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In: Arntzen, Charles J., ed. Encyclopedia of Agricultural
Science. Orlando, FL: Academic Press: 549-561. Vol. 4. October 1994.
Wood Properties JERROLD E. WINANDY, USDA-Forest Service, Forest Products Laboratory,Wisconsin
Wood Structure Physical Properties Mechanical Properties
Factors Affecting Properties of Wood Properties and Grades of Sawn Lumber
Allowable property Value of a property normally published for design use; allowable properties are identified with grade descriptions and standards, and they reflect the orthotropic structure of wood and anticipated end uses Anisotropic Exhibiting different properties along different axes; in general,fibrous materia]s such as wood are anisotropic
Annual growth ring Layer of wood growth put on a tree during a single growing season. In the temperate zone, the annual growth rings of many species (e. g., oaks and pines) are readily distinguished because of differences in the cells formed during the early and late parts of the season; in some temperate zone species
(e. g., black gum and sweetgum) and many tropical species, annual growth rings are not easily recognized Diffuse-porous wood Certain hardwoods in which the pores tend to be uniformly sized and distributed throughout each annual ring or to decrease in size slightly and gradually toward the outer border of the ring Earlywood Portion of the annual growth ring that is formed during the early part of the growing season; it is usually less dense and mechanically weaker than latewood Hardwoods General botanical group of trees that
has broad leaves in contrast to the conifers or soft-The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on offical time, and it is therefore in the public domain and not subject to copyright.
Encyclopedia of Agricultural Science, Volume 4 woods; term has no reference to the actual hardness of the wood Latewood Portion of the annual growth ring that is formed after the earlywood formation has ceased; it is usually denser and mechanically stronger than earlywood Lumber Product of the saw and planing mill manufactured from a log through the process of sawing, resawing to width, passing lengthwise through a standard planing machine, and crosscutting to length
Orthotropic Having unique and independent properties in three mutually orthogonal (perpendicular) planes of symmetry; 3 special case of anisotropy Ring-porous woods Group of hardwoods in which the pores are comparatively large at the beginning of each annual ring and decrease in size more or less abruptly toward the outer portion of the ring, thus forming a distinct inner zone of pores, the earlywood, and an outer zone with smaller pores, the latewood
Softwoods General botanical group of trees that in most cases has needlelike or scalelike leaves (the conifers); term has no reference to the actual hardness of the wood
Wood is an extremely versatile material with a wide range of physical and mechanical properties among the many species of wood. It is also a renewable resource with an exceptional strength-to-weight ratio. Wood is a desirable construction material because the energy requirements of wood for producing a usable end-product are much lower than those of competitive materials, such as steel, concrete, or plastic.
|. Wood Structure
A. Microstructure The primary structural building block of wood is the
cubic centimeter of wood could contain more than 1.5 million wood cells. When packed together they form a strong composite. Each individual wood cell is even more structurally advanced because it is actually a multilayered, fdament-reinforced, closed-end tube (Fig. 1) rather than just a homogeneous-walled,
nonreinforced straw. Each individual cell has four distinct cell wall layers (Primary, S, S, and S). Each layer is composed of a combination of three chemical polymers: cellulose, hemicellulose, and lignin (Fig. 1). The cellulose and hemicellulose are linear polysaccharides (i.e., hydrophilic multiple-sugars), and the lignin is an amorphous phenolic (i. e., a threedimensional hydrophobic adhesive). Cellulose forms long unbranched chains. and hemicellulose forms short branched chains. Lignin encrusts and stiffens these polymers.
Because carbohydrate and phenolic components of wood are assembled in a layered tubular or cellular manner with a large cell cavity, specific gravity of wood can vary immensely. Wood excels as a viable building material because the layered tubular structure provides a large volume of voids (void volume), it has an advantageous strength-to-weight ratio, and it has other inherent advantages, such as corrosion resistance, fatigue resistance, low cost, and ease-ofmodification at the job site.
layer of the bark. Usually, it is 1 to 10 cells wide
The cross-section of a tree is divided into three broad categories consisting of the bark, wood, and cambium
FIGURE 1 Microfibril orientation for each cell wall layer of
Scotch pine with chemical composition as percentage of total weight. Cell wall layers are primary (P), S, S, and S.
(Fig. 2). Bark is the outer layer and is composed of a dead outer phloem of dry corky material and a thin inner phloem of living cells. Its primary functions are protection and nutrient conduction. The thickness and appearance of bark vary substantially depending on the species and age of the tree.
Wood, or xylem, is composed of the inner sections of the trunk. The primary functions of wood are support and nutrient conduction and storage. Wood can be divided into two general classes: sapwood and
It functions primarily in food storage and the mechanical transport of sap. The radial thickness of sapwood is comonly 35 to 50 m but may be 75 to 150 m for some species. Heartwood consists of an inner core of wood cells that have changed, both chemically and physically, from the cells of the outer sapwood. The cell cavities of heartwood may also contain deposits of various materials that frequently give heartwood a much darker color. Extractive deposits formed during the conversion of living sapwood to dead heartwood often make the heartwood of some species more durable in conditions that may induce decay.
The cambium is a continuous ring of reproductive tissue located between the sapwood and the inner depending on the season. All wood and bark cells are aligned or stacked radially because each cell in a radial line originated from the same cambial cell.
Growth in trees is affected by the soil and environmental conditions with which the tree must exist and
FIGURE 2 Elements of microstructure normally visible without magnification.
WOOD PROPERTIES 551 contend. Growth is accomplished by cell division. As new cells form, they are pushed either to the inside to become wood cells or to the outside to become bark cells. As the diameter of the tree increases, new cells are also occasionally retained in the cambium to account for increasing cambial circumference. Also, as the tree diameter increases, additional bark cells are pushed outward, and the outer surface becomes cracked and ridged. forming the bark patterns characteristic of each species.
The type and rate of growth vary between earlywood and latewood cells. Earlywood cells have relatively large cavities and thin walls, whereas latewood cells have smaller cavities and thicker walls. Because void volume is related to density and density is related to lumber strength. latewood is sometimes used to judge the quality or strength of some species. Earlywood is lighter in weight and color, softer, and weaker than latewood; it shrinks less across the grain and more lengthwise along the grain than does latewood.
2. Growth Rings Growth rings vary in width depending on species and site conditions. Rings formed during short or dry seasons are thinner than those formed when growing conditions are more favorable. Also, rings formed in shady conditions areusually thinner than those formed by the same species in sunny conditions. It is commonly believed that the age of a tree may be determined by counting these rings. However, this method can lead to errors because abnormal environmental conditions can cause a tree to produce multiple-growth increments or even prevent growth entirely for a period.
3. Knots As a tree grows, branches develop laterally from the trunk. These branches produce gross deviations in the normal grain of the trunk and result in knots when the log is sawn into lumber or timber. Knots are classified in two categories: intergrown knots and encased or loose knots. Intergrown knots are formed by living branches. Encased knots occur when branches die and the wound is surrounded by the growing trunk. Knots result in grain deviations. which is significant because straight-grained wood is approximately 10 to 20 times stronger parallel to grain than perpendicular to grain. Accordingly, knot size is a major predictor of sawn-timber strength.
4. Reaction Wood Reaction wood is the response of a tree to abnormal environmental or physical stresses associated with leaning trees and crooked limbs. It is generally believed to bean attempt by the tree to return the trunk or limbs to a more natural position. In softwoods, reaction wood is called compression wood and results in the production of wood cells rich in phenolic lignin and poor in carbohydrates. It is found on the lower side of the limb or inclined trunk and effectively results in a higher cell wall packing density and high compressionstrength. ,Many of the anatomical, chemical, physical, and mechanical properties of reac- tion wood differ distinctly from those of normal wood. The specific gravity of compression wood is frequently 30 to 40% greater than that of normal wood, but the tensile strength is many times lower. This is why all grading rules restrict compression wood in any form from graded softwood lumber and timber.
||. Physical Properties
Physical properties are the quantitative characteristics of wood and its behavior to external influences other than applied forces. Included here are directional properties,moisture content, dimensional stability, thermal and pyrolytic (fire) properties, density, and electrical, chemical, and decay resistance. Familiarity with physical properties is important because they can significantly influence the performance and strength of wood used in structural ap- plications. The physical properties of wood most relevant to structural design and performance are discussed in this section. The effects that variations in these properties have on the strength of wood are more fully discussed in Section IV.
A. Directional Properties
Wood is an orthotropic and anisotropic material. Because of the orientation of the wood fibers and the manner in which a tree increases in diameter as it grows, properties vary along three mutually perpendicular axes: longitudinal, radial, and tangential (Fig.
3). The longitudinal axis is parallel to the fiber (grain) direction, the radial axis is perpendicular to the grain direction and normal to the growth rings, and the tangential axis is perpendicular to the grain direction and tangent to the growth rings. Although most wood properties differ in each of these three axis directions, differences between the radial and tangential axes are relatively minor when compared to differences between the radial or tangential axis and the
FIGURE 3 Three principal axes of wood with respect to grain direction and growth rings.
longitudinal axis. Property values tabulated for structural applications arc often given only for axis directions parallel to groin (longitudinal) and perpendicular to grain (radial or tangential).
B. Moisture Content
The moisture content of wood is defined as the weight of water in wood given as a percentage of ovendry weight. In equation form, moisture content (MC) is expressed as follows:
MC = moist weight – dry weight dry weight x 100%. (1)
Water is required for the growth and development of living trees and constitutes a major portion of green wood anatomy. In living trees, moisture content de- pends on the species and the type of-wood, and may range from approximately 25% to more than 250% (two and a half times the weight of the dry wood material). In most species, the moisture content of sapwood is higher than that of heartwood.
Water exists in wood either as bound water (in the cell wall) or free water (in the cell cavity). As bound water, it is bonded (via secondary or hydrogen bonds) within the wood cell walls. As free water, it is simply present in the cell cavities. When wood dries, most free water separates at a faster rate than bound water because of accessibility and the absence of secondary bonding. The moisture content at which the cell walls are still saturated but virtually no water exists in the cell cavities is called the fiber saturation point. The fiber saturation point usually varies between 21 and 28%.
Wood is a hydroscopic material that absorbs moisture in a humid environment and loses moisture in a dry environment. As a result, the moisture content of wood is a function of atmospheric conditions and depends on the relative humidity and temperature of the surrounding air.Under constant conditions of temperature and humidity, wood reaches an equilib- rium moisture content (EMC) at which it is neither gaining nor losing moisture. The EMC represents a balance point where the wood is in equilibrium with its environment.
In structural applications, the moisture content of wood is almost always undergoing some changes as temperature and humidity conditions vary. These changes are usually gradual and short-term fluctuations that influence only the surface of the wood. The time required for wood to reach the EMC depends on the size and permeability of the member, the temperature, and the difference between the moisture content of the member and the EMC potential of that environment. Changes in moisture content cannot be entirely stopped but can be retarded by coatings or treatments applied to the wood surface.
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