APA Engineered Wodd Handbooks Episode 1 ppsx

1.1 CHAPTER ONE INTRODUCTION TO WOOD AS AN ENGINEERING MATERIAL Steven Zylkowski Director, Engineered Wood Systems 1.1 BASIC STRUCTURE OF WOOD The unique characteristics and abundant supply of wood have made it the most desirable building material throughout history. Manufacturing technologies based on the modern understanding of wood have led to a family of engineered wood products that optimize properties to meet the specific needs of design professionals. While there are many types of technically advanced wood products such as machine stress-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 have not been included in this Handbook. Information on these and other non-glued wood 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-resistant adhesives. Engineered wood products are intended for structural applications and include 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 com- posed of longitudinally arranged fibers. This directional aspect of wood imparts directional properties that are well recognized by wood engineers and manufacturers of wood products. While the properties of engineered wood products are largely determined by manufacturing processes that change the configuration of the wood fibers, basic wood characteristics are primary to the end product. Wood character- istics are determined by many factors, such as species, growing conditions, and wood 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 1.2 CHAPTER ONE TABLE 1.1 Prevalent Wood Species of North America Hardwoods Softwoods Alder, red Cedar, eastern red Ash, white Cedar, western red Aspen Cedar, yellow 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 Sweetgum Pine, ponderosa 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 have needles that typically remain green throughout the year. Classification of hardwood and softwood is not based upon the hardness or density of the wood; there are many examples of low-density hardwoods such as basswood or aspen and dense softwoods 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 and softwood 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 the wood. During favorable growing conditions, such as during the spring in temperate climates, trees grow at a faster rate, resulting in lower density fibers. This wood fiber appears as lighter areas in the wood. This portion of the wood is known as earlywood or springwood. As growth rates slow, the wood fibers develop thicker cell walls, resulting in denser fibers that appear as the darker portion of the growth ring. This portion of a growth ring is known as latewood or summerwood. Figure 1.1 depicts the earlywood and latewood portions of a growth ring. INTRODUCTION TO WOOD AS AN ENGINEERING MATERIAL 1.3 Annual Growth Increment (Annual Ring) Latewood (or Summerwood) Earlywood (or Springwood) FIGURE 1.1 Earlywood and latewood. Heartwood Sapwood FIGURE 1.2 Sapwood and heartwood. 1.1.3 Sapwood and Heartwood The inner portion of a cross section of a tree trunk often displays a variation in color 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 various considerably between different tree species. The sapwood portion of a stem is the newer portion and is used by the tree for conduction of water and nutrients for 1.4 CHAPTER ONE 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 nutrients that metabolize into various extractives that discolor the wood. These extractives can include waxes, oils, resins, fats, and tannins, along with aromatic and coloring materials. The color and characteristics of the heartwood is critical to woods used for 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 and Chapter 9. 1.1.4 Anisotrophy The appearance of wood, as well as its properties, are significantly influenced by the 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 longitudinal fiber 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. Wood properties are significantly influenced by the direction relative to the fiber and growth ring orientation. 1.1.5 Chemical Makeup of Wood From the standpoint of basic chemical elements, wood is primarily composed of carbon, hydrogen, and oxygen, as shown in Table 1.2. The basic chemical elements INTRODUCTION TO WOOD AS AN ENGINEERING MATERIAL 1.5 TABLE 1.2 Basic Chemical Composition of Wood Element % of dry weight Carbon 49 Hydrogen 6 Oxygen 44 Nitrogen slight amount Ash a 0.2–1.0 a What remains of wood after complete combus- tion in the presence of abundant oxygen. TABLE 1.3 Organic Compounds in Wood (% of Oven-Dry Weight) Cellulose Hemicellulose Lignin Hardwoods 40–44 15–35 18–25 Softwoods 40–44 20–32 25–35 of wood are incorporated into a number of organic compounds. The primary organic compounds are cellulose, hemicellulose, and lignin. These three compounds account for almost all of the extractive-free dry weight of wood. On average, proportions of cellulose, hemicellulose, and lignin differ slightly between hardwood and soft- wood species as shown in Table 1.3. 1.2 RESOURCE AND THE ENVIRONMENT 1.2.1 Distribution of Forests Throughout the World The northern hemisphere contains mostly softwood timberlands and the southern hemisphere mostly hardwoods. As shown in Table 1.4, excerpted from Ref. 1, North America contains a large source of the world’s softwood forests. North America is nearly 21% forestland. As shown in Fig. 1.4, this percentage has been fairly constant since the 1920s. Each year the amount of timber cut is less 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 engineered wood products. The producers of these products in North America are among the most advanced in the world. Engineered wood products have been readily adopted by the construction industry in North America due to an overall familiarity and preference 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 the percentage of worldwide production. 1.6 CHAPTER ONE TABLE 1.4 Distribution of Forests Throughout the World Region Softwood or coniferous forests Land area % Hardwood or deciduous forests Land area % Combined softwood and hardwood forests Land area % North America 400 30.5 230 13.4 630 20.8 Central America 20 1.5 10 2.3 60 2.0 South America 10 0.8 550 32.0 560 18.5 Africa 2 0.2 188 10.9 190 6.3 Europe 107 8.2 74 4.3 181 6.0 CIS a 697 53.0 233 13.6 930 30.6 Asia 65 5.0 335 19.5 400 13.2 Oceania 11 0.8 69 4.0 80 2.6 Total World 1,312 100.0 1,719 100.0 3,031 100.0 a CIS—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 break- downs have been given for areas in Europe, and the Confederation of Independent States is excluded by law from exploitation. 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 environ- mental impacts of construction materials, the preference for wood products has increased since they are an excellent environmental choice. An environmental study was conducted by the ATHENA Sustainable Materials Institute for the Canadian Wood Council 3 to compare the environmental impact of constructing a house using INTRODUCTION TO WOOD AS AN ENGINEERING MATERIAL 1.7 FIGURE 1.5 Harvest ratio of forest land in the U.S. TABLE 1.5 2000 Volume of Engineered Wood Products from North America Product Units U.S. Canada 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 LVL 10 6 ft 3 44.4 4.4 I-Joist 10 6 ft 693 173 Glulam 10 6 bd ft 356 21 wood framing, steel framing, and concrete. The study used life-cycle analysis to assess the environmental effects at all stages of the product’s life, including resource procurement, manufacturing, on-site construction, building service life, and decom- missioning at the end of the useful life of the building. The study evaluated a typical 2400 ft 2 house designed for the Toronto, Ontario, Canada market. The wood-designed house was framed with lumber, wood I-joists for the floor, and wood roof trusses. The steel house used light-gage steel members for 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 concrete slab. 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 in- cludes potential energy contained in raw or feedstock materials, such as natural gas used in the production of resins. • Global warming potential is a reference measurement using carbon dioxide as a common reference for global warming or greenhouse gas effects. All greenhouse 1.8 CHAPTER ONE TABLE 1.6 Environmental Impacts from Various Building Systems Athena study results Wood Steel Concrete Embodied energy, GJ 255 389 562 Global warming potential, kg CO 2 equivalent 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 Weighted resource use, kg 121,804 138,501 234,996 Solid waste, kg 10,746 8,897 14,056 gases are referred to as having a ‘‘CO 2 equivalence effect.’’ While greenhouse gas emissions are largely a function of energy combustion, some products also emit greenhouse gases during processing of raw materials, such as during the calcination 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 commonly accepted measure is the critical volume method, used to estimate the volume of ambient 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 weighted kilograms,’’ 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 prac- tices than materials and careful planning can significantly reduce waste. Table 1.6 shows the environmental measure of the design house built with each type of construction material. Construction with wood uses less energy, represents less global warming potential, has fewer impacts on air and water, and represents less weighted resource use. The results of this study clearly demonstrate that wood systems have fewer environmental impacts than the other construction systems used in the study. 1.3 PHYSICAL PROPERTIES OF WOOD The wide spectrum of wood species provides a diverse range of properties. Wood properties are largely a result of several primary influences, such as wood species, grade, and moisture content. Other factors that influence wood’s utility for certain applications 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 to optimize 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. INTRODUCTION TO WOOD AS AN ENGINEERING MATERIAL 1.9 1.3.1 Density and Specific Gravity of Wood The density, or weight per volume, of wood varies considerably between and within wood species. Density of wood is always reported in combination with its moisture content due to the significant influence moisture content has on overall density. To standardize comparisons between species or products, it is common to report the specific 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 its strength, expansion and shrinkage, and thermal conductivity. Table 1.7 reports the specific gravity of common commercial species in North America. The relationship between specific gravity and mechanical properties can be represented by the equa- tion n P ϭ kG (1.1) where P ϭ a mechanical property k and n ϭ constants that depend upon the specific mechanical property and spe- cies G ϭ specific gravity The constants that describe the relationship between properties and specific grav- ity are presented in Table 1.8 (from Ref. 4). 1.3.2 Moisture and Wood Similar to other organic materials, wood is hygroscopic in that it absorbs or loses moisture to reach equilibrium with the surrounding environment. Wood can natu- rally hold large quantities of water. Understanding the effects of water in wood is important because it influences many properties of wood and engineered wood products. 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% because the 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 saturation point is important because moisture changes involving the chemically held water result 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 chemically held 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 exceeded when the moisture content exceeds the fiber saturation point of the wood. When wood is acclimated to be in equilibrium with ambient air conditions, yet protected from direct wetting, the moisture content of the wood will be below the fiber saturation point. Under this condition, the moisture content of wood is a 1.10 CHAPTER ONE TABLE 1.7 Specific Gravity of Common Commercial Wood Species Species Specific gravity Softwoods Douglas fir, coast type 0.45 Fir, grand 0.35 Fir, noble 0.37 Fir, Pacific silver 0.39 Fir, white 0.37 Hemlock, western 0.42 Larch, western 0.48 Pine, loblolly 0.47 Pine, longleaf 0.54 Pine, shortleaf 0.47 Cedar, eastern red 0.46 Cedar, western red 0.31 Fir, balsam 0.32 Hemlock, eastern 0.39 Pine, lodgepole 0.39 Pine, ponderosa 0.39 Pine, red 0.42 Pine, sugar 0.34 Pine, western white 0.35 Redwood 0.39 Spruce, black 0.38 Spruce, Engelmann 0.33 Spruce, Sitka 0.38 Hardwoods Alder, red 0.38 Ash, white 0.54 Basswood, American 0.32 Beech, American 0.57 Birch, paper 0.48 Cottonwood, eastern 0.37 Elm, American 0.46 Hackberry 0.49 Maple, sugar 0.57 Maple, silver 0.44 Oak, northern red 0.56 Oak, southern red 0.53 Oak, white 0.60 Sycamore, American 0.46 Sweetgum 0.46 Tupelo, black 0.47 Yellow-poplar 0.40 [...]... 3.4 6.3 6.3 6.2 6.2 6 .1 5.9 5.8 5.6 5.4 5.2 5.0 4.8 7.9 7.9 7.8 7.7 7.6 7.4 7.2 7.0 6.8 6.6 6.3 6 .1 9.5 9.5 9.4 9.2 9 .1 8.9 8.7 8.4 8.2 7.9 7.7 7.4 11 .3 11 .2 11 .1 11. 0 10 .8 10 .5 10 .3 10 .0 9.7 9.4 9 .1 8.8 13 .5 13 .4 13 .3 13 .1 12.9 12 .6 12 .3 12 .0 11 .7 11 .3 11 .0 10 .6 16 .5 16 .4 16 .2 16 .0 15 .7 15 .4 15 .1 14.7 14 .4 14 .0 13 .6 13 .1 21. 0 20.9 20.7 20.5 20.2 19 .8 19 .5 19 .1 18.6 18 .2 17 .7 17 .2 INTRODUCTION TO WOOD... 5839 616 9 6 410 5854 6637 7652 7300 8538 7435 030 18 4 517 420 490 13 0 820 893 688 500 11 8 705 660 15 60 12 50 13 80 14 20 11 61 1307 14 58 14 02 15 86 13 88 649 939 12 51 1073 10 76 997 12 81 1032 11 93 11 77 13 82 10 29 12 30 3784 2939 3 013 314 2 2902 3364 3756 3 511 43 21 3527 3570 2774 26 31 3080 2 610 2450 2730 2459 2434 4 210 2836 218 0 2670 904 739 802 746 756 864 869 863 10 41 905 10 08 7 71 662 848 685 704 686 718 677... Oven-dry 12 % MC 0.63 0.40 0.38 0.68 0.53 0.43 0.54 0.57 0.50 0.66 0.65 0.62 0.72 0.55 0.54 0.54 0.46 0. 41 (0.98) 0 .10 (0.67) 0.092 (0.64) 0 .15 (1. 0) 0 .12 (0.84) 0 .10 (0. 71) 0 .12 (0.86) 0 .13 (0.90) 0 .12 (0.80) 0 .15 (1. 0) 0 .14 (1. 0) 0 .14 (0.96) 0 .16 (1. 1) 0 .13 (0.87) 0 .12 (0.86) 0 .12 (0.86) 0 .11 (0.75) 0 .17 0 .12 0 .11 0 .18 0 .15 0 .12 0 .15 0 .16 0 .14 0 .18 0 .18 0 .17 0 .19 0 .15 0 .15 0 .15 0 .13 (1. 2) (0.80) (0.77) (1. 3)... Heartwood Sapwood 33 58 32 37 88 91 34 55 98 97 85 54 33 41 31 40 32 32 98 62 86 52 51 41 — 249 16 6 11 5 17 3 13 6 11 5 16 4 16 0 11 9 17 0 11 9 11 0 12 0 10 6 14 8 13 4 12 2 219 14 8 210 11 3 17 3 14 2 1. 12 CHAPTER ONE function of relative humidity and to a much smaller degree the temperature Table 1. 10 provides the equilibrium moisture content of wood at given relative humidity and temperatures Engineered wood products and... 700 475 478 414 4 91 457 676 6 61 804 573 700 244 18 7 359 252 282 259 214 19 2 424 242 19 7 279 0.45 0.35 0.37 0.39 0.37 0.42 0.48 0.47 0.54 0.47 0.46 0. 31 0.32 0.39 0.39 0.39 0.42 0.34 0.35 0.39 0.38 0.33 0.38 540 500 960 570 380 260 19 0 480 480 820 300 920 300 470 11 0 040 950 11 67 14 36 10 38 13 81 117 0 10 13 11 14 954 15 46 943 13 53 11 41 1246 10 65 12 01 10 31 1222 2960 3990 2220 3550 2360 2280 2 910 2650 4020... 3.9 4.2 4.8 3.0 4.8 4.0 4.7 4.4 5.3 5.0 5 .1 5.5 4.6 7.3 7.8 6.7 9.3 11 .9 8.6 7 .1 9.2 9.5 8.9 7.2 9.9 8.6 11 .3 8.8 10 .2 8.4 8.7 7.8 8.2 12 .6 13 .3 11 .5 15 .8 17 .2 16 .2 11 .5 13 .9 14 .6 13 .8 12 .0 14 .7 13 .7 16 .1 12.7 15 .8 14 .1 14.4 12 .8 12 .7 From ref 4 1. 13 1. 14 CHAPTER ONE affected by other variables Generally there is greater shrinkage with wood with higher density Engineered wood products that cross-laminate... (1. 3) (1. 3) (0.85) (1. 0) (1. 1) (0.97) (1. 2) (1. 2) (1. 2) (1. 3) (1. 1) (1. 0) (1. 0) (0.90) 0.48 0.33 0. 51 0.37 0. 41 0.42 0.48 0.56 0.54 0.43 0.42 0.46 0.54 0.37 0.40 0 .10 0.43 0.37 0.42 0 .11 (0.77) 0.083 (0.57) 0 .12 (0.82) 0.090 (0.63) 0 .10 (0.68) 0 .10 (0.69) 0 .11 (0.77) 0 .13 (0.88) 0 .12 (0.86) 0 .10 (0. 71) 0 .10 (0.69) 0 .11 (0.75) 0 .12 (0.82) 0.090 (0.63) 0 .10 (0.67) 0 .12 (0.82) 0 .10 (0. 71) 0.090 (0.63) 0 .10 ... 0.090 (0.63) 0 .10 (0.67) 0 .12 (0.82) 0 .10 (0. 71) 0.090 (0.63) 0 .10 (0.69) 0 .14 0 .10 0 .14 0 .11 0 .12 0 .12 0 .14 0 .15 0 .15 0 .12 0 .12 0 .13 0 .15 0 .11 0 .12 0 .12 0 .12 0 .11 0 .12 (0.94) (0.68) (0.99) (0.75) (0.82) (0.84) (0.94) (1. 1) (1. 0) (0.85) (0.84) (0.90) (1. 0) (0.75) (0.80) (0.82) (0.85) (0.75) (0.84) 1. 16 CHAPTER ONE TABLE 1. 14 Thermal Conductivity of Plywood and OSB Panels Panel Btu ⅐ in / hr ⅐ ft2 ⅐... perpendicular (lb / in.2) Shear parallel (lb / in.2) Tension perpendicular (lb / in.2) Side hardness (lbf) 24,760 G1. 01 2.97 G0.84 25.9 G1.34 77.7 G1.39 13 ,590 G0.97 2,390 G1.57 2, 410 G0.85 870 G1 .11 1, 930 G1.5 24,850 G0 .13 2.39 G0.7 31. 8 G1.54 95 .1 G1.65 11 ,030 G0.89 3 ,13 0 G2.09 3 ,17 0 G1 .13 1, 460 G1.3 3,440 G2.09 a Compression parallel to grain is maximum crushing strength; compression perpendicular to grain... n␪ ϩ Q cos n␪ (1. 5) INTRODUCTION TO WOOD AS AN ENGINEERING MATERIAL 1. 21 Load Duration Factor (Based on 10 year "normal" duration) 2.2 2 1. 8 1. 6 10 Years 0.6 1 Year 0.8 2 months - 1 1 Day -7 Days 1. 2 10 Minutes 1. 4 0.4 1. E+00 1. E+02 1. E+04 1. E+06 1. E+08 1. E +10 1. E +12 Time (seconds) FIGURE 1. 6 Relationship between load duration and wood strength TABLE 1. 19 Load Duration . 8.9 10 .5 12 .6 15 .4 19 .8 10 0 2.3 4.2 5.8 7.2 8.7 10 .3 12 .3 15 .1 19.5 11 0 2.2 4.0 5.6 7.0 8.4 10 .0 12 .0 14 .7 19 .1 120 2 .1 3.9 5.4 6.8 8.2 9.7 11 .7 14 .4 18 .6 13 0 2.0 3.7 5.2 6.6 7.9 9.4 11 .3 14 .0 18 .2 14 0. 11 .3 13 .5 16 .5 21. 0 50 2.6 4.6 6.3 7.9 9.5 11 .2 13 .4 16 .4 20.9 60 2.5 4.6 6.2 7.8 9.4 11 .1 13.3 16 .2 20.7 70 2.5 4.5 6.2 7.7 9.2 11 .0 13 .1 16.0 20.5 80 2.4 4.4 6 .1 7.6 9 .1 10.8 12 .9 15 .7 20.2 90. 0.66 0 .15 (1. 0) 0 .18 (1. 2) Oak, northern red 0.65 0 .14 (1. 0) 0 .18 (1. 2) Oak, southern red 0.62 0 .14 (0.96) 0 .17 (1. 2) Oak, white 0.72 0 .16 (1. 1) 0 .19 (1. 3) Sweetgum 0.55 0 .13 (0.87) 0 .15 (1. 1) Sycamore,