In: Arntzen, Charles J., ed. Encyclopedia of AgriculturalScience. Orlando, FL: Academic Press: 549-561. Vol. 4.October 1994.

Wood PropertiesJERROLD E. WINANDY, USDA-Forest Service, Forest Products Laboratory,1Wisconsin

Wood StructurePhysical PropertiesMechanical PropertiesFactors Affecting Properties of WoodProperties and Grades of Sawn Lumber

Glossary

Allowable property Value of a property normallypublished for design use; allowable properties areidentified with grade descriptions and standards, andthey reflect the orthotropic structure of wood andanticipated end usesAnisotropic Exhibiting different properties alongdifferent axes; in general, fibrous materia]s such aswood are anisotropicAnnual growth ring Layer of wood growth put ona tree during a single growing season. In the temperatezone, the annual growth rings of many species (e. g.,oaks and pines) are readily distinguished because ofdifferences in the cells formed during the early andlate parts of the season; in some temperate zone species(e. g., black gum and sweetgum) and many tropicalspecies, annual growth rings are not easily recognizedDiffuse-porous wood Certain hardwoods in whichthe pores tend to be uniformly sized and distributedthroughout each annual ring or to decrease in sizeslightly and gradually toward the outer border ofthe ringEarlywood Portion of the annual growth ring thatis formed during the early part of the growing season;it is usually less dense and mechanically weaker thanlatewoodHardwoods General botanical group of trees thathas broad leaves in contrast to the conifers or soft-

1The Forest Products Laboratory is maintained in cooperation

with the University of Wisconsin. This article was written andprepared by U.S. Government employees on offical time, and itis therefore in the public domain and not subject to copyright.

Encyclopedia of Agricultural Science, Volume 4

woods; term has no reference to the actual hardnessof the woodLatewood Portion of the annual growth ring that isformed after the earlywood formation has ceased; itis usually denser and mechanically stronger than ear-lywoodLumber Product of the saw and planing mill manu-factured from a log through the process of sawing,resawing to width, passing lengthwise through a stan-dard planing machine, and crosscutting to lengthOrthotropic Having unique and independent prop-erties in three mutually orthogonal (perpendicular)planes of symmetry; 3 special case of anisotropyRing-porous woods Group of hardwoods in whichthe pores are comparatively large at the beginning ofeach annual ring and decrease in size more or lessabruptly toward the outer portion of the ring, thusforming a distinct inner zone of pores, the earlywood,and an outer zone with smaller pores, the latewoodSoftwoods General botanical group of trees that inmost cases has needlelike or scalelike leaves (the coni-fers); term has no reference to the actual hardness ofthe wood

Wood is an extremely versatile material with a widerange of physical and mechanical properties amongthe many species of wood. It is also a renewable re-source with an exceptional strength-to-weight ratio.Wood is a desirable construction material because theenergy requirements of wood for producing a usableend-product are much lower than those of competi-tive materials, such as steel, concrete, or plastic.

|. Wood Structure

A. Microstructure

The primary structural building block of wood is the

5 4 9

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cubic centimeter of wood could contain more than1.5 million wood cells. When packed together theyform a strong composite. Each individual wood cellis even more structurally advanced because it is actu-ally a multilayered, fdament-reinforced, closed-endtube (Fig. 1) rather than just a homogeneous-walled,nonreinforced straw. Each individual cell has fourdistinct cell wall layers (Primary, S1, S2, and S3). Eachlayer is composed of a combination of three chemicalpolymers: cellulose, hemicellulose, and lignin (Fig.1). The cellulose and hemicellulose are linear poly-saccharides (i.e., hydrophilic multiple-sugars), andthe lignin is an amorphous phenolic (i. e., a three-dimensional hydrophobic adhesive). Cellulose formslong unbranched chains. and hemicellulose formsshort branched chains. Lignin encrusts and stiffensthese polymers.

Because carbohydrate and phenolic components ofwood are assembled in a layered tubular or cellularmanner with a large cell cavity, specific gravity ofwood can vary immensely. Wood excels as a viablebuilding material because the layered tubular structureprovides a large volume of voids (void volume), ithas an advantageous strength-to-weight ratio, andit has other inherent advantages, such as corrosionresistance, fatigue resistance, low cost, and ease-of-modification at the job site.

layer of the bark. Usually, it is 1 to 10 cells wide

B. Macrostructure

The cross-section of a tree is divided into three broadcategories consisting of the bark, wood, and cambium

FIGURE 1 Microfibril orientation for each cell wall layer ofScotch pine with chemical composition as percentage of totalweight. Cell wall layers are primary (P), S1, S2, and S3.

(Fig. 2). Bark is the outer layer and is composed ofa dead outer phloem of dry corky material and a thininner phloem of living cells. Its primary functionsare protection and nutrient conduction. The thicknessand appearance of bark vary substantially dependingon the species and age of the tree.

Wood, or xylem, is composed of the inner sectionsof the trunk. The primary functions of wood aresupport and nutrient conduction and storage. Woodcan be divided into two general classes: sapwood and

It functions primarily in food storage and the mechan-ical transport of sap. The radial thickness of sapwoodis commonly 35 to 50 mm but may be 75 to 150 mmfor some species. Heartwood consists of an inner coreof wood cells that have changed, both chemically andphysically, from the cells of the outer sapwood. Thecell cavities of heartwood may also contain depositsof various materials that frequently give heartwood amuch darker color. Extractive deposits formed duringthe conversion of living sapwood to dead heartwoodoften make the heartwood of some species more dura-ble in conditions that may induce decay.

The cambium is a continuous ring of reproductivetissue located between the sapwood and the inner

depending on the season. All wood and bark cellsare aligned or stacked radially because each cell in aradial line originated from the same cambial cell.

1. GrowthGrowth in trees is affected by the soil and environ-

mental conditions with which the tree must exist and

FIGURE 2 Elements of microstructure normally visible withoutmagnification.

WOOD PROPERTIES551

contend. Growth is accomplished by cell division. Asnew cells form, they are pushed either to the insideto become wood cells or to the outside to becomebark cells. As the diameter of the tree increases, newcells are also occasionally retained in the cambium toaccount for increasing cambial circumference. Also,as the tree diameter increases, additional bark cellsare pushed outward, and the outer surface becomescracked and ridged. forming the bark patterns charac-teristic of each species.

The type and rate of growth vary between ear-lywood and latewood cells. Earlywood cells have rel-atively large cavities and thin walls, whereas latewoodcells have smaller cavities and thicker walls. Becausevoid volume is related to density and density is relatedto lumber strength. latewood is sometimes used tojudge the quality or strength of some species. Ear-lywood is lighter in weight and color, softer, andweaker than latewood; it shrinks less across the grainand more lengthwise along the grain than doeslatewood.

2. Growth RingsGrowth rings vary in width depending on species

and site conditions. Rings formed during short or dryseasons are thinner than those formed when growingconditions are more favorable. Also, rings formed inshady conditions are usually thinner than thoseformed by the same species in sunny conditions. It iscommonly believed that the age of a tree may bedetermined by counting these rings. However, thismethod can lead to errors because abnormal envi-ronmental conditions can cause a tree to producemultiple-growth increments or even prevent growthentirely for a period.

3. KnotsAs a tree grows, branches develop laterally from

the trunk. These branches produce gross deviationsin the normal grain of the trunk and result in knotswhen the log is sawn into lumber or timber. Knotsare classified in two categories: intergrown knots andencased or loose knots. Intergrown knots are formedby living branches. Encased knots occur whenbranches die and the wound is surrounded by thegrowing trunk. Knots result in grain deviations.which is significant because straight-grained wood isapproximately 10 to 20 times stronger parallel to grainthan perpendicular to grain. Accordingly, knot sizeis a major predictor of sawn-timber strength.

4. Reaction WoodReaction wood is the response of a tree to abnormal

environmental or physical stresses associated with

leaning trees and crooked limbs. It is generally be-lieved to bean attempt by the tree to return the trunkor limbs to a more natural position. In softwoods,reaction wood is called compression wood and resultsin the production of wood cells rich in phenolic ligninand poor in carbohydrates. It is found on the lowerside of the limb or inclined trunk and effectively re-sults in a higher cell wall packing density and highcompression strength. ,Many of the anatomical,chemical, physical, and mechanical properties of reac-tion wood differ distinctly from those of normalwood. The specific gravity of compression wood isfrequently 30 to 40% greater than that of normalwood, but the tensile strength is many times lower.This is why all grading rules restrict compressionwood in any form from graded softwood lumber andtimber.

||. Physical Properties

Physical properties are the quantitative characteris-tics of wood and its behavior to external influencesother than applied forces. Included here are direc-tional properties, moisture content, dimensional sta-bility, thermal and pyrolytic (fire) properties, dens-ity, and electrical, chemical, and decay resistance.Familiarity with physical properties is importantbecause they can significantly influence the perfor-mance and strength of wood used in structural ap-plications.

The physical properties of wood most relevant tostructural design and performance are discussed in thissection. The effects that variations in these propertieshave on the strength of wood are more fully discussedin Section IV.

A. Directional Properties

Wood is an orthotropic and anisotropic material. Be-cause of the orientation of the wood fibers and themanner in which a tree increases in diameter as itgrows, properties vary along three mutually perpen-dicular axes: longitudinal, radial, and tangential (Fig.3). The longitudinal axis is parallel to the fiber (grain)direction, the radial axis is perpendicular to the graindirection and normal to the growth rings, and thetangential axis is perpendicular to the grain directionand tangent to the growth rings. Although mostwood properties differ in each of these three axis direc-tions, differences between the radial and tangentialaxes are relatively minor when compared to differ-ences between the radial or tangential axis and the

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FIGURE 3 Three principal axes of wood with respect to graindirection and growth rings.

longitudinal axis. Property values tabulated for struc-tural applications arc often given only for axis direc-tions parallel to groin (longitudinal) and perpendicularto grain (radial or tangential).

B. Moisture Content

The moisture content of wood is defined as the weightof water in wood given as a percentage of ovendryweight. In equation form, moisture content (MC) isexpressed as follows:

MC =moist weight – dry weight

dry weightx 100%. (1)

Water is required for the growth and developmentof living trees and constitutes a major portion of greenwood anatomy. In living trees, moisture content de-pends on the species and the type of-wood, and mayrange from approximately 25% to more than 250%(two and a half times the weight of the dry woodmaterial). In most species, the moisture content ofsapwood is higher than that of heartwood.

Water exists in wood either as bound water (in thecell wall) or free water (in the cell cavity). As boundwater, it is bonded (via secondary or hydrogen bonds)within the wood cell walls. As free water, it is simplypresent in the cell cavities. When wood dries, mostfree water separates at a faster rate than bound water

because of accessibility and the absence of secondarybonding. The moisture content at which the cell wallsare still saturated but virtually no water exists in thecell cavities is called the fiber saturation point. Thefiber saturation point usually varies between 21and 28%.

Wood is a hydroscopic material that absorbs mois-ture in a humid environment and loses moisture in adry environment. As a result, the moisture contentof wood is a function of atmospheric conditions anddepends on the relative humidity and temperature ofthe surrounding air. Under constant conditions oftemperature and humidity, wood reaches an equilib-rium moisture content (EMC) at which it is neithergaining nor losing moisture. The EMC represents abalance point where the wood is in equilibrium withits environment.

In structural applications, the moisture content of wood is almost always undergoing some changes astemperature and humidity conditions vary. Thesechanges are usually gradual and short-term fluctua-tions that influence only the surface of the wood. Thetime required for wood to reach the EMC dependson the size and permeability of the member, the tem-perature, and the difference between the moisturecontent of the member and the EMC potential of thatenvironment. Changes in moisture content cannot beentirely stopped but can be retarded by coatings ortreatments applied to the wood surface.

C. Dimensional Stability

Above the fiber saturation point, wood will not shrinkor swell from changes in moisture content becausefree water is found only in the cell cavity and is notassociated within the cell walls. However, woodchanges in dimension as moisture content varies be-low the fiber saturation point. Wood shrinks as itloses moisture below the fiber saturation point andswells as it gains moisture up to the fiber saturationpoint. These dimensional changes may result in split-ting, checking, and warping. The phenomena of di-mensional stability and EMC must be understood,

Dimensional stability of wood is one of the fewproperties that significantly differs in each of the threeaxis directions. Dimensional changes in the longitudi-nal direction between the fiber saturation point andovendry are between 0.1 and 0.2% and are of nopractical significance; however, in reaction or juvenilewood, these percentages may be significantly higher.The combined effects of shrinkage in the tangentialand radial axes can distort the shape of wood piecesbecause of the difference in shrinkage and the curva-ture of the annual rings (Fig. 4). Generally, tangentialshrinkage (varying from 4.4 to 7.8% depending onspecies) is twice that of radial shrinkage (from 2.2 to5.6%).

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FIGURE 4 Characteristic shrinkage and distortion of wood asaffected by direction of growth rings. Such distortion can resultin warp, generally classified as bow, twist, crook, and cup.

D. Thermal Expansion

Thermal expansion of dry wood is positive in alldirections; wood expands when heated and contractswhen cooled. Wood that contains moisture reacts totemperature changes differently than dry wood.

The linear expansion coefficients of dry wood par-allel to grain are generally independent of specificgravity and species and range from approximately3 x 10-6 to 4.5 x 10-6 per oC. The linear expansioncoefficients across the grain (tangential and radial) arein proportion to density and range from approxi-mately 5 to 10 times greater than parallel to graincoefficients.

When moist wood is heated, it tends to expandbecause of normal thermal expansion and shrink be-cause of moisture loss from increased temperature.Unless the initial moisture content of the wood isvery low (3 to 4%), the net dimensional change onheating is negative. Wood at intermediate moisturecontents of approximately 8 to 20% will expand whenfirst heated, then gradually shrink to a volume smallerthan the initial volume as moisture is lost in the heatedcondition.

E. Pyrolytic Properties

Under appropriate conditions, wood will undergothermal degradation or pyrolysis. The by-products

of pyrolysis may burn, and if enough heat is generatedand retained by the wood, the wood can be set onfire. In the presence of a pilot flame (independentsource of ignition), the minimum rate of heating nec-essary for ignition is of the order of 0.3 calorie persquare centimeter. In the absence of a pilot flame, theminimum rate of heating necessary for ignition is ofthe order of 0.6 calorie per square centimeter, nearlydouble the rate of the pilot flame situation.

Still, heavy timber construction deserves an ex-tremely favorable fire-insurance rating because it willgenerally not produce sufficient heat energy to main-tain combustion unless an external heat source is pres-ent. Timber will gradually produce a char layer fromthe residue of wood combustion. This char acts as athermal insulator. On heavy timbers, this char layerwill eventually inhibit combustion by establishing athermal barrier between the uncharred wood (interiorto char) and the heat of the fire (exterior to char).Heavy timber is virtually self-extinguishing, but steel,which has a thermal conductivity 100 times that ofwood, will absorb heat until it reaches a temperatureat which it yields under structural load without actu-ally burning.

F. Density and Specific Gravity

The density of a material is the mass per unit volumeat some specified condition. For a hydroscopic mate-rial such as wood, density depends on two factors: theweight of the wood structure and moisture retained inthe wood. Wood density at various moisture contentscan vary significantly and must be given relative toa specific condition to have practical meaning.

Specific gravity provides a relative measure of theamount of wood substance contained in a sample ofwood. It is a dimensionless ratio of the weight of anovendry volume of wood to the weight of an identicalvolume of water. In research activities, specific grav-ity may be reported on the basis of both weight andvolume ovendry. For many engineering applications,the basis for specific gravity is generally the ovendryweight and volume at a moisture content of 12%.For example, a volume of wood at some specifiedmoisture content with a specific gravity of 0.50 wouldhave a density of 500 kg/m3.

G. Electrical Resistance

Wood is a good electrical insulator. However, sig-nificant variations in conductivity do exist. Thesevariations in electrical resistance can be related to vari-

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ations in grain orientation, temperature, and moisturecontent. The conductivity of wood in the longitudinalaxis is approximately twice that in the radial or tan-gential axes. The electrical conductivity of wood gen-erally doubles for each 10 oC increase in temperature.Generally, variations in conductivity related to wooddensity and species are considered minor.

The correlation between electrical resistivity andmoisture content is the basis for electrical resistance-type moisture meters that estimate moisture contentby measuring the resistance of the wood between twoelectrodes. Moisture content meters, as these instru-ments are commonly called, need to be calibrated fortemperature and species and are effective only formoisture content ranges of 5 to 25%. They are gener-ally unreliable for high resistivities at moisture con-tents below 5 or 6%, for estimating the moisturecontent of green timber, or for estimating moisturecontent of treated timbers (most treatments alter con-ductivity).

H. Decay Resistance

Wood decay fungi and wood-destroying organismsrequire oxygen, appropriate temperature, moisture,and a food source. Wood will not decay if kept dry(moisture content less than 20%). On the other ex-tremc, if continuously submerged in water at suffi-cient depths. wood will usually not decay. Wheneverwood is intermediary to either of these two extremes,problems with wood decay can result. To avoid prob-lems with decay where moisture cannot be controlled,the engineer or designer can use either naturally dura-ble species or treated timber.

The natural durability of wood to the mechanismsand processes of deterioration is related to the anatom-ical characteristics and species of wood. In general,the outer zone or sapwood of all species has littleresistance to deterioration and fails rapidly in adverseenvironments. For heartwood, natural durability de-pends on species. Heartwood forms as the living sap-wood cells gradually die. In some species, the sugarspresent in the cells are converted to highly toxic ex-tratives that are deposited in the wood cell wall.Many species produce durable heartwood, includingwestern red cedar, redwood, and black locust; how-ever, durability varies within a tree and between treesof a given species. To enhance durability, wood canbe treated with an EPA-registered, toxic preservativechemical treatment.

1. Chemical Resistance

Wood is highly resistant to many chemicals, whichgives it a significant advantage over many alternative

building materials. Wood is often considered superiorto alternative materials, such as concrete and steel,partly because of its resistance to mild acids (pH morethan 2.0), acidic salt solutions, and corrosive agents.Generally, iron holds up better on exposure to alkalinesolution than does wood, but wood can be treatedwith many of the common wood preservatives (e. g.,creosote) to greatly enhance its performance in thisrespect.

Heartwood is far more durable than sapwood tochemical attack because heartwood is more resistantto penetration by liquids. Many preservative treat-ments, such as creosote or pentachlorophenol inheavy oil, can also significantly increase the ability ofwood to resist liquid or chemical penetration, or both.Chemical solutions may induce two general types ofaction: normal reversible swelling by a liquid andirreversible chemical degradation. With the former,removal of the liquid will return wood to its originalcondition. With the latter, permanent changes occurwithin the wood structure from hydrolysis, oxida-tion, or delignification.

|||. Mechanical Properties

Mechanical properties are the characteristics of a ma-terial in response to externally applied forces. Theyinclude elastic properties, which characterize resis-tance to deformation and distortion, and strengthproperties, which characterize resistance to appliedloads. Mechanical property values are given in termsof stress (force per unit area) and strain (deformationresulting from the applied stress). The mechanicalproperty values of wood are obtained from laboratorytests of lumber of straight-grained clear wood samples(without natural defects that would reduce strength,such as knots, checks, splits, etc.).

A. Elastic Properties

Elastic properties relate the resistance of a material todeformation under an applied stress to the ability ofthe material to regain its original dimensions whenthe stress is removed. For a material with ideal elasticproperties loaded below the proportional (elastic)limit, all deformation is recoverable and the bodyreturns to its original shape when the stress is re-moved. Wood is not ideally elastic in that some defor-mation from loading is not immediately recoveredwhen the load is removed; however, residual defor-mations are generally recoverable over a period oftime. Although technically considered a viscoelastic

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material, wood is usually assumed to behave as anelastic material for most engineering applications.

For an isotropic material with equal property valuesin all directions, elastic properties are measured bythree elastic constants: modulus of elasticity (E), mod-

lowing equation shows the relationship:

where i, j, and k represent the three principal axes.Because wood is orthotropic, 12 constants are re-quired to measure elastic behavior: three moduli ofelasticity, three moduli of rigidity, and six Poisson’sratios.

1. Modulus of ElasticityModulus of elasticity relates the stress applied along

one axis to the strain occurring on the same axis. Thethree moduli of elasticity for wood are denoted EL,ER, and ET to reflect the elastic moduli in the longitu-dinal, radial, and tangential directions, respectively.For example, EL relates the stress in the longitudinaldirection to the strain in the longitudinal direction.

Elastic constants vary within and between speciesand with moisture content and specific gravity. Theonly constant that has been extensively derived fromtest data is EL. Other constants may be available fromlimited test data but are most frequently developedfrom material relationships or by regression equationsthat predict behavior as a function of density. Relativevalues of elastic constants for clearwood of severalcommon wood species are given in Table I.

2. Shear ModulusShear modulus relates shear stress to shear strain.

The three shear moduli for wood are denotedG GLT,LR, and GRT for the longitudinal-radial.longitudinal-tangential, and radial-tangential planes.respectively. For example, GLR is the modulus of ri-gidity based on the shear strain in the LR plane andthe shear stress in the LT and RT planes. The modulusof rigidity for several wood species and for each planeare given in Table 1.

3. Poisson’s RatioPoisson’s ratio relates the strain parallel to an ap-

plied stress to the accompanying strain occurring lat-erally. For wood, the six Poisson’s ratios are denoted

refers to the direction of applied stress; the secondsubscript refers to the direction of the accompanyinglateral strain. For example, ~~~ is the Poisson’s ratiofor stress along the longitudinal axis and strain along

the radial axis. Estimates of Poisson’s ratios for severalwood species and for each orientation are given inTable I.

B. Strength Properties

Strength properties me the ultimate resistance of amaterial to applied loads. With wood, strength variessignificantly depending on species, loading condition,load duration, and a number of assorted material andenvironmental factors.

Because wood is anisotropic, mechanical propertiesalso vary in the three principal axes. Property values inthe longitudinal axis are generally significantly higherthan those in the tangential or radial axes. Strength-related properties in the longitudinal axis are usuallyreferred to as parallel-to-grain properties. For mostengineering design purposes, simply differentiatingbetween parallel- and perpendicular-to-grain proper-ties is sufficient because the relative tangential andradial directions are randomized by the primary saw-ing process (i. e., conversion from logs to boards).

1. CompressionWhen a compression load is applied parallel to

grain, it produces stress that deforms (shortens) woodcells along their longitudinal axis. When wood isstressed in compression parallel to grain, failure ini-tially begins as the microfibrils begin to fold withinthe cell wall, thereby creating planes of weakness orinstability within the cell wall. As stress in compres-sion parallel to grain continues to increase, the wood-cells themselves fold into S shapes. forming visiblewrinkles on the surface. Large deformations occurfrom the internal crushing of the complex cellularstructure. The average strength of green clear woodspecimens of Douglas-fir and loblolly pine in com-pression parallel to grain is approximately 26.1 and24.2 MPa, respectively.

When a compression load is applied perpendicularto grain, it produces stress that deforms the woodcells perpendicular to their length. Once the hollowcell cavities are collapsed, wood is quite strong be-cause no void space exists. In practice, compressivestrength of wood perpendicular to grain is usuallyassumed to be exceeded when deformation exceeds4%. of the proportional limit stress. Using this con-vention, the average strength of green clear woodspecimens of Douglas-fir and loblolly pine in com-pression perpendicular to grain is approximately 4.8and 4.6 MPa, respectively.

Compression applied at an angle to the grain pro-duces stresses that act both parallel and perpendicular

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to grain. The strength at any intermediate angle isintermediate to values of compression parallel andperpendicular to grain and is determined using Han-kinson’s formula.

2. TensionParallel to its grain, wood is very strong in tension.

Failure occurs by a complex combination of twomodes: cell-to-cell slippage and cell wall failure. Slip-page occurs where two adjacent cells slide past oneanother. Cell wail failure involves rupture within thecell wall with little or no visible deformation prior tocomplete failure. Tensile strength parallel to grain forclear wood has been historically difficult to obtain; itis often conservatively estimated from bending testvalues because clear wood normally exhibits initialfailure on the face stressed in tension.

In contrast to tension parallel to grain, wood isrelatively weak when loaded in tension perpendicularto grain. Stresses in this direction act perpendicularto the cell lengths and produce splitting or cleavagealong the grain, which can have a significant effecton structural integrity. Deformations are usually lowprior to failure because of the geometry and structureof the cell wall cross-section. Strength in tensionperpendicular to grain for clear green samplesof Douglas-fir and loblolly pine average 2.1 and1.8 MPa, respectively. However, because of the ex-cessive variability associated with ultimate stress intension perpendicular to grain. design situations thatinduce this stress should be avoided.

3. BendingFlexural (bending) properties are critical. Bending

stresses are induced when a material is used as a beam,such as in a floor or rafter system. The. bendingstrength of clear Dougias-fir and loblolly pine aver-ages 52.6 and 50.3 MPa, respectively, while the mod-ulus of elasticity averages 10.7 and 9.7 GPa, respec-tively. Because tensile and compressive strengthsparallel to grain are different from each other, thestrength in bending is less than in tension but morethan in compression.

4. ShearWhen used as a beam, wood is exposed to compres-

sion stress on one surface of the beam and tensilestress on the other. This opposition of stress resultsin a shearing action through the section of the beam.This parallel-to-grain shearing action is termed hori-zontal shear. The horizontal shear strength of clearDouglas-fir and loblolly pine averages 6.2 and

5.9 MPa, respectively. Conversely, when stress isapplied perpendicular to the cell length in a planeparallel to grain, this action is termed rolling shear.Rolling shear stresses produce a tendency for thewood cells to roll over one another. In general, rollingshear strength values for clear specimens average 18to 28% of the parallel-to-grain shear values.

5. Energy Absorption ResistanceEnergy absorption or shock resistance is a function

of the ability of a material to quickly absorb and thendissipate energy via deformation. Wood is remark-ably resilient in this respect and is often a preferredmaterial for shock loading. Several parameters areused to describe energy absorption depending on theeventual criteria of failure considered. Work to pro-portional limit, work to maximum load, and workto total failure (i. e., toughness) describe the energyabsorption of wood materials at progressively moresevere failure criteria.

6. FatigueThe fatigue resistance of wood is sometimes an

important consideration. Wood, like many fibrousmaterials, is quite resistant to fatigue (i. e., the effectsof repeated loading). In many crystalline metals, re-peated loadings of 1 to 10 million cycles at stresslevels of 10 to 15%. of ultimate can induce fatigue-type failures. At comparable stress levels, the fatiguestrength of wood is often several times that of mostmetals.

7. HardnessHardness represents the resistance of wood to in-

dentation and marring. Hardness is comparativelymeasured by force required to embed a 11.3-mm ballone-half its diameter into the wood.

|V. Factors Affecting Propertieso f W o o d

To this point, our discussions of wood propertieshave mostly been based on tests of straight-grainedspecimens of clear wood. Clear wood properties areimportant, but by no means do they totally representthe engineering performance of solid-sawn lumber,timber, or glulam (glued-laminated timber) con-taining knots, slope of grain, and other strength-reducing characteristics. To understand the propertiesof these end-use products, the user must appreciate

WOOD PROPERTIES5 6 0

the impacts of several anatomical and processing-related factors. The user must also appreciate the in-teractive nature of environmental factors. This sectionwill attempt to briefly relate the importance of manyof these factors independently and in aggregate.

A. Anatomical Factors

The mechanical properties of wood vary between spe-cies; they are often compared via species averages (Ta-ble II). However, because mechanical properties varywithin a species, it is incorrect to think that all materialof Species A is stronger than material of Species B if,for example, average values are 10 to 15% different.

1. Specific Gravity and DensityThe property values of wood increase with increas-

ing specific gravity (SG). While density is a measure ofweight per unit volume often reported with kilogramsper cubic meter, SG is a dimensionless ratio of thedensity of wood at a specified moisture content tothe density of water. Because changes in moisturecontents result in dimensional changes, SG and den-sity should be compared at the same moisture content.Specific gravity is an index of mechanical propertyvalues of wood free from defects; the higher the SG,the higher the appropriate property value. However,SG and density values for lumber are also affected bythe presence of gums, resins, and extratives, whichcontribute little to mechanical properties.

2. KnotsA knot is that portion of a branch that has become

incorporated in the bole of the tree. The influence ofa knot on mechanical properties of a wood memberis due to the interruption of continuity and change indirection of wood fibers associated with a knot. Theinfluence of a knot depends on its size, its location, itsshape, its soundness, and the type of stress measured.

Most mechanical property values are lower at sec-tions containing knots. Knots generally have a greatereffect on tensile strength than on compressivestrength. For this reason, knots have their greatestinfluence in the tension zone when exposed to bendingstress. The effects of knot size, type, and locationare specifically addressed by the grading rules thatspecify limits for each commercially marketedspecies-size–grade combination.

3. Slope of GrainThe mechanical properties of wood are quite

sensitive to fiber and ring orientation. For example,

parallel-to-grain tensile or compressive strengthproperty values are generally 10 to 20 times greaterthan those perpendicular to grain. Deviations fromstraight grain in a typical board are termed slope ofgrain or cross-grain. The terms relate the fiber direc-tion to the edges of the piece. Any form of cross-grain can have detrimental effects on mechanicalproperties.

4. Juvenile WoodDuring the first 5 to 20 years of growth, the imma-

ture cambial tissue produces wood cells with distinctvariations in microfibril orientation throughout theimportant S2 layer of the cell wall. This wood is re-ferred to as juvenile wood. Juvenile wood exhibitsexcessive warpage because of anatomical differenceswithin this S2 layer of the cell wall. It also exhibitslower strength properties and becomes a problemwithin the wood industry because of the trend towardprocessing younger, smaller diameter trees as thelarger diameter, old-growth stock becomes more dif-ficult to obtain.

5. CreepWood is a viscoelastic material. Initially, it will act

elastically, experiencing nearly full recovery of load-induced deformation upon stress removal. However,wood will experience nonrecoverable deformationupon extended loading. This deformation is knownas creep. For example, the magnitude of additionalcreep-related deformation after a 10-year loading willroughly equal the initial deformation caused by thatload. The rate of creep increases with increasing tem-perature and moisture content.

B. Environmental

1. Moisture ContentMechanical property values of wood increase as

wood dries from the fiber saturation point to 10 to15% moisture content. For clear wood, mechanicalproperty values continue to increase as wood driesbelow 10 to 15%. moisture content. For lumber, stud-ies have shown that mechanical property values reacha maximum at about 10 to 15% moisture content.then begin to decrease with decreasing moisture con-tent below 10 to 15%. For either product, the effectsof moisture content are considered to be reversible inthe absence of decay.

2. TemperatureStrength and stiffness decrease when wood is heated

and increase when cooled. The temperature effect is

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immediate and, for the most part, reversible for shortheating durations. However, if wood is exposed toelevated temperatures for an extended time, strengthis permanently reduced because of wood substancedegradation and a corresponding loss in weight. Themagnitude of these permanent effects depends onmoisture content, heating medium, temperature, ex-posure period, and to a lesser extent, species and speci-men size. As a general rule, wood should not beexposed to temperatures above 65OC. The immediateeffect of temperature interacts with the effect of mois-ture content so that neither effect can be completelyunderstood without consideration of the other.

3. Decay and Insect Damage Wood is conducive to decay and insect damage in

moist, warm conditions. Decay within a structurecannot be tolerated because strength is rapidly reducedin even the early stages of decay. It has been esti-mated that a 5% weight loss from decay can resultin strength losses as high as 50%. If the warm, moistconditions required for decay cannot be controlled,then the use of natural]y decay resistant wood speciesor chemical treatments are required to impede decay.Insects, such as termites and certain types of beetles,can be just as damaging to mechanical performance.Insect infestation can be controlled via mechanicalbarriers, naturally durable species, or chemical treat-ments.

V. Properties and Grades ofSawn Lumber

At first, the highest quality level of sawn 1umbermight seem desirable for all uses, and indeed it isneeded for several uses. However, in most situations,such material would be prohibitively expensive anda wasteful use of our timber resource. In practice,the quality level needed for a function can be easilyspecified because lumber and timber are graded in anorderly system developed to serve the interests of theusers and the producers.

The grading system is actually several systems, eachdesigned for specific products. Hardwood lumber ismostly graded for remanufacture, with only smallamounts graded for construction. Softwood is alsograded for both remanufacture and construction, butprimarily for construction.

In practice, an orderly, voluntary but circuitoussystem of responsibilities has evolved in the United

States for the development, manufacture, and mer-chandising of most stress-graded lumber and tim-ber. In general, stress-grading principles are de-veloped from research findings and engineeringconcepts, often within committees and subcommit-tees of the American Society for Testing and Mate-rials.

For lumber, the National Institute for Standardsand Technology cooperates with producers, dis-tributors, users, and regional grade-rules-writingagencies through the American Lumber StandardCommittee (ALSC). The ALSC has assembled a vol-untary softwood standard of manufacture, called theAmerican Softwood Lumber Standard. The Ameri-can Softwood Lumber Standard and its related Na-tional Grading Rule prescribe the ways in whichstress-grading principles can be used to formulategrading rules for dimension lumber (nominal 2 to4 in. thick). This lumber standard is the basis forcommercially marketing structural lumber in theUnited States.

For timbers (more than 5 in. nominal), the NationalGrading Rule does not apply. Thus, each regionalgrade-rules-writing agency publishes grade rules fortimbers following the general principles of the Na-tional Grading Rule, but each differs slightly in even-tual grade requirements and names. For further spe-cifics on the various characteristics for the individualspecies-grade combinations, contact the individualgrade-rules-writing organizations directly. In NorthAmerica, those agencies are National Lumber GradesAuthority (Vancouver, BC, Canada), NortheasternLumber Manufacturers Association (Cumberland,ME), Redwood Inspection Service (Mill Valley, CA),Southern Pine Inspection Bureau (Pensacola, FL),West Coast Lumber Inspection Bureau (Portland,OR), and Western Wood Products Association (Port-land, OR). [See FOREST TREE, GENETIC IMPROVEMENT . ]

Bibliography

American Society for Testing and Materials (1991). “An-nual Book of Standards, ” Vol. D.09 Wood. Philadel-phia, PA.

Forest Products Laboratory (1987). “Wood Handbook:Wood as an Engineering Material. ” Agric. Handb. 72.U.S. Department of Agriculture, Forest Service, Wash-ington, DC.

Panshin, A. J., and deZeeuw, C. (1980). “Textbook ofWood Technology, ” 4th ed., p. 705. McGraw-Hill, NewYork.

U,S. Department of Commerce. (1986). American LumberStandard PS20-70. Washington, DC.