Structural Composite Lumber

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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