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Manufacturing technologies based on the modern understanding of wood have led to a family of engineered woodproducts that optimize properties to meet the specific needs of design profess

CHAPTER ONE INTRODUCTION TO WOOD AS

AN ENGINEERING MATERIAL

Steven Zylkowski

Director, Engineered Wood Systems

The unique characteristics and abundant supply of wood have made it the mostdesirable building material throughout history Manufacturing technologies based

on the modern understanding of wood have led to a family of engineered woodproducts that optimize properties to meet the specific needs of design professionals.While there are many types of technically advanced wood products such as machinestress-rated (MSR) lumber and metal-plated connected wood trusses that are con-sidered to be part of the broad definition of engineered wood products, they havenot been included in this Handbook Information on these and other non-gluedwood products may be found in other manuals and literature sources For the pur-poses of this Handbook, engineered wood products are defined as products manu-factured from various forms of wood fiber bonded together with water-resistantadhesives Engineered wood products are intended for structural applications andinclude such products as structural plywood, oriented strand board (OSB), glued-laminated timber, laminated veneer lumber (LVL), and wood I-joists These prod-ucts are also known as wood composites

As an organic material derived from trees, wood has a cellular structure posed of longitudinally arranged fibers This directional aspect of wood impartsdirectional properties that are well recognized by wood engineers and manufacturers

com-of wood products While the properties com-of engineered wood products are largelydetermined by manufacturing processes that change the configuration of the woodfibers, basic wood characteristics are primary to the end product Wood character-istics are determined by many factors, such as species, growing conditions, andwood quality

1.1.1 Softwoods and Hardwoods

One fundamental characterization of wood is based upon whether the wood is from

a hardwood or softwood tree Hardwood, or deciduous, trees are those that lose

TABLE 1.1 Prevalent Wood Species of North America

Alder, red Cedar, eastern red Ash, white Cedar, western red

Basswood, American Douglas fir, coast type Beech, American Fir, balsam

Birch, paper Fir, grand Cherry, black Fir, noble Cottonwood Fir, Pacific silver Elm, American Fir, white Hackberry Hemlock, eastern Maple, silver Hemlock, western Maple, sugar Larch, western Oak, northern red Pine, loblolly Oak, southern red Pine, lodgepole Oak, white Pine, longleaf

Sycamore, American Pine, red Tupelo, black Pine, shortleaf Tupelo, swamp Pine, sugar Walnut, black Pine, western white Yellow-poplar Redwood

Spruce, black Spruce, Engelmann Spruce, Sitka

their leaves during the fall Softwoods, or coniferous, trees are those that haveneedles that typically remain green throughout the year Classification of hardwoodand softwood is not based upon the hardness or density of the wood; there aremany examples of low-density hardwoods such as basswood or aspen and densesoftwoods such as southern pines Rather, the classification is based upon the tax-onomy of the tree Table 1.1 lists some of the prevalent commercial hardwood andsoftwood wood species in North America

1.1.2 Earlywood and Latewood Formation

Trees grow at different rates throughout the year, resulting in growth rings in thewood During favorable growing conditions, such as during the spring in temperateclimates, trees grow at a faster rate, resulting in lower density fibers This woodfiber appears as lighter areas in the wood This portion of the wood is known asearlywood or springwood As growth rates slow, the wood fibers develop thickercell walls, resulting in denser fibers that appear as the darker portion of the growthring This portion of a growth ring is known as latewood or summerwood Figure1.1 depicts the earlywood and latewood portions of a growth ring

Annual Growth Increment (Annual Ring)

Latewood (or Summerwood)

Earlywood (or Springwood)

FIGURE 1.1 Earlywood and latewood.

Heartwood Sapwood

1.1.3 Sapwood and Heartwood

The inner portion of a cross section of a tree trunk often displays a variation incolor compared to the outer portion of the trunk The lighter colored outer portion

of the cross section is the sapwood As shown in Fig 1.2, the darker inner portion

is the heartwood The width of the sapwood and the color of the heartwood variousconsiderably between different tree species The sapwood portion of a stem is thenewer portion and is used by the tree for conduction of water and nutrients for

Longitudinal direction

Tangential

direction

Radial direction

FIGURE 1.3 Directional orientation of wood.

growth As trees age, the center portion of the stem can collect excess nutrientsthat metabolize into various extractives that discolor the wood These extractivescan include waxes, oils, resins, fats, and tannins, along with aromatic and coloringmaterials The color and characteristics of the heartwood is critical to woods usedfor decorative uses such as furniture, but less critical for engineered wood uses.Most properties of sapwood and heartwood are identical, except that the heartwood

of some species is resistant to decay fungi as discussed further in Section 1.5 andChapter 9

1.1.4 Anisotrophy

The appearance of wood, as well as its properties, are significantly influenced bythe surface orientation relative to its location in the tree stem As shown in Fig.1.3, wood has different orientations relative to the growth rings and longitudinalfiber arrangement The cross section is perpendicular to the longitudinal direction

of the fibers The surface from the center of the stem outward is the radial surface.The outer surface, parallel to the growth rings, is called the tangential surface Woodproperties are significantly influenced by the direction relative to the fiber andgrowth ring orientation

1.1.5 Chemical Makeup of Wood

From the standpoint of basic chemical elements, wood is primarily composed ofcarbon, hydrogen, and oxygen, as shown in Table 1.2 The basic chemical elements

TABLE 1.2 Basic Chemical Composition

of cellulose, hemicellulose, and lignin differ slightly between hardwood and wood species as shown in Table 1.3

1.2.1 Distribution of Forests Throughout the World

The northern hemisphere contains mostly softwood timberlands and the southernhemisphere mostly hardwoods As shown in Table 1.4, excerpted from Ref 1, NorthAmerica contains a large source of the world’s softwood forests

North America is nearly 21% forestland As shown in Fig 1.4, this percentagehas been fairly constant since the 1920s Each year the amount of timber cut isless than the growth of standing timber, as shown in Fig 1.5 (from Ref 1)

1.2.2 Volume of Engineered Wood Products from North America

The United States and Canada are major manufacturers and exporters of engineeredwood products The producers of these products in North America are among themost advanced in the world Engineered wood products have been readily adopted

by the construction industry in North America due to an overall familiarity andpreference for wood Table 1.5 (from Ref 2) shows the volume of plywood, OSB,I-joists, LVL, and glulam produced by North American producers as well as thepercentage of worldwide production

TABLE 1.4 Distribution of Forests Throughout the World

Region

Softwood or coniferous forests Land

Hardwood or deciduous forests Land

Combined softwood and hardwood forests Land

aCIS—Confederation of Independent States of the former Soviet Union.

Note: The totals for combined softwood and hardwood forests do not always add up because no downs have been given for areas in Europe, and the Confederation of Independent States is excluded by law from exploitation.

break-Source: R Sedjo and K Lyon, The Long Term Adequacy of World Timber Supply, Resources for the

Future, Washington, DC, 1990 Excerpted from ref 1.

FIGURE 1.4 Percent forestland in the U.S from ref 1.

1.2.3 Environmental Advantages of Wood Construction

As construction professionals have become increasingly interested in the mental impacts of construction materials, the preference for wood products hasincreased since they are an excellent environmental choice An environmental studywas conducted by the ATHENA Sustainable Materials Institute for the CanadianWood Council3to compare the environmental impact of constructing a house using

environ-FIGURE 1.5 Harvest ratio of forest land in the U.S.

TABLE 1.5 2000 Volume of Engineered Wood Products from

North America

Structural plywood 10 6 ft 2 - 3 ⁄ 8 in basis 17,475 2,200

OSB 10 6 ft 2 - 3 ⁄ 8 in basis 11,910 8,740

The study evaluated a typical 2400 ft2house designed for the Toronto, Ontario,Canada market The wood-designed house was framed with lumber, wood I-joistsfor the floor, and wood roof trusses The steel house used light-gage steel membersfor wall and floor framing The concrete house used insulated concrete forms (ICF)for walls and a composite floor system with open web steel joists and concreteslab The study considered the following environmental aspects

• Embodied energy measures the total amount of direct and indirect energy used

to extract, manufacture, transport, and install the construction materials It cludes potential energy contained in raw or feedstock materials, such as naturalgas used in the production of resins

in-• Global warming potential is a reference measurement using carbon dioxide as a

common reference for global warming or greenhouse gas effects All greenhouse

TABLE 1.6 Environmental Impacts from Various Building Systems

Athena study results

Global warming potential, kg CO2equivalent 62,183 76,453 93,573 Air toxicity, critical volume measurement 3,236 5,628 6,971 Water toxicity, critical volume measurement 407,787 1,413,784 876,189

gases are referred to as having a ‘‘CO2 equivalence effect.’’ While greenhousegas emissions are largely a function of energy combustion, some products alsoemit greenhouse gases during processing of raw materials, such as during thecalcination of limestone during the production of cement

• Air and water toxicity indices represent the human health effects of substances

emitted during the various stages of the life cycle of the materials The commonlyaccepted measure is the critical volume method, used to estimate the volume ofambient air or water required to dilute contaminants to acceptable levels

• Weighted resource use is the sum of the weighted resource requirements for all

products used in each design This can be thought of as ‘‘ecologically weightedkilograms,’’ which reflect the relative ecological carrying capacity effects of ex-tracting resources

• Solid waste is reported on a mass basis according to general life-cycle conventions

that tend to favor lighter materials Solid waste is more related to building tices than materials and careful planning can significantly reduce waste.Table 1.6 shows the environmental measure of the design house built with eachtype of construction material Construction with wood uses less energy, representsless global warming potential, has fewer impacts on air and water, and representsless weighted resource use The results of this study clearly demonstrate that woodsystems have fewer environmental impacts than the other construction systems used

prac-in the study

The wide spectrum of wood species provides a diverse range of properties Woodproperties are largely a result of several primary influences, such as wood species,grade, and moisture content Other factors that influence wood’s utility for certainapplications are permeability, decay resistance, thermal properties, chemical resis-tance, and shrinkage and expansion characteristics Engineered wood products rely

on a combination of underlying wood properties and manufacturing techniques tooptimize the desirable characteristics of wood and minimize undesirable character-istics This section provides a review of the most important physical characteristic

of wood as they affect engineered wood products

1.3.1 Density and Specific Gravity of Wood

The density, or weight per volume, of wood varies considerably between and withinwood species Density of wood is always reported in combination with its moisturecontent due to the significant influence moisture content has on overall density Tostandardize comparisons between species or products, it is common to report thespecific gravity, or density relative to that of water, on the basis of oven dry weight

of wood and volume at a specified moisture content

In general, the greater the density or specific gravity of wood, the greater itsstrength, expansion and shrinkage, and thermal conductivity Table 1.7 reports thespecific gravity of common commercial species in North America The relationshipbetween specific gravity and mechanical properties can be represented by the equa-tion

n

where P⫽a mechanical property

k and n ⫽constants that depend upon the specific mechanical property and

spe-cies

G ⫽specific gravity

The constants that describe the relationship between properties and specific ity are presented in Table 1.8 (from Ref 4)

grav-1.3.2 Moisture and Wood

Similar to other organic materials, wood is hygroscopic in that it absorbs or losesmoisture to reach equilibrium with the surrounding environment Wood can natu-rally hold large quantities of water Understanding the effects of water in wood isimportant because it influences many properties of wood and engineered woodproducts

The measure of water in wood is called the moisture content and is reported as

the weight of water per weight of oven-dry, or moisture-free, wood As can be seen

in Table 1.9, the moisture content of freshly cut wood can exceed 100% becausethe weight of water in a given volume of wood can exceed the weight of the oven-dry wood

With a cellular structure, wood can hold water both in the cell cavity and in the

cell wall itself Water is held in the cavity as liquid, or free, water Water held in

the cell walls is chemically bound water Cell walls can chemically hold about 30%

moisture The term fiber saturation point describes the conceptual point where the

cell walls are saturated while no water exists in the cavity The fiber saturationpoint is important because moisture changes involving the chemically held waterresult in a different behavior of the wood product than with the loss of free water

in the cell cavities Section 1.3.3 describes how moisture changes in chemicallyheld water lead to changes in strength properties and the dimensions of the wood,respectively As discussed in Chapter 9, the threshold for wood decay is exceededwhen the moisture content exceeds the fiber saturation point of the wood.When wood is acclimated to be in equilibrium with ambient air conditions, yetprotected from direct wetting, the moisture content of the wood will be below thefiber saturation point Under this condition, the moisture content of wood is a

TABLE 1.7 Specific Gravity of Common Commercial Wood Species

Species Specific gravity

TABLE 1.8 Mechanical Properties as a Function of Specific Gravity for Wood

Compression parallel (lb / in 2 ) 13,590 G 0.97 11,030 G 0.89

Compression perpendicular (lb / in 2 ) 2,390 G 1.57 3,130 G 2.09

Shear parallel (lb / in 2 ) 2,410 G 0.85 3,170 G 1.13

Tension perpendicular (lb / in 2 ) 870 G 1.11 1,460 G 1.3

aCompression 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 oven-dry weight and green volume; for dry wood, use specific gravity based on oven-dry weight and volume at 12% moisture.

TABLE 1.9 Average Moisture Content of Freshly Cut Wood

Hardwoods

Moisture content (%) Heartwood Sapwood Softwoods

Moisture content (%) Heartwood Sapwood

Basswood, American 81 133 Douglas fir, coast type 37 115

TABLE 1.10 Equilibrium Moisture Content of Wood (%)

Engineered wood products and most lumber products are dried to a relativelylow moisture content as part of the manufacturing process One objective of dryingwood during manufacturing is to lower the moisture to conditions near that of in-service conditions Drying of wood used for engineered wood products is alsonecessary for effective bonding using most wood adhesives

Wood products in service are always undergoing changes in moisture content,

as products respond to exposure to seasonal and short-term weather changes To alesser degree, the equilibrium moisture content of engineered wood products is alsosomewhat dependent upon the manufacturing process At a specific relative humid-ity, the moisture content of products such as glued laminated timber is similar tothe moisture content of the lumber constituents Products that involve elevated tem-peratures during manufacturing, such as plywood and OSB, have reduced equilib-rium moisture contents compared to wood at the same relative humidity, as shown

in Table 1.11

1.3.3 Movement of Wood and Engineered Wood Products

Wood shrinks and expands when the moisture content changes while below thefiber saturation point As chemically held moisture in the cell wall is lost, the woodshrinks As moisture is added to the cell wall, the wood expands The amount ofwood shrinkage or expansion is highly dependent upon the direction relative to thewood grain As previously described in Section 1.1.4, the three principal grainorientations are a function of the growth of fibers around the perimeter of the treestem The expansion / shrinkage rate longitudinal to the grain is between 0.10% and0.20% from green to oven dry and is therefore considered negligible in most cases.The expansion / shrinkage across the grain, that is, in the radial and tangential di-rections, is a function of wood species and other variables Table 1.12 presents theshrinkage rate of common commercial species as the moisture changes from thegreen (freshly cut) condition to oven dry The rate of shrinkage or expansion is