Engineering and Construction

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Wood in panel form is advantageous for many building applications (Figure 3.28). The panel dimensions are usually 4 by 8 feet (1220 by 2440 mm).

Panels require less labor for installation than individual boards because  fewer pieces must be handled, and  wood panel products are fabricated  in such a way that they minimize  many of the limitations of boards and
dimension lumber: Panels are more nearly equal in strength in their  two principal directions than solid  wood; shrinking, swelling, checking,  and splitting are greatly reduced.

Additionally, panel products make  more efficient use of forest resources  than solid wood products through  less wasteful ways of reducing logs  to building products and through  utilization in some types of panels  of material that would otherwise be  thrown awayÑbranches, undersized trees, and mill wastes. Many wood  panel products are made largely  from recovered or recycled wood  waste, and panels made from rapidly  renewable vegetable fibers are also  available.

Figure 3.28 Plywood is made of veneers selected to give the optimum combination of economy and performance for each application. This sheet of roof sheathing plywood is faced with a D veneer on the underside and a C veneer on the top side. These veneers, though unattractive, perform well structurally and are much less costly than the higher grades incorporated into plywood made for uses where appearance is important.

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Of the major building materials, wood is the only one that is organic in origin. This accounts for much of its uniqueness as a material of construction. Trees grow naturally in the forest, and most of the work of ÒmanufacturingÓ wood is done for us by the solar-powered processes of growth within the tree itself. This makes wood a renewable resource and inexpensive to produce: Trees need only be harvested, cut to size, and dried before they are ready for use in construction. On the other hand, we have only limited control ofthe quality and properties of wood. Unlike steel, concrete, or masonry, we can do little to refine solid wood tosuit our needs. Rather, we must accept its natural strengths and limitations.

Wood is well suited to our building structures. The tree itself is a wooden structure, a tower, erected for the purpose of displaying leaves to the sun.* The tree is sub-ject to the same forces as the buildings we erect: It supports itself against the pull of gravity; it withstands the pressures of wind and the accumulation of snow and ice on its limbs; and it resists the natural stresses of the envi-ronment, such as changes in temperature and moisture, and attack by other organisms. Wood has evolved to per-form these jobs well in the tree. Not surprisingly, it also performs them well in our buildings.

In nature, the greatest forces acting on a tree, such as the pressure of the wind, tend to act in bursts of relatively short duration. In response, wood has evolved to be strongest when resisting forces that act brieß y rather than those that act over longer periods of time. When we build with wood, we recognize this unique property. In the engineering design of a wooden structure, the maximum loads that the structure may carry are increased as the length of time these loads are expected to act decreases.

This increase, called the load duration factor, ranges from 15 to 100 percent over the basic design strength of the material and is applied to wooden structures when considering the effects of wind, snow, earthquake, and impactÑ the same types of loads the tree has evolved to withstand most effectively.

The structural form of the tree is also well suited to its environment. Tree branches are supported at one end onlyÑan arrangement called a  cantilever. The joint sup-porting a cantilevered branch must be strong and stiff, efficiently utilizing the materials structural capabilities to the utmost. Yet despite the stiffness of thejoint, the branch it supports can deß ect relatively large amounts since it is fastened at one end only. Underheavy loads, such as wet snow, the branch can droop and shed its load. Likewise, under strong winds,the tree and its branches can sway and shed much of the force acting upon them.

The trees capacity to bend and sway helps it to survive forces that might otherwise break it. When we build with wood, we cannot directly exploit the efficient structural form of the tree. The large deß  ections characteristic of this form are unsuitable for buildings. They would cause discomfort to building occupants and place undue stress on other building components.

Nor can we exploit the naturally strong joints that join a trees limbs, despite their benefits in stiffness and economy of material. In preparing wood for our uses, we cut the tree into pieces of convenient size and shape, destroying the continuity between its parts. When we reassemble these pieces, we must devise new ways to join them. Despite the many methods developed for fabricating wood connections, joints withstrength and stiffness comparable to the trees are rarely achievable. Rather, we rely on much simpler types of connections that suffice when a beam is supported at both ends.

Wood in a tree performs many functions that wood in a building does not, because wood is involved with all aspects of the trees life processes and growth. In order to meet numerous and specialized tasks, wood has evolved a complex internal structure that is highly directional.

The fact that wood is anisotropic, that is, it has different properties in different directions, affects virtually all of its properties. The direction of the grain in a piece of wood influences its production, shaping, fastening, strength, durability, aging, and beauty. No other major building material, perhaps with the exception of some specialized reinforced concrete systems, has such directional characteristics.

Consider two wood building systems, the log cabin and the heavy timber frame. Log cabin construction is appropriate to the most primitive construction technology. Logs found on or near the site are prepared for use with a minimum of labor and simple tools. Once assembled, the logs provide structural support and act as interior finish, exterior cladding, and insulation. Though a practical solution to providing shelter, log cabins use wood inefficiently and result in a structure  that is dimensionally unstable due to the large stacks of  cross-grain lumber. Where more sophisticated tools and methods are available, heavy timber takes the place of log cabin construction. In this system, wood members are assembled in ways that more closely resemble the tree form. The resulting structure is stronger, requires less material, and is more stable. A simple change in the orientation of the wood members produces a completely different building system, one that is more efficient, durable, and comfortable.

Modern developments in the wood construction materials industry reß ect efforts to overcome the limitations of wood in its natural state and to make the most  efficient use of the material. Wood panels made of ply- wood are larger and more stable than those that can  be produced from solid boards while minimizing the
adverse effects of wood grain and natural defects. Panel  products such as OSB and hardboard make use of wood waste that might otherwise be discarded. Glue-laminated and composite lumber beams can be produced in sizes and of a quality exceeding those available in solid timbers. Chemical treatments can protect wood from fire or decay. Thus, the trend is toward using wood less in its natural state and more as a raw material in sophisticated manufacturing processes to produce more refined and higher-quality construction components. Yet despite such advances, the tree itself remains an inspiration in its grace and strength, as wellas in the lessons  it offers for our understanding of building materials and methods.

*Brayton F. Wilson and Robert R. Archer, ÒTree Design: Some  Biological Solutions to Mechanical Problems,Ó BioScience, Vol. 29, No. 5, p. 293, 1979.

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Wood-plastic composite (WPCs) products are made from wood lumbers and  plastics of various types, mixed with  other ingredients, such as ultraviolet stabilizers, pigments, lubricants,  and biocides, which are then heated  and pressed, extruded, or injection-molded into final form. In comparison to their solid wood counterparts,

WPC materials offer more consistent  material quality, freedom from defects and distortion, and, depending  on their formulation, superior resistance to moisture. They are used  most notably for exterior decking,  as well as for exterior railing systems  and finish trim, both interior and  exterior. Like structural composites,  WPC products make productive use  of rapidly renewable or waste materials. Some WPCs also have high recycled materials content.

WPC decking, most commonly  made from blends of polyethylene  or polypropylene and wood lumber,  is  available in sizes matching conven-tional solid wood decking in lengths  up to 20 feet (6.1 m). It may be fastened with corrosion-resistant nails or  screws, or with concealed hardware  that engages the edges of the boards.

A variety of maintenance-free colors  and textured finishes are available,  some remarkably similar in appearance to genuine hardwoods.

Ingredients and manufacturing processes used in the making of  composite wood trim vary widely, as do  the workability, surface qualities, and  durability of the finished  products.

Blends of plastic and wood similar  to those used for the manufacture  of composite decking may be used.

Alternatively, formulations with a  higher proportion of wood lumberÑ more similar in composition to traditional wood panel products, such as plywood, OSB, and fiberboard,  discussed later in this part may be  used. Products may be preÞ  nished, factory primed, encapsulated within a dense plastic outer shell, or coated with a resin-impregnated paper that  improves the quality of field-applied finishes.

In comparison to solid lumber, nonstructural composite lumber expands and contracts more with changes in temperature, so greater allowance for thermal movement must be made during installation.

And in the case of spanning members such as decking, the lesser stiffness of composites necessitates closer spacing of joists or other members on which the members are supported.

Wood trim made up from shorter lengths of finger-jointed  and  glued solid wood material is also available as an alternative to conventional finish lumber. In comparison to solid pieces, finger-joint stock is more stable and more consistently free of defects. It also makes use of short-length scraps that might otherwise be treated as waste.

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Structural composite lumber, also called engineered lumber, products are substitutes for solid lumber made from wood veneers or wood fiber  strands and glue.  Laminated strand lum-ber (LSL) and  oriented strand lumber (OSL) are made from shredded wood strands, coated with adhesive, pressed into a rectangular cross section, and cured under heat and pressure (the wood strands used in the manufacture of LSL are longer than those used in OSL). LSL and OSL are the least strong and least expensive of the composite lumber products. They are used mainly for rim boards and short-span headers.  Laminated veneer lumber (LVL) is made from thin wood veneer sheets, as wide as the member is deep, that are glued and laminated into thicker members. LVL is similar in appearance to plywood except without crossbands (Figure 3.26).

Figure 3.26 An LVL beam, made of veneers similar to those used in the manufacture of plywood, resting on its side in preparation for installation on a concrete foundation wall. When placed in its fi nal position, the beam will be rotated so that the laminations are oriented vertically in the beam. Also visible in the photograph are a preservative- treated wood sill (left) and an LSL rim board (right), both of which are discussed later

Parallel strand lumber (PSL) is made from long, thin strips of wood veneer glued and pressed in a process similar to that for LSL and OSL, but with the veneer strips arranged more uniformly parallel than the strands in those other products. PSL is the heaviest, strongest, and most expensive of the composite lumber products (Figure 3.27). LVL and PSL are most commonly used for longer-span headers and oor beams.

Figure 3.27 A view of corner framing for a  residential garage. The lower beam is a PSL. A high-strength member such as this  is needed to span the roughly 9-foot (2.7-m)-wide garage opening while  supporting the fl oor load above. There are two solid wood top plates above  the PSL. Above the top plates is an LSL rim board that encloses and steadies  the fl oor framing behind it. This lower-strength member is adequate for  this nonstructural function. Several manufactured wood products discussed  later in this chapter are also in view: Above the rim board, the laminated edge  of the plywood subfl oor is visible, and to the left of the rim board, an I-joist is  exposed where the fl oor framing projects beyond the plane of the wall below.

Studs and posts may also be made of any of the composite lumber types described in the previous paragraph, as well as from structural finger-jointedlumber in which short lengths of solid  lumber scrap are finger-jointed  and glued end-to-end into longer lengths.

Composite studs and posts are well  suited for use in the framing of walls that are especially tall or that have very large openings, or wherever else long, straight, and especially strong, stiff members are needed.

Structural composite lumber products make productive use of wood materials that are rapidly renewable or that might otherwise be treated as waste, and they offer many of the benefits of glue-laminated wood: dimensional stability, structural strength up to three times that of con-ventional solid material, availability of large sizes and long lengths, and consistent quality. Particularly where these products are used in exposed interior applications, however, attention should also be paid to the types of adhesives and binders used in the products and the potential for offgassing of volatile organic compounds or formaldehyde.

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Lumber sizes in the United States are given as nominal dimensions in inches,  such as 1 x 2 (one by two), 2 x 10, and so on. At one time, sawn lumber may have approached these actual dimensions. Today however, subsequent to sawing, seasoning, and surfacing,  true sizes are less. By the time a kilndried 2 10 reaches the lumberyard, its actual dimensions are close to 1½ by 9¼ inches (38 by 235 mm). The relationship between nominal lumber  dimensions (which are always written without inch marks) and actual dimensions (which are written with inch marks) is given in simplified form in Figure 3.22 and in more complete  form in Figure 3.23. Anyone who designs or constructs wooden buildings soon commits the simpler of these  relationships to memory. Because of  changing moisture content and manufacturing tolerances, however, it is  never wise to assume that a piece of  lumber will conform precisely to its  expected dimensions. Wood members vary in size seasonally with changes in  humidity and temperature. In hot, dry  locations such as attics, wood framing  may shrink to dimensions substantially below its original measurements.

Figure 3.22 The relationship between nominal and actual dimensions for the most common sizes of kiln-dried lumber is given in this simplifi ed chart, which is extracted from the complete chart in Figure 3.23.

Figure 3.23 A complete chart of nominal and actual dimensions for both framing lumber and fi  nish lumber.

Members in older buildings may have been manufactured to full nominal  dimensions or to earlier standards of  actual dimensions such as 15/ 8 inches or 1¾ inches (41 or 44 mm) for a  nominal 2-inch member.

Pieces of lumber less than 2 inches in nominal thickness (38 mm  actual thickness) are called  boards.
Pieces ranging from 2 to 4 inches  in nominal thickness (38 to 89 mm  actual thickness) are referred to collectively as  dimension lumber. Pieces  nominally 5 inches (actual size 114  mm) and more in thickness are  termed timbers.  Dimension lumber is usually supplied in 2-foot (610-mm) increments  of length. The most commonly used  lengths are 8, 10, 12, 14, and 16 feet  (2.44, 3.05, 3.66, 4.27, and 4.88 m),  but retailers frequently stock rafter  material in lengths to 24 feet (7.32 m).

Actual lengths are usually a fraction of  an inch longer than nominal lengths.

Lumber in the United States is  priced by the  board foot. Board foot measurement is based on nominal  dimensions, not actual dimensions.

A board foot of lumber is deÞ  ned as  a solid volume 12 square inches in  nominal cross-sectional area and 1  foot long. A 1 x 12 or 2 x 6 10 feet long contains 10 board feet. A 2 x  4 10 feet long contains [(2 x 4)/12] 10 6.67 board feet, and so on. Prices of dimension lumber and timbers  in the United States are usually quoted on the basis of dollars per thou- sand board feet. In other parts of the world, lumber is sold by the cubic meter. Because the board foot is based on lumber nominal dimensions and  metric sizes use actual dimensions,  there is no direct conversion between  board feet and cubic meters.

The architect and engineer specify lumber for a particular construction use by designating its species, grade,
seasoning, surfacing, nominal size, and chemical treatment, if any. When ordering lumber, the contractor must additionally give the required lengths of pieces and the required number of pieces of each length.