Past Issue

Product Information
Engineered Wood Products

Because modern forestry has focused on forcing faster growth of trees for commercial harvesting, the structural capability of lumber has declined. This has been reflected in lower allowable stresses for lumber. (Old beams really are stronger.) Therefore, natural lumber is being replaced by “engineered lumber”. This term that didn’t exist a few decades ago refers to manufactured framing lumber such as glulams, wood I-joists, laminated veneer lumber, parallel strand lumber, and laminated strand lumber. “Engineered wood” is a broader category that includes structural panels such as plywood and oriented strand board.

The resulting products offer high strength-to-weight ratios and are free of annoying, if endearing, idiosyncrasies of traditional lumber: Dimensionally stable, they do not twist, check, cup, split shrink, or warp.

Developed more than a century ago in Europe and introduced in this country in 1934, glulams are the best-known engineered lumber products. Formed of solid-sawn wood glued end-to-end and then face-bonded in layered laminations (lams), a glulam is stronger than comparable dimensional lumber and is easily cambered.

Wood I-joists were introduced in 1969 by Trus Joist, now a partnership known as Trus Joist MacMillan. Requiring 50 percent less wood than a comparable dimensioned solid-sawn member, I-joists are fabricated with laminated veneer lumber or solid-sawn flanges and oriented strand board or plywood webs.

Put into production in 1970, laminated veneer lumber (Trus Joist MacMillan’s “Microllam”, for example) is made from dried, graded veneers that, unlike plywood, are laid up with parallel grain.

Trus Joist MacMillan, which bills itself as the first forest products company exploiting technology rather than timberland, has also been the innovator of products known as structural composite lumber – laminated veneer lumber (LVL), parallel strand lumber (PSL), and laminated strand lumber (LSL). Currently Louisiana-Pacific, Georgia-Pacific, Boise Cascade, and Willamette have similar engineered lumber.

Parallel strand lumber, such as Trus Joist MacMillan’s “Parallam”, was introduced in the late 1980s and is gaining market share as an alternative to glulams and steel.

While these products efficiently use a higher percentage of the tree and are structurally efficient there are limitations.
• Limited fire testing has been done.
• Exterior or higher humidity use may be a problem.
• Wood I-joists are subject to vibration problems.

Contact Laura Champion for further information at:
860 388.1224 ext.127 or champion@gncbengineers.com

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