The Mechanical Properties of Wood
by Samuel J. Record · English · Gutenberg #12299 · 386,178 chars · public domain (US)
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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
[Illustration: Frontispiece. _Photo by the author_.
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. X 35.]
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
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_, in FUNDAMENTAL
CONSIDERATIONS AND DEFINITIONS, above) 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.
[Footnote 1: This is in accordance with the discovery made in
1678 by Robert Hooke, and is known as _Hooke's law_.]
Stress is measured in pounds (or other unit of weight or force).
A ~unit stress~ is the stress on a unit of the sectional
{ P }
area. { Unit stress = --- } For instance, if a load (P) of one
{ A }
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
l 0.1
strain is --- = ----- = 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.
[Illustration: FIG. 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.)
[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.)]
~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
{ unit stress }
length. { E = ------------- } It is a number indicative of
{ unit strain }
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 |
|------------------------------------------------------------------------------|
| | Sp. | Modulus of | Tensile | Crushing | Modulus |
| MATERIAL | gr., | elasticity | strength | strength | of |
| | dry | in bending | | | rupture |
|-------------------------+----- +------------+----------+----------+----------|
| | | Lbs. per | Lbs. per | Lbs. per | Lbs. per |
| | | sq. in. | sq. in. | sq. in. | sq. in. |
| | | | | | |
| Cast iron, cold blast | | | | | |
| (Hodgkinson) | 7.1 | 17,270,000 | 16,700 | 106,000 | 38,500 |
| Bessenger steel, | | | | | |
| high grade (Fairbain) | 7.8 | 29,215,000 | 88,400 | 225,600 | |
| Longleaf pine, | | | | | |
| 3.5% moisture (U.S.) | .63 | 2,800,000 | | 13,000 | 21,000 |
| Redspruce, | | | | | |
| 3.5% moisture (U.S.) | .41 | 1,800,000 | | 8,800 | 14,500 |
| Pignut hickory, | | | | | |
| 3.5% moisture (U.S.) | .86 | 2,370,000 | | 11,130 | 24,000 |
|------------------------------------------------------------------------------|
| 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) |
|--------------------------------------------------------------|
| | Ratio: | A stick 1 square inch in |
| | | cross section. |
| | Tensile | |
| KIND OF WOOD | strength | Weight required to-- |
| | R = ----------- +----------------------------|
| | compressive | Pull apart | Crush endwise |
| | strength | | |
|---------------+-----------------+------------+---------------|
| Hickory | 3.7 | 32,000 | 8,500 |
| Elm | 3.8 | 29,000 | 7,500 |
| Larch | 2.3 | 19,400 | 8,600 |
| Longleaf Pine | 2.2 | 17,300 | 7,400 |
|--------------------------------------------------------------|
| 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]
[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.]
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 | When |
| COMMON NAME | surface of | surface of |
| OF SPECIES | failure is | failure is |
| | radial | tangential |
|------------------+------------+------------|
| | Lbs. per | Lbs. per |
| | sq. inch | sq. inch |
| | | |
| Hardwoods | | |
| | | |
| Ash, white | 645 | 671 |
| Basswood | 226 | 303 |
| Beech | 633 | 969 |
| Birch, yellow | 446 | 526 |
| Elm, slippery | 765 | 832 |
| Hackberry | 661 | 786 |
| Locust, honey | 1,133 | 1,445 |
| Maple, sugar | 610 | 864 |
| Oak, post | 714 | 924 |
| red | 639 | 874 |
| swamp white | 757 | 909 |
| white | 622 | 749 |
| yellow | 728 | 929 |
| Sycamore | 540 | 781 |
| Tupelo | 472 | 796 |
| | | |
| Conifers | | |
| | | |
| Arborvitæ | 241 | 235 |
| Cypress, bald | 242 | 251 |
| Fir, white | 213 | 304 |
| Hemlock | 271 | 323 |
| Pine, longleaf | 240 | 298 |
| red | 179 | 205 |
| sugar | 239 | 304 |
| western yellow | 230 | 252 |
| 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.
[Illustration: FIG. 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.
[Illustration: FIG. 3.--Side view of failures in compression
across the grain, showing crushing of blocks under bearing
plate. Specimen at right shows splitting at ends.]
[Illustration: FIG. 4.--End view of failures in compression
across the grain, showing splitting of the ends of the test
specimens.]
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.
[Illustration: FIG. 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.
[Illustration: FIG. 6.--Unequal distribution of stress in a long
column due to lateral bending.]
|-------------------------------------------------------|
| TABLE IV |
|-------------------------------------------------------|
| RESULTS OF COMPRESSION TESTS ACROSS THE GRAIN ON |
| 51 WOODS IN GREEN CONDITION, AND COMPARISON WITH |
| WHITE OAK |
| (U. S. Forest Service) |
|-------------------------------------------------------|
| | Fibre stress | Fiber stress |
| COMMON NAME | at elastic | in per cent |
| OF SPECIES | limit | of white oak, |
| | perpendicular | or 853 pounds |
| | to grain | per sq. in. |
|-----------------------+---------------+---------------|
| | Lbs. per | |
| | sq. inch | Per cent |
| | | |
| Osage orange | 2,260 | 265.0 |
| Honey locust | 1,684 | 197.5 |
| Black locust | 1,426 | 167.2 |
| Post oak | 1,148 | 134.6 |
| Pignut hickory | 1,142 | 133.9 |
| Water hickory | 1,088 | 127.5 |
| Shagbark hickory | 1,070 | 125.5 |
| Mockernut hickory | 1,012 | 118.6 |
| Big shellbark hickory | 997 | 116.9 |
| Bitternut hickory | 986 | 115.7 |
| Nutmeg hickory | 938 | 110.0 |
| 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 maple | 742 | 87.0 |
| Rock elm | 696 | 81.6 |
| Beech | 607 | 71.2 |
| Slippery elm | 599 | 70.2 |
| Redwood | 578 | 67.8 |
| Bald cypress | 548 | 64.3 |
| Red maple | 531 | 62.3 |
| Hackberry | 525 | 61.6 |
| Incense cedar | 518 | 60.8 |
| Hemlock | 497 | 58.3 |
| Longleaf pine | 491 | 57.6 |
| Tamarack | 480 | 56.3 |
| Silver maple | 456 | 53.5 |
| Yellow birch | 454 | 53.2 |
| Tupelo | 451 | 52.9 |
| Black cherry | 444 | 52.1 |
| Sycamore | 433 | 50.8 |
| Douglas fir | 427 | 50.1 |
| Cucumber tree | 408 | 47.8 |
| Shortleaf pine | 400 | 46.9 |
| Red pine | 358 | 42.0 |
| Sugar pine | 353 | 41.1 |
| White elm | 351 | 41.2 |
| Western yellow pine | 348 | 40.8 |
| Lodgepole pine | 348 | 40.8 |
| Red spruce | 345 | 40.5 |
| White pine | 314 | 36.8 |
| Engelman spruce | 290 | 34.0 |
| Arborvitæ | 288 | 33.8 |
| Largetooth aspen | 269 | 31.5 |
| White spruce | 262 | 30.7 |
| Butternut | 258 | 30.3 |
| Buckeye (yellow) | 210 | 24.6 |
| Basswood | 209 | 24.5 |
| Black willow | 193 | 22.6 |
|-------------------------------------------------------|
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
{ P }
by the area of the cross-section (A). { C = --- }
{ A }
[Illustration: FIG. 7.--Endwise compression of a short column.]
It has been demonstrated[4] 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
V.) 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.
[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.]
|-------------------------------------------------------------------------------|
| 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 |
|-------------------------------------------------------------------------------|
| | Soaked | Green | 14 | 11.5 | 9.5 | Kiln-dry |
| MOISTURE CONDITION | 50 per | 23 per | per | per | per | 6.2 per |
| | cent | cent | cent | cent | cent | cent |
| -------------------------+--------+--------+-------+-------+-------+----------|
| Number of tests averaged | 5 | 5 | 5 | 5 | 4 | 5 |
| _r_ in bending | 4,920 | 5,944 | 6,924 | 7,852 | 9,280 | 11,550 |
| _C_ in compression | 4,668 | 5,100 | 6,466 | 7,466 | 8,985 | 10,910 |
| Per cent _r_ is in | | | | | | |
| excess of _C_ | 5.5 | 16.5 | 7.1 | 5.2 | 3.3 | 5.9 |
|-------------------------------------------------------------------------------|
| SPRUCE |
|-------------------------------------------------------------------------------|
| | Soaked | Green | 10 | 8.1 | Kiln-dry |
| MOISTURE CONDITION | 30 per | 30 per | per | per | 3.9 per |
| | cent | cent | cent | cent | cent |
|----------------------------------+--------+--------+-------+-------+----------|
| Number of tests averaged | 5 | 4 | 5 | 3 | 4 |
| _r_ in bending | 3,002 | 3,362 | 6,458 | 8,400 | 10,170 |
| _C_ in compression | 2,680 | 3,025 | 6,120 | 7,610 | 9,335 |
| Per cent _r_ | | | | | |
| is in excess of _C_ | 12.0 | 11.1 | 5.5 | 10.4 | 9.0 |
|-------------------------------------------------------------------------------|
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]
[Footnote 5: See Bulletin 70, _op. cit._, p. 129.]
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 Jaccard[6] 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.
[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.]
|-------------------------------------------------------|
| TABLE VI |
|-------------------------------------------------------|
| RESULTS OF ENDWISE COMPRESSION TESTS ON SMALL CLEAR |
| PIECES OF 40 WOODS IN GREEN CONDITION |
| (Forest Service Cir. 213) |
|-------------------------------------------------------|
| | Fibre | | Modulus |
| COMMON NAME | stress at | Crushing | of |
| OF SPECIES | elastic | strength | elasticity |
| | limit | | |
|-------------------+-----------+----------+------------|
| | Lbs. per | Lbs. per | Lbs. per |
| | sq. inch | sq. inch | sq. 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, yellow | 2,570 | 3,400 | 1,915,000 |
| Elm, slippery | 3,410 | 3,990 | 1,453,000 |
| Hackberry | 2,730 | 3,310 | 1,068,000 |
| Hickory, | | | |
| big shellbark | 3,570 | 4,520 | 1,658,000 |
| bitternut | 4,330 | 4,570 | 1,616,000 |
| mockernut | 3,990 | 4,320 | 1,359,000 |
| 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, honey | 4,300 | 4,970 | 1,536,000 |
| Maple, sugar | 3,040 | 3,670 | 1,463,000 |
| Oak, post | 2,780 | 3,330 | 1,062,000 |
| red | 2,290 | 3,210 | 1,295,000 |
| swamp white | 3,470 | 4,360 | 1,489,000 |
| white | 2,400 | 3,520 | 946,000 |
| yellow | 2,870 | 3,700 | 1,465,000 |
| Osage orange | 3,980 | 5,810 | 1,331,000 |
| 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, incense | 2,710 | 3,030 | 868,000 |
| Cypress, bald | 3,560 | 3,960 | 1,738,000 |
| 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, lodgepole | 2,290 | 2,530 | 1,219,000 |
| 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 yellow | 2,100 | 2,420 | 1,271,000 |
| white | 2,370 | 2,720 | 1,318,000 |
| Redwood | 3,420 | 3,820 | 1,175,000 |
| Spruce, Engelmann | 1,880 | 2,170 | 1,021,000 |
| 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.
[Illustration: FIG. 8.--Failures of short columns of green
spruce.]
[Illustration: FIG. 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.
[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._]
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.
[Footnote 8: Shear should not be confused with ordinary cutting
or incision.]
[Illustration: FIG. 10.--Example of shear along the grain.]
[Illustration: FIG. 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) |
|---------------------------------------------|
| | When | When |
| COMMON NAME | surface of | surface of |
| OF SPECIES | failure is | failure is |
| | radial | tangential |
|-------------------+------------+------------|
| | Lbs. per | Lbs. per |
| | sq. inch | sq. inch |
| | | |
| Hardwoods | | |
| | | |
| Ash, black | 876 | 832 |
| white | 1,360 | 1,312 |
| Basswood | 560 | 617 |
| Beech | 1,154 | 1,375 |
| Birch, yellow | 1,103 | 1,188 |
| Elm, slippery | 1,197 | 1,174 |
| white | 778 | 872 |
| Hackberry | 1,095 | 1,161 |
| Hickory, | | |
| big shellbark | 1,134 | 1,191 |
| bitternut | 1,134 | 1,348 |
| mockernut | 1,251 | 1,313 |
| nutmeg | 1,010 | 1,053 |
| pignut | 1,334 | 1,457 |
| shagbark | 1,230 | 1,297 |
| water | 1,390 | 1,490 |
| Locust, honey | 1,885 | 2,096 |
| Maple, red | 1,130 | 1,330 |
| sugar | 1,193 | 1,455 |
| Oak, post | 1,196 | 1,402 |
| red | 1,132 | 1,195 |
| swamp white | 1,198 | 1,394 |
| white | 1,096 | 1,292 |
| yellow | 1,162 | 1,196 |
| Sycamore | 900 | 1,102 |
| Tupelo | 978 | 1,084 |
| | | |
| Conifers | | |
| | | |
| Arborvitæ | 617 | 614 |
| Cedar, incense | 613 | 662 |
| Cypress, bald | 836 | 800 |
| Fir, alpine | 573 | 654 |
| amabilis | 517 | 639 |
| Douglas | 853 | 858 |
| white | 742 | 723 |
| Hemlock | 790 | 813 |
| Pine, lodgepole | 672 | 747 |
| longleaf | 1,060 | 953 |
| red | 812 | 741 |
| sugar | 702 | 714 |
| western yellow | 686 | 706 |
| white | 649 | 639 |
| Spruce, Engelmann | 607 | 624 |
| 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.
[Illustration: FIG. 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
P
with the axis of the specimen. In this case, shear = -----. When
2 A
the value of the angle [Greek: theta] is less than 45 degrees,
P
the shear along the plane = --- sin [Greek: theta] cos [Greek:
A
theta]. (See Fig. 13.) The effect of oblique shear is often
visible in the failures of short columns. (See Fig. 14.)
[Illustration: FIG. 13.--Oblique shear in a short column.]
[Illustration: FIG. 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) |
|---------------------------------------------------------------------------|
| KIND OF WOOD | Lbs. per | KIND OF WOOD | Lbs. per |
| | sq. inch | | sq. inch |
|-----------------------+----------+-----------------------------+----------|
| Ash | 6,280 | Hickory | 7,285 |
| Beech | 5,223 | Locust | 7,176 |
| Birch | 5,595 | Maple | 6,355 |
| Cedar (white) | 1,372 | Oak | 4,425 |
| Cedar (white) | 1,519 | Oak (live) | 8,480 |
| Cedar (Central Amer.) | 3,410 | Pine (white) | 2,480 |
| Cherry | 2,945 | Pine (northern yellow) | 4,340 |
| Chestnut | 1,536 | Pine (southernyellow) | 5,735 |
| Dogwood | 6,510 | Pine (very resinous yellow) | 5,053 |
| 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 directly[9] proportional to their distances
from a plane intermediate between them--the ~neutral plane~.
(N_{1} P 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.
[Footnote 9: While in reality this relationship does not exactly
hold, the formulæ for beams are based on its assumption.]
[Illustration: FIG. 15.--Diagram of a simple beam. N_{1} P =
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.)
[Illustration: FIG. 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]
[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.]
P' l^{3}
E = -------------
4 D b h^{3}
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 h^{3}
Necessary weight (P') = -------------
l^{3}
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 h{^2}
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 (P_{1}) 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 |
| COMMON NAME | stress at | Modulus | Modulus |-------------------------------|
| OF SPECIES | elastic | of | of | To | To | |
| | limit | rupture | elasticity | elastic | maximum | Total |
| | | | | limit | load | |
|-----------------+-----------+----------+------------+----------+----------+---------|
| | | | | In.-lbs. | In.-lbs. | In.-lb. |
| | Lbs. per | Lbs. per | Lbs. per | per cu. | per cu. | per |
| | sq. in. | sq. in. | sq. in. | inch | 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, yellow | 4,190 | 8,390 | 1,597,000 | .62 | 14.2 | 31.5 |
| 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 shellbark | 6,370 | 11,110 | 1,562,000 | 1.47 | 24.3 | 78.0 |
| bitternut | 5,470 | 10,280 | 1,399,000 | 1.22 | 20.0 | 75.5 |
| mockernut | 6,550 | 11,110 | 1,508,000 | 1.50 | 31.7 | 84.4 |
| 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, honey | 6,020 | 12,360 | 1,732,000 | 1.28 | 17.3 | 64.4 |
| 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 white | 5,380 | 9,860 | 1,593,000 | 1.05 | 14.5 | 37.6 |
| 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 orange | 7,760 | 13,660 | 1,329,000 | 2.53 | 37.9 | 101.7 |
| 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, incense | 3,950 | 6,040 | 754,000 | | | |
| Cypress, bald | 4,430 | 7,110 | 1,378,000 | .96 | 5.1 | 15.4 |
| 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, lodgepole | 3,080 | 5,130 | 1,015,000 | .54 | 5.1 | 7.4 |
| 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, yellow | 3,180 | 5,180 | 1,111,000 | .52 | 4.3 | 15.6 |
| White | 3,410 | 5,310 | 1,073,000 | .62 | 5.9 | 13.3 |
| Redwood | 4,530 | 6,560 | 1,024,000 | | | |
| Spruce, | | | | | | |
| Engelmann | 2,740 | 4,550 | 866,000 | .50 | 4.8 | 6.1 |
| 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]
[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.]
~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. (Factor of safety = ultimate strength / 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 three[12] general methods in which loads may be
applied to beams, namely:
[Footnote 12: A fourth might be added, namely, ~vibratory~, or
~harmonic repetition~, which is frequently serious in the case
of bridges.]
(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]
[Footnote 13: Johnson, J.B.: The materials of construction, pp.
81-82.]
(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.
[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.]
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) |
|-------------------------------------------------------|
| | Fibre | | Work in |
| COMMON NAME | stress at | Modulus of | bending |
| OF SPECIES | elastic | elasticity | to |
| | limit | | elastic |
| | | | limit |
|-------------------+-----------+------------+----------|
| | | | In.-lbs. |
| | Lbs. per | Lbs. per | per cu. |
| | sq. in. | sq. 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, yellow | 11,080 | 1,812,000 | 3.79 |
| Elm, rock | 12,090 | 1,367,000 | 6.52 |
| slippery | 11,700 | 1,569,000 | 4.86 |
| white | 9,910 | 1,138,000 | 4.82 |
| Hackberry | 10,420 | 1,398,000 | 4.48 |
| Locust, honey | 13,460 | 2,114,000 | 4.76 |
| 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 | 10,580 | 1,506,000 | 4.16 |
| swamp white | 13,280 | 2,048,000 | 4.79 |
| white | 9,860 | 1,414,000 | 3.84 |
| yellow | 10,840 | 1,479,000 | 4.44 |
| Osage orange | 15,520 | 1,498,000 | 8.92 |
| 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, bald | 8,290 | 1,431,000 | 2.71 |
| 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, lodgepole | 6,870 | 1,142,000 | 2.31 |
| 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 yellow | 7,070 | 1,115,000 | 2.51 |
| white | 6,490 | 1,156,000 | 2.06 |
| Spruce, Engelmann | 6,300 | 1,076,000 | 2.09 |
| 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]
[Footnote 15: See Proc. Int. Assn. for Testing Materials, 1912,
XXIII_{2}, pp. 12-13.]
Within the elastic limit the elongations and shortenings are
equal, and the neutral plane lies in the middle of the beam.
(See TRANSVERSE OR BENDING STRENGTH: BEAMS, above.) 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.
[Illustration: FIG. 17.--Characteristic failures of simple
beams.]
(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.
~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.
[Illustration: FIG. 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 | Per cent of total failing by |
| COMMON NAME | number |---------+-------------+-------|
| OF SPECIES | of | 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" x 10" to 8" x 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.
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.
[Illustration: FIG. 19.--Torsion of a shaft.]
[Illustration: FIG. 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 NAME OF SPECIES | Average | End | Radial | Tangential |
| | | surface | surface | surface |
|------------------------+---------+---------+---------+------------|
| | Pounds | Pounds | Pounds | Pounds |
| | | | | |
| Hardwoods | | | | |
| | | | | |
| 1 Osage orange | 1,971 | 1,838 | 2,312 | 1,762 |
| 2 Honey locust | 1,851 | 1,862 | 1,860 | 1,832 |
| 3 Swamp white oak | 1,174 | 1,205 | 1,217 | 1,099 |
| 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 maple | 937 | 992 | 918 | 901 |
| 11 Rock elm | 910 | 954 | 883 | 893 |
| 12 Hackberry | 799 | 829 | 795 | 773 |
| 13 Slippery elm | 788 | 919 | 757 | 687 |
| 14 Yellow birch | 778 | 827 | 768 | 739 |
| 15 Tupelo | 738 | 814 | 666 | 733 |
| 16 Red maple | 671 | 766 | 621 | 626 |
| 17 Sycamore | 608 | 664 | 560 | 599 |
| 18 Black ash | 551 | 565 | 542 | 546 |
| 19 White elm | 496 | 536 | 456 | 497 |
| 20 Basswood | 239 | 273 | 226 | 217 |
| | | | | |
| Conifers | | | | |
| | | | | |
| 1 Longleaf pine | 532 | 574 | 502 | 521 |
| 2 Douglas fir | 410 | 415 | 399 | 416 |
| 3 Bald cypress | 390 | 460 | 355 | 354 |
| 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 yellow pine | 328 | 334 | 307 | 342 |
| 9 Lodgepole pine | 318 | 316 | 318 | 319 |
| 10 White pine | 299 | 304 | 294 | 299 |
| 11 Engelmann pine | 266 | 272 | 253 | 274 |
| 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.
[Footnote 16: See articles by Gabriel Janka listed in
bibliography, pages 151-152.]
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.
[Illustration: FIG. 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 |
| COMMON NAME | surface of | surface of |
| OF SPECIES | failure is | failure is |
| | radial | tangential |
|-------------------+------------+------------|
| | Lbs. per | Lbs. per |
| | sq. inch | sq. 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 |
| Hackberry | 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, Engelmann | 110 | 135 |
| 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.
[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.]
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.)
[Illustration: FIG. 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). X 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.)
[Illustration: FIG. 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. X 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.
[Footnote 18: Bul. 88: Properties and uses of Douglas fir, p.
29.]
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]
[Footnote 19: Bul. 108, U. S. Forest Service: Tests of
structural timbers, p. 37.]
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]
[Footnote 20: Bul. 80: The commercial hickories, pp. 48-50.]
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]
[Footnote 21: Bul. 53: Chestnut in southern Maryland, pp.
20-21.]
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.
[Footnote 22: Bul. 108: Tests of structural timber, p. 35.]
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]
[Footnote 23: Bul. 80: The commercial hickories, p. 50.]
[Footnote 24: _Loc. cit._]
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]
[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.]
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]
[Footnote 26: Bul. 108, U.S. Forest Service, p. 36.]
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]
[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.]
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 / 62.43 = 0.561. To find the weight
per cubic foot when the specific gravity is given, simply
multiply by 62.43. Thus, 0.561 X 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 gravity | Shrinkage from green to |
| | Mois- | oven-dry, based on | oven-dry condition |
| COMMON NAME | ture |--------------------+---------------------------|
| OF SPECIES | content | Volume | Volume | In | | Tangen- |
| | | when | when | volume | Radial | tial |
| | | green | oven-dry | | | |
|-----------------+---------+---------+----------+--------+--------+---------|
| | Per | | | Per | Per | Per |
| | cent | | | 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 shellbark | 64 | .601 | | 17.6 | 7.4 | 11.2 |
| " " | 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 gravity | Shrinkage from green to |
| | | oven-dry, based on | oven-dry condition |
| COMMON NAME | |--------------------+---------------------------|
| OF SPECIES | Mois- | Volume | Volume | In | | Tangen- |
| | ture | when | when | volume | Radial | tial |
| | content | green | oven-dry | | | |
|-----------------+---------+---------+----------+--------+--------+---------|
| | Per | | | Per | Per | Per |
| | cent | | | 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 |
| " | 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
D
density of oven-dry wood, then ---------- = 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
w W V - v
oven-dry, then, d = --- ; D = --- ; S = ------- X 100, and
v V V
V - v
s = ------- X 100, in which S is the percentage of shrinkage
v
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,
above.) Some experiments[28] 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.
[Footnote 28: Bul. 70, U.S. Forest Service, p. 92; also p. 126,
appendix.]
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]
[Footnote 29: See Burke, H.E.: Black check in western hemlock.
Cir. No. 61, U.S. Bu. Entomology, 1905.]
[Footnote 30: See McAtee, W.L.: Woodpeckers in relation to trees
and wood products. Bul. No. 39, U.S. Biol. Survey, 1911.]
[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.]
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]
[Footnote 32: See Standard classification of structural timber.
Yearbook Am. Soc. for Testing Materials, 1913, pp. 300-303.
Contains three plates showing standard defects.]
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 Service[33] indicate
the following effects of knots on structural timbers:
[Footnote 33: Bul. 108, pp. 52 _et seq._]
(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]
[Footnote 34: Bul. 115, U.S. Forest Service: Mechanical
properties of western hemlock, p. 20.]
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. Hartig[35] 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.
[Footnote 35: Hartig, R.: The diseases of trees (trans. by
Somerville and Ward), London and New York, 1894, pp. 282-294.]
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 Busse[36] 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.
[Footnote 36: Busse, W.: Frost-, Ring- und Kernrisse. Forstwiss.
Centralb., XXXII, 2, 1910, pp. 74-81.]
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 INJURIES[37]
[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.]
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.
[Footnote 38: See Smith, C. Stowell: Preservation of piling
against marine wood borers. Cir. 128, U.S. Forest Service, 1908,
pp. 15.]
FUNGOUS INJURIES[39]
[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.]
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 stated[40] that in the three
years (1911-1913) more than $100,000 was required to repair
damage due to dry rot.
[Footnote 40: Dry rot in factory timbers, by Inspection Dept.
Associated Factory Mutual Fire Insurance Cos., 31 Milk Street,
Boston, 1913.]
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.
[Footnote 41: Falck, Richard: Die Meruliusfaüle des Bauholzes,
Hausschwammforschungen, 6. Heft., Jena, 1912.]
[Footnote 42: Mez, Carl: Der Hausschwamm. Dresden, 1908, p. 63.]
All fungi require moisture and air[43] 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.
[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.]
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]
[Footnote 44: Experiments in kiln-drying _Eucalyptus_ in
Berkeley, U.S. Forest Service.]
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]
[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.]
PARASITIC PLANT INJURIES.[46]
[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.]
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.
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]
[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.]
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 CONTENT[48]
[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.]
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) |
|----------------------------------------------------------------------|
| KIND OF STRENGTH | Longleaf | Spruce | Chestnut |
| | pine | | |
|----------------------------+-------------+-------------+-------------|
| | (1) (2) | (1) (2) | (1) (2) |
| | | | |
| Crushing strength parallel | | | |
| to grain | 2.89 2.60 | 3.71 3.41 | 2.83 2.55 |
| Elastic limit in | | | |
| compression | | | |
| parallel to grain | 2.60 2.34 | 3.80 3.49 | 2.40 2.26 |
| Modulus of rupture in | | | |
| bending | 2.50 2.20 | 2.81 2.50 | 2.09 1.82 |
| Stress at elastic limit in | | | |
| bending | 2.90 2.55 | 2.90 2.58 | 2.30 2.00 |
| Crushing strength at right | | | |
| angles to grain | | 2.58 2.48 | |
| Shearing strength parallel | | | |
| to grain | 2.01 1.91 | 2.03 1.95 | 1.55 1.47 |
| Modulus of elasticity in | | | |
| compression parallel to | | | |
| grain | 1.63 1.47 | 2.26 2.08 | 1.43 1.29 |
| Modulus of elasticity in | | | |
| bending | 1.59 1.35 | 1.43 1.23 | 1.44 1.21 |
|----------------------------------------------------------------------|
| 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 dry[49]. 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.
[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.]
[Illustration: FIG. 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.
[Illustration: FIG. 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.
[Illustration: FIG. 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.)
[Illustration: FIG. 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.
[Illustration: FIG. 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
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