Much of the wood used in construction is processed into manufactured products such as laminated wood, wood panels, or composites of various types. These products were originally designed to overcome various shortcomings of solid wood structural members. With diminishing forest quality and a new awareness of sustainability, however, these products have assumed new importance. Emphasis in the forest products industry is steadily shifting away from dimension lumber and focusing on maximum utilization of wood Þ ber from each tree. Year by year, a larger and larger percentage of the wood Þ ber used in buildings is in the form of manufactured wood products.
Large structural members are often produced by joining many smaller strips of wood together with glue to form glue-laminated wood (called glulam for short). There are three major reasons to laminate: size, shape, and quality. Any desired size of structural member can be laminated, up to the capacities of the hoisting and transportation machinery needed to deliver and erect it, without having to search for a tree of sufÞ cient girth and height. Wood can be laminated into shapes that cannot be obtained in nature: curves, angles, and varying cross sections (Figure 3.24). Quality can be speciÞ ed and closely controlled in laminated members because defects can be cut out of the wood before laminating. Seasoning is carried out before the wood is laminated (largely eliminating the checks and distortions that characterize solid timbers), and the strongest, highest-quality wood can be placed in the parts of the member that will be subjected to the highest structural stresses.
The fabrication of laminated members adds to the cost per board foot but provides structural members that are smaller in size than solid timbers of equal load-carrying capacity. In many cases, solid timbers are simply not available at any price in the required size, shape, or quality. Individual laminations are most commonly 1½ inches (38 mm) thick except in curved members with small bending radii, where ¾-inch (19-mm) stock is used. End joints between individual pieces are either fi nger jointed or scarf jointed. These types of joints allow the glue to transmit tensile and compressive forces longitudinally from one piece to the next within a lamination. Adhesives are chosen according to the moisture conditions under which the member will serve. Any size member can be laminated. For use in residential construction, standard sizes range from 318 to 6¾ inches (79 to 171 mm) in width and from 9 to 36 inches (229 to 914 mm) in depth. For larger buildings, standard sizes up to 14¼ inches (362 mm) wide and 75 inches (1905 mm) deep are not uncommon Where glue-laminated beams will be exposed to the weather or to high levels of moisture in the completed construction, the laminations may be preservative-treated to protect against decay.
Structural Composite Lumber:
Structural composite lumber, also called engineered lumber, products are substitutes for solid lumber made from wood veneers or wood Þ ber strands and glue. Laminated strand lumber (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. 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. LVL and PSL are most commonly used for longer-span headers and ß oor beams.
Decking and Nonstructural
Wood-plastic composite (WPCs) products are made from wood Þ bers 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 injectionmolded into Þ nal 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 Þ nish 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 Þ ber, is available in sizes matching conventional 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 Þ nishes 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 Þ nished 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 Þ berÑ more similar in composition to traditional wood panel products, such as plywood, OSB, and Þ berboard, discussed
later in this chapterÑmay 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 Þ eld-applied Þ nishes.
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 Þ nger-jointed and glued solid wood material is also available as an alternative to conventional Þ nish lumber. In comparison to solid pieces, Þ nger-joint stock is more stable and more consistently free of defects. It also makes use of shortlength scraps that might otherwise be treated as waste.