail driven in the beam on the neutral
plane midway between the end nails. All three nails should be in place before the beam is put into the machine.
The indicator is adjustable by means of a thumb-screw at the base and is set at zero before the load is applied.
Deflections are read to the nearest 0.001 inch. For rate of application of load see Speed of Testing Machine, page
92. The speed should be uniform from start to finish without stopping. Readings must be made "on the fly."
Log of the test: The log sheets used for small beams (see Fig. 32) are the same as for large sizes and the procedure
is practically identical. The stress-strain diagram is continued to or beyond the maximum load, and in a portion of
the tests should be continued to six-inch deflection or until the specimen fails to support a load of 200 pounds.
Deflection readings for equal increments of load are taken until well beyond the elastic limit, after which the scale
beam is kept balanced and the load read for each 0.1 inch deflection. The load and deflection at first failure, the
maximum load, and any points of sudden change should be shown on the diagram, even though they do not occur
at one of the regular points. A brief description of the failure and the nature of any defects is entered on the log
sheet.

Figure 32
Sample log sheet, giving full details of a transverse
bending test on a small pine beam.

Calculating the results: The formulæ used in calculating the results of tests on small rectangular simple beams are
as follows:

0.75 P
(1) J = --------
bh

1.5 P1 l
(2) r = ------------
b h2

1.5 P l
(3) R = ---------
b h2

P1 l 3
(4) E = ------------
4 D b h3

P1 D
(5) S = ---------
2V

The same legend is used as on page 98. The weight of the beam itself is disregarded.

ENDWISE COMPRESSION

Apparatus: An ordinary static testing machine and a compressometer are required. (See Fig. 33.)

Figure 33
Endwise compression test, showing method of
measuring the deformation by means of a
compressometer.

Preparing the material: Two classes of specimens are commonly used, namely, (1) posts 24 inches in length, and
(2) small clear blocks approximately 2" × 2" × 8". The specimens are surfaced on all four sides and both ends
squared smoothly and evenly. They are carefully weighed, measured, rate of growth and proportion of late wood
determined, as in bending tests. After the test a moisture section is cut and weighed. Ordinarily these specimens
should be free from defects.
Sketching: Sketches are made of each end of the specimens to show the character of the growth. After testing, the
manner of failure is shown for all four sides, and the various parts of the failure are numbered in the order of their
occurrence.
Adjusting specimen in machine: The compressometer collars are adjusted, the distance between them being 20
inches for the posts and 6 inches for the blocks. If the two ends of the blocks are not exactly parallel a ball-and-
socket block can be placed between the upper end of the specimen and the movable head of the machine to
overcome the irregularity. If the blocks are true they can simply be stood on end upon the platform and the
movable head allowed to press directly upon the upper end.
Measuring the deformation: The deformation is measured by a compressometer. (See Fig. 33.) The latter registers
to 0.001 inch. In the case of posts the compression between the collars is communicated to the four points on the
arms by means of brass rods; with short blocks, as in Fig. 33, the points of the arms are in direct contact with the
collars. The operator lowers the fulcrum of the apparatus by moving the micrometer screws at such a rate that the
set-screw in the rear end of the upper lever is kept barely touching the fixed arm below it, being guided by a bell
operated by electric contact.
Log of the test: The load is applied continuously at a uniform rate of speed. (See Speed of Testing Maching, page
92.) Readings are taken from the scale of the compressometer at regular increments of either load or compression.
The stress-strain diagram is continued to at least one deformation point beyond the maximum load, and in event of
sudden failure, the direction of the curve beyond the maximum point is indicated. A brief description of the failure
is entered on the log sheet. (See Fig. 34.)

Figure 34
Sample log sheet of an endwise compression test on a
short pine column.

In short specimens the failure usually occurs in one or several planes diagonal to the axis of the specimen. If the
ends are more moist than the middle a crushing may occur on the extreme ends in a horizontal plane. Such a test is
not valid and should always be culled. If the grain is diagonal or the stress is unevenly applied a diagonal shear
may occur from top to bottom of the test specimen. Such tests are also invalid and should be culled. When the
plane (or several planes) of failure occurs through the body of the specimen the test is valid. It may sometimes be
advantageous to allow the extreme ends to dry slightly before testing in order to bring the planes of failure within
the body. This is a perfectly legitimate procedure provided no drying is allowed from the sides of the specimen,
and the moisture disk is cut from the region of failure.
Calculating the results: The formulæ used in calculating the results of tests on endwise compression are as
follows:

P
(1) C = -----
A

P1
(2) c = -------
A

P1 l
(3) E = ---------
AD

PD
(4) S = -----
2V

C = crushing strength, pounds per square inch.
c = fibre strength at elastic limit, pounds per square inch.
A = area of cross section, square inches.
l = distance between centres of collars, inches.
D = total shortening at elastic limit, inches.
V = volume of specimen, cubic inches.

Remainder of legend as on page 98.

COMPRESSION ACROSS THE GRAIN

Apparatus: An ordinary static testing machine, a bearing plate, and a deflectometer are required. (See Fig. 35.)

Figure 35
Compression across the grain. Note method of
measuring the deformation by means of a deflectomoter.

Preparing the material: Two classes of specimens are used, namely, (1) sections of commercial sizes of ties,
beams, and other timbers, and (2) small, clear specimens with the length several times the width. Sometimes small
cubes are tested, but the results are hardly applicable to conditions in practice. In (2) the sides are surfaced and the
ends squared. The specimens are then carefully measured and weighed, defects noted, rate of growth and

proportion of late wood determined, as in bending tests. (See page 95.) After the test a moisture section is cut and
weighed.
Sketching: Sketches are made as in endwise compression tests. (See page 102.)
Adjusting specimen in machine: The specimen is laid horizontally upon the platform of the machine and a steel
bearing plate placed on its upper surface immediately beneath the centre of the movable head. For the larger
specimens this plate is six inches wide; for the smaller sizes, two inches wide. The plate in all cases projects over
the edges of the test piece, and in no case should the length of the latter be less than four times the width of the
plate.
Measuring the deformation: The compression is measured by means of a deflectometer (see Fig. 35), which, after
the first increment of load is applied, is adjusted (by means of a small set screw) to read zero. The actual
downward motion of the movable head (corresponding to the compression of the specimen) is multiplied ten times
on the scale from which the readings are made.
Log of the test: The load is applied continuously and at uniform speed (see Speed of Testing Machine, page 92),
until well beyond the elastic limit. The compression readings are taken at regular load increments and entered on
the cross-section paper in the usual way. Usually there is no real maximum load in this case, as the strength
continually increases as the fibres are crushed more compactly together.
Calculating the results: Ordinarily only the fibre stress at the elastic limit (c) is computed. It is equal to the load at
elastic limit (P1) divided by the area under the plate (B).

P1
( c = ------- )
B

SHEAR ALONG THE GRAIN

Apparatus: An ordinary static testing machine and a special tool designed for producing single shear are required.
(See Figs. 36 and 37.) This shearing apparatus consists of a solid steel frame with set screws for clamping the
block within it firmly in a vertical position. In the centre of the frame is a vertical slot in which a square-edged
steel plate slides freely. When the testing block is in position, this plate impinges squarely along the upper surface
of the tenon or lip, which, as vertical pressure is applied, shears off.

Figure 36

Vertical section of shearing tool.

Figure 37
Front view of shearing tool with test specimen and steel
plate in position for testing.

Preparing the material: The specimens are usually in the form of small, clear, straight-grained blocks with a
projecting tenon or lip to be sheared off. Two common forms and sizes are shown in Figure 38. Part of the blocks
are cut so that the shearing surface is parallel to the growth rings, or tangential; others at right angles to the growth
rings, or radial. It is important that the upper surface of the tenon or lip be sawed exactly parallel to the base of the
block. When the form with a tenon is used the under cut is extended a short distance horizontally into the block to
prevent any compression from below.

Figure 38
Two forms of shear test specimens.

In designing a shearing specimen it is necessary to take into consideration the proportions of the area of shear,
since, if the length of the portion to be sheared off is too great in the direction of the shearing face, failure would
occur by compression before the piece would shear. Inasmuch as the endwise compressive strength is sometimes
not more than five times the shearing strength, the shearing surface should be less than five times the surface to
which the load is applied. This condition is fulfilled in the specimens illustrated.
Shearing specimens are frequently cut from beams after testing. In this case the specific gravity (dry), proportion
of late wood, and rate of growth are assumed to be the same as already recorded for the beams. In specimens not

so taken, these quantities are determined in the usual way. The sheared-off portion is used for a moisture section.
Adjusting specimen in machine: The test specimen is placed in the shearing apparatus with the tenon or lip under
the sliding plate, which is centred under the movable head of the machine. (See Fig. 39.) In order to reduce to a
minimum the friction due to the lateral pressure of the plate against the bearings of the slot, the apparatus is
sometimes placed upon several parallel steel rods to form a roller base. A slight initial load is applied to take up the
lost motion of the machinery, and the beam balanced.

Figure 39
Making a shearing test.

Log of the test: The load is applied continuously and at a uniform rate until failure, but no deformations are
measured. The points noted are the maximum load and the length of time required to reach it. Sketches are made of
the failure. If the failure is not pure shear the test is culled.
The shearing strength per square inch is found by dividing the maximum load by the cross-sectional area.

P
( Q = --- )
A

IMPACT TEST

Apparatus: There are several types of impact testing machines.59 One of the simplest and most efficient for use
with wood is illustrated in Figure 40. The base of the machine is 7 feet long, 2.5 feet wide at the centre, and weighs
3,500 pounds. Two upright columns, each 8 feet long, act as guides for the striking head. At the top of the column
is the hoisting mechanism for raising or lowering the striking weights. The power for operating the machine is
furnished by a motor set on the top. The hoisting-mechanism is all controlled by a single operating lever, shown on
the side of the column, whereby the striking weight may be raised, lowered, or stopped at the will of the operator.
There is an automatic safety device for stopping the machine when the weight reaches the top.

Figure 40
Impact testing machine.

The weight is lifted by a chain, one end of which passes over a sprocket wheel in the hoisting mechanism. On the
lower end of the chain is hung an electro-magnet of sufficient magnetic strength to support the heaviest striking
weights. When it is desired to drop the striking weight the electric current is broken and reversed by means of an
automatic switch and current breaker. The height of drop may be regulated by setting at the desired height on one
of the columns a tripping pin which throws the switch on the magnet and so breaks and reverses the current.
There are four striking weights, weighing respectively 50, 100, 250, and 500 pounds, any one of which may be
used, depending upon the desired energy of blow. When used for compression tests a flat steel head six inches in
diameter is screwed into the lower end of the weight. For transverse tests, a well-rounded knife edge is screwed
into the weight in place of the flat head. Knife edges for supporting the ends of the specimen to be tested, are
securely bolted to the base of the machine.
The record of the behavior of the specimen at time of impact is traced upon a revolving drum by a pencil fixed in
the striking head. (See Fig. 41.) When a drop is made the pencil comes in contact with the drum and is held in
place by a spring. The drum is revolved very slowly, either automatically or by hand. The speed of the drum can be
recorded by a pencil in the end of a tuning fork which gives a known number of vibrations per second.

Figure 41

Drum record of impact bending test.

One size of this machine will handle specimens for transverse tests 9 inches wide and 6-foot span; the other, 12
inches wide and 8-foot span. For compression tests a free fall of about 6.5 feet may be obtained. For transverse
tests the fall is a little less, depending upon the size of the specimen.
The machine is calibrated by dropping the hammer upon a copper cylinder. The axial compression of the plug is
noted. The energy used in static tests to produce this axial compression under stress in a like piece of metal is
determined. The external energy of the blow (i.e., the weight of the hammer × the height of drop) is compared with
the energy used in static tests at equal amounts of compression. For instance:

Energy delivered, impact test 35,000 inch-pounds
Energy computed from static test 26,400 inch-pounds
Efficiency of blow of hammer 75.3 per cent.

Preparing the material: The material used in making impact tests is of the same size and prepared in the same way
as for static bending and compression tests. Bending in impact tests is more commonly used than compression, and
small beams with 28-inch span are usually employed.
Method: In making an impact bending test the hammer is allowed to rest upon the specimen and a zero or datum
line is drawn. The hammer is then dropped from increasing heights and drum records taken until first failure. The
first drop is one inch and the increase is by increments of one inch until a height of ten inches is reached, after
which increments of two inches are used until complete failure occurs or 6-inch deflection is secured.
The 50-pound hammer is used when with drops up to 68 inches it is reasonably certain it will produce complete
failure or 6-inch deflection in the case of all specimens of a species; for all other species a 100-pound hammer is
used.
Results: The tracing on the drum (see Fig. 41) represents the actual deflection of the stick and the subsequent
rebounds for each drop. The distance from the lowest point in each case to the datum line is measured and its
square in tenths of a square inch entered as an abscissa on cross-section paper, with the height of drop in inches as
the ordinate. The elastic limit is that point on the diagram where the square of the deflection begins to increase
more rapidly than the height of drop. The difference between the datum line and the final resting point after each
drop represents the set the material has received.
The formulæ used in calculating the results of impact tests in bending when the load is applied at the centre up to
the elastic limit are as follows:

3WHl
(1) r = -----------
D b h2

F S l2
(2) E = -----------
6Dh

WH
(3) S = -------
lbh

H = height of drop of hammer, including deflection, inches.
S = modulus of elastic resilience, inch-pounds per cubic inch.

W = weight of hammer, pounds.

Remainder of legend as on page 98.

HARDNESS TEST: ABRASION AND INDENTATION

Abrasion: The machine used by the U.S. Forest Service is a modified form of the Dorry abrasion machine. (See
Fig. 42.) Upon the revolving horizontal disk is glued a commercial sandpaper, known as garnet paper, which is
commonly employed in factories in finishing wood.

Figure 42
Abrasion machine for testing the wearing qualities of
woods.

A small block of the wood to be tested is fixed in one clamp and a similar block of some wood chosen as a
standard, as sugar maple, at 10 per cent moisture, in the opposite, and held against the same zone of sandpaper by a
weight of 26 pounds each. The size of the section under abrasion for each specimen is 2" × 2". The conditions for
wear are the same for both specimens. The speed of rotation is 68 revolutions a minute.
The test is continued until the standard specimen is worn a specified amount, which varies with the kind of wood
under test. A comparison of the wear of the two blocks affords a fair idea of their relative resistance to abrasion.

Another method makes use of a sand blast to abrade the woods and is the one employed in New South Wales.60
The apparatus consists essentially of a nozzle through which sand can be propelled at a high velocity against the
test specimen by means of a steam jet.
The wood to be tested is cut into blocks 3" × 3" × 1', and these are weighed to the nearest grain just before placing
in the apparatus. Steam from the boiler at a pressure of about 43 pounds per square inch is ejected from a nozzle in
such a way that particles of fine quartz sand are caught up and thrown violently against the block which is being
rotated. Only superheated steam strikes the block, thus leaving the wood dry. The test is continued for two minutes,
after which the specimen is removed and immediately weighed.
By comparison with the original weight the loss from abrasion is determined, and by comparison with a certain
wood chosen as a standard, a coefficient of wear-resistance can be obtained. The amount of wear will vary more or
less according to the surface exposed, and in these tests quarter-sawed material was used with the edge grain to the
blast.

Indentation: The tool used for this test consists of a punch with a hemispherical end or steel ball having a diameter
of 0.444 inch, giving a surface area of one-fourth square inch. It is fitted with a guard plate, which works loosely
until the penetration has progressed to a depth of 0.222 inch, whereupon it tightens. (See Fig. 43.) The effect is that
of sinking a ball half its diameter into the specimen. This apparatus is fitted into the movable head of the static
testing machine.

Figure 43
Design of tool for testing the hardness of woods by
indentation.

The wood to be tested is cut square with the grain into rectangular blocks measuring 2" × 2" × 6". A block is
placed on the platform and the end of the punch forced into the wood at the rate of 0.25 inch per minute. The
operator keeps moving the small handle of the guard plate back and forth until it tightens. At this instant the load is
read and recorded.
Two penetrations each are made on the tangential and radial surfaces, and one on each end of every specimen
tested.
In choosing the places on the block for the indentations, effort should be made to get a fair average of heartwood
and sapwood, fine and coarse grain, early and late wood.
Another method of testing by indentation involves the use of a right-angled cone instead of a ball. For details of
this test as used in New South Wales see loc. cit., pp. 86-87.

CLEAVAGE TEST

A static testing machine and a special cleavage testing device are required. (See Fig. 44.) The latter consists
essentially of two hooks, one of which is suspended from the centre of the top of the cage, the other extended
above the movable head.

Figure 44
Design of tool for cleavage test.

The specimens are 2" × 2" × 3.75". At one end a one-inch hole is bored, with its centre equidistant from the two
sides and 0.25 inch from the end. (See Fig. 45.) This makes the cross section to be tested 2" × 3". Some of the
blocks are cut radially and some tangentially, as indicated in the figure.

Figure 45
Design of cleavage test specimen.

The free ends of the hooks are fitted into the notch in the end of the specimen. The movable head of the machine is
then made to descend at the rate of 0.25 inch per minute, pulling apart the hooks and splitting the block. The
maximum load only is taken and the result expressed in pounds per square inch of width. A piece one-half inch
thick is split off parallel to the failure and used for moisture determination.

TENSION TEST PARALLEL TO THE GRAIN

Since the tensile strength of wood parallel to the grain is greater than the compressive strength, and exceedingly
greater than the shearing strength, it is very difficult to make satisfactory tension tests, as the head and shoulders of
the test specimen (which is subjected to both compression and shear) must be stronger than the portion subjected
to a pure tensile stress.

Various designs of test specimens have been made. The one first employed by the Division of Forestry61 was
prepared as follows: Sticks were cut measuring 1.5" × 2.5" × 16". The thickness at the centre was then reduced to
three-eighths of an inch by cutting out circular segments with a band saw. This left a breaking section of 2.5" ×
0.375". Care was taken to cut the specimen as nearly parallel to the grain as possible, so that its failure would
occur in a condition of pure tension. The specimen was then placed between the plane wedge-shaped steel grips of
the cage and the movable head of the static machine and pulled in two. Only the maximum load was recorded.
(See Fig. 46, No. 1.)

Figure 46
Designs of tension test specimens used in United States.

The difficulty of making such tests compared with the minor importance of the results is so great that they are at
present omitted by the U.S. Forest Service. A form of specimen is suggested, however, and is as follows: "A rod of
wood about one inch in diameter is bored by a hollow drill from the stick to be tested. The ends of this rod are
inserted and glued in corresponding holes in permanent hardwood wedges. The specimen is then submitted to the
ordinary tension test. The broken ends are punched from the wedges."62 (See Fig. 46, No. 2.)
The form used by the Department of Forestry of New South Wales63 is as shown in Fig. 47. The specimen has a
total length of 41 inches and is circular in cross section. On each end is a head 4 inches in diameter and 7 inches
long. Below each head is a shoulder 8.5 inches long, which tapers from a diameter of 2.75 inches to 1.25 inches. In
the middle is a cylindrical portion 1.25 inches in diameter and 10 inches long.

Figure 47
Design of tension test specimen used in New South
Wales.

In making the test the specimen is fitted in the machine, and an extensometer attached to the middle portion and
arranged to record the extension between the gauge points 8 inches apart. The area of the cross section then is
1.226 square inches, and the tensile strength is equal to the total breaking load applied divided by this area.

TENSION TEST AT RIGHT ANGLES TO THE GRAIN

A static testing machine and a special testing device (see Fig. 48) are required. The latter consists essentially of
two double hooks or clamps, one of which is suspended from the centre of the top of the cage, the other extended
above the movable head. The specimens are 2" × 2" × 2.5". At each end a one-inch hole is bored with its centre
equidistant from the two sides and 0.25 inch from the ends. This makes the cross section to be tested 1" × 2".

Figure 48
Design of tool and specimen for testing tension at right
angles to the grain.

The free ends of the clamps are fitted into the notches in the ends of the specimen. The movable head of the
machine is then made to descend at the rate of 0.25 inch per minute, pulling the specimen in two at right angles to
the grain. The maximum load only is taken and the result expressed in pounds per inch of width. A piece one-half
inch thick is split off parallel to the failure and used for moisture determination.

TORSION TEST64

Apparatus: The torsion test is made in a Riehle-Miller torsional testing machine or its equivalent. (See Fig. 49.)

Figure 49
Making a torsion test on hickory.

Preparation of material: The test pieces are cylindrical, 1.5 inches in diameter and 18 inches gauge length, with
squared ends 4 inches long joined to the cylindrical portion with a fillet. The dimensions are carefully measured,
and the usual data obtained in regard to the rate of growth, proportion of late wood, location and kind of defects.
The weight of the cylindrical portion of the specimen is obtained after the test.
Making the test: After the specimen is fitted in the machine the load is applied continuously at the rate of 22° per
minute. A troptometer is used in measuring the deformation. Readings are made until failure occurs, the points
being entered on the cross-section paper. The character of the failure is described. Moisture determinations are
made by the disk method.
Results: The conditions of ultimate rupture due to torsion appear not to be governed by definite mathematical laws;
but where the material is not overstrained, laws may be assumed which are sufficiently exact for practical cases.
The formulæ commonly used for computations are as follows:

5.1 M
(1) T = -------
c3

114.6 T f
(2) G = -----------
ac

a = angle measured by troptometer at elastic limit, in degrees.
c = diameter of specimen, inches.
f = gauge length of specimen, inches.
G = modulus of elasticity in shear across the grain, pounds per square inch.
M = moment of torsion at elastic limit, inch-pounds.
T = outer fibre torsional stress at elastic limit, pounds per square inch.

SPECIAL TESTS

Spike-pulling Test

Spike-pulling tests apply to problems of railroad maintenance, and the results are used to compare the spike-
holding powers of various woods, both untreated and treated with different preservatives, and the efficiency of
various forms of spikes. Special tests are also made in which the spike is subjected to a transverse load applied
repetitively by a blow.
For details of tests and results see:

Cir. 38, U.S.F.S.: Instructions to engineers of timber tests, p. 26.
Cir. 46, U.S.F.S.: Holding force of railroad spikes in wooden ties.
Bul. 118, U.S.F.S.: Prolonging the life of cross-ties, pp. 37-40.

Packing Boxes

Special tests on the strength of packing boxes of various woods have been made by the U.S. Forest Service to
determine the merits of different kinds of woods as box material with the view of substituting new kinds for the
more expensive ones now in use. The methods of tests consisted in applying a load along the diagonal of a box, an
action similar to that which occurs when a box is dropped on one of its corners. The load was measured at each
one-fourth inch in deflection, and notes were made of the primary and subsequent failures.
For details of tests and results, see:

Cir. 47, U.S.F.S.: Strength of packing boxes of various woods.
Cir. 214, U.S.F.S.: Tests of packing boxes of various forms.

Vehicle and Implement Woods

Tests were made by the U.S. Forest Service to obtain a better knowledge of the mechanical properties of the woods
at present used in the manufacture of vehicles and implements and of those which might be substituted for them.
Tests were made upon the following materials: hickory buggy spokes (see Fig. 5); hickory and red oak buggy
shafts; wagon tongues; Douglas fir and southern pine cultivator poles.
Details of the tests and results may be found in:

Cir. 142, U.S.F.S.: Tests on vehicle and implement woods.

Cross-arms

In tests by the U.S. Forest Service on cross-arms a special apparatus was devised in which the load was distributed
along the arm as in actual practice. The load was applied by rods passing through the pinholes in the arms. Nuts on
these rods pulled down on the wooden bearing-blocks shaped to fit the upper side of the arm. The lower ends of
these rods were attached to a system of equalizing levers, so arranged that the load at each pinhole would be the
same. In all the tests the load was applied vertically by means of the static machine.

Cir. 204, U.S.F.S.: Strength tests of cross-arms.

Other Tests

Many other kinds of tests are made as occasion demands. One kind consists of barrels and liquid containers,
match-boxes, and explosive containers. These articles are subjected to shocks such as they would receive in transit
and in handling, and also to hydraulic pressure.
One of the most important tests from a practical standpoint is that of built-up structures such as compounded
beams composed of small pieces bolted together, mortised joints, wooden trusses, etc. Tests of this kind can best
be worked out according to the specific requirements in each case.

APPENDIX

SAMPLE WORKING PLAN OF THE U.S. FOREST SERVICE

MECHANICAL PROPERTIES OF WOODS GROWN IN THE UNITED STATES

Working Plan No. 124

PURPOSE OF WORK

It is the general purpose of the work here outlined to provide:
(a) Reliable data for comparing the mechanical properties of various species;
(b) Data for the establishment of correct strength functions or working stresses;
(c) Data upon which may be based analyses of the influence on the mechanical properties of such factors as:
Locality;
Distance of timber from the pith of the tree;
Height of timber in the tree;
Change from the green to the air-dried condition, etc.
The mechanical properties which will be considered and the principal tests used to determine them are as follows:
Strength and stiffness—
Static bending;
Compression parallel to grain;
Compression perpendicular to grain;
Shear.
Toughness—
Impact bending;
Static bending;
Work to maximum load and total work.
Cleavability—
Cleavage test.
Hardness—
Modification of Janka ball test for surface hardness.

MATERIAL

Selection and Number of Trees

The material will be from trees selected in the forest by one qualified to determine the species. From each locality,
three to five dominant trees of merchantable size and approximately average age will be so chosen as to be
representative of the dominant trees of the species. Each species will eventually be represented by trees from five
to ten localities. These localities will be so chosen as to be representative of the commercial range of the species.
Trees from one to three localities will be used to represent each species until most of the important species have
been tested.
The 16-foot butt log will be taken from each tree selected and the entire merchantable hole of one average tree for
each species.

Field Notes and Shipping Instructions

Field notes as outlined in Form—a Shipment Description, Manual of the Branch of Products, will be fully and
carefully made by the collector. The age of each tree selected will be recorded and any other information likely to
be of interest or importance will also be made a part of these field notes. Each log will have the bark left on. It will
be plainly marked in accordance with directions given under Detailed Instructions. All material will be shipped to
the laboratory immediately after being cut. No trees will be cut until the collector is notified that the laboratory is
ready to receive the material.

DETAILED INSTRUCTIONS

Part of Tree to be Tested

(a) For determining the value of tree and locality and the influence on the mechanical properties of distance from
the pith, a 4-foot bolt will be cut from the top end of each 16-foot butt log.
(b) For investigating the variation of properties with the height of timber in the tree, all the logs from one average
tree will be used.
(c) For investigating the effect of drying the wood, the bolt next below that provided for in (a) will be used in the
case of one tree from each locality.

Marking and Grouping of Material

The marking will be standard except as noted. Each log will be considered a "piece." The piece numbers will be
plainly marked upon the butt end of each log by the collector. The north side of each log will also be marked.
When only one bolt from a tree is used it will be designated by the number of the log from which it is cut.
Whenever more than one bolt is taken from a tree, each 4-foot bolt or length of trunk will be given a letter (mark),
a, b, c, etc., beginning at the stump.
All bolts will be sawed into 2-1/2" × 2-1/2" sticks and the sticks marked according to the sketch, Fig. 50. The
letters N, E, S, and W indicate the cardinal points when known; when these are unknown, H, K, L, and M will be
used. Thus, N5, K8, S7, M4 are stick numbers, the letter being a part of the stick number.

Figure 50
Method of cutting and marking test specimens.

Only straight-grained specimens, free from defects which will affect their strength, will be tested.

Care of Material

No material will be kept in the bolt or log long enough to be damaged or disfigured by checks, rot, or stains.
Green material: The material to be tested green will be kept in a green state by being submerged in water until near
the time of test. It will then be surfaced, sawed to length, and stored in damp sawdust at a temperature of 70°F. (as
nearly as practicable) until time of test. Care should be taken to avoid as much as possible the storage of green
material in any form.
Air-dry material: The material to be air-dried will be cut into sticks 2-1/2" × 2-1/2" × 4'. The ends of these sticks
will be paraffined to prevent checking. This material will be so piled as to leave an air space of at least one-half
inch on each side of each stick, and in such a place that it will be protected from sunshine, rain, snow, and moisture
from the ground. The sticks will be surfaced and cut to length just previous to test.

Order of Tests

The order of tests in all cases will be such as to eliminate so far as possible from the comparisons the effect of
changes of condition of the specimens due to such factors as storage and weather conditions.
The material used for determining the effect of height in tree will be tested in such order that the average time
elapsing from time of cutting to time of test will be approximately the same for all bolts from any one tree.

Tests on Green Material

The tests on all bolts, except those from which a comparison of green and dry timber is to be gotten, will be as
follows:
Static bending: One stick from each pair. A pair consists of two adjacent sticks equidistant from the pith, as N7 and
N8, or H5 and H6.
Impact bending: Four sticks; one to be taken from near the pith; one from near the periphery; and two
representative of the cross section.
Compression parallel to grain: One specimen from each stick. These will be marked "1" in addition to the number
of the stick from which they are taken.

Compression perpendicular to grain: One specimen from each of 50 per cent of the static bending sticks. These
will be marked "2" in addition to the number of the stick from which they are cut.
Hardness: One specimen from each of the other 50 per cent of the static bending sticks. These specimens will be
marked "4."
Shear: Six specimens from sticks not tested in bending or from the ends cut off in preparing the bending
specimens. Two specimens will be taken from near the pith; two from near the periphery; and two that are
representative of the average growth. One of each two will be tested in radial shear and the other in tangential
shear. These specimens will have the mark "3."
Cleavage: Six specimens chosen and divided just as those for shearing. These specimens will have the mark "5."
(For sketches showing radial and tangential cleavage, see Fig. 45.)
When it is impossible to secure clear specimens for all of the above tests, tests will have precedence in the order in
which they are named.

Tests to Determine the Effect of Air-drying

These tests will be made on material from the adjacent bolts mentioned in "c" under Part of Tree to be Tested. Both
bolts will be cut as outlined above. One-half the sticks from each bolt will be tested green, the other half will be
air-dried and tested. The division of green and air-dry will be according to the following scheme:

STICK NUMBERS
Lower bolt, 1, 4, 5, 8, 9,
Upper bolt, 2, 3, 6, 7, 10,
etc. }
Tested green

Lower bolt, 2, 3, 6, 7, 10,
Upper bolt, 1, 4, 5, 8, 9, }
etc. Air-dried and tested

All green sticks from these two bolts will be tested as if they were from the same bolt and according to the plan
previously outlined for green material from single bolts. The tests on the air-dried material will be the same as on
the green except for the difference of seasoning.
The material will be tested at as near 12 per cent moisture as is practicable. The approximate weight of the air-
dried specimens at 12 per cent moisture will be determined by measuring while green 20 per cent of the sticks to
be air-dried and assuming their dry gravity to be the same as that of the specimens tested green. This 20 per cent
will be weighed as often as is necessary to determine the proper time of test.

Methods of Test

All tests will be made according to Circular 38 except in case of conflict with the instructions given below:
Static bending: The tests will be on specimens 2" × 2" × 30" on 28-inch span. Load will be applied at the centre.
In all tests the load-deflection curve will be carried to or beyond the maximum load. In one-third of the tests the
load-deflection curve will be continued to 6-inch deflection, or till the specimen fails to support a 200-pound load.
Deflection readings for equal increments of load will be taken until well past the elastic limit, after which the scale
beam will be kept balanced and the load read for each 0.1-inch deflection. The load and deflection at first failure,
maximum load and points of sudden change, will be shown on the curve sheet even if they do not occur at one of
the regular load or deflection increments.
Impact bending: The impact bending tests will be on specimens of the same size as those used in static bending.
The span will be 28 inches.
The tests will be by increment drop. The first drop will be 1 inch and the increase will be by increments of 1 inch
till a height of 10 inches is reached, after which increments of 2 inches will be used until complete failure occurs
or 6-inch deflection is secured.

A 50-pound hammer will be used when with drops up to 68 inches it is practically certain that it will produce
complete failure or 6-inch deflection in the case of all specimens of a species. For all other species, a 100-pound
hammer will be used.
In all cases drum records will be made until first failure. Also the height of drop causing complete failure or 6-inch
deflection will be noted.
Compression parallel to grain: This test will be on specimens 2" × 2" × 8" in size. On 20 per cent of these tests
load-compression curves for a 6-inch centrally located gauge length will be taken. Readings will be continued until
the elastic limit is well passed. The other 80 per cent of the tests will be made for the purpose of obtaining the
maximum load only.
Compression perpendicular to grain: This test will be on specimens 2" × 2" × 6" in size. The bearing plates will be
2 inches wide. The rate of descent of the moving head will be 0.024 inch per minute. The load-compression curve
will be plotted to 0.1 inch compression and the test will then be discontinued.
Hardness: The tool shown in Fig. 43 (an adaptation of the apparatus used by the German investigator, Janka) will
be used. The rate of descent of the moving head will be 0.25 inch per minute. When the penetration has progressed
to the point at which the plate "a" becomes tight, due to being pressed against the wood, the load will be read and
recorded.
Two penetrations will be made on a tangential surface, two on a radial, and one on each end of each specimen
tested. The choice between the two radial and between the two tangential surfaces and the distribution of the
penetrations over the surfaces will be so made as to get a fair average of heart and sap, slow and fast growth, and
spring and summer wood. Specimens will be 2" × 2" × 6".
Shear: The tests will be made with a tool slightly modified from that shown in Circular 38. The speed of descent of
head will be 0.015 inch per minute. The only measurements to be made are those of the shearing area. The offset
will be 1/8 inch. Specimens will be 2" × 2" × 2-1/2" in size. (For definition of offset and form of test specimen, see
Fig. 38.)
Cleavage: The cleavage tests will be made on specimens of the form and size shown in Fig. 45. The apparatus will
be as shown in Fig. 44. The maximum load only will be taken and the result expressed in pounds per inch of width.
The speed of the moving head will be 0.25 inch per minute.

Moisture Determinations

Moisture determinations will be made on all specimens tested except those to be photographed or kept for exhibit.
A 1-inch disk will be cut from near the point of failure of bending and compression parallel specimens, from the
portion under the plate in the case of the compression perpendicular specimens, and from the centre of the
hardness test specimens. The beads from the shear specimens will be used as moisture disks. In the case of the
cleavage specimens a piece 1/2 inch thick will be split off parallel to the failure and used as a moisture disk.

RECORDS

All records will be standard.

PHOTOGRAPHS

Cross Sections

Just before cutting into sticks, the freshly cut end of at least one bolt from each tree will be photographed. A scale
of inches will be shown in this photograph.

Specimens

Three photographs will be made of a group consisting of four 2" × 2" × 30" specimens chosen from the material
from each locality. Two of these specimens will be representative of average growth, one of fast and one of slow

growth. These photographs will show radial, tangential, and end surfaces for each specimen.

Failures

Typical and abnormal failures of material from each site will be photographed.

Disposition of Material

The specimens photographed to show typical and abnormal failures will be saved for purposes of exhibit until
deemed by the person in charge of the laboratory to be of no further value.

SHRINKAGE AND SPECIFIC GRAVITY

Appendix to Working Plan 124

PURPOSE OF WORK

It is the purpose of this work to secure data on the shrinkage and specific gravity of woods tested under Project
124. The figures to be obtained are for use as average working values rather than as the basis for a detailed study
of the principles involved.

MATERIAL

The material will be taken from that provided for mechanical tests.

RADIAL AND TANGENTIAL SHRINKAGE

Specimens

Preparation: Two specimens 1 inch thick, 4 inches wide, and 1 inch long will be obtained from near the periphery
of each "d" bolt. These will be cut from the sector-shaped sections left after securing the material for the
mechanical tests or from disks cut from near the end of the bolt. They will be taken from adjoining pieces chosen
so that the results will be comparable for use in determining radial and tangential shrinkage. (When a disk is used,
care must be taken that it is green and has not been affected by the shrinkage and checking near the end of the
bolt.)
One of these specimens will be cut with its width in the radial direction and will be used for the determination of
radial shrinkage. The other will have its width in the tangential direction and will be used for tangential shrinkage.
These specimens will not be surfaced.
Marking: The shrinkage specimens will retain the shipment and piece numbers and marks of the bolts from which
they are taken, and will have the additional mark 7R or 7T according as their widths are in the radial or tangential
direction.
Shrinkage measurements: The shrinkage specimens will be carefully weighed and measured soon after cutting.
Rings per inch, per cent sap, and per cent summer wood will be measured. They will then be air-dried in the
laboratory to constant weight, and afterward oven-dried at 100°C. (212°F.), when they will again be weighed and
measured.

VOLUMETRIC SHRINKAGE AND SPECIFIC GRAVITY

Specimens

Selection and preparation: Four 2" × 2" × 6" specimens will be cut from the mechanical test sticks of each "d"
bolt; also from each of the composite bolts used in getting a comparison of green and air-dry. One of these
specimens will be taken from near the pith and one from near the periphery; the other two will be representative of
the average growth of the bolt. The sides of these specimens will be surfaced and the ends smooth sawn.
Marking: Each specimen will retain the shipment, piece, and stick numbers and mark of the stick from which it is
cut, and will have the additional mark "S."
Manipulation: Soon after cutting, each specimen will be weighed and its volume will be determined by the method
described below. The rings per inch and per cent summer wood, where possible, will be determined, and a carbon
impression of the end of the specimen made. It will then be air-dried in the laboratory to a constant weight and
afterward oven-dried at 100°C. When dry, the specimen will be taken from the oven, weighed, and a carbon
impression of its end made. While still warm the specimen will be dipped in hot paraffine. The volume will then be
determined by the following method:
On one pan of a pair of balances is placed a container having in it water enough for the complete submersion of the
test specimen. This container and water is balanced by weights placed on the other scale pan. The specimen is then
held completely submerged and not touching the container while the scales are again balanced. The weight
required to balance is the weight of water displaced by the specimen, and hence if in grams is numerically equal to
the volume of the specimen in cubic centimetres. A diagrammatic sketch of the arrangement of this apparatus is
shown in Fig. 51.

Figure 51
Diagram of specific gravity apparatus, showing a
balance with container (c) filled with water in which the
test block (b) is held submerged by a light rod (a) which
is adjustable vertically and provided with a sharp point to
be driven into the specimen.

Air-dry specimens will be dipped in water and then wiped dry after the first weighing and just before being
immersed for weighing their displacement. All displacement determinations will be made as quickly as possible in
order to minimize the absorption of water by the specimen.

STRENGTH VALUES FOR STRUCTURAL TIMBERS

(From Cir. 189, U.S. Forest Service)

The following tables bring together in condensed form the average strength values resulting from a large number
of tests made by the Forest Service on the principal structural timbers of the United States. These results are more
completely discussed in other publications of the Service, a list of which is given on pages 157-159.

The tests were made at the laboratories of the U.S. Forest Service, in cooperation with the following institutions:
Yale Forest School, Purdue University, University of California, University of Oregon, University of Washington,
University of Colorado, and University of Wisconsin.
Tables XVIII and XIX give the average results obtained from tests on green material, while Tables XX and XXI
give average results from tests on air-seasoned material. The small specimens, which were invariably 2" × 2" in
cross section, were free from defects such as knots, checks, and cross grain; all other specimens were
representative of material secured in the open market. The relation of stresses developed in different structural
forms to those developed in the small clear specimens is shown for each factor in the column headed "Ratio to 2" ×
2"." Tests to determine the mechanical properties of different species are often confined to small, clear specimens.
The ratios included in the tables may be applied to such results in order to approximate the strength of the species
in structural sizes, and containing the defects usually encountered, when tests on such forms are not available.
A comparison of the results of tests on seasoned material with those from tests on green material shows that,
without exception, the strength of the 2" × 2" specimens is increased by lowering the moisture content, but that
increase in strength of other sizes is much more erratic. Some specimens, in fact, show an apparent loss in strength
due to seasoning. If structural timbers are seasoned slowly, in order to avoid excessive checking, there should be
an increase in their strength. In the light of these facts it is not safe to base working stresses on results secured from
any but green material. For a discussion of factors of safety and safe working stresses for structural timbers see the
Manual of the American Railway Engineering Association, Chicago, 1911. A table from that publication, giving
working unit stresses for structural timber, is reproduced in this book, see Table XXII.

TABLE XVIII
BENDING TESTS ON GREEN MATERIAL
Calcul
Sizes F.S. at E.L. M. of R. M. of E.
she
Numbe Per cent Rings
Ratio Ratio Ratio
r of of per Average Average Average Average
to 2" to 2" to 2"
Species Cross Section Span tests moisture inch per sq. per sq. per sq. per sq.
by by by
inch inch inch inch
2" 2" 2"
1,000
Inches Ins. Lbs. Lbs. Lbs.
lbs.
Longleaf pine 12 by 12 138 4 28.6 9.7 4,029 0.83 6,710 0.74 1,523 0.99 261
10 by 16 168 4 26.8 16.7 6,453 .85 6,453 .71 1,626 1.05 306
8 by 16 156 7 28.4 14.6 3,147 .64 5,439 .60 1,368 .89 390
6 by 16 132 1 40.3 21.8 4,120 .83 6,460 .71 1,190 .77 378
6 by 10 180 1 31.0 6.2 3,580 .72 6,500 .72 1,412 .92 175
6 by 8 180 2 27.0 8.2 3,735 .75 5,745 .63 1,282 .83 121
2 by 2 30 15 33.9 14.1 4,950 1.00 9,070 1.00 1,540 1:00 303
Douglas fir 8 by 16 180 191 31.5 11.0 3,968 .76 5,983 .72 1,517 .95 269
5 by 8 180 84 30.1 10.8 3,693 .71 5,178 .63 1,533 .96 172
2 by 12 180 27 35.7 20.3 3,721 .71 5,276 .64 1,642 1.03 256
2 by 10 180 26 32.9 21.6 3,160 .60 4,699 .57 1,593 1.00 189
2 by 8 180 29 33.6 17.6 3,593 .69 5,352 .65 1,607 1.01 171
2 by 2 24 568 30.4 11.6 5,227 1.00 9,070 1.00 1,540 1.00 303
Douglas fir (fire-
8 by 16 180 30 36.8 10.9 3,503 .80 4,994 .64 1,531 .94 330
killed)
2 by 12 180 32 34.2 17.7 3,489 .80 5,085 .66 1,624 .99 247
2 by 10 180 32 38.9 18.1 3,851 .88 5,359 .69 1,716 1.05 216
2 by 8 180 31 37.0 15.7 3,403 .78 5,305 .68 1,676 1.02 169
2 by 2 30 290 33.2 17.2 4,360 1.00 7,752 1.00 1,636 1.00 277
Shortleaf pine 8 by 16 180 12 39.5 12.1 3,185 .73 5,407 .70 1,438 1.03 362
8 by 14 180 12 45.8 12.7 3,234 .74 5,781 .75 1,494 1.07 338
8 by 12 180 24 52.2 11.8 3,265 .75 5,503 .71 1,480 1.06 277

5 by 8 180 24 47.8 11.5 3,519 .81 5,732 .74 1,485 1.06 185
2 by 2 30 254 51.7 13.6 4,350 1.00 7,710 1.00 1,395 1.00 258
Western larch 8 by 16 180 32 51.0 25.3 3,276 .77 4,632 .64 1,272 .97 298
8 by 12 180 30 50.3 23.2 3,376 .79 5,286 .73 1,331 1.02 254
5 by 8 180 14 56.0 25.6 3,528 .83 5,331 .74 1,432 1.09 169
2 by 2 28 189 46.2 26.2 4,274 1.00 7,251 1.00 1,310 1.00 269
Loblolly pine 8 by 16 180 17 15.8 6.1 3,094 .75 5,394 .69 1,406 .98 383
5 by 12 180 94 60.9 5.9 3,030 .74 5,028 .64 1,383 .96 221
2 by 2 30 44 70.9 5.4 4,100 1.00 7,870 1.00 1,440 1.00 265
Tamarack 6 by 12 162 15 57.6 16.6 2,914 .75 4,500 .66 1,202 1.05 255
4 by 10 162 15 43.5 11.4 2,712 .70 4,611 .68 1,238 1.08 209
2 by 2 30 82 38.8 14.0 3,875 1.00 6,820 1.00 1,141 1.00 229
Western
8 by 16 180 39 42.5 15.6 3,516 .80 5,296 .73 1,445 1.01 261
hemlock
2 by 2 28 52 51.8 12.1 4.406 1.00 7,294 1.00 1,428 1.00 284
Redwood 8 by 16 180 14 86.5 19.9 3,734 .79 4,492 .64 1,016 .96 300
6 by 12 180 14 87.3 17.8 3,787 .80 4,451 .64 1,068 1.00 224
7 by 9 180 14 79.8 16.7 4,412 .93 5,279 .76 1,324 1.25 199
3 by 14 180 13 86.1 23.7 3,506 .74 4,364 .62 947 .89 255
2 by 12 180 12 70.9 18.6 3,100 .65 3,753 .54 1,052 .99 187
2 by 10 180 13 55.8 20.0 3,285 .69 4,079 .58 1,107 1.04 169
2 by 8 180 13 63.8 21.5 2,989 .63 4,063 .58 1,141 1.08 134
2 by 2 28 157 75.5 19.1 4,750 1.00 6,980 1.00 1,061 1.00 248
Norway pine 6 by 12 162 15 50.3 12.5 2,305 .82 3,572 .69 987 1.03 201
4 by 12 162 18 47.9 14.7 2,648 .94 4,107 .79 1,255 1.31 238
4 by 10 162 16 45.7 13.3 2,674 .95 4,205 .81 1,306 1.36 198
2 by 2 30 133 32.3 11.4 2,808 1.00 5,173 1.00 960 1.00 172
Red spruce 2 by 10 144 14 32.5 21.9 2,394 .66 3,566 .60 1,180 1.02 181
2 by 2 26 60 37.3 21.3 3,627 1.00 5,900 1.00 1,157 1.00 227
White spruce 2 by 10 144 16 40.7 9.3 2,239 .72 3,288 .63 1,081 1.08 166
2 by 2 26 83 58.3 10.2 3.090 1.00 5,185 1.00 998 1.00 199
Note.—Following is an explanation of the abbreviations used in the foregoing tables:
F.S. at E.L. = Fiber stress at elastic limit.
M. of E. = Modulus of elasticity.
M. of R. = Modulus of rupture.
Cr. str. at E.L. = Crushing strength at elastic limit.
Cr. str. at max. ld. = Crushing strength at maximum load.

TABLE XIX
COMPRESSION AND SHEAR TESTS ON GREEN MATERIAL
Compression perpendicular to
Compression parallel to grain Shear
grain
Cr. Cr.
Cr.
str. at str. at
str. at M. of
No. Per cent max. No. Per cent max. No. Per cent s
Size of E. L. E. per Stress
Species of of ld., Height of of ld., of of
specimen per square area
tests moisture per tests moisture per tests moisture
square inch
square square
inch
inch inch
1,000
Inches Lbs. Lbs. Inches Inches Lbs.
lbs.
Longleaf pine 4 by 4 46 26.3 3,480 4,800 4 by 4 4 22 25.3 568 44 21.8

2 by 2 14 34.7 4,400
Douglas fir 6 by 6 515 30.7 2,780 1,181 3,500 4 by 8 16 259 30.3 570 531 29.7
5 by 6 170 30.9 2,720 2,123 3,490
2 by 2 902 29.8 3,500 1,925 4,030
Douglas fir (fire-
6 by 6 108 34.8 2,620 1,801 3,290 6 by 8 16 24 33.7 368 77 35.8
killed)
2 by 2 204 37.9 3,430
Shortleaf pine 6 by 6 95 41.2 2,514 1,565 3,436 5 by 8 16 12 37.7 361 179 47.0
5 by 8 23 43.5 2,241 1,529 3,423 5 by 8 14 12 42.8 366
2 by 2 281 51.4 3,570 5 by 8 12 24 53.0 325
5 by 5 8 24 47.0 344
2 by 2 2 277 48.5 400
Western larch 6 by 6 107 49.1 2,675 1,575 3,510 6 by 8 16 22 43.6 417 179 40.7
2 by 2 491 50.6 3,026 1,545 3,696 6 by 8 12 20 40.2 416
4 by 6 6 53 52.8 478
4 by 4 4 30 50.4 472
Loblolly pine 8 by 8 14 63.4 1,560 365 2,140 8 by 4 8 16 67.2 392 121 83.2
4 by 8 18 60.0 2,430 691 3,560 4 by 4 8 38 44.6 546
2 by 2 53 74.0 3,240
Tamarack 6 by 7 4 49.9 2,332 1,432 3,032 24 39.2
4 by 7 6 27.7 2,444 1,334 3,360
2 by 2 165 36.8 3,190
Western
6 by 6 82 46.6 2,905 1,617 3,355 6 by 4 6 30 48.7 434 54 65.7
hemlock
2 by 2 131 55.6 2,938 1,737 3,392
Redwood 6 by 6 34 83.6 3,194 1,240 3,882 6 by 8 16 13 86.7 473 148 84.2
2 by 2 143 36.8 3,490 1,222 3,980 6 by 6 12 14 83.0 424
6 by 7 9 13 74.7 477
6 by 3 14 13 75.6 411
6 by 2 12 12 66.5 430
6 by 2 10 11 55.0 423
6 by 2 8 12 56.7 396
2 by 2 2 186 75.5 569
Norway pine 6 by 7 5 29.0 1,928 905 2,404 20 26.7
4 by 7 8 28.4 2,154 1,063 2,652
2 by 2 178 26.8 2,504
Red spruce 2 by 2 58 35.4 2,750 2 by 2 2 43 31.8 310 30 32.0
White spruce 2 by 2 84 61.0 2,370 2 by 2 2 46 50.4 270 40 58.0
Note.—Following is an explanation of the abbreviations used in the foregoing tables:
F.S. at E.L. = Fiber stress at elastic limit.
M. of E. = Modulus of elasticity.
M. of R. = Modulus of rupture.
Cr. str. at E.L. = Crushing strength at elastic limit.
Cr. str. at max. ld. = Crushing strength at maximum load.

TABLE XX
BENDING TESTS ON AIR-SEASONED MATERIAL
Species Numbe Per cent Rings Calcu
Sizes F.S. at E.L. M. of R. M. of E.
r of of per she
tests moisture inch

Ratio Ratio Ratio
Average Average Average Average
to 2" to 2" to 2"
Cross Section Span per sq. per sq. per sq. per sq.
by by by
inch inch inch inch
2" 2" 2"
1,000
Inches Ins. Lbs. Lbs. Lbs.
lbs.
Longleaf pine 8 by 16 180 5 22.2 16.0 3,390 0.50 4,274 0.37 1,747 1.00 288
6 by 16 132 1 23.4 17.1 3,470 .51 6,610 .57 1,501 .86 388
6 by 10 177 2 19.0 8.8 4,560 .68 7,880 .68 1,722 .99 214
4 by 11 180 1 18.4 23.9 3,078 .46 8,000 .69 1,660 .95 251
6 by 8 177 6 20.0 13.7 4,227 .63 8,196 .71 1,634 .94 177
2 by 2 30 17 15.9 13.9 6,750 1.00 11,520 1.00 1,740 1.00 383
Douglas fir 8 by 16 180 91 20.8 13.1 4,563 .68 6,372 .61 1,549 .91 269
5 by 8 180 30 14.9 12.2 5,065 .76 6,777 .65 1,853 1.09 218
2 by 2 24 211 19.0 16.4 6,686 1.00 10,378 1.00 1,695 1.00 419
Shortleaf pine 8 by 16 180 3 17.0 12.3 4,220 .54 6,030 .50 1,517 .85 398
8 by 14 180 3 16.0 12.3 4,253 .55 5,347 .44 1,757 .98 307
8 by 12 180 7 16.0 12.4 5,051 .65 7,331 .60 1,803 1.01 361
5 by 8 180 6 12.2 22.5 7,123 .92 9,373 .77 1,985 1.11 301
2 by 2 30 67 14.2 13.7 7,780 1.00 12,120 1.00 1,792 1.00 404
Western larch 8 by 16 180 23 18.3 21.9 3,343 .57 5,440 .53 1,409 .90 349
8 by 12 180 29 17.8 23.4 3,631 .62 6,186 .60 1,549 .99 295
5 by 8 180 10 13.6 27.6 4,730 .80 7,258 .71 1,620 1.04 221
2 by 2 30 240 16.1 26.8 5,880 1.00 10,254 1.00 1,564 1.00 364
Loblolly pine 8 by 16 180 14 20.5 7.4 4,195 .81 6,734 .72 1,619 1.10 462
6 by 16 126 4 20.2 5.0 2,432 .47 4,295 .46 1,324 .90 266
6 by 10 174 3 21.3 4.7 3,100 .60 6,167 .66 1,449 .99 173
4 by 12 174 4 19.8 4.7 2,713 .52 5,745 .61 1,249 .85 185
8 by 8 180 9 22.9 4.9 2,903 .56 4,557 .48 1,136 .77 93
6 by 7 144 2 21.1 5.0 2,990 .58 4,968 .53 1,286 .88 116
4 by 8 132 8 19.5 9.1 3,384 .65 6,194 .66 1,200 .82 196
2 by 2 30 123 17.6 6.6 5,170 1.00 9,400 1.00 1,467 1.00 318
Tamarack 6 by 12 162 5 23.0 15.1 3,434 .45 5,640 .43 1,330 .82 318
4 by 10 162 4 14.4 9.7 4,100 .54 5,320 .41 1,386 .84 252
2 by 2 30 47 11.3 16.2 7,630 1.00 13,080 1.00 1,620 1.00 425
Western hemlock 8 by 16 180 44 17.7 17.8 4,398 .69 6,420 .62 1,737 1.04 406
2 by 2 28 311 17.9 19.4 6,333 1.00 10,369 1.00 1,666 1.00 382
Redwood 8 by 16 180 6 26.3 22.4 3,797 .79 4,428 .57 1,107 .96 294
6 by 12 180 6 16.1 17.7 3,175 .66 3,353 .43 728 .64 167
7 by 9 180 6 15.9 15.2 3,280 .69 4,002 .51 1,104 .96 147
3 by 14 180 6 13.1 24.4 5,033 .64 291
2 by 12 180 5 13.8 14.4 3,928 .82 5,336 .68 1,249 1.09 260
2 by 10 180 5 13.8 24.8 3,757 .79 4,606 .59 1,198 1.05 186
2 by 8 180 6 13.7 20.7 4,314 .90 5,050 .65 1,313 1.15 166
2 by 2 28 122 15.2 18.8 4,777 1.00 7,798 1.00 1,146 1.00 279
Norway pine 6 by 12 162 5 16.7 8.1 2,968 .56 5,204 .61 1,123 .97 286
4 by 10 162 5 13.7 12.0 5,170 .98 6,904 .82 1,712 1.48 317
2 by 2 30 60 14.9 11.2 5,280 1.00 8,470 1.00 1,158 1.00 281
Note.—Following is an explanation of the abbreviations used in the foregoing tables:
F.S. at E.L. = Fiber stress at elastic limit.
M. of E. = Modulus of elasticity.
M. of R. = Modulus of rupture.

Cr. str. at E.L. = Crushing strength at elastic limit.
Cr. str. at max. ld. = Crushing strength at maximum load.

TABLE XXI
COMPRESSION AND SHEAR TESTS ON AIR-SEASONED MATERIAL
Compression perpendicular to
Compression parallel to grain Shear
grain
Cr. Cr.
Cr.
str. at str. at
str. at M. of
No. Per cent max. No. Per cent max. No. Per cent
Size of E. L. E. per Stress
Species of of ld., Height of of ld., of of
specimen per square area
tests moisture per tests moisture per tests moisture
square inch
square square
inch
inch inch
1,000
Inches Lbs. Lbs. Inches Inches Lbs.
lbs.
Longleaf pine 4 by 5 46 26.3 3,480 4,800 4 by 5 4 22 25.1 572 52 20.2
Douglas fir 6 by 6 259 20.3 3,271 1,038 4,258 4 by 8 16 44 20.8 732 465 22.1
2 by 2 247 18.7 3,842 1,084 5,002 4 by 8 10 32 18.1 584
4 by 4 8 51 20.2 638
4 by 4 6 49 24.0 613
4 by 4 4 29 24.8 603
Shortleaf pine 6 by 6 29 15.7 4,070 1,951 6,030 8 by 5 16 4 17.8 725 85
2 by 2 57 14.2 6,380 8 by 5 14 3 16.3 757
8 by 5 12 5 15.1 730
5 by 5 8 6 13.0 918
2 by 2 2 57 13.9 926
Western larch 6 by 6 112 16.0 5,445 8 by 6 16 17 18.8 491 193 15.0
4 by 4 81 14.7 6,161 8 by 6 12 18 17.6 526
2 by 2 270 14.8 5,934 5 by 4 8 22 13.3 735
Loblolly pine 6 by 6 23 3,357 1,693 5,005 8 by 5 16 12 19.8 602 156 11.3
5 by 5 10 22.4 2,217 545 2,950 8 by 5 8 7 22.9 679
4 by 8 8 19.4 3,010 633 3,920 4 by 5 8 8 19.5 715
2 by 2 69 5,547
Tamarack 6 by 7 3 15.7 2,257 1,042 3,323 2 by 2 2 57 16.2 697 60 14.0
4 by 7 3 13.6 3,780 1,301 4,823
4 by 4 57 14.9 3,386 1,353 4,346
2 by 2 66 14.6 4,790
Western hemlock 6 by 6 102 18.6 4,840 2,140 5,814 7 by 6 15 25 18.2 514 131 17.7
2 by 2 463 17.0 4,560 1,923 5,403 6 by 6 6 26 16.8 431
4 by 4 4 6 15.9 488
Redwood 6 by 6 18 16.9 4,276 8 by 6 16 5 25.4 548 95 12.4
2 by 2 115 14.6 5,119 6 by 6 12 6 14.7 610
7 by 6 9 5 14.8 500
3 by 6 14 2 12.6 470
2 by 6 12 2 16.2 498
2 by 6 10 4 14.3 511
2 by 6 8 2 13.2 429
2 by 2 2 145 13.8 564
Norway pine 6 by 7 4 15.2 2,670 1,182 4,212 2 by 2 2 36 10.0 924 44 11.9
4 by 7 2 22.2 3,275 1,724 4,575
4 by 4 55 16.6 3,048 1,367 4,217
2 by 2 44 11.2 7,550

Note.—Following is an explanation of the abbreviations used in the foregoing tables:
F.S. at E.L. = Fiber stress at elastic limit.
M. of E. = Modulus of elasticity.
M. of R. = Modulus of rupture.
Cr. str. at E.L. = Crushing strength at elastic limit.
Cr. str. at max. ld. = Crushing strength at maximum load.

TABLE XXII
[b]WORKING UNIT-STRESSES FOR STRUCTURAL TIMB
EXPRESSED IN POUNDS PER SQUARE INCH
(From Manual of the American Railway Engineering Assn., 1911,
NOTE.—The working unit-stresses given in the table are intended for railroad bridges and trestles. For highway brid
twenty-five (25) per cent. For buildings and similar structures, in which the timber is protected from the weather and
increased fifty (50) per cent. To compute the deflection of a beam under long-continued loading instead of that when
corresponding modulus of elasticity given in the table is to be employed.
BENDING SHEARING
Modulus
Extreme fibre Parallel to the Longitudinal Perpendicular Parallel
of
KIND OF TIMBER stress grain shear in beams to the grain grai
elasticity
Average Working Average Working Average Working Elastic Working Average W
Average
ultimate stress ultimate stress ultimate stress limit stress ultimate
Douglas fir 6100 1200 1,510,000 690 170 270 110 630 310 3600
Longleaf pine 6500 1300 1,610,000 720 180 300 120 520 260 3800
Shortleaf pine 5600 1100 1,480,000 710 170 330 130 340 170 3400
White pine 4400 900 1,130,000 400 100 180 70 290 150 3000
Spruce 4800 1000 1,310,000 600 150 170 70 370 180 3200
Norway pine 4200 800 1,190,000 590[d] 130 250 100 150 2600[d]
Tamarack 4600 900 1,220,000 670 170 260 100 220 3200[d]
Western hemlock 5800 1100 1,480,000 630 160 270[d] 100 440 220 3500
Redwood 5000 900 800,000 300 80 400 150 3300
Bald cypress 4800 900 1,150,000 500 120 340 170 3900
Red cedar 4200 800 800,000 470 230 2800
White oak 5700 1100 1,150,000 840 210 270 110 920 450 3500
These unit-stresses are for a green condition of timber and are to be used without increasing the live load stresses for
[Footnote b: Adopted, Vol. 1909, pp. 537, 564, 609-611.]
[Footnote c: Green timber in exposed work.]
[Footnote d: Partially air-dry]

BIBLIOGRAPHY

Part I: Some general works on mechanics, materials of construction, and testing of materials.

Part II: Publications and articles on the mechanical properties of wood, and timber testing.

Part III: Publications of the U.S. Government on the mechanical properties of wood, and timber testing.

PART I.
SOME GENERAL WORKS ON MECHANICS, MATERIALS OF CONSTRUCTION, AND
TESTING OF MATERIALS

ALLAN, WILLIAM: Strength of beams under transverse loads. New York, 1893.
ANDERSON, SIR JOHN: The strength of materials and structures. London, 1902.
BARLOW, PETER: Strength of materials, 1st ed. 1817; rev. 1867.
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CHURCH, IRVING P.: Mechanics of engineering. New York, 1911.
HATFIELD, R.G.: Theory of transverse strain. 1877.
HATT, W.K., and SCOFIELD, H.H.: Laboratory manual of testing materials. New York, 1913.
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1911.
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KENT, WILLIAM: The strength of materials. New York, 1890.
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LANZA, GAETANO: Applied mechanics. New York, 1901.
MERRIMAN, MANSFIELD: Mechanics of materials. New York, 1912.
MURDOCK, H.E.: Strength of materials. New York, 1911.
RANKINE, WILLIAM J.M.: A manual of applied mechanics. London, 1901.
THIL, A.: Conclusion de l'étude présentée à la Commission des méthodes d'essai des matériaux de construction.
Paris, 1900.
THURSTON, ROBERT H.: A treatise on non-metallic materials of engineering: stone, timber, fuel, lubricants, etc.
(Materials of engineering, Part I.) New York, 1899.
UNWIN, WILLIAM C.: The testing of materials of construction. London, 1899.

WATERBURY, L.A.: Laboratory manual for testing materials of construction. New York, 1912.
WOOD, DEVOLSON: A treatise on the resistance of materials. New York, 1897.

PART II.
PUBLICATIONS AND ARTICLES ON THE MECHANICAL PROPERTIES OF WOOD, AND
TIMBER TESTING

ABBOT, ARTHUR V.: Testing machines, their history, construction and use. Van Nostrand's Eng. Mag., Vol. XXX,
1884, pp. 204-214; 325-344; 382-397; 477-490.
ADAMS, E.E.: Tests to determine the strength of bolted timber joints. Cal. Jour, of Technology, Sept., 1904.
ALVAREZ, ARTHUR C.: The strength of long seasoned Douglas fir and redwood. Univ. of Cal. Pub. in Eng., Vol.
I, No. 2, Berkeley, 1913, pp. 11-20.
BARLOW, PETER: An essay on the strength and stress of timber. London, 1817; 3d ed., 1826.
——: Experiments on the strength of different kinds of wood made in the carriage department, Royal Arsenal,
Woolwich. Jour. Franklin Inst., Vol. X, 1832, pp. 49-52. Reprinted from Philosophical Mag. and Annals of Philos.,
No. 63, Mch., 1832.
BATES, ONWARD: Pine stringers and floorbeams for bridges. Trans. Am. Soc. C.E., Vol. XXIII.
BAUSCHINGER, JOHANN: Untersuchungen über die Elasticität und Festigkeit von Fichten- und
Kiefernbauhölzern. Mitt. a. d. mech.-tech. Laboratorium d. k. techn. Hochschule in München, 9. Hft., München,
1883.
——: Verhandlungen der Münchener Conferenz und der von ihr gewählten ständigen Commission zur
Vereinbarung einheitlicher Prüfungsmethoden für Bau- und Constructions-material. Ibid., 14. Hft., 1886.
——: Untersuchungen über die Elasticität und Festigkeit verschiedener Nadelhölzer. Ibid., 16. Hft., 1887.
BEARE, T. HUDSON: Timber: its strength and how to test it. Engineering, London, Dec. 9, 1904.
BEAUVERIE, J.: Le bois. I. Paris, 1905, pp. 105-185.
——: Les bois industriels. Paris, 1910, pp. 55-77. Bending tests with wood, executed at the Danish State Testing
Laboratory, Copenhagen. Proc. Int. Assn. Test. Mat., 1912, XXIII2, pp. 17. See also Eng. Record, Vol. LXVI, 1912,
p. 269.
BERG, WALTER G.: Berg's complete timber test record. Chicago, 1899. Reprint from Am. By. Bridges and
Buildings. BOULGER, G.S.: Wood. London, 1908, pp. 112-121.
BOUNICEAU,—: Note et expériences sur la torsion des bois. [N.p., n.d.]
BOVEY, HENRY T.: Results of experiments at McGill University, Montreal, on the strength of Canadian Douglas
fir, red pine, white pine, and spruce. Trans. Can. Soc. C.E., Vol. IX, Part I, 1895, pp. 69-236.
BREUIL, M. PIERRE: Contribution to the discussion on the testing of wood. Proc. Int. Assn. Test. Mat., 1906,
Disc, 1e, pp. 2.
BROWN, T.S.: An Account of some experiments made by order of Col. Totten, at Fort Adams, Newport, R.I., to
ascertain the relative stiffness and strength of the following kinds of timber, viz.: white pine (Pinus strobus),
spruce (Abies nigra), and southern pine (Pinus australis), also called long-leaved pine. Jour. Franklin Inst., Vol.
VII (n.s.), 1831, pp. 230-238.
BUCHANAN, C.P.: Some tests of old timber. Eng. News, Vol. LXIV, No. 23, 1910, p. 67.
BUSGEN, M.: Zur Bestimmung der Holzhärten. Zeitschrift f. Forst- und Jagdwesen. Berlin, 1904, pp. 543-562.

CHEVANDIER, E., et WERTHEIM, G.: Mémoire sur les propriétés mécaniques du bois. Paris, 1846.
CIESLAR, A.: Studien über die Qualität rasch erwachsenen Fichtenholzes. Centralblatt f. d. ges. Forstwesen,
Wien, 1902, pp. 337-403.
CLINE, McGARVEY: Forest Service investigations of American woods with special reference to investigations of
mechanical properties. Proc. Int. Assn. Test. Mat., 1912, XXIII5, pp. 17.

——: Forest Service tests to determine the influence of different methods and rates of loading on the strength and
stiffness of timber. Proc. Am. Soc. Test. Mat., Vol. VIII, 1908, pp. 535-540.
——: The Forest Products Laboratory: its purpose and work. Proc. Am. Soc. Test. Mat., Vol. X, 1910, pp. 477-489.
——: Specifications and grading rules for Douglas fir timber: an analysis of Forest Service tests on structural
timbers. Proc. Am. Soc. Test. Mat., Vol. XI, 1911, pp. 744-766.
Comparative strength and resistance of various tie timbers. Elec. Traction Weekly, Chicago, June 15, 1912.
DAY, FRANK M.: Microscopic examination of timber with regard to its strength. 1883, pp. 6.
DEWELL, H.D.: Tests of some joints used in heavy timber framing. Eng. News, Mch. 19, 1914, pp. 594-598; et
seq.
DÖRR, KARL: Die Festigkeit von Fichten- und Kiefernholz. Deutsche Bauzeitung, Berlin, Aug. 17, 1910. See
also Zeitschrift d. ver. deutsch. Ing., Bd. 54, Nr. 36, 1910, p. 1503.
DUPIN, CHARLES: Expériences sur la flexibilité, la force, et l'élasticité des bois. Jour, de l'École Polytechnique,
Vol. X, 1815.
DUPONT, ADOLPHE, et BOUQUET DE LA GRYE: Les bois indigènes et étrangers. Paris, 1875, pp. 273-352.
ESTRADA, ESTEBAN DUQUE: On the strength and other properties of Cuban woods. Van Nostrand's Eng.
Mag., Vol. XXIX, 1883, pp. 417-426; 443-449.
EVERETT, W.H.: Memorandum on mechanical tests of some Indian timbers. Govt. Bul. No. 6 (o.s.), Calcutta.
EXNER, WILHELM FRANZ: Die mechanische Technologie des Holzes. Wien, 1871. (A translation and revision
of Chevandier and Wertheim's Mémoire sur les propriétés mécaniques du bois.)
——: Die technischen Eigenschaften der Hölzer. Lorey's Handbuch der Forstwissenschaft, II. Bd., 6. Kap.,
Tübingen, 1903.
FERNOW, B.E.: Scientific timber testing. Digest of Physical Tests, Vol. I, No. 2, 1896, pp. 87-95.
FOWKE, FRANCIS: Experiments on British colonial and other woods. 1867.
GARDNER, ROLAND: I. Mechanical tests, properties, and uses of thirty Philippine woods. II. Philippine
sawmills, lumber market and prices. Bul. 4, Bu. For., P.I., 1906. (2d ed., 1907, contains tests of 34 woods.)
GAYER, KARL: Forest utilization. (Vol. V, Schlich's Manual of Forestry. Translation of Die Forstbenutzung,
Berlin, 1894.) London, 1908.
GOLLNER, H.: Ueber die Festigkeit des Schwarzföhrenholzes. Mitt. a. d. forstl. Versuchswesen Oesterreichs. II.
Bd., 3. Hft., Wien, 1881.
GOTTGETREU, RUDOLPH: Physische und chemische Beschaffenheit der Baumaterialien. 3d ed., Berlin, 1880.
GREEN, A.O.: Tasmanian timbers: their qualities and uses. Hobart, Tasmania, 1903, pp. 63.
GREGORY, W.B.: Tests of creosoted timber. Trans. Am. Soc. C.E., Vol. LXXVI, 1913, pp. 1192-1203. See also
ibid., Vol. LXX, p. 37.
GRISARD, JULES, et VANDENBERGHE, MAXIMILIEN: Les bois industriels, indigènes et exotiques;
synonymie et description des espèces, propriétés physiques des bois, qualités, défauts, usages et emplois. Paris,

189-. From Bul. de la Société nationale d'acclimatation de France, Vols. XXXVIII-XL.
Hardwoods of Western Australia. Engineering, Vol. LXXXIII, Jan. 11, 1907, pp. 35-37.
HATT, WILLIAM KENDRICK: A Preliminary program for the timber test work to be undertaken by the Bureau of
Forestry, United States Department of Agriculture. Proc. Am. Soc. Test. Mat., Vol. III, 1903, pp. 308-343.
Appendix I: Method of determining the effect of the rate of application of load on the strength of timber, pp. 325-
327; App. II: A discussion on the effect of moisture on strength and stiffness of timber, together with a plan of
procedure for future tests, pp. 328-334.
HATT, WILLIAM KENDRICK: Relation of timber tests to forest products. Proc. Int. Assn. Test. Mat., 1906, C 2
e, pp. 6.
——: Structural timber. Proc. Western Ry. Club, St. Louis, Mch. 17, 1908.
——: Abstract of report on the present status of timber tests in the Forest Service, United States Department of
Agriculture. Proc. Int. Assn. Test. Mat., 1909, XVL, pp. 10.
—— and TURNER, W.P.: The Purdue University impact machine. Proc. Am. Soc. Test. Mat., Vol. VI, 1906, pp.
462-475.
HAUPT, HERMAN: formulæ for the strain upon timber. Center of gravity of an ungula and semi-cylinder. Jour.
Franklin Inst., Vol. XIX, 3d series, 1850, pp. 408-413.
HEARDING, W.H.: Report upon experiments ... upon the compressive power of pine and hemlock timber.
Washington, 1872, pp. 12.
HOWE, MALVERD A.: Wood in compression; bearing values for inclined cuts. Eng. News, Vol. LXVIII, 1912,
pp. 190-191.
HOYER, EGBERT: Lehrbuch der vergleichenden mechanischen Technologie. 1878.
IHLSENG, MANGUS C.: On the modulus of elasticity of some American woods as determined by vibration. Van
Nostrand's Eng. Mag., Vol. XIX, 1878, pp. 8-9.
——: On a mode of measuring the velocity of sounds in woods. Am. Jour. Sci. and Arts, Vol. XVII, 1879.
JACCARD, P.: Étude anatomique des bois comprimés. Mitt. d. Schw. Centralanstalt f. d. forst. Versuchswesen. X.
Bd., 1. Hft., Zurich, 1910, pp. 53-101.
JANKA, GABRIEL: Untersuchungen über die Elasticität und Festigkeit der österreichischen Bauhölzer. I. Fichte
Südtirols; II. Fichte von Nordtirol vom Wienerwalde und Erzgebirge; III. Fichte aus den Karpaten, aus dem
Böhmerwalde, Ternovanerwalde und den Zentralalpen. Technische Qualität des Fichtenholzes im allgemeinen; IV.
Lärche aus dem Wienerwalde, aus Schlesien, Nord- und Südtirol. Mitt. a. d. forst. Untersuchungswesen
Oesterreichs, Wien, 1900-13.
——: Untersuchungen über Holzqualität. Centralblatt f. d. ges. Forstwesen. Wien, 1904, pp. 95-115.
——: Ueber neuere holztechnologische Untersuchungen. Oesterr. Vierteljahresschrift für Forstwesen, Wien, 1906,
pp. 248-269.
——: Die Härte des Holzes. Centralblatt f. d. ges. Forstwesen, Wien, 1906, pp. 193-202; 241-260.
JANKA, GABRIEL: Die Einwirkung von Süss- und Salzwässern auf die gewerblichen Eigenschaften der
Hauptholzarten. I. Teil. Untersuchungen u. Ergebnisse in mechanisch-technischer Hinsicht. Mitt. a. d. forst.
Versuchswesen Oesterreichs, 33. Hft., Wien, 1907.
——: Results of trials with timber carried out at the Austrian forestry testing-station at Mariabrunn. Proc. Int.
Assn. Test. Mat., 1906, Disc. 2 e, pp. 7.
——: Ueber die an der k. k. forstlichen Versuchsanstalt Mariabrunnen gewonnenen Resultate der
Holzfestigkeitsprüfungen. Zeitschrift d. Oesterr. Ing. u. Arch. Ver., Wien, Aug. 9, 1907.

——: Ueber Holzhärteprufüng. Centralblatt f. d. ges. Forstwesen, Wien, 1908, pp. 443-456.
——: Testing the hardness of wood by means of the ball test. Proc. Int. Assn. Test. Mat., 1912, XXIII3.

JENNY, K.: Untersuchungen über die Festigkeit der Hölzer aus den Ländern der ungarischen Krone. Budapest,
1873.
JOHNSON, J.B.: Time tests of timber in endwise compression. Paper before Section D, Am. Assn. for Adv. of Sci.,
Aug., 1898.
JOHNSON, WALTER B.: Experiments on the adhesion of iron spikes of various forms when driven into different
species of timbers. Jour. Franklin Inst., Vol. XIX (n.s.), 1837, pp. 281-292.
JULIUS, G.A.: Western Australia timber tests, 1906. The physical characteristics of the hardwoods of Western
Australia. Perth, 1906, pp. 36.
——: Supplement to the Western Australia timber tests, 1906. The hardwoods of Australia. Perth, 1907, pp. 6.
KARMARSH, CARL: Handbuch der mechanischen Technologie. I. Aufl., 1837; V. Aufl., 1875; verm. von H.
Fisher, 1888.
KIDDER, F.E.: Experiments on the transverse strength of southern and white pine. Van Nostrand's Eng. Mag., Vol.
XXII, 1880, pp. 166-168.
——: Experiments on the strength and stiffness of small spruce beams. Ibid., Vol. XXIV, 1881, pp. 473-477.
——: Experiments on the fatigue of small spruce beams. Jour. Franklin Inst., Vol. CXIV, 1882, pp. 261-279.
KIDWELL, EDGAR: The efficiency of built-up wooden beams. Trans. Am. Inst. Min. Eng., Feb., June, 1898.
KIRKALDY, WM. G.: Illustrations of David Kirkaldy's system of mechanical testing. London, 1891.
KUMMER, FREDERICK A.: The effects of preservative treatment on the strength of timber. Proc. Am. Soc. Test.
Mat., Vol. IV, 1904, pp. 434-438.
LABORDÈRE, P., and ANSTETT, F.: Contribution to the study of means for improving the strength of wood for
pavements. Proc. Int. Assn. Test. Mat., 1912, XXIII4, pp. 12.

LANZA, GAETANO: An account of certain tests on the transverse strength and stiffness of large spruce beams.
Trans. Am. Soc. Mech. Eng., Vol. IV, 1882, pp. 119-135. See also Jour. Franklin Inst., Vol. XCV, 1883, pp. 81-94.
LASLETT, T.: Properties and characteristics of timber. Chatham, 1867.
——: Timber and timber trees, native and foreign. (2d ed. revised and enlarged by H. Marshall Ward.) London and
New York, 1894.
LEA, W.: Tables of strength and deflection of timber. London, 1861.
LEDEBUR, A.: Die Verarbeitung des Holzes auf mechanischem Wege. 1881.
LORENZ, N. VON: Analytische Untersuchung des Begriffes der Holzhärte. Centralblatt f. d. ges. Forstwesen,
Wien, 1909, pp. 348-387.
LUDWIG, PAUL: Die Regelprobe. Ein neues Verfahren zur Härtebestimmung von Materialien. Berlin. 1908.
MACFARLAND, H.B.: Tests of longleaf pine bridge timbers. Bul. 149, Am. Ry. Eng. Assn., Sept., 1912. See also
Eng. News, Dec. 12, 1912, p. 1035.
McKAY, DONALD: On the weight and strength of American ship-timber. Jour. Franklin Inst., Vol. XXXIX (3d
series), 1860, p. 322.
MALETTE, J.: Essais des bois de construction. Revue Technique, Apr. 25, 1905.
MANN, JAMES: Australian timber: its strength, durability, and identification. Melbourne, 1900.

MARTIN, CLARENCE A.: Tests on the relation between cross-bending and direct compressive strength in timber.
Railroad Gazette, Mch. 13, 1903.
Methods of testing metals and alloys ... Recommended by the Fourth Congress of the International Association for
Testing Materials, held at Brussels, Sept. 3-6, 1906. London, 1907, pp. 54. Methods of testing wood, pp. 39-49.
MIKOLASCHEK, CARL: Untersuchungen über die Elasticität und Festigkeit der wichtigsten Bau- und
Nutzhölzer. Mitt. a. d. forstl. Versuchswesen Oesterreiches, II. Bd., 1. Hft., Wien, 1879.
MOELLER, JOSEPH: Die Rohstoffe des Tischler- und Drechslergewerbes. I. Theil: Das Holz. Kassel, 1883, pp.
68-122.
MOLESWORTH, G.L.: Graphic diagrams of strength of teak beams. Roorke, 1881.
MORGAN, J.J.: Bending strength of yellow pine timber. Eng. Record, Vol. LXVII, 1913, pp. 608-609.
MOROTO, K.: Untersuchungen über die Biegungselasticität und -Festigkeit der japanischen Bauhölzer.
Centralblatt f. d. ges. Forstwesen, Wien, 1908, pp. 346-355.
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Gewerbetreibende. Stuttgart, 1860.
——: Druckfestigkeit des Holzes. 1882.
——: Die gewerblichen Eigenschaften der Hölzer. Stuttgart, 1890.
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World Review, May 25, 1914, pp. 27-29.
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Nostrand's Eng. Mag., Vol. XVII, 1877, pp. 531-535.
PACCINOTTI E PERI: [Investigations into the elasticity of timbers.] Il Cimento, Vol. LVIII, 1845.
PALACIO, E.: Tensile tests of timber. La Ingenieria, Buenos Aires, May 31, 1903, et seq.
PARENT,—: Expériences sur la résistance des bois de chêne et de sapin. Mémoires de l'Académie des Sciences,
1707-08.
Propositions relatives à l'établissement d'un precédé uniforme pour l'essai des qualités techniques des bois. Proc.
Int. Assn. Test. Mat., 1901, Annexe, pp. 13-28.
ROGERS, CHARLES G.: A manual of forest engineering for India. Vol. I, Calcutta, 1900, pp. 50-91.
RUDELOFF, M.: Der heutige Stand der Holzuntersuchungen. Mitt. a. d. königlichen tech. Versuchsanstalt, Berlin,
IV, 1899.
——: Principles of a standard method of testing wood. Proc. Int. Assn. Test. Mat., 1906, 23 C, pp. 16.
——: Large vs. small test-pieces in testing wood. Proc. Int. Soc. Test. Mat., 1912, XXIII1, pp. 7.

SARGENT, CHARLES SPRAGUE: Woods of the United States, with an account of their structure, qualities, and
uses. New York, 1885.
SCHNEIDER, A.: Zusammengesetzte Träger. Zeitschrift d. Oesterr. Ing. u. Arch. Ver., Nov. 24; Dec. 9, 1899.
SCHWAPPACH, A.F.: Beiträge zur Kenntniss der Qualität des Rotbuchenholzes. Zeitschrift f. Forst- und
Jagdwesen, Berlin, 1894, pp. 513-539.
——: Untersuchungen über Raumgewicht und Druckfestigkeit des Holzes wichtiger Waldbäume. Berlin, 1897-98.
——: Etablissement de méthodes uniformes pour l'essai á la compression des bois. Proc. Int. Assn. Test. Mat.,
1901, Rapport 23, pp. 28.

SEBERT, H.: Notice sur les bois de la Nouvelle Calédonie suivie de considerations génerates sur les propriétés
mécaniques des bois et sur les precédés employés pour les mesurer. Paris.
SHERMAN, EDWARD C.: Crushing tests on water-soaked timbers. Eng. News, Vol. LXII, 1909, p. 22.
SNOW, CHARLES H.: The principal species of wood: their characteristic properties. New York, 1908.
STAUFFER, OTTMAR: Untersuchungen über specifisches Trockengewicht, sowie anatomisches Verhalten des
Holzes der Birke. München, 1892.
STENS, D.: Ueber die Eigenschaftenimprägnierter Grubenholzer, insbesondere über ihre Festigkeit. Glückauf,
Essen, Mch. 6, 1907.
Strength of wood for pavements. Can. Eng., Toronto, Sept. 12, 1912.
STÜBSCHEN-KISCHNER: Karmarsch-Heerins technisches Wröterbuch. 3. Aufl., 1886.
TALBOT, ARTHUR N.: Tests of timber beams. Bul. 41, Eng. Exp. Sta., Univ. of Ill., Urbana, 1910.
Tests of wooden beams made at the Massachusetts Institute of Technology on spruce, white pine, yellow pine, and
oak beams of commercial sizes. Technology Quarterly, Boston, Vol. VII, 1894.
TETMAJER, L. v.: Zur Frage der Knickungsfestigkeit der Bauhölzer. Schweizerische Bauzeitung, Bd. 11, Nr. 17.
——: Methoden und Resultate der Prüfung der schweizerischen Bauhölzer. Mitt. d. Anstalt z. Prüfung v.
Baumaterialien am eidgenössischen Polytechnicum in Zürich. 2. Hft., 1884.
——: Methoden und Resultate der Prüfung der schweizerischen Bauhölzer. Mitt. d. Materialprüfungs-Anstalt am
Schweiz. Polytechnikum in Zürich. Landesaustellungs-Ausgabe, 2. Hft., Zürich, 1896.
THELEN, ROLF: The structural timbers of the Pacific Coast. Proc. Am. Soc. Test. Mat., Vol. VIII, 1908, pp. 558-
567.
THURSTON, R.H.: Torsional resistance of materials determined by a new apparatus with automatic registry. Jour.
Franklin Inst., Vol. LXV, 1873, pp. 254-260.
——: On the strength of American timber. Ibid., Vol. LXXVIII, 1879, pp. 217-235.
——: Experiments on the strength of yellow pine. Ibid., Vol. LXXIX, 1880, pp. 157-163.
——: Influence of time on bending strength and elasticity. Proc. Am. Assn. for Adv. Sci., 1881. Also Proc. Inst.
C.E., Vol. LXXI.
——: On the effect of prolonged stress upon the strength and elasticity of pine timber. Jour. Franklin Inst., Vol.
LXXX, 1881, pp. 161-169.
THURSTON, R.H.: On Flint's investigations of Nicaraguan woods. Ibid., Vol. XCIV, 1887, pp. 289-315.
TIEMANN, HARRY DONALD: The effect of moisture and other extrinsic factors upon the strength of wood.
Proc. Am. Soc. Test. Mat., Vol. VII, 1907, pp. 582-594.
——: The effect of the speed of testing upon the strength of wood and the standardization of tests for speed. Ibid.,
Vol. VIII, 1908, pp. 541-557.
——: The theory of impact and its application to testing materials. Jour. Franklin Inst., Vol. CLXVIII, 1909, pp.
235-259; 336-364.
——: Some results of dead load bending tests of timber by means of a recording deflectometer. Proc. Am. Soc.
Test. Mat., Vol. IX, 1909, pp. 534-548.
TJADEN, M.E.H.: Het Indrukken van Paalkoppen in Kespen. De Ingenieur, Sept. 11, 1909.
——: Weerstand van Hout loodrecht op de Vezelrichting. Ibid., May, 1911.
——: Buigvastheid van Hout. Ibid., May 31, 1913.

TRAUTWINE, JOHN C.: Shearing strength of some American woods. Jour. Franklin Inst., Vol. CIX, 1880, pp.
105-106.
TREDGOLD, THOMAS: Elementary principles of carpentry. London, 1870.
TURNBULL, W.: A practical treatise on the strength and stiffness of timber. London, 1833.
Untersuchungen über den Einfluss des Blauwerdens auf die Festigkeit von Kiefernholz. Mitt. a. d. könig. techn.
Versuchsanstalten, I, 1897.
Verfahren zur Prüfung v. Metallen und Legierungen, von hydraulischen Bindemitteln, von Holz, von Ton-,
Steinzeug- und Zementröhren. Empfohlen v. dem in Brüssel v. 3-6, IX, 1906, abgeh. IV. Kongress des
internationalen Verbandes f. die Materialprüfungen der Technik. Wien, 1907.
WARREN, W.H.: Australian timbers. Sydney, 1892.
——: The strength, elasticity, and other properties of New South Wales hardwood timbers. Sydney, 1911.
——: The strength, elasticity, and other properties of New South Wales hardwood timbers. Proc. Int. Assn. Test.
Mat., 1912, XXIII6, pp. 9.

——: The properties of New South Wales hardwood timbers. Builder, London, Nov. 1, 1912.
——: The hardwood timbers of New South Wales, Australia. Jour. Soc. of Arts, London, Dec. 6, 1912.
WELLINGTON, A.M.: Experiments on impregnated timber. Railroad Gazette, 1880.
WIJKANDER, ——: Untersuchung der Festigkeitseigenschaften schwedischer Holzarten in der
Materialprüfungsanstalt des Chalmers'schen Institutes ausgeführt. 1897.
WING, CHARLES B.: Transverse strength of the Douglas fir. Eng. News, Vol. XXXIII, Mch. 14, 1895.

PART III.
PUBLICATIONS OF THE U.S. GOVERNMENT ON THE MECHANICAL PROPERTIES OF
WOOD, AND TIMBER TESTING

MISCELLANEOUS

House Misc. Doc. 42, pt. 9, 47th Cong., 2d sess., 1884. (Vol. IX, Tenth Census report.) Report on the forests of
North America (exclusive of Mexico). Part II, The Woods of the United States.
House Report No. 1442, 53d Cong., 2d sess. Investigations and tests of American timber. 1894, pp. 4.
War Dept. Doc. 1. Resolutions of the conventions held at Munich, Dresden, Berlin, and Vienna, for the purpose of
adopting uniform methods for testing construction materials with regard to their mechanical properties. By J.
Bauschinger. Translated by O.M. Carter and E.A. Gieseler. 1896, pp. 44.
War Dept. Doc. 11. On tests of construction materials. Translations from the French and from the German. By
O.M. Carter and E.A. Gieseler. 1896, pp. 84.
House Doc. No. 181, 55th Cong., 3d sess. Report upon the forestry investigations of the U.S. Department of
Agriculture, 1877-1898. By B.E. Fernow, 1899, pp. 401. Contains chapter on The work in timber physics in the
Division of Forestry, by Filibert Roth, pp. 330-395.

FOREST SERVICE

Cir. 7—The Government timber tests [189-], pp. 4.
Cir. 8—Strength of "boxed" or "turpentine" timber. 1892, pp. 4.

Bul. 6—Timber Physics. Pt. I. Preliminary report. 1. Need of the investigation. 2. Scope and historical
development of the science of "timber physics." 3. Organization and methods of timber examinations in the
Division of Forestry. By B.E. Fernow, 1892, pp. 57.
Unnumbered Cir.—Instructions for the collection of test pieces of pines for timber investigations [1893], pp. 4.
Cir. 9—Effect of turpentine gathering on the timber of longleaf pine. By B.E. Fernow [1893], p. 1.
Bul. 8—Timber physics. Pt. II. Progress report. Results of investigations on longleaf pine. 1893, pp. 92.
Bul. 10—Timber: an elementary discussion of the characteristics and properties of wood. By Filibert Roth. 1895,
pp. 88.
Bul. 12—Economical designing of timber trestle bridges. By A.L. Johnson, 1896, pp. 57.
Cir. 12—Southern pine, mechanical and physical properties. 1896, pp. 12.
Cir. 15—Summary of mechanical tests on thirty-two species of American woods. 1897, pp. 12.
Cir. 18—Progress in timber physics. 1898, pp. 20.
Cir. 19—Progress in timber physics: Bald cypress (Taxodium distichum). By Filibert Roth, 1898, pp. 24.
Y.B. Extr. 288—Tests on the physical properties of woods. By F.E. Olmstead, 1902, pp. 533-538.
Unnumbered Cir.—Timber tests. [1903], pp. 15.
Unnumbered Cir.—Timber preservation and timber testing at the Louisiana Purchase Exposition. 1904, pp. 6.
Cir. 32—Progress report on the strength of structural timber. By W.K. Hatt, 1904, pp. 28.
Bul. 58—The red gum. By Alfred Chittenden. Includes a discussion of The mechanical properties of red gum
wood, by W.K. Hatt. 1905, pp. 56.
Cir. 38—Instructions to engineers of timber tests. By W.K. Hatt, 1906, pp. 55. Revised edition, 1909, pp. 56.
Cir. 39—Experiments on the strength of treated timber. By W.K. Hatt, 1906, pp. 31. Revised edition, 1908.
Bul. 70—Effect of moisture upon the strength and stiffness of wood. By H.D. Tiemann, 1906, pp. 144.
Cir. 46—Holding force of railroad spikes in wooden ties. By W.K. Hatt, 1906, pp. 7.
Cir. 47—Strength of packing boxes of various woods. By W.K. Hatt, 1906, pp. 7.
Cir. 108—The strength of wood as influenced by moisture. By H.D. Tiemann, 1907, pp. 42.
Cir. 115—Second progress report on the strength of structural timber. By W.K. Hatt, 1907, pp. 39.
Cir. 142—Tests of vehicle and implement woods. By H.B. Holroyd and H.S. Betts, 1908, pp. 29.
Cir. 146—Experiments with railway cross-ties. By H.B. Eastman, 1908, pp. 32.
Cir. 179—Utilization of California eucalypts. By H.S. Betts and C. Stowell Smith, 1910, pp. 30.
Bul. 75—California tanbark oak. Part II, Utilization of the wood of tanbark oak, by H.S. Betts, 1911, pp. 24-32.
Bul. 88—Properties and uses of Douglas fir. By McGarvey Cline and J.B. Knapp, 1911, pp. 75.
Cir. 189—Strength values for structural timbers. By McGarvey Cline, 1912, pp. 8.
Cir. 193—Mechanical properties of redwood. By A.L. Heim, 1912, pp. 32.
Bul. 108—Tests of structural timbers. By McGarvey Cline and A.L. Heim, 1912, pp. 1231.
Bul. 112—Fire-killed Douglas fir: a study of its rate of deterioration, usability, and strength. By J.B. Knapp, 1912,
pp. 18.

Bul. 115—Mechanical properties of western hemlock. By O.P.M. Goss, 1913, pp. 45.
Bul. 122—Mechanical properties of western larch. By O.P.M. Goss, 1913, pp. 45.
Cir. 213—Mechanical properties of woods grown in the United States. 1913, pp. 4.
Cir. 214—Tests of packing boxes of various forms. By John A. Newlin, 1913, pp. 23.
Review Forest Service Investigations. 1913. [Outline of investigations.] Vol. I, pp. 17-21. A microscopic study of
the mechanical failure of wood, by Warren D. Brush. Vol. II, pp. 33-38.
Bul. 67, U.S.D.A.—Tests of Rocky Mountain woods for telephone poles. By Norman deW. Betts and A.L. Heim,
1914, pp. 28.
Bul. 77, U.S.D.A.—Rocky Mountain mine timbers. By Norman deW. Betts, 1914, pp. 34.
Bul. 86, U.S.D.A.—Tests of wooden barrels. By J.A. Newlin, 1914, pp. 12.

REPORTS OF TESTS ON THE STRENGTH OF STRUCTURAL MATERIAL, MADE AT THE
WATERTOWN ARSENAL, MASS.

House Ex. Doc. No. 12, 47th Cong., 1st sess., 1882. Strength of wood grown on the Pacific slope, pp. 19-93.
Senate Ex. Doc. No. 1, 47th Cong., 2d sess., 1883. Resistance of white and yellow pines to forces of compression
in the direction of the fibers, as used for columns, or posts, pp. 239-395.
Senate Ex. Doc. No. 5, 48th Cong., 1st sess., 1884. Tests of California laurel wood by compression, indentation,
shearing, transverse tension, pp. 223-236. Tests of North American woods (under supervision of Prof. C.S. Sargent
in charge of the forestry division of the Tenth Census), with 16 photographs of fractures of American woods, pp.
237-347.
Senate Ex. Doc. No. 35, 49th Cong., 1st sess., 1885. Adhesion of nails, spikes, and screws in various woods.
Experiments on the resistance of cut nails, wire nails (steel), wood screws, lag screws in white pine, yellow pine,
chestnut, white oak, and laurel, pp. 448-471.
House Ex. Doc. No. 14, 51st Cong., 1st sess., 1890. Adhesion of spikes and bolts in railroad ties, pp. 595-617.
House Ex. Doc. No. 161, 52d Cong., 1st sess., 1892. Adhesion of nails in wood, pp. 744-745.
House Ex. Doc. No. 92, 53d Cong., 3d sess., 1895. Woods—compression tests (endwise compression), pp. 471-
476.
House Doc. No. 54, 54th Cong., 1st sess., 1896. Compression tests on Douglas fir wood, pp. 536-563. Expansion
and contraction of oak and pine wood, pp. 567-574.
House Doc. No. 164, 55th Cong., 2d sess., 1898. Compression tests of timber posts, pp. 405-411. New posts of
yellow pine and spruce, pp. 413-450; Old yellow pine posts from Boston Fire Brick Co. building, No. 394 Federal
St., Boston, Mass., pp. 451-473.
House Doc. No. 143, 55th Cong., 3d sess., 1899. Fire-proofed wood (endwise and transverse tests), pp. 676-681.
House Doc. No. 190, 56th Cong., 2d sess., 1901. Cypress wood for United States Engineer Corps; compression
and transverse tests, pp. 1121-1126. Old white pine and red oak from roof trusses of Old South Church, Boston,
Mass., pp. 1127-1130. Compression of rubber, balata, and wood buffers, pp. 1149-1158.
House Doc. No. 335, 57th Cong., 2d sess., 1903. Douglas fir and white oak woods. Transverse and shearing tests;
also observations on heat conductivity of sticks over wood fires and a stick exposed to low temperature. Expansion
crosswise the grain of wood after submersion, pp. 519-561. Adhesion of lag screws and bolts in wood, pp. 563-
578.

INDEX
Abrasion, 39, 114-117
Annual rings, 44
Apparatus, testing, 94, 99, 102, 104, 107, 110-111, 114, 117, 118, 121, 122, 132, 133, 136
Arborvitæ, 6, 9, 13, 16, 20, 27, 32, 42, 57
Ash, 15, 22, 44, 48, 51, 66, 78
black, 20, 27, 32, 40, 42, 56
white, 9, 13, 16, 20, 27, 32, 40, 42, 45, 56
Aspen, largetooth, 13
Axis, neutral, 23
Basswood, 6, 13, 16, 20, 27, 32, 40, 42, 44, 56
Beams, 15, 24-37, 92, 94
cantilever, 24
continuous, 24
simple, 24
Beech, 9, 13, 16, 20, 22, 27, 32, 40, 42, 51, 57
Bending large beams, 94-99
small beams, 99-102, 132
strength, 2, 22-37, 26, 75
Bibliography, 145-160
Birch, 22, 44
yellow, 9, 13, 16, 20, 27, 32, 40, 42, 57
Bird-peck, 59, 72
Black check, 59
Boiling, effect of, 6, 85
Bow, flexure of a, 4
Boxheart, 54, 82
Brash, 6
Breaking strength of beams, 15
Brittleness, 6, 34, 37, 38
Buckeye, 13, 44

Buckling of fibres, 15, 77
Butternut, 13
Cantilever, 24
Calibration of testing machines, 92, 112
Case-hardening, 80
Catalpa, 44
Cedar, Central American, 22
incense, 13, 16, 20, 27, 57
red, 144
white, 22
Checking, 54, 61, 74, 75-84
Chelura, 67
Cherry, black, 13, 22
Chestnut, 15, 22, 44, 49, 50, 51, 66, 78
Cleavability, 2, 41
Cleavage, 41, 118, 133
Coefficient of elasticity (see Modulus of elasticity)
expansion, 84
Cold, effect of, 86
Color, 50, 58-59
Column, long, 12, 14, 144
short, 15, 102-104
Compression across the grain, 94, 104-107, 133
endwise, 12, 92, 94, 102, 132
failure, 34, 104
parallel to grain (see C. endwise)
perpendicular to grain (see C. right angles to grain)
right angles to grain, 94, 133
Compressive strength, 1, 9, 23
Compressometer, 103
Coniferous wood, 44
Cottonwood, 44, 45
Creosote, effect of, 87
Cross-arms, testing, 124

Cross grain, 8, 59-61
Cross-grained tension failure, 34
Crushing strength, 2, 9, 54, 75
formula for, 104
Cucumber tree, 13
Cup shake, 64
Cypress, bald, 9, 13, 16, 20, 27, 32, 40, 42, 57, 68, 144
Dead Load (see Load)
Definitions, 2-7
Deflection, 25, 30
measuring, 96-97, 100
Deflectometer, 99, 106-107
Deformation, measuring, 103, 107
Density, 54-58
Diffuse-porous, 44, 50
Dogwood, 22
Drying, 75-84
effect of, 75-78, 138-139
Dry rot, 69
Durability, 53, 54, 75
Early Wood, 44, 82
Ease of working, factors affecting, 48, 50
Ebony, 22
Elasticity, modulus of, 6, 25, 89
formulæ for, 98, 102, 104, 114, 123
Elastic limit, 2, 5, 22, 62
resilience, 6
formulæ for, 98, 102, 104
Elm, 8, 38, 44
rock, 9, 13, 16, 20, 27, 32, 40, 42, 56
slippery, 13, 27, 32, 40, 56
white, 13, 20, 27, 32, 40, 42, 56
Elongation, 3, 7, 33
Eucalyptus globulus, 54, 78, 82

viminalis, 54
Factor of Safety, 29, 139
Failures, bending, 33-37, 77, 78
compression, endwise, 12, 15-19, 104
cross-grained, 10, 76, 77, 107
shearing, 19
tension, 8
torsion, 38, 39
Fibre-saturation point, 78
Fibre strain, rate of, 92-93
stress, 15
at elastic limit, 54, 62, 75, 123
formulæ, 98, 102, 104, 107, 114
Fir, Alpine, 16, 20, 27, 32, 40, 42, 57
amabilis, 16, 20, 27, 57
Douglas, 13, 16, 20, 27, 32, 36, 40, 42, 48, 55, 57, 124, 140, 141, 142, 143, 144
white, 9, 16, 20, 27, 32, 40, 42, 57
Flexibility, 5, 37
Flexure, 4, 12, 60
Formulæ, 98, 102, 104, 107, 110, 114, 123
Frost splits, 62-64
Fungi 59, 68-70, 75
Grain, cross, 8, 59-61
diagonal, 60
spiral, 60
Growth, in diameter, 43, 44
locality of, effect, 70-73
rate of, effect, 43, 50, 72
rings, 44, 52
measuring, 95-96
Gum 22, 44, 51
red, 27, 45, 56
Hackberry, 9, 13, 16, 20, 27, 32, 42, 51, 56
Hardness, 2, 39-41, 54, 114-118, 133

Heart break, 65
shake, 64
Heartwood, 50-54, 58, 73, 75
Heat, effect of, 84-86
Hemlock, 9, 13, 15, 16, 20, 22, 27, 32, 40, 42, 44, 45, 56, 78
western, 48, 59, 140, 141, 142, 143, 144
Hickory, 8, 22, 38, 40, 43, 44, 49, 51, 53, 55, 59, 65, 66, 72, 124
big shellbark, 13, 16, 20, 27, 56
bitternut, 13, 16, 20, 27, 56
mockernut, 13, 16, 20, 27, 56
nutmeg, 13, 16, 20, 27, 56
pignut, 6, 7, 13, 16, 20, 27, 48, 56
shagbark, 13, 16, 20, 27, 56
water, 13, 16, 20, 27, 56
Hollow-horning, 80
Honey-combing, 54, 80
Impact, 30-33, 110-114, 132
Implement woods, testing, 124
Indentation, 39, 117-118
Injuries, fungous, 52, 59, 68-70
insect, 52, 66-67, 72
marine wood-borer, 67-68
parasitic plant, 70
Kerosene, effect of, 87
Knots, 51, 52, 61-62, 89
Larch, 8
western, 36, 48, 64, 140, 141, 142, 143
Late wood, 44, 59, 82
measuring, 96
relation to strength, 47-49
Limit of elasticity, 5
Limnoria, 67
Live load, 28
Load, application of, 29

concentrated, 28
dead, 28, 144
immediate, 28
kinds of, 26
live, 28
maximum, 29
permanent, 28
safe, 29
uniform, 26
Loading, centre, 97, 100
static, 29
sudden, 29
third-point, 97
vibratory, 29
Locust, 22
black, 13, 40, 44, 51
honey, 9, 13, 16, 20, 27, 32, 40, 42, 56
Log of tests, 97-98, 100-102, 103-104, 107, 110, 114
Machine for static tests, 90-92 Maple, 22, 44, 51
red, 13, 20, 27, 32, 40, 42, 56
silver, 13
sugar, 9, 13, 16, 20, 27, 32, 40, 42, 56
Marking test specimens, 94-95, 100, 129-131
Material for tests, 88-90, 94, 99-100, 102, 106, 107-110, 113, 115-116, 117, 118, 119-120, 121, 122, 123-125, 128-
134, 135, 136
Mechanical properties, definition of, 1
factors affecting, 43-87
Medullary rays (see Rays)
Mistletoe, 70

Modulus of elasticity, 6, 25, 89
formulæ, 98, 102, 104, 114, 123
of rupture, 26, 54, 62, 75
formulæ, 98, 102
speed-strength, 94
Moisture, determination, 90, 133-134
effect of, 6, 8, 17, 33, 75-84, 138-139
Mulberry, 44, 51
Natural shape and size, 2
Neutral axis, 23
plane, 23, 33
Oak, 15, 22, 43, 48, 49, 55, 60, 66, 71, 84
black, 40
bur, 13
live, 22
post, 9, 13, 16, 20, 27, 32, 40, 42, 56
red, 9, 13, 16, 20, 27, 32, 40, 42, 56, 124
southern, 52, 71
swamp white, 9, 16, 20, 27, 32, 42, 56
tanbark, 56
white, 13, 16, 20, 27, 32, 40, 42, 54, 56, 72, 144
yellow, 9, 13, 16, 20, 27, 32, 42, 56

Osage orange, 13, 16, 27, 32, 40, 51, 56
Oven-dry, 57
Packing Boxes, testing, 124
Permanent set, 5
Permeability, 54
Pitch pockets, 65
Pith rays (see Rays)
Pine, 44, 45, 55
Cuban, 71
loblolly, 36, 48, 51, 54, 71, 85, 140, 141, 142, 143
lodgepole, 13, 16, 20, 27, 32, 40, 42, 57
longleaf, 6, 7, 8, 9, 13, 14, 16, 20, 27, 32, 36, 40, 42, 46, 57,
59, 65, 71, 78, 140, 141, 142, 143, 144
northern yellow, 22
Norway (see Red pine)
red, 9, 13, 16, 20, 27, 32, 36, 40, 42, 48, 57, 140, 141, 142,
143, 144
shortleaf, 13, 27, 36, 48, 57, 71, 140, 141, 142, 143, 144
southern yellow, 22, 124
sugar, 9, 13, 16, 20, 27, 32, 42, 57
western yellow, 9, 13, 16, 20, 27, 32, 40, 42, 57
white, 13, 15, 16, 20, 22, 27, 32, 40, 42, 51, 57, 144
Plane, neutral, 23, 33

Plasticity, 6
Pliability, 5, 38, 85
Poplar, 22, 44
yellow, 44
Pores, 44
Preservatives, effect of, 86
Rays, 60
effect on compression failure, 17, 18
shrinkage, 81-82
Redwood, 13, 16, 27, 36, 48, 57, 140, 141, 142, 143, 144
Resilience, 2, 5, 49
elastic, 6
formulæ for, 98, 102, 104, 114
Resin, effect of, 59
pockets, 65
Rind-gall, 65
Ring, annual, 44
growth, 44, 52
-shake, 64-65
-porous, 44, 59
Rot, 68, 69
Rupture, modulus of, 26, 54, 62, 75
formulæ, 98, 102

Safe Load, 29
Sap, 73, 74
-stain, 59
-wood, 50-54, 73, 74
Sassafras, 51
Season checks, 61, 78-84
of cutting, effect of, 73-75
Seasoning, 55, 74, 75-84
Second-growth, 49, 50
Set, 5
Shake, 64-66, 72
cup, 64
heart, 64
ring, 64
star, 64
Shear 3, 19-22, 133
across the grain, 19, 21, 22
along the grain, 19, 21, 76, 94, 107-110
formulæ, 98, 102
horizontal, 24
failure, 35-37
longitudinal, 21, 24
oblique, 21, 22

transverse, 23
vertical, 23
Shearing strength, 2, 19
Shipping dry, 57
Shipworms, 67
Shock, 30, 49
Shortening, 3, 33
Shrinkage, 54, 58, 74, 76, 78-82, 135-137
S-irons, 83
Site, effect on wood, 48, 49, 70-73
Size of test specimens, effect of, 89-90, 138-139
Sketching test specimens, 94-95, 100, 102, 106
Softwood, 44, 60
Span, 25
Specific gravity, 55, 135-137
Speed of testing machine, 93-94
-strength modulus, 94
Sphæeroma, 67
Spike-pulling test, 123
Spiral grain, 60
Splintering tension failure, 34
Splitting, 41, 60
Spring wood (see Early wood)

Spruce, 14, 15, 22, 44, 59, 75, 84, 144
Engelmann, 13, 16, 20, 27, 32, 40, 42, 57
red, 7, 13, 27, 57, 140, 141
white, 13, 27, 57, 140, 140
Static tests, machine for, 90-92
Steaming, effect of, 85, 87
Stiffness, 1, 4, 5, 6, 25, 26, 62, 76
Strain, definition of, 2
unit, 3
Stress, compressive, 3
definition of, 2
due to impact, 31, 32
external, 2, 33
internal, 2
shearing, 3, 21
tensile, 3, 62
torsional, 38
Stress, unit, 3
-strain diagram, 3, 97-98, 100
Structural timbers, strength of, 138-144
Summer wood, (see Late wood)
Sycamore, 9, 13, 16, 20, 27, 32, 42, 57, 64
Tamarack, 9, 13, 16, 20, 27, 32, 36, 40, 42, 48, 57, 140, 141, 142, 143, 144

Temperature, effect of, 84-86
Tensile strength, 1, 7, 23, 78
parallel to grain, 7, 8
right angles to grain, 8
Tension, 7
failures, 34
tests, 118-122
Teredo, 67
Tests, impact, 31-33
timber, 88-136
Test specimens, size of, 89-90
Timber testing, 88-136
Vehicle woods, testing, 124
Variability of wood, 1, 2, 43
Walnut, black, 22
common, 22
Water content, 55, 73, 74
effect of, 06, 08, 17, 33, 75-84
Wear, resistance to (see Abrasion)
Weight, relation to mechanical properties, 54-55
Willow, 44
black, 13
Work (see Resilience), 30, 54

Working plan, 88, 127-137
Xylotrya, 67
Yellow Poplar, 44
Zinc Chloride, effect of, 87

FOOTNOTES
return
[Footnote 1: This is in accordance with the discovery made in 1678 by
Robert Hooke, and is known as Hooke's law.]
return
[Footnote 2: If the straight portion does not pass through the origin, a
parallel line should be drawn through the origin, and the load at elastic limit
taken from this line. (See Fig. 32.)]
return
[Footnote 3: See Brush, Warren D.: A microscopic study of the mechanical
failure of wood. Vol. II, Rev. F.S. Investigations, Washington, D.C., 1912, p.
35.]
return
[Footnote 4: See Circular No. 18, U.S. Division of Forestry: Progress in
timber physics, pp. 13-18; also Bulletin 70, U.S. Forest Service: Effect of
moisture on the strength and stiffness of wood, pp. 42, 89-90.]
return
[Footnote 5: See Bulletin 70, op. cit., p. 129.]
return
[Footnote 6: Jaccard, P.: Étude anatomique des bois comprimés. Mit. d.
Schw. Centralanstalt f.d. forst. Versuchswesen. X. Band, 1. Heft. Zurich,
1910, p. 66.]
return

[Footnote 7: This does not correspond exactly with the conclusions of A.
Thil, who says ("Constitution anatomique du bois," pp. 140-141): "The
sides of the medullary rays sometimes produce planes of least resistance
varying in size with the height of the rays. The medullary rays assume a
direction more or less parallel to the lumen of the cells on which they
border; the latter curve to the right or left to make room for the ray and then
close again beyond it. If the force acts parallel to the axis of growth, the
tracheids are more likely to be displaced if the marginal cells of the
medullary rays are provided with weak walls that are readily compressed.
This explains why on the radial surface of the test blocks the plane of
rupture passes in a direction nearly following a medullary ray, whereas on
the tangential surface the direction of the plane of rupture is oblique—but
with an obliquity varying with the species and determined by the pitch of
the spirals along which the medullary rays are distributed in the stem." See
Jaccard, op. cit., pp. 57 et seq.]
return
[Footnote 8: Shear should not be confused with ordinary cutting or
incision.]
return
[Footnote 9: While in reality this relationship does not exactly hold, the
formulæ for beams are based on its assumption.]
return
[Footnote 10: Only this form of beam is considered since it is the simplest.
For cantilever and continuous beams, and beams rigidly fixed at one or both
ends, as well as for different methods of loading, different forms of cross
section, etc., other formulæ are required. See any book on mechanics.]
return
[Footnote 11: See Tiemann, Harry D.: Some results of dead load bending
tests of timber by means of a recording deflectometer. Proc. Am. Soc. for
Testing Materials. Phila. Vol. IX, 1909, pp. 534-548.]
return

[Footnote 12: A fourth might be added, namely, vibratory, or harmonic
repetition, which is frequently serious in the case of bridges.]
return
[Footnote 13: Johnson, J.B.: The materials of construction, pp. 81-82.]
return
[Footnote 14: See Tiemann, Harry D.: The theory of impact and its
application to testing materials. Jour. Franklin Inst., Oct., Nov., 1909, pp.
235-259, 336-364.]
return
[Footnote 15: See Proc. Int. Assn. for Testing Materials, 1912, XXIII2, pp.
12-13.]
return
[Footnote 16: See articles by Gabriel Janka listed in bibliography, pages
151-152.]
return
[Footnote 17: For details regarding the structure of wood see Record,
Samuel J.: Identification of the economic woods of the United States. New
York, John Wiley & Sons, 1912.]
return
[Footnote 18: Bul. 88: Properties and uses of Douglas fir, p. 29.]
return
[Footnote 19: Bul. 108, U. S. Forest Service: Tests of structural timbers, p.
37.]
return
[Footnote 20: Bul. 80: The commercial hickories, pp. 48-50.]
return

[Footnote 21: Bul. 53: Chestnut in southern Maryland, pp. 20-21.]
return
[Footnote 22: Bul. 108: Tests of structural timber, p. 35.]
return
[Footnote 23: Bul. 80: The commercial hickories, p. 50.]
return
[Footnote 24: Loc. cit.]
return
[Footnote 25: Although the factor of heart or sapwood does not influence
the mechanical properties of the wood and there is usually no difference in
structure observable under the microscope, nevertheless sapwood is
generally decidedly different from heartwood in its physical properties. It
dries better and more easily than heartwood, usually with less shrinkage and
little checking or honeycombing. This is especially the case with the more
refractory woods, such as white oaks and Eucalyptus globulus and
viminalis. It is usually much more permeable to air, even in green wood,
notably so in loblolly pine and even in white oak. As already stated, it is
much more subject to decay. The sapwood of white oak may be
impregnated with creosote with comparative ease, while the heartwood is
practically impenetrable. These facts indicate a difference in its chemical
nature.—H.D. Tiemann.]
return
[Footnote 26: Bul. 108, U.S. Forest Service, p. 36.]
return
[Footnote 27: The oaks for some unknown reason fall below the normal
strength for weight, whereas the hickories rise above. Certain other woods
also are somewhat exceptional to the normal relation of strength and
density.]
return

[Footnote 28: Bul. 70, U.S. Forest Service, p. 92; also p. 126, appendix.]
return
[Footnote 29: See Burke, H.E.: Black check in western hemlock. Cir. No.
61, U.S. Bu. Entomology, 1905.]
return
[Footnote 30: See McAtee, W.L.: Woodpeckers in relation to trees and
wood products. Bul. No. 39, U.S. Biol. Survey, 1911.]
return
[Footnote 31: See Von Schrenck, Hermann: The "bluing" and the "red rot"
of the western yellow pine, with special reference to the Black Hills forest
reserve. Bul. No. 36, U.S. Bu. Plant Industry, Washington, 1903, pp. 13-14.
Weiss, Howard, and Barnum, Charles T.: The prevention of sapstain in
lumber. Cir. 192, U.S. Forest Service, Washington, 1911, pp. 16-17.]
return
[Footnote 32: See Standard classification of structural timber. Yearbook
Am. Soc. for Testing Materials, 1913, pp. 300-303. Contains three plates
showing standard defects.]
return
[Footnote 33: Bul. 108, pp. 52 et seq.]
return
[Footnote 34: Bul. 115, U.S. Forest Service: Mechanical properties of
western hemlock, p. 20.]
return
[Footnote 35: Hartig, R.: The diseases of trees (trans. by Somerville and
Ward), London and New York, 1894, pp. 282-294.]
return

[Footnote 36: Busse, W.: Frost-, Ring- und Kernrisse. Forstwiss. Centralb.,
XXXII, 2, 1910, pp. 74-81.]
return
[Footnote 37: For detailed information regarding insect injuries, the reader
is referred to the various publications of the U.S. Bureau of Entomology,
Washington, D.C.]
return
[Footnote 38: See Smith, C. Stowell: Preservation of piling against marine
wood borers. Cir. 128, U.S. Forest Service, 1908, pp. 15.]
return
[Footnote 39: See Von Schrenck, H.: The decay of timber and methods of
preventing it. Bul. 14, U.S. Bu. Plant Industry, Washington, D.C., 1902.
Also Buls. 32, 114, 214, 266.
Meineoke, E.P.: Forest tree diseases common in California and Nevada,
U.S. Forest Service, Washington, D.C., 1914.
Hartig, R.: The diseases of trees. London and New York, 1894.]
return
[Footnote 40: Dry rot in factory timbers, by Inspection Dept. Associated
Factory Mutual Fire Insurance Cos., 31 Milk Street, Boston, 1913.]
return
[Footnote 41: Falck, Richard: Die Meruliusfaüle des Bauholzes,
Hausschwammforschungen, 6. Heft., Jena, 1912.]
return
[Footnote 42: Mez, Carl: Der Hausschwamm. Dresden, 1908, p. 63.]
return
[Footnote 43: A culture of fungus placed in a glass jar and the air pumped
out ceases to grow, but will start again as soon as oxygen is admitted.]

return
[Footnote 44: Experiments in kiln-drying Eucalyptus in Berkeley, U.S.
Forest Service.]
return
[Footnote 45: See Anderson, Paul J.: The morphology and life history of the
chestnut blight fungus. Bul. No. 7, Penna. Chestnut Tree Blight Com.,
Harrisburg, 1914, p. 17.]
return
[Footnote 46: See York, Harlan H.: The anatomy and some of the biological
aspects of the "American mistletoe." Bul. 120, Sci. Ser. No. 13, Univ. of
Texas, Austin, 1909.
Bray, Wm. L.: The mistletoe pest in the Southwest. Bul. 166, U.S. Bu. Plant
Ind., Washington, 1910.
Meinecke, E.P.: Forest tree diseases common in California and Nevada.
U.S. Forest Service, Washington, 1914, pp. 54-58.]
return
[Footnote 47: See Record, S.J.: Sap in relation to the properties of wood.
Proc. Am. Wood Preservers' Assn., Baltimore, Md., 1913, pp. 160-166.
Kempfer, Wm. H.: The air-seasoning of timber. In Bul. 161, Am. Ry. Eng.
Assn., 1913, p. 214.]
return
[Footnote 48: See Tiemann, H.D.: Effect of moisture upon the strength and
stiffness of wood. Bul. 70, U.S. Forest Service, Washington, D.C., 1906;
also Cir. 108, 1907.]
return
[Footnote 49: The wood of Eucalyptus globulus (blue gum) appears to be an
exception to this rule. Tiemann says: "The wood of blue gum begins to
shrink immediately from the green condition, even at 70 to 90 per cent

moisture content, instead of from 30 or 25 per cent as in other species of
hardwoods." Proc. Soc. Am. For., Washington, Vol. VIII, No. 3, Oct., 1913,
p. 313.]
return
[Footnote 50: See Schlich's Manual of Forestry, Vol. V. (rev. ed.), p. 75.]
return
[Footnote 51: Cir. 39. Experiments on the strength of treated timber, p. 18.]
return
[Footnote 52: Ibid., p. 21. See also Cir. 108, p. 19, table 5.]
return
[Footnote 53: Hatt, W. K.: Experiments on the strength of treated timber.
Cir. 39, U.S. Forest Service, 1906, p. 31.]
return
[Footnote 54: Teesdale, Clyde II.: The absorption of creosote by the cell
walls of wood. Cir. 200, U. S. Forest Service, 1912, p. 7.]
return
[Footnote 55: Tiemann, H.D.: Effect of moisture upon the strength and
stiffness of wood. Bul. 70, U. S. Forest Service, 1907, pp. 122-123, tables
43-44.]
return
[Footnote 56: The methods of timber testing described here are for the most
part those employed by the U. S. Forest Service. See Cir. 38 (rev. ed.),
1909.]
return
[Footnote 57: Bul. 108, U. S. Forest Service: Tests of structural timbers, pp.
53-54.]

return
[Footnote 58: See Tiemann, Harry Donald: The effect of the speed of
testing upon the strength and the standardization of tests for speed. Proc.
Am. Soc. for Testing Materials, Vol. VIII, Philadelphia, 1908.]
return
[Footnote 59: For description of U.S. Forest Service automatic and
autographic impact testing machine, see Proc. Am. Soc. for Testing
Materials, Vol. VIII, 1908, pp. 538-540.]
return
[Footnote 60: See Warren, W.H.: The strength, elasticity, and other
properties of New South Wales hardwood timbers. Dept. For., N.S.W.,
Sydney, 1911, pp. 88-95.]
return
[Footnote 61: Bul. No. 8: Timber physics, Part II., 1893, p. 7.]
return
[Footnote 62: Cir. 38: Instructions to engineers of timber tests, 1906, p. 24.]
return
[Footnote 63: Warren, W.H.: The strength, elasticity, and other properties of
New South Wales hardwood timbers, 1911, pp. 58-62.]
return
[Footnote 64: Wood is so seldom subjected to a pure stress of this kind that
the torsion test is usually omitted.]

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