Wood Properties

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2–1

Chapter

Structure of Wood

Regis B. Miller

Contents

Bark, Wood, Branches, and Cambium 2–1

Sapwood and Heartwood 2–2

Growth Rings 2–2

Wood Cells 2–3

Chemical Composition 2–3

Species Identification 2–4

References 2–4

he fibrous nature of wood strongly influences how it

is used. Wood is primarily composed of hollow,

elongate, spindle-shaped cells that are arranged

parallel to each other along the trunk of a tree. When lumber

and other products are cut from the tree, the characteristics of

these fibrous cells and their arrangement affect such properties

as strength and shrinkage as well as the grain pattern of the

wood. This chapter briefly describes some elements of wood

structure.

Bark, Wood, Branches,

and Cambium

A cross section of a tree (Fig. 2–1) shows the following well-

defined features (from outside to center): bark, which may be

divided into an outer corky dead part (A), whose thickness

varies greatly with species and age of trees, and an inner thin

living part (B), which carries food from the leaves to growing

parts of the tree; wood, which in merchantable trees of most

species is clearly differentiated into sapwood (D) and heart-

wood (E); and pith (F), a small core of tissue located at the

center of tree stems, branches, and twigs about which initial

wood growth takes place. Sapwood contains both living and

dead tissue and carries sap from the roots to the leaves.

Heartwood is formed by a gradual change in the sapwood and

is inactive. The wood rays (G), horizontally oriented tissue

through the radial plane of the tree, vary in size from one cell

wide and a few cells high to more than 15 cells wide and

several centimeters high. The rays connect various layers

from pith to bark for storage and transfer of food. The cam-

bium layer (C), which is inside the inner bark and forms

wood and bark cells, can be seen only with a microscope.

As the tree grows in height, branching is initiated by lateral

bud development. The lateral branches are intergrown with

the wood of the trunk as long as they are alive. After a branch

dies, the trunk continues to increase in diameter and sur-

rounds that portion of the branch projecting from the trunk

when the branch died. If the dead branches drop from the tree,

the dead stubs become overgrown and clear wood is formed.

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Most growth in thickness of bark and wood is caused by cell

division in the cambium (Fig. 2–1C). No growth in diame-

ter takes place in wood outside the cambial zone; new

growth is purely the addition and growth of new cells, not

the further development of old ones. New wood cells are

formed on the inside of the cambium and new bark cells on

the outside. Thus, new wood is laid down to the outside of

old wood and the diameter of the woody trunk increases.

In most species, the existing bark is pushed outward by the

formation of new bark, and the outer bark layers become

stretched, cracked, and ridged and are finally sloughed off.

Sapwood and Heartwood

Sapwood is located between the cambium and heartwood

(Fig. 2–1D). Sapwood contains both living and dead cells

and functions primarily in the storage of food; in the outer

layers near the cambium, sapwood handles the transport of

water or sap. The sapwood may vary in thickness and num-

ber of growth rings. Sapwood commonly ranges from 4 to

6 cm (1-1/2 to 2 in.) in radial thickness. In certain species,

such as catalpa and black locust, the sapwood contains few

growth rings and usually does not exceed 1 cm (1/2 in.) in

thickness. The maples, hickories, ashes, some southern

pines, and ponderosa pine of North America and cativo

(Prioria copaifera), ehie (Guibourtia ehie), and courbaril

(Hymenaea courbaril) of tropical origin may have sapwood

8 to 15 cm (3 to 6 in.) or more in thickness, especially in

second-growth trees. As a rule, the more vigorously growing

trees have wider sapwood. Many second-growth trees of

merchantable size consist mostly of sapwood.

In general, heartwood consists of inactive cells that do not

function in either water conduction or food storage. The

transition from sapwood to heartwood is accompanied by an

increase in extractive content. Frequently, these extractives

darken the heartwood and give species such as black walnut

and cherry their characteristic color. Lighter colored heart-

wood occurs in North American species such as the spruces

(except Sitka spruce), hemlocks, true firs, basswood, cotton-

wood, and buckeye, and in tropical species such as ceiba

(Ceiba pentandra), obeche (Triplochiton scleroxylon), and

ramin (Gonystylus bancanus). In some species, such as black

locust, western redcedar, and redwood, heartwood extractives

make the wood resistant to fungi or insect attack. All dark-

colored heartwood is not resistant to decay, and some nearly

colorless heartwood is decay resistant, as in northern white-

cedar. However, none of the sapwood of any species is resis-

tant to decay. Heartwood extractives may also affect wood by

(a) reducing permeability, making the heartwood slower to

dry and more difficult to impregnate with chemical preserva-

tives, (b) increasing stability in changing moisture condi-

tions, and (c) increasing weight (slightly). However, as

sapwood changes to heartwood, no cells are added or taken

away, nor do any cells change shape. The basic strength of

the wood is essentially not affected by the transition from

sapwood cells to heartwood cells.

In some species, such as the ashes, hickories, and certain

oaks, the pores (vessels) become plugged to a greater or

lesser extent with ingrowths known as tyloses. Heartwood in

which the pores are tightly plugged by tyloses, as in white

oak, is suitable for tight cooperage, because the tyloses

prevent the passage of liquid through the pores. Tyloses also

make impregnation of the wood with liquid preservatives

difficult.

Growth Rings

In most species in temperate climates, the difference between

wood that is formed early in a growing season and that

formed later is sufficient to produce well-marked annual

growth rings (Fig. 2–2). The age of a tree at the stump or the

age at any cross section of the trunk may be determined by

counting these rings. However, if the growth in diameter is

interrupted, by drought or defoliation by insects for example,

more than one ring may be formed in the same season. In

such an event, the inner rings usually do not have sharply

defined boundaries and are termed false rings. Trees that have

only very small crowns or that have accidentally lost most of

their foliage may form an incomplete growth layer, some-

times called a discontinuous ring.

The inner part of the growth ring formed first in the growing

season is called earlywood and the outer part formed later in

the growing season, latewood. Actual time of formation of

these two parts of a ring may vary with environmental and

weather conditions. Earlywood is characterized by cells with

relatively large cavities and thin walls. Latewood cells have

smaller cavities and thicker walls. The transition from early-

wood to latewood may be gradual or abrupt, depending on

Figure 2–1. Cross section of white oak tree trunk:

(A) outer bark (dry dead tissue), (B) inner bark (living

tissue), (C) cambium, (D) sapwood, (E) heartwood,

(F) pith, and (G) wood rays.

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the kind of wood and the growing conditions at the time it

was formed.

Growth rings are most readily seen in species with sharp

contrast between latewood formed in one year and earlywood

formed in the following year, such as in the native ring-

porous hardwoods ash and oak, and in softwoods like south-

ern pines. In some other species, such as water tupelo, aspen,

and sweetgum, differentiation of earlywood and latewood is

slight and the annual growth rings are difficult to recognize.

In many tropical regions, growth may be practically continu-

ous throughout the year, and no well-defined growth rings

are formed.

When growth rings are prominent, as in most softwoods and

ring-porous hardwoods, earlywood differs markedly from late-

wood in physical properties. Earlywood is lighter in weight,

softer, and weaker than latewood. Because of the greater

density of latewood, the proportion of latewood is sometimes

used to judge the strength of the wood. This method is

useful with such species as the southern pines, Douglas-fir,

and the ring-porous hardwoods (ash, hickory, and oak).

Wood Cells

Wood cells—the structural elements of wood tissue—are of

various sizes and shapes and are quite firmly cemented to-

gether. Dry wood cells may be empty or partly filled with

deposits, such as gums and resins, or with tyloses. The

majority of wood cells are considerably elongated and

pointed at the ends; these cells are customarily called fibers

or tracheids. The length of wood fibers is highly variable

within a tree and among species. Hardwood fibers average

about 1 mm (1/25 in.) in length; softwood fibers range from

3 to 8 mm (1/8 to 1/3 in.) in length.

In addition to fibers, hardwoods have cells of relatively large

diameter known as vessels or pores. These cells form the

main conduits in the movement of sap. Softwoods do not

contain vessels for conducting sap longitudinally in the tree;

this function is performed by the tracheids.

Both hardwoods and softwoods have cells (usually grouped

into structures or tissues) that are oriented horizontally in the

direction from pith toward bark. These groups of cells con-

duct sap radially across the grain and are called rays or wood

rays (Fig. 2–1G). The rays are most easily seen on edge-

grained or quartersawn surfaces, and they vary greatly in size

in different species. In oaks and sycamores, the rays are

conspicuous and add to the decorative features of the wood.

Rays also represent planes of weakness along which season-

ing checks readily develop.

Another type of wood cells, known as longitudinal or axial

parenchyma cells, function mainly in the storage of food.

Chemical Composition

Dry wood is primarily composed of cellulose, lignin, hemi-

celluloses, and minor amounts (5% to 10%) of extraneous

materials. Cellulose, the major component, constitutes

approximately 50% of wood substance by weight. It is a

high-molecular-weight linear polymer consisting of chains of

1 to more than 4

-linked glucose monomers. During

growth of the tree, the cellulose molecules are arranged into

ordered strands called fibrils, which in turn are organized into

the larger structural elements that make up the cell wall of

wood fibers. Most of the cell wall cellulose is crystalline.

Delignified wood fibers, which consist mostly of cellulose,

have great commercial value when formed into paper. Delig-

nified fibers may also be chemically altered to form textiles,

films, lacquers, and explosives.

Lignin constitutes 23% to 33% of the wood substance in

softwoods and 16% to 25% in hardwoods. Although lignin

occurs in wood throughout the cell wall, it is concentrated

toward the outside of the cells and between cells. Lignin is

often called the cementing agent that binds individual cells

together. Lignin is a three-dimensional phenylpropanol

polymer, and its structure and distribution in wood are still

not fully understood. On a commercial scale, it is necessary

to remove lignin from wood to make high-grade paper or

other paper products.

Theoretically, lignin might be converted to a variety of

chemical products, but in commercial practice a large per-

centage of the lignin removed from wood during pulping

operations is a troublesome byproduct, which is often burned

for heat and recovery of pulping chemicals. One sizable

commercial use for lignin is in the formulation of oil-well

drilling muds. Lignin is also used in rubber compounding

and concrete mixes. Lesser amounts are processed to yield

Figure 2–2. Cross section of ponderosa pine log

showing growth rings. Light bands are earlywood,

dark bands latewood. An annual (growth) ring is

composed of an inner earlywood zone and outer

latewood zone.

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vanillin for flavoring purposes and to produce solvents.

Current research is examining the potential of using lignin in

the manufacture of wood adhesives.

The hemicelluloses are associated with cellulose and are

branched, low-molecular-weight polymers composed of

several different kinds of pentose and hexose sugar mono-

mers. The relative amounts of these sugars vary markedly

with species. Hemicelluloses play an important role in fiber-

to-fiber bonding in the papermaking process. The component

sugars of hemicellulose are of potential interest for conversion

into chemical products.

Unlike the major constituents of wood, extraneous materials

are not structural components. Both organic and inorganic

extraneous materials are found in wood. The organic compo-

nent takes the form of extractives, which contribute to such

wood properties as color, odor, taste, decay resistance, den-

sity, hygroscopicity, and flammability. Extractives include

tannins and other polyphenolics, coloring matter, essential

oils, fats, resins, waxes, gum starch, and simple metabolic

intermediates. This component is termed extractives because

it can be removed from wood by extraction with solvents,

such as water, alcohol, acetone, benzene, or ether. Extractives

may constitute roughly 5% to 30% of the wood substance,

depending on such factors as species, growth conditions, and

time of year when the tree is cut.

The inorganic component of extraneous material generally

constitutes 0.2% to 1.0% of the wood substance, although

greater values are occasionally reported. Calcium, potassium,

and magnesium are the more abundant elemental constitu-

ents. Trace amounts (<100 parts per million) of phosphorus,

sodium, iron, silicon, manganese, copper, zinc, and perhaps

a few other elements are usually present.

Valuable nonfibrous products produced from wood include

naval stores, pulp byproducts, vanillin, ethyl alcohol, char-

coal, extractives, and products made from bark.

Species Identification

Many species of wood have unique physical, mechanical, or

chemical properties. Efficient utilization dictates that species

should be matched to end-use requirements through an un-

derstanding of their properties. This requires identification of

the species in wood form, independent of bark, foliage, and

other characteristics of the tree.

General wood identification can often be made quickly on the

basis of readily visible characteristics such as color, odor,

density, presence of pitch, or grain pattern. Where more

positive identification is required, a laboratory investigation

must be made of the microscopic anatomy of the wood.

Identifying characteristics are described in publications such

as the Textbook of Wood Technology by Panshin and de

Zeeuw and Identifying Wood: Accurate Results With Simple

Tools by R.B. Hoadley.

References

Bratt, L.C. 1965. Trends in the production of silvichemi-

cals in the United States and abroad. Tappi Journal.

48(7): 46A–49A.

Browning, B.L. 1975. The chemistry of wood. Huntington,

NY: Robert E. Krieger Publishing Company.

Core, H.A.; Côté, W.A.; Day, A.C. 1979. Wood structure

and identification. 7th ed. Syracuse, NY: Syracuse

University Press.

Desch, H.E.; revised by Dinwoodie, J.M. 1996. Timber,

structure, properties, conversion, and use. 7th ed. London:

MacMillan Press, Ltd.

Fengel, D.; Wegener, G. 1984. Wood: Chemistry, ultras-

tructure, reactions. Berlin and New York: W. deGruyter.

Hamilton, J.K.; Thompson, N.C. 1959. A comparison of

the carbohydrates of hardwoods and softwoods. Tappi

Journal. 42: 752–760.

Hoadley, R.B. 1980. Identifying wood: Accurate results

with simple tools. Newtown, CT: Taunton Press.

Hoadley, R.B. 1990. Understanding wood: A craftsmen’s

guide to wood technology. Newtown, CT: Taunton Press.

Kribs, D.A. 1968. Commercial woods on the American

market. New York: Dover Publications.

Panshin, A.J.; de Zeeuw, C. 1980. Textbook of wood

technology. 4th ed. New York: McGraw–Hill.

Rowell, R.M. 1984. The chemistry of solid wood. Advances

in Chemistry Series No. 207. Washington, DC: American

Chemical Society.

Sarkanen, K.V.; Ludwig, C.H. (eds.). 1971. Lignins:

occurrence, formation, structure and reactions. New York:

Wiley–Interscience.

Sjöström, E. 1981. Wood chemistry: fundamentals and

applications. New York: Academic Press.

Stamm, A.J. 1964. Wood and cellulose science. New York:

Ronald Press Company.

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4–1

Chapter

Mechanical Properties of Wood

David W. Green, Jerrold E. Winandy, and David E. Kretschmann

Contents

Orthotropic Nature of Wood 4–1

Elastic Properties 4–2

Modulus of Elasticity 4–2

Poisson’s Ratio 4–2

Modulus of Rigidity 4–3

Strength Properties 4–3

Common Properties 4–3

Less Common Properties 4–24

Vibration Properties 4–25

Speed of Sound 4–25

Internal Friction 4–26

Mechanical Properties of Clear Straight-Grained Wood 4–26

Natural Characteristics Affecting Mechanical Properties 4–27

Specific Gravity 4–27

Knots 4–27

Slope of Grain 4–28

Annual Ring Orientation 4–30

Reaction Wood 4–31

Juvenile Wood 4–32

Compression Failures 4–33

Pitch Pockets 4–33

Bird Peck 4–33

Extractives 4–33

Properties of Timber From Dead Trees 4–33

Effects of Manufacturing and Service Environments 4–34

Moisture Content 4–34

Temperature 4–35

Time Under Load 4–37

Aging 4–41

Exposure to Chemicals 4–41

Chemical Treatment 4–41

Nuclear Radiation 4–43

Mold and Stain Fungi 4–43

Decay 4–43

Insect Damage 4–43

References 4–44

he mechanical properties presented in this chapter

were obtained from tests of small pieces of wood

termed “clear” and “straight grained” because they

did not contain characteristics such as knots, cross grain,

checks, and splits. These test pieces did have anatomical

characteristics such as growth rings that occurred in consis-

tent patterns within each piece. Clear wood specimens are

usually considered “homogeneous” in wood mechanics.

Many of the mechanical properties of wood tabulated in this

chapter were derived from extensive sampling and analysis

procedures. These properties are represented as the average

mechanical properties of the species. Some properties, such

as tension parallel to the grain, and all properties for some

imported species are based on a more limited number of

specimens that were not subjected to the same sampling and

analysis procedures. The appropriateness of these latter prop-

erties to represent the average properties of a species is uncer-

tain; nevertheless, the properties represent the best informa-

tion available.

Variability, or variation in properties, is common to all

materials. Because wood is a natural material and the tree is

subject to many constantly changing influences (such as

moisture, soil conditions, and growing space), wood proper-

ties vary considerably, even in clear material. This chapter

provides information, where possible, on the nature and

magnitude of variability in properties.

This chapter also includes a discussion of the effect of growth

features, such as knots and slope of grain, on clear wood

properties. The effects of manufacturing and service environ-

ments on mechanical properties are discussed, and their

effects on clear wood and material containing growth features

are compared. Chapter 6 discusses how these research results

have been implemented in engineering standards.

Orthotropic Nature of Wood

Wood may be described as an orthotropic material; that is, it

has unique and independent mechanical properties in the

directions of three mutually perpendicular axes: longitudinal,

radial, and tangential. The longitudinal axis L is parallel to

the fiber (grain); the radial axis R is normal to the growth

rings (perpendicular to the grain in the radial direction); and

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4–2

the tangential axis T is perpendicular to the grain but tangent

to the growth rings. These axes are shown in Figure 4–1.

Elastic Properties

Twelve constants (nine are independent) are needed to de-

scribe the elastic behavior of wood: three moduli of elasticity

E, three moduli of rigidity G, and six Poisson’s ratios µ

The moduli of elasticity and Poisson’s ratios are related by

expressions of the form

E E

i j i, j L,R,T

(4–1)

General relations between stress and strain for a homogene-

ous orthotropic material can be found in texts on anisotropic

elasticity.

Modulus of Elasticity

Elasticity implies that deformations produced by low stress

are completely recoverable after loads are removed. When

loaded to higher stress levels, plastic deformation or failure

occurs. The three moduli of elasticity, which are denoted by

, E

, and E

, respectively, are the elastic moduli along the

longitudinal, radial, and tangential axes of wood. These

moduli are usually obtained from compression tests; how-

ever, data for E

and E

are not extensive. Average values of

and E

for samples from a few species are presented in

Table 4–1 as ratios with E

; the Poisson’s ratios are shown

in Table 4–2. The elastic ratios, as well as the elastic con-

stants themselves, vary within and between species and with

moisture content and specific gravity.

The modulus of elasticity determined from bending, E

rather than from an axial test, may be the only modulus of

elasticity available for a species. Average E

values obtained

from bending tests are given in Tables 4–3 to 4–5. Represen-

tative coefficients of variation of E

determined with bending

tests for clear wood are reported in Table 4–6. As tabulated,

includes an effect of shear deflection; E

from bending can

be increased by 10% to remove this effect approximately.

This adjusted bending E

can be used to determine E

and E

based on the ratios in Table 4–1.

Poisson’s Ratio

When a member is loaded axially, the deformation perpen-

dicular to the direction of the load is proportional to the

deformation parallel to the direction of the load. The ratio of

the transverse to axial strain is called Poisson’s ratio. The

Poisson’s ratios are denoted by µ

, µ

, and

. The first letter of the subscript refers to direction of

applied stress and the second letter to direction of lateral

deformation. For example, µ

is the Poisson’s ratio for

deformation along the radial axis caused by stress along the

longitudinal axis. Average values of Poisson’s ratios for

samples of a few species are given in Table 4–2. Values for

and µ

are less precisely determined than are those for

the other Poisson’s ratios. Poisson’s ratios vary within and

between species and are affected by moisture content and

specific gravity.

Radial

Tangential

Longitudinal

Fiber direction

Figure 4–1. Three principal axes of wood with

respect to grain direction and growth rings.

Table 4–1. Elastic ratios for various species at

approximately 12% moisture content

Species

Hardwoods

Ash, white

0.080 0.125 0.109 0.077

—

Balsa

0.015 0.046 0.054 0.037 0.005

Basswood

0.027 0.066 0.056 0.046

—

Birch, yellow

0.050 0.078 0.074 0.068 0.017

Cherry, black

0.086 0.197 0.147 0.097

—

Cottonwood, eastern

0.047 0.083 0.076 0.052

—

Mahogany, African

0.050 0.111 0.088 0.059 0.021

Mahogany, Honduras

0.064 0.107 0.066 0.086 0.028

Maple, sugar

0.065 0.132 0.111 0.063

—

Maple, red

0.067 0.140 0.133 0.074

—

Oak, red

0.082 0.154 0.089 0.081

—

Oak, white

0.072 0.163 0.086

—

Sweet gum

0.050 0.115 0.089 0.061 0.021

Walnut, black

0.056 0.106 0.085 0.062 0.021

Yellow-poplar

0.043 0.092 0.075 0.069 0.011

Softwoods

Baldcypress

0.039 0.084 0.063 0.054 0.007

Cedar, northern white 0.081 0.183 0.210 0.187 0.015

Cedar, western red

0.055 0.081 0.087 0.086 0.005

Douglas-fir

0.050 0.068 0.064 0.078 0.007

Fir, subalpine

0.039 0.102 0.070 0.058 0.006

Hemlock, western

0.031 0.058 0.038 0.032 0.003

Larch, western

0.065 0.079 0.063 0.069 0.007

PineLoblolly

0.078 0.113 0.082 0.081 0.013

Lodgepole

0.068 0.102 0.049 0.046 0.005

Longleaf

0.055 0.102 0.071 0.060 0.012

Pond

0.041 0.071 0.050 0.045 0.009

Ponderosa

0.083 0.122 0.138 0.115 0.017

Red

0.044 0.088 0.096 0.081 0.011

Slash

0.045 0.074 0.055 0.053 0.010

Sugar

0.087 0.131 0.124 0.113 0.019

Western white

0.038 0.078 0.052 0.048 0.005

Redwood

0.089 0.087 0.066 0.077 0.011

Spruce, Sitka

0.043 0.078 0.064 0.061 0.003

Spruce, Engelmann

0.059 0.128 0.124 0.120 0.010

may be approximated by increasing modulus of elasticity values

in Table 4–3 by 10%.

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4–3

Modulus of Rigidity

The modulus of rigidity, also called shear modulus, indi-

cates the resistance to deflection of a member caused by shear

stresses. The three moduli of rigidity denoted by G

, G

and G

are the elastic constants in the LR, LT, and RT

planes, respectively. For example, G

is the modulus of

rigidity based on shear strain in the LR plane and shear

stresses in the LT and RT planes. Average values of shear

moduli for samples of a few species expressed as ratios with

are given in Table 4–1. As with moduli of elasticity, the

moduli of rigidity vary within and between species and with

moisture content and specific gravity.

Strength Properties

Common Properties

Mechanical properties most commonly measured and repre-

sented as “strength properties” for design include modulus of

rupture in bending, maximum stress in compression parallel

to grain, compressive stress perpendicular to grain, and shear

strength parallel to grain. Additional measurements are often

made to evaluate work to maximum load in bending, impact

bending strength, tensile strength perpendicular to grain, and

hardness. These properties, grouped according to the broad

forest tree categories of hardwood and softwood (not corre-

lated with hardness or softness), are given in Tables 4–3 to

4–5 for many of the commercially important species. Average

coefficients of variation for these properties from a limited

sampling of specimens are reported in Table 4–6.

Modulus of rupture—Reflects the maximum load-

carrying capacity of a member in bending and is propor-

tional to maximum moment borne by the specimen.

Modulus of rupture is an accepted criterion of strength, al-

though it is not a true stress because the formula by which

it is computed is valid only to the elastic limit.

Work to maximum load in bending—Ability to absorb

shock with some permanent deformation and more or less

injury to a specimen. Work to maximum load is a meas-

ure of the combined strength and toughness of wood under

bending stresses.

Compressive strength parallel to grain—Maximum

stress sustained by a compression parallel-to-grain speci-

men having a ratio of length to least dimension of less

than 11.

Compressive stress perpendicular to grain—Reported

as stress at proportional limit. There is no clearly defined

ultimate stress for this property.

Shear strength parallel to grain—Ability to resist inter-

nal slipping of one part upon another along the grain.

Values presented are average strength in radial and tangen-

tial shear planes.

Impact bending—In the impact bending test, a hammer

of given weight is dropped upon a beam from successively

increased heights until rupture occurs or the beam deflects

152 mm (6 in.) or more. The height of the maximum

drop, or the drop that causes failure, is a comparative value

that represents the ability of wood to absorb shocks that

cause stresses beyond the proportional limit.

Tensile strength perpendicular to grain—Resistance of

wood to forces acting across the grain that tend to split a

member. Values presented are the average of radial and

tangential observations.

Hardness—Generally defined as resistance to indentation

using a modified Janka hardness test, measured by the load

required to embed a 11.28-mm (0.444-in.) ball to one-half

its diameter. Values presented are the average of radial and

tangential penetrations.

Tensile strength parallel to grain—Maximum tensile

stress sustained in direction parallel to grain. Relatively

few data are available on the tensile strength of various

species of clear wood parallel to grain. Table 4–7 lists av-

erage tensile strength values for a limited number of

specimens of a few species. In the absence of sufficient ten-

sion test data, modulus of rupture values are sometimes

substituted for tensile strength of small, clear, straight-

grained pieces of wood. The modulus of rupture is consid-

ered to be a low or conservative estimate of tensile strength

for clear specimens (this is not true for lumber).

Table 4–2. Poisson’s ratios for various species at

approximately 12% moisture content

Species

Hardwoods

Ash, white

0.371 0.440 0.684 0.360 0.059 0.051

Aspen, quaking

0.489 0.374

—

0.496 0.054 0.022

Balsa

0.229 0.488 0.665 0.231 0.018 0.009

Basswood

0.364 0.406 0.912 0.346 0.034 0.022

Birch, yellow

0.426 0.451 0.697 0.426 0.043 0.024

Cherry, black

0.392 0.428 0.695 0.282 0.086 0.048

Cottonwood, eastern 0.344 0.420 0.875 0.292 0.043 0.018

Mahogany, African

0.297 0.641 0.604 0.264 0.033 0.032

Mahogany, Honduras 0.314 0.533 0.600 0.326 0.033 0.034

Maple, sugar

0.424 0.476 0.774 0.349 0.065 0.037

Maple, red

0.434 0.509 0.762 0.354 0.063 0.044

Oak, red

0.350 0.448 0.560 0.292 0.064 0.033

Oak, white

0.369 0.428 0.618 0.300 0.074 0.036

Sweet gum

0.325 0.403 0.682 0.309 0.044 0.023

Walnut, black

0.495 0.632 0.718 0.378 0.052 0.035

Yellow-poplar

0.318 0.392 0.703 0.329 0.030 0.019

Softwoods

Baldcypress

0.338 0.326 0.411 0.356

—

Cedar, northern white 0.337 0.340 0.458 0.345

—

Cedar, western red

0.378 0.296 0.484 0.403

—

Douglas-fir

0.292 0.449 0.390 0.374 0.036 0.029

Fir, subalpine

0.341 0.332 0.437 0.336

—

Hemlock, western

0.485 0.423 0.442 0.382

—

Larch, western

0.355 0.276 0.389 0.352

—

PineLoblolly

0.328 0.292 0.382 0.362

—

Lodgepole

0.316 0.347 0.469 0.381

—

Longleaf

0.332 0.365 0.384 0.342

—

Pond

0.280 0.364 0.389 0.320

—

Ponderosa

0.337 0.400 0.426 0.359

—

Red

0.347 0.315 0.408 0.308

—

Slash

0.392 0.444 0.447 0.387

—

Sugar

0.356 0.349 0.428 0.358

—

Western white

0.329 0.344 0.410 0.334

—

Redwood

0.360 0.346 0.373 0.400

—

Spruce, Sitka

0.372 0.467 0.435 0.245 0.040 0.025

Spruce, Engelmann

0.422 0.462 0.530 0.255 0.083 0.058

Page 8

4–4

Table 4–3a. Strength properties

of some commercially important woods grown in the United States (metric)

Static bending

Com-

Modulus

Work to

maxi-

mum Impact

Com-

pression

parallel

pression

perpen-

dicular

Shear

parallel

Tension

perpen-

dicular Side

hard-

Common species

names

Moisture

content Specific

gravity

rupture

(kPa) elasticity

(MPa)

load

(kJ/m

) bending

(mm) to grain

(kPa) to grain

(kPa) grain

(kPa) to grain

(kPa) ness

(N)

Hardwoods

Alder, red

Green

0.37 45,000

8,100

560 20,400 1,700 5,300 2,700 2,000

12%

0.41 68,000

9,500

510 40,100 3,000 7,400 2,900 2,600

AshBlack

Green

0.45 41,000

7,200

840 15,900 2,400 5,900 3,400 2,300

12%

0.49 87,000 11,000

103

890 41,200 5,200 10,800 4,800 3,800

Blue

Green

0.53 66,000

8,500

101

—

24,800 5,600 10,600 —

—

12%

0.58 95,000

9,700

—

48,100 9,800 14,000 —

—

Green

0.53 66,000

9,700

890 29,000 5,000 8,700 4,100 3,900

12%

0.56 97,000 11,400

810 48,800 9,000 13,200 4,800 5,300

Oregon

Green

0.50 52,000

7,800

990 24,200 3,700 8,200 4,100 3,500

12%

0.55 88,000

9,400

840 41,600 8,600 12,300 5,000 5,200

White

Green

0.55 66,000

9,900

108

970 27,500 4,600 9,300 4,100 4,300

12%

0.60 103,000 12,000

115

1,090 51,100 8,000 13,200 6,500 5,900

Aspen

Bigtooth

Green

0.36 37,000

7,700

—

17,200 1,400 5,000 —

—

12%

0.39 63,000

9,900

—

36,500 3,100 7,400 —

—

Quaking

Green

0.35 35,000

5,900

560 14,800 1,200 4,600 1,600 1,300

12%

0.38 58,000

8,100

530 29,300 2,600 5,900 1,800 1,600

Basswood, American Green

0.32 34,000

7,200

410 15,300 1,200 4,100 1,900 1,100

12%

0.37 60,000 10,100

410 32,600 2,600 6,800 2,400 1,800

Beech, American

Green

0.56 59,000

9,500

1,090 24,500 3,700 8,900 5,000 3,800

12%

0.64 103,000 11,900

104

1,040 50,300 7,000 13,900 7,000 5,800

Birch

Paper

Green

0.48 44,000

8,100

112

1,240 16,300 1,900 5,800 2,600 2,500

12%

0.55 85,000 11,000

110

860 39,200 4,100 8,300 — 4,000

Sweet

Green

0.60 65,000 11,400

108

1,220 25,800 3,200 8,500 3,000 4,300

12%

0.65 117,000 15,000

124

1,190 58,900 7,400 15,400 6,600 6,500

Yellow

Green

0.55 57,000 10,300

111

1,220 23,300 3,000 7,700 3,000 3,600

12%

0.62 114,000 13,900

143

1,400 56,300 6,700 13,000 6,300 5,600

Butternut

Green

0.36 37,000

6,700

610 16,700 1,500 5,200 3,000 1,700

12%

0.38 56,000

8,100

610 36,200 3,200 8,100 3,000 2,200

Cherry, black

Green

0.47 55,000

9,000

840 24,400 2,500 7,800 3,900 2,900

12%

0.50 85,000 10,300

740 49,000 4,800 11,700 3,900 4,200

Chestnut, American Green

0.40 39,000

6,400

610 17,000 2,100 5,500 3,000 1,900

12%

0.43 59,000

8,500

480 36,700 4,300 7,400 3,200 2,400

Cottonwood

Balsam poplar

Green

0.31 27,000

5,200

—

11,700 1,000 3,400 —

—

12%

0.34 47,000

7,600

—

27,700 2,100 5,400 —

—

Black

Green

0.31 34,000

7,400

510 15,200 1,100 4,200 1,900 1,100

12%

0.35 59,000

8,800

560 31,000 2,100 7,200 2,300 1,600

Eastern

Green

0.37 37,000

7,000

530 15,700 1,400 4,700 2,800 1,500

12%

0.40 59,000

9,400

510 33,900 2,600 6,400 4,000 1,900

ElmAmerican

Green

0.46 50,000

7,700

970 20,100 2,500 6,900 4,100 2,800

12%

0.50 81,000

9,200

990 38,100 4,800 10,400 4,600 3,700

Rock

Green

0.57 66,000

8,200

137

1,370 26,100 4,200 8,800 —

—

12%

0.63 102,000 10,600

132

1,420 48,600 8,500 13,200 —

—

Slippery

Green

0.48 55,000

8,500

106

1,190 22,900 2,900 7,700 4,400 2,900

12%

0.53 90,000 10,300

117

1,140 43,900 5,700 11,200 3,700 3,800

Hackberry

Green

0.49 45,000

6,600

100

1,220 18,300 2,800 7,400 4,300 3,100

12%

0.53 76,000

8,200

1,090 37,500 6,100 11,000 4,000 3,900

Page 9

4–23

Table 4–5b. Mechanical properties of some woods imported into the United States other than Canadian imports

(inch–pound)

—con.

Static bending

Com-

Common and botanical

Moisture Specific

Modulus

rupture

Modulus

elasticity

Work to

maximum

load

pression

parallel

to grain

Shear

parallel

to grain

Side

hard-

ness Sample

names of species

content gravity (lbf/in

) (

lbf/in

) (in-lbf/in

) (lbf/in

) (lbf) origin

Shorea (Shorea spp.,

Green 0.68 11,700

2.1

—

5,380 1,440 1,350 AS

bullau group)

12%

18,800

2.61

—

10,180 2,190 1,780

Shorea, lauan–meranti group

Dark red meranti

Green 0.46

9,400

1.5

8.6

4,720 1,110 700 AS

12%

12,700

1.77

13.8

7,360 1,450 780

Light red meranti

Green 0.34

6,600

1.04

6.2

3,330

710 440 AS

12%

9,500

1.23

8.6

5,920

970 460

White meranti

Green 0.55

9,800

1.3

8.3

5,490 1,320 1,000 AS

15%

12,400

1.49

11.4

6,350 1,540 1,140

Yellow meranti

Green 0.46

8,000

1.3

8.1

3,880 1,030 750 AS

12%

11,400

1.55

10.1

5,900 1,520 770

Spanish-cedar (Cedrela spp.)

Green 0.41

7,500

1.31

7.1

3,370

990 550 AM

12%

— 11,500

1.44

9.4

6,210 1,100 600

Sucupira (Bowdichia spp.)

Green 0.74 17,200

2.27

—

9,730

—

15%

19,400

—

11,100

—

Sucupira (Diplotropis purpurea)

Green 0.78 17,400

2.68

8,020 1,800 1,980 AM

12%

20,600

2.87

14.8

12,140 1,960 2,140

Teak (Tectona grandis)

Green 0.55 11,600

1.37

13.4

5,960 1,290 930 AS

12%

14,600

1.55

8,410 1,890 1,000

Tornillo (Cedrelinga

Green 0.45

8,400

—

4,100 1,170 870 AM

cateniformis)

12%

—

Wallaba (Eperua spp.)

Green 0.78 14,300

2.33

—

8,040

—

1,540 AM

12%

— 19,100

2.28

—

10,760

—

2,040

Results of tests on small, clear, straight-grained specimens. Property values were taken from world literature

(not obtained from experiments conducted at the Forest Products Laboratory). Other species may be reported

in the world literature, as well as additional data on many of these species. Some property values have been

adjusted to 12% moisture content.

AF is Africa; AM, America; AS, Asia.

Table 4–6. Average coefficients of variation for some mechanical properties

of clear wood

Coefficient of variation

Property

(%)

Static bending

Modulus of rupture

Modulus of elasticity

Work to maximum load

Impact bending

Compression parallel to grain

Compression perpendicular to grain

Shear parallel to grain, maximum shearing strength

Tension parallel to grain

Side hardness

Toughness

Specific gravity

Values based on results of tests of green wood from approximately 50 species.

Values for wood adjusted to 12% moisture content may be assumed to be

approximately of the same magnitude.

Page 10

4–24

Less Common Properties

Strength properties less commonly measured in clear wood

include torsion, toughness, rolling shear, and fracture tough-

ness. Other properties involving time under load include

creep, creep rupture or duration of load, and fatigue strength.

Torsion strength—Resistance to twisting about a longi-

tudinal axis. For solid wood members, torsional shear

strength may be taken as shear strength parallel to grain.

Two-thirds of the value for torsional shear strength may be

used as an estimate of the torsional shear stress at the pro-

portional limit.

Toughness—Energy required to cause rapid complete

failure in a centrally loaded bending specimen. Tables 4–8

and 4–9 give average toughness values for samples of a few

hardwood and softwood species. Average coefficients of

variation for toughness as determined from approximately

50 species are shown in Table 4–6.

Creep and duration of load—Time-dependent deforma-

tion of wood under load. If the load is sufficiently high and

the duration of load is long, failure (creep–rupture) will

eventually occur. The time required to reach rupture is

commonly called duration of load. Duration of load is an

important factor in setting design values for wood. Creep

and duration of load are described in later sections of this

chapter.

Fatigue—Resistance to failure under specific combinations

of cyclic loading conditions: frequency and number of

cycles, maximum stress, ratio of maximum to minimum

stress, and other less-important factors. The main factors

affecting fatigue in wood are discussed later in this chapter.

The discussion also includes interpretation of fatigue data

and information on fatigue as a function of the service

environment.

Rolling shear strength—Shear strength of wood where

shearing force is in a longitudinal plane and is acting per-

pendicular to the grain. Few test values of rolling shear in

solid wood have been reported. In limited tests, rolling

shear strength averaged 18% to 28% of parallel-to-grain

shear values. Rolling shear strength is about the same in

the longitudinal–radial and longitudinal–tangential planes.

Fracture toughness—Ability of wood to withstand flaws

that initiate failure. Measurement of fracture toughness

helps identify the length of critical flaws that initiate failure

in materials.

To date there is no standard test method for determining

fracture toughness in wood. Three types of stress fields, and

associated stress intensity factors, can be defined at a crack

tip: opening mode (I), forward shear mode (II), and transverse

shear mode (III) (Fig. 4–2a). A crack may lie in one of these

Table 4–7. Average parallel-to-grain tensile strength of

some wood species

Tensile strength

Species

(kPa (lb/in

))

Hardwoods

Beech, American

86,200

(12,500)

Elm, cedar

120,700

(17,500)

Maple, sugar

108,200

(15,700)

Oak

Overcup

77,900

(11,300)

112,400

(16,300)

Poplar, balsam

51,000

(7,400)

Sweetgum

93,800

(13,600)

Willow, black

73,100

(10,600)

Yellow-poplar

109,600

(15,900)

Softwoods

Baldcypress

58,600

(8,500)

Cedar

Port-Orford

78,600

(11,400)

Western redcedar

45,500

(6,600)

Douglas-fir, interior north

107,600

(15,600)

Fir

California red

77,900

(11,300)

Pacific silver

95,100

(13,800)

Hemlock, western

89,600

(13,000)

Larch, western

111,700

(16,200)

Pine

Eastern white

73,100

(10,600)

Loblolly

80,000

(11,600)

Ponderosa

57,900

(8,400)

Virginia

94,500

(13,700)

Redwood

Virgin

64,800

(9,400)

Young growth

62,700

(9,100)

Spruce

Engelmann

84,800

(12,300)

Sitka

59,300

(8,600)

Results of tests on small, clear, straight-grained specimens tested

green. For hardwood species, strength of specimens tested at

12% moisture content averages about 32% higher; for softwoods,

about 13% higher.

Table 4–8. Average toughness values for a few hardwood

species

Toughness

Moisture Specific

Radial

Tangential

Species

content gravity (J (in-lbf))

(J (in-lbf))

Birch, yellow

12%

0.65

8,100 (500) 10,100 (620)

Hickory (mocker-

Green

0.64 11,400 (700) 11,700 (720)

nut, pignut, sand)

12%

0.71 10,100 (620) 10,700 (660)

Maple, sugar

14%

0.64

6,000 (370)

5,900 (360)

Oak, red

12%

0.64

7,000 (430)

Scarlet

11%

0.66

8,300 (510)

7,200 (440)

Oak, white

Overcup

Green

0.56 11,900 (730) 11,100 (680)

13%

0.62

5,500 (340)

5,000 (310)

Sweetgum

Green

0.48

5,500 (340)

5,400 (330)

13%

0.51

4,200 (260)

Willow, black

Green

0.38

5,000 (310)

5,900 (360)

11%

0.4

3,400 (210)

3,700 (230)

Yellow-poplar

Green

0.43

5,200 (320)

4,900 (300)

12%

0.45

3,600 (220)

3,400 (210)

Page 11

4–25

three planes and may propagate in one of two directions in

each plane. This gives rise to six crack-propagation systems

(RL, TL, LR, TR, LT, and RT) (Fig. 4–2b). Of these crack-

propagation systems, four systems are of practical impor-

tance: RL, TL, TR, and RT. Each of these four systems allow

for propagation of a crack along the lower strength path

parallel to the grain. The RL and TL orientations in wood

(where R or T is perpendicular to the crack plane and L is the

direction in which the crack propagates) will predominate as

a result of the low strength and stiffness of wood perpendicu-

lar to the grain. It is therefore one of these two orientations

that is most often tested. Values for Mode I fracture

toughness range from 220 to 550 kPa m (200 to

500 lbf in in

) and for Mode II range from 1,650 to

2,400 kPa m (1,500 to 2,200 lbf in in

). Table 4–10

summarizes selected mode I and mode II test results at 10%

to 12% moisture content available in the literature. The

limited information available on moisture content effects on

fracture toughness suggests that fracture toughness is either

insensitive to moisture content or increases as the material

dries, reaching a maximum between 6% and 15% moisture

content; fracture toughness then decreases with further drying.

Vibration Properties

The vibration properties of primary interest in structural

materials are speed of sound and internal friction (damping

capacity).

Speed of Sound

The speed of sound in a structural material is a function of

the modulus of elasticity and density. In wood, the speed of

sound also varies with grain direction because the transverse

modulus of elasticity is much less than the longitudinal

value (as little as 1/20); the speed of sound across the grain

is about one-fifth to one-third of the longitudinal value.

For example, a piece of wood with a longitudinal modulus

of elasticity of 12.4 GPa (1.8 ×

lbf/in

) and density of

Table 4–9. Average toughness values for a few softwood

species

Toughness

Moisture Specific

Radial

Tangential

Species

content gravity

(J (in-lbf))

Cedar

Western red

0.33 1,500 (90) 2,100 (130)

Yellow

10%

0.48 3,400 (210) 3,700 (230)

Douglas-fir

Coast

Green

12%

0.44

0.47 3,400

3,300 (210)

(200) 5,900

5,900 (360)

(360)

Interior west

Green

13%

0.48

0.51 3,300

3,400 (200)

(210) 4,900

5,500 (300)

(340)

Interior north

Green

14%

0.43

0.46 2,800

2,600 (170)

(160) 3,900

4,100 (240)

(250)

Interior south

Green

14%

0.38

0.4

2,100

2,000 (130)

(120) 2,900

2,900 (180)

(180)

Fir

California red

Green

12%

0.36

0.39 2,100

2,000 (130)

(120) 2,900

2,800 (180)

(170)

Noble

Green

0.36

—

— 3,900 (240)

12%

0.39

—

— 3,600 (220)

Pacific silver

Green

0.37 2,400 (150) 3,700 (230)

13%

0.4

2,800 (170) 4,200 (260)

White

Green

0.36 2,300 (140) 3,600 (220)

13%

0.38 2,100 (130) 3,300 (200)

Hemlock

Mountain

Green

0.41 4,100 (250) 4,600 (280)

14%

0.44 2,300 (140) 2,800 (170)

Western

Green

0.38 2,400 (150) 2,800 (170)

12%

0.41 2,300 (140) 3,400 (210)

Larch, western

Green

0.51 4,400 (270) 6,500 (400)

12%

0.55 3,400 (210) 5,500 (340)

PineEastern white Green 0.33 2,000 (120) 2,600 (160)

12%

0.34 1,800 (110) 2,000 (120)

Jack

Green

0.41 3,300 (200) 6,200 (380)

12%

0.42 2,300 (140) 3,900 (240)

Loblolly

Green

0.48 5,000 (310) 6,200 (380)

12%

0.51 2,600 (160) 4,200 (260)

Lodgepole

Green

0.38 2,600 (160) 3,400 (210)

Ponderosa

Green

0.38 3,100 (190) 4,400 (270)

11%

0.43 2,400 (150) 3,100 (190)

Red

Green

0.4 3,400 (210) 5,700 (350)

12%

0.43 2,600 (160) 4,700 (290)

Shortleaf

Green

0.47 4,700 (290) 6,500 (400)

13%

0.5 2,400 (150) 3,700 (230)

Slash

Green

0.55 5,700 (350) 7,300 (450)

12%

0.59 3,400 (210) 5,200 (320)

Virginia

Green

0.45 5,500 (340) 7,600 (470)

12%

0.49 2,800 (170) 4,100 (250)

Redwood

Old-growth

Green

0.39 1,800 (110) 3,300 (200)

11%

0.39 1,500

(90) 2,300 (140)

Young-growth

Green

0.33 1,800 (110) 2,300 (140)

12%

0.34 1,500

(90) 1,800 (110)

Spruce,

Green

0.34 2,400 (150) 3,100 (190)

Engelmann

12%

0.35 1,800 (110) 2,900 (180)

Results of tests on small, clear, straight-grained specimens.

Figure 4–2. Possible crack propagation systems for

wood.

Page 12

4–26

480 kg/m

(30 lb/ft

) would have a speed of sound in the

longitudinal direction of about 3,800 m/s (12,500 ft/s).

In the transverse direction, modulus of elasticity would be

about 690 MPa (100 ×

lbf/in

) and the speed of sound

approximately 890 m/s (2,900 ft/s).

The speed of sound decreases with increasing temperature or

moisture content in proportion to the influence of these

variables on modulus of elasticity and density. The speed of

sound decreases slightly with increasing frequency and am-

plitude of vibration, although for most common applications

this effect is too small to be significant. There is no recog-

nized independent effect of species on the speed of sound.

Variability in the speed of sound in wood is directly related

to the variability of modulus of elasticity and density.

Internal Friction

When solid material is strained, some mechanical energy is

dissipated as heat. Internal friction is the term used to denote

the mechanism that causes this energy dissipation. The

internal friction mechanism in wood is a complex function of

temperature and moisture content. In general, there is a value

of moisture content at which internal friction is minimum.

On either side of this minimum, internal friction increases as

moisture content varies down to zero or up to the fiber satu-

ration point. The moisture content at which minimum inter-

nal friction occurs varies with temperature. At room tempera-

ture (23ºC (73ºF)), the minimum occurs at about 6%

moisture content; at -

20ºC (-

4ºF), it occurs at about 14%

moisture content, and at 70ºC (158ºF), at about 4%. At

90ºC (194ºF), the minimum is not well defined and occurs

near zero moisture content.

Similarly, there are temperatures at which internal friction is

minimum, and the temperatures of minimum internal friction

vary with moisture content. The temperatures of minimum

internal friction are higher as the moisture content is de-

creased. For temperatures above 0ºC (32ºF) and moisture

content greater than about 10%, internal friction increases

strongly as temperature increases, with a strong positive

interaction with moisture content. For very dry wood, there

is a general tendency for internal friction to decrease as the

temperature increases.

The value of internal friction, expressed by logarithmic

decrement, ranges from about 0.1 for hot, moist wood to less

than 0.02 for hot, dry wood. Cool wood, regardless of mois-

ture content, would have an intermediate value.

Mechanical Properties of

Clear Straight-Grained Wood

The mechanical properties listed in Table 4–1 through

Table 4–9 are based on a variety of sampling methods.

Generally, the most extensive sampling is represented in

Tables 4–3 and 4–4. The values in Table 4–3 are averages

derived for a number of species grown in the United States.

The tabulated value is an estimate of the average clear wood

property of the species. Many values were obtained from test

specimens taken at a height of 2.4 to 5 m (8 to 16 ft) above

the stump of the tree. Values reported in Table 4–4 represent

estimates of the average clear wood properties of species

grown in Canada and commonly imported into the United

States.

Methods of data collection and analysis changed over the

years during which the data in Tables 4–3 and 4–4 were

collected. In addition, the character of some forests has

changed with time. Because not all the species were reevalu-

ated to reflect these changes, the appropriateness of the data

should be reviewed when used for critical applications such

as stress grades of lumber.

Values reported in Table 4–5 were collected from the world

literature; thus, the appropriateness of these properties to

represent a species is not known. The properties reported in

Tables 4–1, 4–2, 4–5, 4–7, 4–8, 4–9 and 4–10 may not

necessarily represent average species characteristics because of

inadequate sampling; however, they do suggest the relative

influence of species and other specimen parameters on the

mechanical behavior recorded.

Variability in properties can be important in both production

and consumption of wood products. The fact that a piece

may be stronger, harder, or stiffer than the average is often of

less concern to the user than if the piece is weaker; however,

this may not be true if lightweight material is selected for a

specific purpose or if harder or tougher material is difficult to

work. Some indication of the spread of property values is

therefore desirable. Average coefficients of variation for many

mechanical properties are presented in Table 4–6.

Table 4–10. Summary of selected fracture toughness

results

Fracture toughness (

kPa m (

lbf/in in.

))

Mode I

Mode II

Species

Douglas-fir

320

(290)

360

(330)

2,230

(2,030)

Western hemlock

375

(340)

2,240

(2,040)

PineWestern white

250

(225)

260

(240)

Scots

440

(400)

500

(455)

2,050

(1,860)

Southern

375

(340)

2,070

(1,880)

Ponderosa

290

(265)

Red spruce

420

(380)

2,190

(1,990)

1,665

(1,510)

Northern red oak

410

(370)

Sugar maple

480

(430)

Yellow-poplar

517

(470)

Page 13

4–27

The mechanical properties reported in the tables are signifi-

cantly affected by specimen moisture content at time of test.

Some tables include properties that were evaluated at differ-

ing moisture levels; these moisture levels are reported. As

indicated in the tables, many of the dry test data were ad-

justed to a common moisture content base of 12%.

Specific gravity is reported in many tables because this

property is used as an index of clear wood mechanical proper-

ties. The specific gravity values given in Tables 4–3 and 4–4

represent the estimated average clear wood specific gravity of

the species. In the other tables, the specific gravity values

represent only the specimens tested. The variability of spe-

cific gravity, represented by the coefficient of variation de-

rived from tests on 50 species, is included in Table 4–6.

Mechanical and physical properties as measured and reported

often reflect not only the characteristics of the wood but also

the influence of the shape and size of the test specimen and

the test mode. The test methods used to establish properties

in Tables 4–3, 4–4, 4–7, 4–8 and 4–9 are based on standard

procedures (ASTM D143). The test methods for properties

presented in other tables are referenced in the selected bibli-

ography at the end of this chapter.

Common names of species listed in the tables conform to

standard nomenclature of the U.S. Department of Agriculture,

Forest Service. Other names may be used locally for a spe-

cies. Also, one common name may be applied to groups of

species for marketing.

Natural Characteristics

Affecting Mechanical Properties

Clear straight-grained wood is used for determining funda-

mental mechanical properties; however, because of natural

growth characteristics of trees, wood products vary in specific

gravity, may contain cross grain, or may have knots and

localized slope of grain. Natural defects such as pitch pockets

may occur as a result of biological or climatic elements

influencing the living tree. These wood characteristics must

be taken into account in assessing actual properties or esti-

mating the actual performance of wood products.

Specific Gravity

The substance of which wood is composed is actually heav-

ier than water; its specific gravity is about 1.5 regardless of

wood species. In spite of this, the dry wood of most species

floats in water, and it is thus evident that part of the volume

of a piece of wood is occupied by cell cavities and pores.

Variations in the size of these openings and in the thickness

of the cell walls cause some species to have more wood

substance per unit volume than other species and therefore

higher specific gravity. Thus, specific gravity is an excellent

index of the amount of wood substance contained in a piece

of wood; it is a good index of mechanical properties as long

as the wood is clear, straight grained, and free from defects.

However, specific gravity values also reflect the presence of

gums, resins, and extractives, which contribute little to

mechanical properties.

Approximate relationships between various mechanical

properties and specific gravity for clear straight-grained wood

of hardwoods and softwoods are given in Table 4–11 as

power functions. Those relationships are based on average

values for the 43 softwood and 66 hardwood species pre-

sented in Table 4–3. The average data vary around the rela-

tionships, so that the relationships do not accurately predict

individual average species values or an individual specimen

value. In fact, mechanical properties within a species tend to

be linearly, rather than curvilinearly, related to specific grav-

ity; where data are available for individual species, linear

analysis is suggested.

Knots

A knot is that portion of a branch that has become incorpo-

rated in the bole of a tree. The influence of a knot on the

mechanical properties of a wood member is due to the inter-

ruption of continuity and change in the direction of wood

fibers associated with the knot. The influence of knots de-

pends on their size, location, shape, and soundness; atten-

dant local slope of grain; and type of stress to which the

wood member is subjected.

The shape (form) of a knot on a sawn surface depends upon

the direction of the exposing cut. A nearly round knot is

produced when lumber is sawn from a log and a branch is

sawn through at right angles to its length (as in a flatsawn

board). An oval knot is produced if the saw cut is diagonal

to the branch length (as in a bastard-sawn board) and a

“spiked” knot when the cut is lengthwise to the branch (as

in a quartersawn board).

Knots are further classified as intergrown or encased

(Fig. 4–3). As long as a limb remains alive, there is con-

tinuous growth at the junction of the limb and the bole of the

tree, and the resulting knot is called intergrown. After the

branch has died, additional growth on the trunk encloses the

dead limb, resulting in an encased knot; bole fibers are not

continuous with the fibers of the encased knot. Encased knots

and knotholes tend to be accompanied by less cross-grain

than are intergrown knots and are therefore generally less

problematic with regard to most mechanical properties.

Most mechanical properties are lower in sections containing

knots than in clear straight-grained wood because (a) the clear

wood is displaced by the knot, (b) the fibers around the knot

are distorted, resulting in cross grain, (c) the discontinuity of

wood fiber leads to stress concentrations, and (d) checking

often occurs around the knots during drying. Hardness and

strength in compression perpendicular to the grain are excep-

tions, where knots may be objectionable only in that they

cause nonuniform wear or nonuniform stress distributions at

contact surfaces.

Knots have a much greater effect on strength in axial tension

than in axial short-column compression, and the effects on

bending are somewhat less than those in axial tension.

Page 14

4–28

For this reason, in a simply supported beam, a knot on the

lower side (subjected to tensile stresses) has a greater effect

on the load the beam will support than does a knot on the

upper side (subjected to compressive stresses).

In long columns, knots are important because they affect

stiffness. In short or intermediate columns, the reduction in

strength caused by knots is approximately proportional to

their size; however, large knots have a somewhat greater

relative effect than do small knots.

Knots in round timbers, such as poles and piles, have less

effect on strength than do knots in sawn timbers. Although

the grain is irregular around knots in both forms of timber,

the angle of the grain to the surface is smaller in naturally

round timber than in sawn timber. Furthermore, in round

timbers there is no discontinuity in wood fibers, which

results from sawing through both local and general slope of

grain.

The effects of knots in structural lumber are discussed in

Chapter 6.

Slope of Grain

In some wood product applications, the directions of impor-

tant stresses may not coincide with the natural axes of fiber

orientation in the wood. This may occur by choice in

design, from the way the wood was removed from the log, or

because of grain irregularities that occurred while the tree was

growing.

Table 4–11a. Functions relating mechanical properties to specific gravity of clear, straight-grained wood (metric)

Specific gravity–strength relationship

Green wood

Wood at 12% moisture content

Property

Softwoods

Hardwoods

Softwoods

Hardwoods

Static bending

MOR (kPa)

109,600 G

1.01

118,700 G

1.16

170,700 G

1.01

171,300 G

0.13

MOE (MPa)

16,100 G

0.76

13,900 G

0.72

20,500 G

0.84

16,500 G

0.7

WML (kJ/m

)

147 G

1.21

229 G

1.52

179 G

1.34

219 G

1.54

Impact bending (N)

353 G

1.35

422 G

1.39

346 G

1.39

423 G

1.65

Compression parallel

(kPa)

49,700 G

0.94

49,000 G

1.11

93,700 G

0.97

76,000 G

0.89

Compression perpendicular (kPa)

8,800 G

1.53

18,500 G

2.48

16,500 G

1.57

21,600 G

2.09

Shear parallel (kPa)

11,000 G

0.73

17,800 G

1.24

16,600 G

0.85

21,900 G

1.13

Tension perpendicular (kPa)

3,800 G

0.78

10,500 G

1.37

6,000 G

1.11

10,100 G

1.3

Side hardness (N)

6,230 G

1.41

16,550 G

2.31

85,900 G

1.5

15,300 G

2.09

Compression parallel to grain is maximum crushing strength; compression perpendicular to grain is fiber stress at

proportional limit. MOR is modulus of rupture; MOE, modulus of elasticity; and WML, work to maximum load. For green

wood, use specific gravity based on ovendry weight and green volume; for dry wood, use specific gravity based on

ovendry weight and volume at 12% moisture content.

Table 4–11b. Functions relating mechanical properties to specific gravity of clear, straight-grained wood (inch–pound)

Specific gravity–strength relationship

Green wood

Wood at 12% moisture content

Property

Softwoods

Hardwoods

Softwoods

Hardwoods

Static bending

MOR (lb/in

)

15,890 G

1.01

17,210 G

1.16

24,760 G

1.01

24,850 G

0.13

MOE (

lb/in

)

2.33 G

0.76

2.02 G

0.72

2.97 G

.0.84

2.39 G

0.7

WML (in-lbf/in

)

21.33 G

1.21

33.2 G

1.52

25.9 G

1.34

31.8 G

1.54

Impact bending (lbf)

79.28 G

1.35

94.9 G

1.39

77.7 G

1.39

95.1 G

1.65

Compression parallel (lb/in

)

7,210 G

0.94

7,110 G

1.11

13,590 G

0.97

11,030 G

0.89

Compression perpendicular (lb/in

)

1,270 G

1.53

2,680 G

2.48

2,390 G

1.57

3,130 G

2.09

Shear parallel (lb/in

)

1,590 G

0.73

2,580 G

1.24

2,410 G

.0.85

3,170 G

1.13

Tension perpendicular (lb/in

)

550 G

0.78

1,520 G

1.37

870 G

1.11

1,460 G

1.3

Side hardness (lbf)

1,400 G

1.41

3,720 G

2.31

1,930 G

1.5

3,440 G

2.09

Compression parallel to grain is maximum crushing strength; compression perpendicular to grain is fiber stress at

proportional limit. MOR is modulus of rupture; MOE, modulus of elasticity; and WML, work to maximum load. For green

wood, use specific gravity based on ovendry weight and green volume; for dry wood, use specific gravity based on

ovendry weight and volume at 12% moisture content.

Page 15

4–29

Elastic properties in directions other than along the natural

axes can be obtained from elastic theory. Strength properties

in directions ranging from parallel to perpendicular to the

fibers can be approximated using a Hankinson-type formula

(Bodig and Jayne 1982):

sin

cos

(4–2)

where N is strength at angle ?

from fiber direction,

Q strength perpendicular to grain, P strength parallel to

grain, and n an empirically determined constant.

This formula has been used for modulus of elasticity as well

as strength properties. Values of n and associated ratios of

Q/P tabulated from available literature are as follows:

Property

Q/P

Tensile strength

1.5–2

0.04–0.07

Compression strength

2–2.5

0.03–0.40

Bending strength

1.5–2

0.04–0.10

Modulus of elasticity

0.04–0.12

Toughness

1.5–2

0.06–0.10

The Hankinson-type formula can be graphically depicted as a

function of Q/P and n. Figure 4–4 shows the strength in any

direction expressed as a fraction of the strength parallel to

fiber direction, plotted against angle to the fiber direction ?

The plot is for a range of values of Q/P and n.

The term slope of grain relates the fiber direction to the edges

of a piece. Slope of grain is usually expressed by the ratio

between 25 mm (1 in.) of the grain from the edge or long

axis of the piece and the distance in millimeters (inches)

within which this deviation occurs (tan ?

). The effect of grain

slope on some properties of wood, as determined from tests,

is shown in Table 4–12. The values for modulus of rupture

fall very close to the curve in Figure 4–4 for Q/P = 0.1 and

n = 1.5. Similarly, the impact bending values fall close to

the curve for Q/P = 0.05 and n =1.5, and the compression

values for the curve for Q/P = 0.1, n = 2.5.

The term cross grain indicates the condition measured by

slope of grain. Two important forms of cross grain are spiral

and diagonal (Fig. 4–5). Other types are wavy, dipped,

interlocked, and curly.

Spiral grain is caused by winding or spiral growth of wood

fibers about the bole of the tree instead of vertical growth. In

sawn products, spiral grain can be defined as fibers lying in

the tangential plane of the growth rings, rather than parallel

to the longitudinal axis of the product (see Fig. 4–5 for a

simple case). Spiral grain in sawn products often goes unde-

tected by ordinary visual inspection. The best test for spiral

grain is to split a sample section from the piece in the radial

direction. A visual method of determining the presence of

spiral grain is to note the alignment of pores, rays, and resin

ducts on a flatsawn face. Drying checks on a flatsawn surface

follow the fibers and indicate the slope of the fiber. Relative

Figure 4–3. Types of knots. A, encased knot;

B, intergrown.

1.0

Fraction of property parallel to the

fiber direction

N/P

0.8

0.6

0.4

0.2

Q/P =

0.20

0.10

0.05

Angle to fiber direction (deg)

Figure 4–4. Effect of grain angle on mechanical property

of clear wood according to Hankinson-type formula.

Q/P is ratio of mechanical property across the grain (Q)

to that parallel to the grain (P); n is an empirically

determined constant.

Page 16

4–30

change in electrical capacitance is an effective technique for

measuring slope of grain.

Diagonal grain is cross grain caused by growth rings that are

not parallel to one or both surfaces of the sawn piece. Diago-

nal grain is produced by sawing a log with pronounced taper

parallel to the axis (pith) of the tree. Diagonal grain also

occurs in lumber sawn from crooked logs or logs with butt

swell.

Cross grain can be quite localized as a result of the distur-

bance of a growth pattern by a branch. This condition,

termed local slope of grain, may be present even though the

branch (knot) may have been removed by sawing. The degree

of local cross grain may often be difficult to determine. Any

form of cross grain can have a deleterious effect on mechanical

properties or machining characteristics.

Spiral and diagonal grain can combine to produce a more

complex cross grain. To determine net cross grain, regardless

of origin, fiber slopes on the contiguous surface of a piece

must be measured and combined. The combined slope of

grain is determined by taking the square root of the sum of

the squares of the two slopes. For example, assume that the

spiral grain slope on the flat-grained surface of Figure 4–5D

is 1 in 12 and the diagonal-grain slope is 1 in 18. The com-

bined slope is

( / ) ( / )

1 18

1 12 1 10

or a slope of 1 in 10.

A regular reversal of right and left spiraling of grain in a tree

stem produces the condition known as interlocked grain.

Interlocked grain occurs in some hardwood species (Ch. 3,

Table 3–9) and markedly increases resistance to splitting in

the radial plane. Interlocked grain decreases both the static

bending strength and stiffness of clear wood specimens. The

data from tests of domestic hardwoods shown in Table 4–3

do not include pieces that exhibited interlocked grain. Some

mechanical property values in Table 4–5 are based on speci-

mens with interlocked grain because that is a characteristic of

some species. The presence of interlocked grain alters the

relationship between bending strength and compressive

strength of lumber cut from tropical hardwoods.

Annual Ring Orientation

Stresses perpendicular to the fiber (grain) direction may be

at any angle from 0°

(T ) to 90

(R) to the growth rings

(Fig. 4–6). Perpendicular-to-grain properties depend some-

what upon orientation of annual rings with respect to the

direction of stress. The compression perpendicular-to-grain

values in Table 4–3 were derived from tests in which the

load was applied parallel to the growth rings (T direction);

shear parallel-to-grain and tension perpendicular-to-grain

values are averages of equal numbers of specimens with 0

and 90

growth ring orientations. In some species, there is

no difference in 0

and 90

orientation properties. Other

species exhibit slightly higher shear parallel or tension per-

pendicular-to-grain properties for the 0

orientation than for

Table 4–12. Strength of wood members with various

grain slopes compared with strength of a straight-

grained member

Maximum slope

of grain in

member

Modulus

of rupture

(%)

Impact

bending

(%)

Compression

parallel to grain

(%)

Straight-grained

100

1 in 25

100

1 in 20

100

1 in 15

100

1 in 10

1 in 5

Impact bending is height of drop causing complete

failure (0.71-kg (50-lb) hammer); compression parallel

to grain is maximum crushing strength.

Figure 4–5. Relationship of fiber orientation (O-O) to

axes, as shown by schematic of wood specimens

containing straight grain and cross grain. Specimens A

through D have radial and tangential surfaces;

E through H do not. Specimens A and E contain no

cross grain; B, D, F, and H have spiral grain;

C, D, G, and H have diagonal grain.

Page 17

4–31

the 90

orientation; the converse is true for about an equal

number of species.

The effects of intermediate annual ring orientations have been

studied in a limited way. Modulus of elasticity, compressive

perpendicular-to-grain stress at the proportional limit, and

tensile strength perpendicular to the grain tend to be about

the same at 45

and 0

, but for some species these values are

40% to 60% lower at the 45

orientation. For those species

with lower properties at 45

ring orientation, properties tend

to be about equal at 0

and 90

orientations. For species with

about equal properties at 0

and 45

orientations, properties

tend to be higher at the 90

orientation.

Reaction Wood

Abnormal woody tissue is frequently associated with leaning

boles and crooked limbs of both conifers and hardwoods. It

is generally believed that such wood is formed as a natural

response of the tree to return its limbs or bole to a more

normal position, hence the term reaction wood. In soft-

woods, the abnormal tissue is called compression wood; it

is common to all softwood species and is found on the lower

side of the limb or inclined bole. In hardwoods, the abnor-

mal tissue is known as tension wood; it is located on the

upper side of the inclined member, although in some in-

stances it is distributed irregularly around the cross section.

Reaction wood is more prevalent in some species than in

others.

Many of the anatomical, chemical, physical, and mechanical

properties of reaction wood differ distinctly from those of

normal wood. Perhaps most evident is the increase in den-

sity compared with that of normal wood. The specific gravity

of compression wood is commonly 30% to 40% greater than

that of normal wood; the specific gravity of tension wood

commonly ranges between 5% and 10% greater than that of

normal wood, but it may be as much as 30% greater.

Compression wood is usually somewhat darker than normal

wood because of the greater proportion of latewood, and it

frequently has a relatively lifeless appearance, especially in

woods in which the transition from earlywood to latewood is

abrupt. Because compression wood is more opaque than

normal wood, intermediate stages of compression wood can

be detected by transmitting light through thin cross sections;

however, borderline forms of compression wood that merge

with normal wood can commonly be detected only by mi-

croscopic examination.

Tension wood is more difficult to detect than is compression

wood. However, eccentric growth as seen on the transverse

section suggests its presence. Also, because it is difficult to

cleanly cut the tough tension wood fibers, the surfaces of

sawn boards are “woolly,” especially when the boards are

sawn in the green condition (Fig. 4–7). In some species,

tension wood may be evident on a smooth surface as areas of

contrasting colors. Examples of this are the silvery appear-

ance of tension wood in sugar maple and the darker color of

tension wood in mahogany.

Reaction wood, particularly compression wood in the green

condition, may be stronger than normal wood. However,

compared with normal wood with similar specific gravity,

reaction wood is definitely weaker. Possible exceptions to

this are compression parallel-to-grain properties of compres-

sion wood and impact bending properties of tension wood.

Figure 4–6. Direction of load in relation to direction of

annual growth rings: 90

or perpendicular (R), 45°

, 0°

or parallel (T).

Figure 4–7. Projecting tension wood fibers on sawn

surface of mahogany board.

Page 18

4–32

Because of the abnormal properties of reaction wood, it may

be desirable to eliminate this wood from raw material. In

logs, compression wood is characterized by eccentric growth

about the pith and the large proportion of latewood at the

point of greatest eccentricity (Fig. 4–8A). Fortunately, pro-

nounced compression wood in lumber can generally be

detected by ordinary visual examination.

Compression and tension wood undergo extensive longitu-

dinal shrinkage when subjected to moisture loss below the

fiber saturation point. Longitudinal shrinkage in compression

wood may be up to 10 times that in normal wood and in

tension wood, perhaps up to 5 times that in normal wood.

When reaction wood and normal wood are present in the

same board, unequal longitudinal shrinkage causes internal

stresses that result in warping. In extreme cases, unequal

longitudinal shrinkage results in axial tension failure over a

portion of the cross section of the lumber (Fig. 4–8B). Warp

sometimes occurs in rough lumber but more often in planed,

ripped, or resawn lumber (Fig. 4–8C).

Juvenile Wood

Juvenile wood is the wood produced near the pith of the tree;

for softwoods, it is usually defined as the material 5 to

20 rings from the pith depending on species. Juvenile wood

has considerably different physical and anatomical properties

than that of mature wood (Fig. 4–9). In clear wood, the

properties that have been found to influence mechanical

behavior include fibril angle, cell length, and specific gravity,

the latter a composite of percentage of latewood, cell wall

thickness, and lumen diameter. Juvenile wood has a high

fibril angle (angle between longitudinal axis of wood cell

and cellulose fibrils), which causes longitudinal shrinkage

that may be more than 10 times that of mature wood. Com-

pression wood and spiral grain are also more prevalent in

juvenile wood than in mature wood and contribute to longi-

tudinal shrinkage. In structural lumber, the ratio of modulus

of rupture, ultimate tensile stress, and modulus of elasticity

for juvenile to mature wood ranges from 0.5 to 0.9, 0.5 to

0.95, and 0.45 to 0.75, respectively. Changes in shear

strength resulting from increases in juvenile wood content

can be adequately predicted by monitoring changes in den-

sity alone for all annual ring orientations. The same is true

for perpendicular-to-grain compressive strength when the load

is applied in the tangential direction. Compressive strength

perpendicular-to-grain for loads applied in the radial direc-

tion, however, is more sensitive to changes in juvenile wood

content and may be up to eight times less than that sug-

gested by changes in density alone. The juvenile wood to

mature wood ratio is lower for higher grades of lumber than

for lower grades, which indicates that juvenile wood has

greater influence in reducing the mechanical properties of

high-grade structural lumber. Only a limited amount of

research has been done on juvenile wood in hardwood

species.

Figure 4–8. Effects of compression wood. A, eccentric

growth about pith in cross section containing compres-

sion wood—dark area in lower third of cross section is

compression wood; B, axial tension break caused by

excessive longitudinal shrinkage of compression wood;

C, warp caused by excessive longitudinal shrinkage.

Fibril angle

Longitudinal shrinkage

Moisture content

Spiral grain

Specific gravity

Cell length

Strength

Cell wall thickness

Transverse shrinkage

Percentage latewood

Juvenile

wood

Mature wood

Juvenile

wood

Mature wood

Pith 5-20 rings Bark

Figure 4–9. Properties of juvenile wood.

Page 19

4–33

Compression Failures

Excessive compressive stresses along the grain that produce

minute compression failures can be caused by excessive

bending of standing trees from wind or snow; felling of trees

across boulders, logs, or irregularities in the ground; or

rough handling of logs or lumber. Compression failures

should not be confused with compression wood. In some

instances, compression failures are visible on the surface of

a board as minute lines or zones formed by crumpling or

buckling of cells (Fig. 4–10A), although the failures usually

appear as white lines or may even be invisible to the naked

eye. The presence of compression failures may be indicated

by fiber breakage on end grain (Fig. 4–10B). Since compres-

sion failures are often difficult to detect with the unaided eye,

special efforts, including optimum lighting, may be required

for detection. The most difficult cases are detected only by

microscopic examination.

Products containing visible compression failures have low

strength properties, especially in tensile strength and shock

resistance. The tensile strength of wood containing compres-

sion failures may be as low as one-third the strength of

matched clear wood. Even slight compression failures, visi-

ble only under a microscope, may seriously reduce strength

and cause brittle fracture. Because of the low strength associ-

ated with compression failures, many safety codes require

certain structural members, such as ladder rails and scaffold

planks, to be entirely free of such failures.

Pitch Pockets

A pitch pocket is a well-defined opening that contains free

resin. The pocket extends parallel to the annual rings; it is

almost flat on the pith side and curved on the bark side.

Pitch pockets are confined to such species as the pines,

spruces, Douglas-fir, tamarack, and western larch.

The effect of pitch pockets on strength depends upon their

number, size, and location in the piece. A large number of

pitch pockets indicates a lack of bond between annual growth

layers, and a piece with pitch pockets should be inspected for

shake or separation along the grain.

Bird Peck

Maple, hickory, white ash, and a number of other species are

often damaged by small holes made by woodpeckers.

These bird pecks often occur in horizontal rows, sometimes

encircling the tree, and a brown or black discoloration known

as a mineral streak originates from each hole. Holes for tap-

ping maple trees are also a source of mineral streaks. The

streaks are caused by oxidation and other chemical changes

in the wood. Bird pecks and mineral streaks are not generally

important in regard to strength of structural lumber, although

they do impair the appearance of the wood.

Extractives

Many wood species contain removable extraneous materials

or extractives that do not degrade the cellulose–lignin struc-

ture of the wood. These extractives are especially abundant in

species such as larch, redwood, western redcedar, and black

locust.

A small decrease in modulus of rupture and strength in

compression parallel to grain has been measured for some

species after the extractives have been removed. The extent to

which extractives influence strength is apparently a function

of the amount of extractives, the moisture content of the

piece, and the mechanical property under consideration.

Properties of Timber From Dead Trees

Timber from trees killed by insects, blight, wind, or fire may

be as good for any structural purpose as that from live trees,

provided further insect attack, staining, decay, or drying

degrade has not occurred. In a living tree, the heartwood is

entirely dead and only a comparatively few sapwood cells are

alive. Therefore, most wood is dead when cut, regardless of

Figure 4–10. Compression failures. A, compression

failure shown by irregular lines across grain; B, fiber

breakage in end-grain surfaces of spruce lumber caused

by compression failures below dark line.

Page 20

4–34

whether the tree itself is living or not. However, if a tree

stands on the stump too long after its death, the sapwood is

likely to decay or to be attacked severely by wood-boring

insects, and eventually the heartwood will be similarly

affected. Such deterioration also occurs in logs that have been

cut from live trees and improperly cared for afterwards. Be-

cause of variations in climatic and other factors that affect

deterioration, the time that dead timber may stand or lie in

the forest without serious deterioration varies.

Tests on wood from trees that had stood as long as 15 years

after being killed by fire demonstrated that this wood was as

sound and strong as wood from live trees. Also, the heart-

wood of logs of some more durable species has been found to

be thoroughly sound after lying in the forest for many years.

On the other hand, in nonresistant species, decay may cause

great loss of strength within a very brief time, both in trees

standing dead on the stump and in logs cut from live trees

and allowed to lie on the ground. The important considera-

tion is not whether the trees from which wood products are

cut are alive or dead, but whether the products themselves are

free from decay or other degrading factors that would render

them unsuitable for use.

Effects of Manufacturing and

Service Environments

Moisture Content

Many mechanical properties are affected by changes in mois-

ture content below the fiber saturation point. Most properties

reported in Tables 4–3, 4–4, and 4–5 increase with decrease

in moisture content. The relationship that describes these

changes in clear wood property at about 21ºC (70ºF)

P P

12 12g

12 12

(4–3)

where P is the property at moisture content M (%), P

the

same property at 12% MC, P

the same property for green

wood, and M

moisture content at the intersection of a

horizontal line representing the strength of green wood and

an inclined line representing the logarithm of the strength–

moisture content relationship for dry wood. This assumed

linear relationship results in an M

value that is slightly less

than the fiber saturation point. Table 4–13 gives values of M

for a few species; for other species, M

= 25 may be assumed.

Average property values of P

and P

are given for many

species in Tables 4–3 to 4–5. The formula for moisture

content adjustment is not recommended for work to maxi-

mum load, impact bending, and tension perpendicular to

grain. These properties are known to be erratic in their

response to moisture content change.

The formula can be used to estimate a property at any mois-

ture content below M

from the species data given. For

example, suppose you want to find the modulus of rupture of

white ash at 8% moisture content. Using information from

Tables 4–3a and 4–13,

4 12

103 000 103000

66 000

119 500

kPa

Care should be exercised when adjusting properties below

12% moisture. Although most properties will continue to

increase while wood is dried to very low moisture content

levels, for most species some properties may reach a

maximum value and then decrease with further drying

(Fig. 4–11). For clear Southern Pine, the moisture content

at which a maximum property has been observed is given

in Table 4–14.

This increase in mechanical properties with drying assumes

small, clear specimens in a drying process in which no

deterioration of the product (degrade) occurs. For 51-mm-

(2-in.-) thick lumber containing knots, the increase in prop-

erty with decreasing moisture content is dependent upon

lumber quality. Clear, straight-grained lumber may show

increases in properties with decreasing moisture content that

approximate those of small, clear specimens. However, as the

frequency and size of knots increase, the reduction in strength

resulting from the knots begins to negate the increase in

property in the clear wood portion of the lumber. Very low

quality lumber, which has many large knots, may be insensi-

tive to changes in moisture content. Figures 4–12 and 4–13

illustrate the effect of moisture content on the properties of

lumber as a function of initial lumber strength (Green and

others 1989). Application of these results in adjusting allow-

able properties of lumber is discussed in Chapter 6.

Additional information on influences of moisture content

on dimensional stability is included in Chapter 12.

Table 4–13. Intersection moisture content values for

selected species

Species

(%)

Ash, white

Birch, yellow

Chestnut, American

Douglas-fir

Hemlock, western

Larch, western

Pine, loblolly

Pine, longleaf

Pine, red

Redwood

Spruce, red

Spruce, Sitka

Tamarack

Intersection moisture content is point at which

mechanical properties begin to change when wood

is dried from the green condition.

Page 21

4–35

Temperature

Reversible Effects

In general, the mechanical properties of wood decrease when

heated and increase when cooled. At a constant moisture

content and below approximately 150ºC (302ºF), mechanical

properties are approximately linearly related to temperature.

The change in properties that occurs when wood is quickly

heated or cooled and then tested at that condition is termed

an immediate effect. At temperatures below 100ºC (212ºF),

the immediate effect is essentially reversible; that is, the

property will return to the value at the original temperature

if the temperature change is rapid.

Figure 4–14 illustrates the immediate effect of temperature on

modulus of elasticity parallel to grain, modulus of rupture,

and compression parallel to grain, 20

C (68

F), based on a

composite of results for clear, defect-free wood. This figure

represents an interpretation of data from several investigators.

The width of the bands illustrates variability between and

within reported trends.

Table 4–15 lists changes in clear wood properties at -

F) and 50

C (122

F) relative to those at 20

C (68

F) for

a number of moisture conditions. The large changes at

C (-

F) for green wood (at fiber saturation point or

wetter) reflect the presence of ice in the wood cell cavities.

The strength of dry lumber, at about 12% moisture content,

may change little as temperature increases from -

F) to 38

C (100

F). For green lumber, strength gener-

ally decreases with increasing temperature. However, for

temperatures between about 7

C (45

F) and 38

C (100

F),

the changes may not differ significantly from those at room

temperature. Table 4–16 provides equations that have been

Property (MPa)

Property (x10

lbf/in

)

22.0

150

120

Moisture content (%)

16.5

11.0

5.5

Figure 4–11. Effect of moisture content on wood

strength properties. A, tension parallel to grain;

B, bending; C, compression parallel to grain;

D, compression perpendicular to grain; and

E, tension perpendicular to grain.

Table 4–14. Moisture content for maximum property

value in drying clear Southern Pine from green to

4% moisture content

Property

Moisture content

at which peak

property occurs

(%)

Ultimate tensile stress

parallel to grain

12.6

Ultimate tensile stress

perpendicular to grain

10.2

MOE tension perpendicular to grain

4.3

MOE compression parallel to grain

4.3

Modulus of rigidity, G

10.0

120

Ultimate tensile stress (MPa)

Ultimate tensile stress (x10

lbf/in

)

Moisture content (%)

Figure 4–12. Effect of moisture content on tensile

strength of lumber parallel to grain.

0 8

Ultimate compressive strength (x10

lbf/in

)

Ultimate compressive strength (MPa)

Moisture content (%)

Figure 4–13. Effect of moisture content on

compressive strength of lumber parallel to grain.

Page 22

4–36

used to adjust some lumber properties for the reversible

effects of temperature.

Irreversible Effects

In addition to the reversible effect of temperature on wood,

there is an irreversible effect at elevated temperature. This

permanent effect is one of degradation of wood substance,

which results in loss of weight and strength. The loss de-

pends on factors that include moisture content, heating me-

dium, temperature, exposure period, and to some extent,

species and size of piece involved.

The permanent decrease of modulus of rupture caused by

heating in steam and water is shown as a function of tempera-

ture and heating time in Figure 4–15, based on tests of clear

pieces of Douglas-fir and Sitka spruce. In the same studies,

heating in water affected work to maximum load more than

modulus of rupture (Fig. 4–16). The effect of heating dry

wood (0% moisture content) on modulus of rupture and

modulus of elasticity is shown in Figures 4–17 and 4–18,

respectively, as derived from tests on four softwoods and two

hardwoods.

200

150

100

Relative modulus of elasticity (%)

-200

-100

100

200

300

12% moisture content

0% moisture content

(a)

Relative modulus of rupture (%)

250

200

150

100

-200

-100

-150

-50

100

150

18% moisture content

moisture content

12% moisture content

(b)

300

250

200

150

100

0-200

-100

100

200

300

12% moisture content

0% moisture content

Temperature (

°C)

Relative compressive strength (%)

(c)

Figure 4–14. Immediate effect of temperature at two

moisture content levels relative to value at 20°

C (68°

for clear, defect-free wood: (a) modulus of elasticity

parallel to grain, (b) modulus of rupture in bending,

(c) compressive strength parallel to grain. The plot is a

composite of results from several studies. Variability

in reported trends is illustrated by width of bands.

Table 4–15. Approximate middle-trend effects of

temperature on mechanical properties of clear wood

at various moisture conditions

Relative change in

mechanical property

from 20°

C (68°

F) at

Moisture

condition

50°

58°

+50°

(+122°

Property

(%)

MOE parallel to grain

+11

+17

>FSP

+50

—

MOE perpendicular to grain

—

Shear modulus

>FSP

—

Bending strength

+18

11–15

+35

18–20

+60

>FSP

+110

Tensile strength parallel to grain

0–12

—

Compressive strength parallel

+20

to grain

12–45

+50

Shear strength parallel to grain

>FSP

—

Tensile strength perpendicular

4–6

—

to grain

11–16

—

Compressive strength perpen-

dicular to grain at proportional

limit

0–6

—

FSP indicates moisture content greater than fiber saturation point.

Page 23

4–37

Figure 4–19 illustrates the permanent loss in bending

strength of Spruce–Pine–Fir standard 38- by 89-mm

(nominal 2- by 4-in.) lumber heated at 66

C (150

F) and

about 12% moisture content. During this same period,

modulus of elasticity barely changed. Most in-service

exposures at 66°

C (150°

F) would be expected to result in

much lower moisture content levels. Additional results for

other lumber products and exposure conditions will be re-

ported as Forest Products Laboratory studies progress.

The permanent property losses discussed here are based on

tests conducted after the specimens were cooled to room

temperature and conditioned to a range of 7% to 12% mois-

ture content. If specimens are tested hot, the percentage of

strength reduction resulting from permanent effects is based

on values already reduced by the immediate effects. Repeated

exposure to elevated temperature has a cumulative effect on

wood properties. For example, at a given temperature the

property loss will be about the same after six 1-month expo-

sure as it would be after a single 6-month exposure.

The shape and size of wood pieces are important in analyzing

the influence of temperature. If exposure is for only a short

time, so that the inner parts of a large piece do not reach the

temperature of the surrounding medium, the immediate effect

on strength of the inner parts will be less than that for the

outer parts. However, the type of loading must be consid-

ered. If the member is to be stressed in bending, the outer

fibers of a piece will be subjected to the greatest stress and

will ordinarily govern the ultimate strength of the piece;

hence, under this loading condition, the fact that the inner

part is at a lower temperature may be of little significance.

For extended noncyclic exposures, it can be assumed that the

entire piece reaches the temperature of the heating medium

and will therefore be subject to permanent strength losses

throughout the volume of the piece, regardless of size and

mode of stress application. However, in ordinary construc-

tion wood often will not reach the daily temperature extremes

of the air around it; thus, long-term effects should be based

on the accumulated temperature experience of critical

structural parts.

Time Under Load

Rate of Loading

Mechanical property values, as given in Tables 4–3, 4–4,

and 4–5, are usually referred to as static strength values.

Static strength tests are typically conducted at a rate of load-

ing or rate of deformation to attain maximum load in about

5 min. Higher values of strength are obtained for wood

loaded at a more rapid rate and lower values are obtained at

slower rates. For example, the load required to produce

failure in a wood member in 1 s is approximately 10%

higher than that obtained in a standard static strength test.

Over several orders of magnitude of rate of loading, strength

is approximately an exponential function of rate. See

Chapter 6 for application to treated woods.

Figure 4–20 illustrates how strength decreases with time to

maximum load. The variability in the trend shown is based

on results from several studies pertaining to bending, com-

pression, and shear.

Creep and Relaxation

When initially loaded, a wood member deforms elastically.

If the load is maintained, additional time-dependent deforma-

tion occurs. This is called creep. Creep occurs at even very

low stresses, and it will continue over a period of years. For

sufficiently high stresses, failure eventually occurs. This

failure phenomenon, called duration of load (or creep

rupture), is discussed in the next section.

At typical design levels and use environments, after several

years the additional deformation caused by creep may

approximately equal the initial, instantaneous elastic

deformation. For illustration, a creep curve based on creep as

a function of initial deflection (relative creep) at several stress

levels is shown in Figure 4–21; creep is greater under higher

stresses than under lower ones.

Table 4–16. Percentage change in bending properties of lumber with change in temperature

Lumber

Moisture

((P–P

) / P

)100 = A + BT + CT

Temperature range

Property

grade

content

min

max

MOE

All

Green

22.0350

0.4578

Green

13.1215

0.1793

150

12%

7.8553

0.1108

150

MOR

Green

34.13

0.937

0.0043

Green

100

12%

100

No. 2

Green

56.89

1.562

0.0072

or less

Green

100

Dry

100

For equation, P is property at temperature T in °

F; P

, property at 21°

C (70°

F).

SS is Select Structural.

Page 24

4–38

Ordinary climatic variations in temperature and humidity

will cause creep to increase. An increase of about 28

C (50

in temperature can cause a two- to threefold increase in creep.

Green wood may creep four to six times the initial deforma-

tion as it dries under load.

Unloading a member results in immediate and complete

recovery of the original elastic deformation and after time, a

recovery of approximately one-half the creep at deformation as

well. Fluctuations in temperature and humidity increase the

magnitude of the recovered deformation.

Relative creep at low stress levels is similar in bending,

tension, or compression parallel to grain, although it may be

somewhat less in tension than in bending or compression

under varying moisture conditions. Relative creep across the

grain is qualitatively similar to, but likely to be greater than,

creep parallel to the grain. The creep behavior of all species

studied is approximately the same.

If instead of controlling load or stress, a constant deformation

is imposed and maintained on a wood member, the initial

stress relaxes at a decreasing rate to about 60% to 70% of its

original value within a few months. This reduction of stress

with time is commonly called relaxation.

120

°C (250

°F)

100

Modulus of rupture (percentage of

value at 26.7

°C (80

° F))

°C (200

°F)

150

°C (300

°F)

175

°C (350

°F)

Heating period (h)

Figure 4–15. Permanent effect of heating in water

(solid line) and steam (dashed line) on modulus of rup-

ture of clear, defect-free wood. All data based on tests

of Douglas-fir and Sitka spruce at room temperature.

Modulus of rupture

Work

°C (150

°F)

°C (200

°F)

Property (percentage of untreated controls)

100

150

200

250

300

Heating period (days)

Figure 4–16. Permanent effect of heating in water on

work to maximum load and modulus of rupture of clear,

defect-free wood. All data based on tests of Douglas-fir

and Sitka spruce at room temperature.

100

Modulus of rupture (percentage of

untreated controls)

400

100

150

200

250

300

Time of exposure (days)

155

°C (310

°F)

115

°C (240

°F)

135

°C (275

°F)

175

°C (350

°F)

Figure 4–17. Permanent effect of oven heating at four

temperatures on modulus of rupture, based on clear

pieces of four softwood and two hardwood species.

All tests conducted at room temperature.

115

°C (240

°F)

135

°C (275

°F)

155

°C (310

°F)

175

°C (350

°F)

104

102

100

88 0

100

150

200

250

300

Time of exposure (days)

Modulus of elasticity (percentage of

untreated controls)

Figure 4–18. Permanent effect of oven heating at four

temperatures on modulus of elasticity, based on clear

pieces of four softwood and two hardwood species.

All tests conducted at room temperature.

Page 25

4–39

In limited bending tests carried out between approximately

C (64

F) and 49

C (120

F) over 2 to 3 months, the curve

of stress as a function of time that expresses relaxation is

approximately the mirror image of the creep curve

(deformation as a function of time). These tests were carried

out at initial stresses up to about 50% of the bending

strength of the wood. As with creep, relaxation is markedly

affected by fluctuations in temperature and humidity.

Duration of Load

The duration of load, or the time during which a load acts on

a wood member either continuously or intermittently, is an

important factor in determining the load that the member can

safely carry. The duration of load may be affected by changes

in temperature and relative humidity.

The constant stress that a wood member can sustain is ap-

proximately an exponential function of time to failure, as

illustrated in Figure 4–22. This relationship is a composite

of results of studies on small, clear wood specimens, con-

ducted at constant temperature and relative humidity.

1.0

0.9

0.8

0.7

0.6

0.5

MOR after exposure/MOR of controls

0 12 24 36 48 60 72

Exposure time (months)

2100f-1.8E

1650f-1.5E

Figure 4–19. Permanent effect of heating at 66°

C (150°

on modulus of rupture for two grades of machine-stress-

rated Spruce–Pine–Fir lumber at 12% moisture content.

All tests conducted at room temperature.

140

120

100

-2

Ultimate stress (percentage of static strength)

Time to ultimate stress (s)

˜12% moisture content

Figure 4–20. Relationship of ultimate stress at short-

time loading to that at 5-min loading, based on com-

posite of results from rate-of-load studies on bending,

compression, and shear parallel to grain. Variability

in reported trends is indicated by width of band.

Creep deflection (multiple of

initial deflection)

MPa x10

lbf/in

3.4 0.5

6.9 1.0

13.8 2.0

27.6 4.0

100

200

300

400

500

Time under load (days)

Stress

Figure 4–21. Influence of four levels of stress on creep

(Kingston 1962).

6% and 12% moisture content

Time to failure (h)

Constant stress (percentage of

static strength)

-6

-4

-2

120

100

Figure 4–22. Relationship between stress due to constant

load and time to failure for small clear wood specimens,

based on 28 s at 100% stress. The figure is a composite

of trends from several studies; most studies involved

bending but some involved compression parallel to grain

and bending perpendicular to grain. Variability in

reported trends is indicated by width of band.

Page 26

4–40

For a member that continuously carries a load for a long

period, the load required to produce failure is much less than

that determined from the strength properties in Tables 4–3 to

4–5. Based on Figure 4–22, a wood member under the

continuous action of bending stress for 10 years may carry

only 60% (or perhaps less) of the load required to produce

failure in the same specimen loaded in a standard bending

strength test of only a few minutes duration. Conversely, if

the duration of load is very short, the load-carrying capacity

may be higher than that determined from strength properties

given in the tables.

Time under intermittent loading has a cumulative effect. In

tests where a constant load was periodically placed on a

beam and then removed, the cumulative time the load was

actually applied to the beam before failure was essentially

equal to the time to failure for a similar beam under the same

load applied continuously.

The time to failure under continuous or intermittent loading

is looked upon as a creep–rupture process; a member has to

undergo substantial deformation before failure. Deformation at

failure is approximately the same for duration of load tests as

for standard strength tests.

Changes in climatic conditions increase the rate of creep and

shorten the duration during which a member can support a

given load. This effect can be substantial for very small wood

specimens under large cyclic changes in temperature and

relative humidity. Fortunately, changes in temperature and

relative humidity are moderate for wood in the typical service

environment.

Fatigue

In engineering, the term fatigue is defined as the progressive

damage that occurs in a material subjected to cyclic loading.

This loading may be repeated (stresses of the same sign; that

is, always compression or always tension) or reversed

(stresses of alternating compression and tension). When

sufficiently high and repetitious, cyclic loading stresses can

result in fatigue failure.

Fatigue life is a term used to define the number of cycles that

are sustained before failure. Fatigue strength, the maximum

stress attained in the stress cycle used to determine fatigue

life, is approximately exponentially related to fatigue life;

that is, fatigue strength decreases approximately linearly as

the logarithm of number of cycles increases. Fatigue strength

and fatigue life also depend on several other factors: frequency

of cycling; repetition or reversal of loading; range factor (ratio

of minimum to maximum stress per cycle); and other factors

such as temperature, moisture content, and specimen size.

Negative range factors imply repeated reversing loads,

whereas positive range factors imply nonreversing loads.

Results from several fatigue studies on wood are given in

Table 4–17. Most of these results are for repeated loading

with a range ratio of 0.1, meaning that the minimum stress

per cycle is 10% of the maximum stress. The maximum

stress per cycle, expressed as a percentage of estimated static

strength, is associated with the fatigue life given in millions

of cycles. The first three lines of data, which list the same

cyclic frequency (30 Hz), demonstrate the effect of range ratio

on fatigue strength (maximum fatigue stress that can be

maintained for a given fatigue life); fatigue bending strength

decreases as range ratio decreases. Third-point bending re-

sults show the effect of small knots or slope of grain on

fatigue strength at a range ratio of 0.1 and frequency of

8.33 Hz. Fatigue strength is lower for wood containing small

knots or a 1-in-12 slope of grain than for clear straight-

grained wood and even lower for wood containing a combi-

nation of small knots and a 1-in-12 slope of grain. Fatigue

strength is the same for a scarf joint in tension as for tension

parallel to the grain, but a little lower for a finger joint in

tension. Fatigue strength is slightly lower in shear than in

tension parallel to the grain. Other comparisons do not have

much meaning because range ratios or cyclic frequency differ;

however, fatigue strength is high in compression parallel to

the grain compared with other properties. Little is known

about other factors that may affect fatigue strength in wood.

Creep, temperature rise, and loss of moisture content occur in

tests of wood for fatigue strength. At stresses that cause

failure in about 106 cycles at 40 Hz, a temperature rise of

Table 4–17. Summary of reported results on cyclic

fatigue

Range

Cyclic

fre-

quency

Maximum

stress per

cycle

Approxi-

mate

fatigue

life

Property

ratio

(Hz)

(%)

(×

cycles)

Bending, clear,

straight grain

Cantilever

0.45

Cantilever

1.0

Center-point

1.0

Rotational

1.0

—

Third-point

0.1

8-1/3

Bending, third-point

Small knots

0.1

8-1/3

Clear, 1:12 slope

of grain

0.1

8-1/3

Small knots, 1:12

slope of grain

0.1

8-1/3

Tension parallel

to grain

Clear, straight grain

0.1

Clear, straight grain

3.5

Scarf joint

0.1

Finger joint

0.1

Compression parallel

to grain

Clear, straight grain 0.1

3.5

Shear parallel to grain

Glue-laminated

0.1

Initial moisture content about 12% to 15%.

Percentage of estimated static strength.

Page 27

4–41

C (27

F) has been reported for parallel-to-grain compres-

sion fatigue (range ratio slightly greater than zero), parallel-

to-grain tension fatigue (range ratio = 0), and reversed bend-

ing fatigue (range ratio = -

1). The rate of temperature rise is

high initially but then diminishes to moderate; a moderate

rate of temperature rise remains more or less constant during

a large percentage of fatigue life. During the latter stages of

fatigue life, the rate of temperature rise increases until failure

occurs. Smaller rises in temperature would be expected for

slower cyclic loading or lower stresses. Decreases in mois-

ture content are probably related to temperature rise.

Aging

In relatively dry and moderate temperature conditions where

wood is protected from deteriorating influences such as de-

cay, the mechanical properties of wood show little change

with time. Test results for very old timbers suggest that

significant losses in clear wood strength occur only after

several centuries of normal aging conditions. The soundness

of centuries-old wood in some standing trees (redwood, for

example) also attests to the durability of wood.

Exposure to Chemicals

The effect of chemical solutions on mechanical properties

depends on the specific type of chemical. Nonswelling liq-

uids, such as petroleum oils and creosote, have no apprecia-

ble effect on properties. Properties are lowered in the presence

of water, alcohol, or other wood-swelling organic liquids

even though these liquids do not chemically degrade the

wood substance. The loss in properties depends largely on

the amount of swelling, and this loss is regained upon re-

moval of the swelling liquid. Anhydrous ammonia markedly

reduces the strength and stiffness of wood, but these proper-

ties are regained to a great extent when the ammonia is

removed. Heartwood generally is less affected than sapwood

because it is more impermeable. Accordingly, wood treat-

ments that retard liquid penetration usually enhance natural

resistance to chemicals.

Chemical solutions that decompose wood substance (by

hydrolysis or oxidation) have a permanent effect on strength.

The following generalizations summarize the effect of

chemicals:

•

Some species are quite resistant to attack by dilute

mineral and organic acids.

•

Oxidizing acids such as nitric acid degrade wood more

than do nonoxidizing acids.

•

Alkaline solutions are more destructive than are acidic

solutions.

•

Hardwoods are more susceptible to attack by both acids

and alkalis than are softwoods.

•

Heartwood is less susceptible to attack by both acids and

alkalis than is sapwood.

Because both species and application are extremely impor-

tant, reference to industrial sources with a specific history of

use is recommended where possible. For example, large

cypress tanks have survived long continuous use where

exposure conditions involved mixed acids at the boiling

point. Wood is also used extensively in cooling towers

because of its superior resistance to mild acids and solutions

of acidic salts.

Chemical Treatment

Wood is often treated with chemicals to enhance its fire

performance or decay resistance in service. Each set of

treatment chemicals and processes has a unique effect on the

mechanical properties of the treated wood.

Fire-retardant treatments and treatment methods distinctly

reduce the mechanical properties of wood. Some fire-

retardant-treated products have experienced significant in-

service degradation on exposure to elevated temperatures

when used as plywood roof sheathing or roof-truss lumber.

New performance requirements within standards set by the

American Standards for Testing and Materials (ASTM) and

American Wood Preservers’ Association (AWPA) preclude

commercialization of inadequately performing fire-retardant-

treated products.

Although preservative treatments and treatment methods

generally reduce the mechanical properties of wood, any

initial loss in strength from treatment must be balanced

against the progressive loss of strength from decay when

untreated wood is placed in wet conditions. The effects of

preservative treatments on mechanical properties are directly

related to wood quality, size, and various pretreatment,

treatment, and post-treatment processing factors. The key

factors include preservative chemistry or chemical type,

preservative retention, initial kiln-drying temperature, post-

treatment drying temperature, and pretreatment incising (if

required). North American design guidelines address the

effects of incising on mechanical properties of refractory wood

species and the short-term duration-of-load adjustments for

all treated lumber. These guidelines are described in

Chapter 6.

Oil-Type Preservatives

Oil-type preservatives cause no appreciable strength loss

because they do not chemically react with wood cell wall

components. However, treatment with oil-type preservatives

can adversely affect strength if extreme in-retort seasoning

parameters are used (for example, Boultonizing, steaming, or

vapor drying conditions) or if excessive temperatures or

pressures are used during the treating process. To preclude

strength loss, the user should follow specific treatment proc-

essing requirements as described in the treatment standards.

Waterborne Preservatives

Waterborne preservative treatments can reduce the mechanical

properties of wood. Treatment standards include specific

processing requirements intended to prevent or limit strength

reductions resulting from the chemicals and the waterborne

preservative treatment process. The effects of waterborne

preservative treatment on mechanical properties are related to

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species, mechanical properties, preservative chemistry or

type, preservative retention, post-treatment drying tempera-

ture, size and grade of material, product type, initial kiln-

drying temperature, incising, and both temperature and

moisture in service.

Species

—

The magnitude of the effect of various water-

borne preservatives on mechanical properties does not

appear to vary greatly between different species.

Mechanical property

—

Waterborne preservatives affect

each mechanical property differently. If treated according to

AWPA standards, the effects are as follows: modulus of

elasticity (MOE), compressive strength parallel to grain,

and compressive stress perpendicular to grain are unaffected

or slightly increased; modulus of rupture (MOR) and ten-

sile strength parallel to grain are reduced from 0% to 20%,

depending on chemical retention and severity of redrying

temperature; and energy-related properties (for example,

work to maximum load and impact strength) are reduced

from 10% to 50%.

Preservative chemistry or type

—

Waterborne preservative

chemical systems differ in regard to their effect on strength,

but the magnitude of these differences is slight compared

with the effects of treatment processing factors. Chemistry-

related differences seem to be related to the reactivity of the

waterborne preservative and the temperature during the

fixation/precipitation reaction with wood.

Retention

—

Waterborne preservative retention levels of

16 kg/m

1.0 lb/ft

) have no effect on MOE or compres-

sive strength parallel to grain and a slight negative effect

5% to -

10%) on tensile or bending strength. However,

energy-related properties are often reduced from 15% to

30%. At a retention level of 40 kg/m

(2.5 lb/ft

MOR and energy-related properties are further reduced.

Post-treatment drying temperature

—

Air drying after

treatment causes no significant reduction in the static

strength of wood treated with waterborne preservative at a

retention level of 16 kg/m

(1.0 lb/ft

). However, energy-

related properties are reduced. The post-treatment redrying

temperature used for material treated with waterborne pre-

servative has been found to be critical when temperatures

exceed 75

C (167

F). Redrying limitations in treatment

standards have precluded the need for an across-the-board

design adjustment factor for waterborne-preservative-treated

lumber in engineering design standards. The limitation on

post-treatment kiln-drying temperature is set at 74

(165

F).

Size of material

—

Generally, larger material, specifically

thicker, appears to undergo less reduction in strength than

does smaller material. Recalling that preservative treat-

ments usually penetrate the treated material to a depth of

only 6 to 51 mm (0.25 to 2.0 in.), depending on species

and other factors, the difference in size effect appears to be

a function of the product’s surface-to-volume ratio, which

affects the relative ratio of treatment-induced weight gain

to original wood weight.

Grade of material

—

The effect of waterborne preservative

treatment is a quality-dependent phenomenon. Higher

grades of wood are more affected than lower grades. When

viewed over a range of quality levels, higher quality lum-

ber is reduced in strength to a proportionately greater

extent than is lower quality lumber.

Product type

—

The magnitude of the treatment effect on

strength for laminated veneer lumber conforms closely to

effects noted for higher grades of solid-sawn lumber. The

effects of waterborne preservative treatment on plywood

seem comparable to that on lumber. Fiber-based composite

products may be reduced in strength to a greater extent

than is lumber. This additional effect on fiber-based com-

posites may be more a function of internal bond damage

caused by waterborne-treatment-induced swelling rather

than actual chemical hydrolysis.

Initial kiln-drying temperature

—

Although initial kiln

drying of some lumber species at 100

C to 116

C (212

to 240

F) for short durations has little effect on structural

properties, such drying results in more hydrolytic degrada-

tion of the cell wall than does drying at lower temperature

kiln schedules. Subsequent preservative treatment and

redrying of material initially dried at high temperatures

causes additional hydrolytic degradation. When the mate-

rial is subsequently treated, initial kiln drying at 113

(235

F) has been shown to result in greater reductions over

the entire bending and tensile strength distributions than

does initial kiln drying at 91

C (196

F). Because Southern

Pine lumber, the most widely treated product, is most of-

ten initially kiln dried at dry-bulb temperatures near or

above 113

C (235

F), treatment standards have imposed a

maximum redrying temperature limit of 74

C (165

F) to

preclude the cumulative effect of thermal processing.

Incising—Incising, a pretreatment mechanical process in

which small slits (incisions) are punched in the surface of

the wood product, is used to improve preservative penetra-

tion and distribution in difficult-to-treat species. Incising

may reduce strength; however, because the increase in

treatability provides a substantial increase in biological

performance, this strength loss must be balanced against

the progressive loss in strength of untreated wood from the

incidence of decay. Most incising patterns induce some

strength loss, and the magnitude of this effect is related to

the size of material being incised and the incision depth

and density (that is, number of incisions per unit area).

In less than 50 mm (2 in.) thick, dry lumber, incising and

preservative treatment induces losses in MOE of 5% to

15% and in static strength properties of 20% to 30%. In-

cising and treating timbers or tie stock at an incision den-

sity of =

1,500 incisions/m

140 incisions/ft

) and to a

depth of 19 mm (0.75 in.) reduces strength by 5% to 10%.

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In-service temperature

—

Both fire-retardant and preserva-

tive treatments accelerate the thermal degradation of

bending strength of lumber when exposed to temperatures

above 54°C (130°F).

In-service moisture content—Current design values apply

to material dried to =

19% maximum (15% average) mois-

ture content or to green material. No differences in strength

have been found between treated and untreated material

when tested green or at moisture contents above 12%.

When very dry treated lumber of high grade was tested at

10% moisture content, its bending strength was reduced

compared with that of matched dry untreated lumber.

Duration of load—When subjected to impact loads,

wood treated with chromated copper arsenate (CCA) does

not exhibit the same increase in strength as that exhibited

by untreated wood. However, when loaded over a long

period, treated and untreated wood behave similarly.

Polymerization

Wood is also sometimes impregnated with monomers, such

as methyl methacrylate, which are subsequently polymerized.

Many of the mechanical properties of the resultant wood–

plastic composite are higher than those of the original wood,

generally as a result of filling the void spaces in the wood

structure with plastic. The polymerization process and both

the chemical nature and quantity of monomers influence

composite properties.

Nuclear Radiation

Wood is occasionally subjected to nuclear radiation. Exam-

ples are wooden structures closely associated with nuclear

reactors, the polymerization of wood with plastic using

nuclear radiation, and nondestructive estimation of wood

density and moisture content. Very large doses of gamma

rays or neutrons can cause substantial degradation of wood.

In general, irradiation with gamma rays in doses up to about

1 megarad has little effect on the strength properties of wood.

As dosage exceeds 1 megarad, tensile strength parallel to

grain and toughness decrease. At a dosage of 300 megarads,

tensile strength is reduced about 90%. Gamma rays also

affect compressive strength parallel to grain at a dosage above

1 megarad, but higher dosage has a greater effect on tensile

strength than on compressive strength; only approximately

one-third of compressive strength is lost when the total dose

is 300 megarads. Effects of gamma rays on bending and shear

strength are intermediate between the effects on tensile and

compressive strength.

Mold and Stain Fungi

Mold and stain fungi do not seriously affect most mechanical

properties of wood because such fungi feed on substances

within the cell cavity or attached to the cell wall rather than

on the structural wall itself. The duration of infection and the

species of fungi involved are important factors in determining

the extent of degradation.

Although low levels of biological stain cause little loss in

strength, heavy staining may reduce specific gravity by 1%

to 2%, surface hardness by 2% to 10%, bending and crushing

strength by 1% to 5%, and toughness or shock resistance by

15% to 30%. Although molds and stains usually do not

have a major effect on strength, conditions that favor these

organisms also promote the development of wood-destroying

(decay) fungi and soft-rot fungi (Ch. 13). Pieces with mold

and stain should be examined closely for decay if they are

used for structural purposes.

Decay

Unlike mold and stain fungi, wood-destroying (decay) fungi

seriously reduce strength by metabolizing the cellulose

fraction of wood that gives wood its strength.

Early stages of decay are virtually impossible to detect. For

example, brown-rot fungi may reduce mechanical properties

in excess of 10% before a measurable weight loss is observed

and before decay is visible. When weight loss reaches 5% to

10%, mechanical properties are reduced from 20% to 80%.

Decay has the greatest effect on toughness, impact bending,

and work to maximum load in bending, the least effect on

shear and hardness, and an intermediate effect on other prop-

erties. Thus, when strength is important, adequate measures

should be taken to (a) prevent decay before it occurs,

(b) control incipient decay by remedial measures (Ch. 13), or

believed to exist in a critical section. Decay can be prevented

from starting or progressing if wood is kept dry (below 20%

moisture content).

No method is known for estimating the amount of reduction

in strength from the appearance of decayed wood. Therefore,

when strength is an important consideration, the safe proce-

dure is to discard every piece that contains even a small

amount of decay. An exception may be pieces in which decay

occurs in a knot but does not extend into the surrounding

wood.

Insect Damage

Insect damage may occur in standing trees, logs, and undried

(unseasoned) or dried (seasoned) lumber. Although damage

is difficult to control in the standing tree, insect damage can

be eliminated to a great extent by proper control methods.

Insect holes are generally classified as pinholes, grub holes,

and powderpost holes. Because of their irregular burrows,

powderpost larvae may destroy most of a piece’s interior

while only small holes appear on the surface, and the

strength of the piece may be reduced virtually to zero. No

method is known for estimating the reduction in strength

from the appearance of insect-damaged wood. When strength

is an important consideration, the safe procedure is to elimi-

nate pieces containing insect holes.

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