Parallel-strand lumber

Parallel-strand lumber (PSL) is a form of engineered wood made from parallel wood strands bonded together with adhesive. It is used for beams, headers, columns, and posts, among others uses. The strands in PSL are clipped veneer elements having a least dimension of not more than 0.25 inches (6.4 mm) and an average length of at least 300 times this least dimension (around 6 ft or 1.8 m).[1] It is a member of the structural composite lumber (SCL) family of engineered wood products.[1]

A PSL block

The design strength of PSL is greater than that of sawn lumber as the strands are glued together in a directional manner and under high pressure. This results in a much denser and stronger material. Because knots and other imperfections are randomly dispersed throughout the product (and filled up and fortified with glue) strength variability from one piece of PSL to another is less than in solid-sawn wooden beams. Since wooden construction materials are commonly graded to the lowest 5th percentile of the material's strength curve, PSL has much higher usable values for bending, tension parallel to grain, and compression parallel to grain.

Parallam is the brand name for the product invented, developed, commercialized and patented by MacMillan Bloedel (now Weyerhaeuser). It is the world's only commercially manufactured and marketed parallel-strand lumber product. PSL can be made from any wood species, but Douglas fir, southern pine, western hemlock, and yellow poplar are commonly chosen[2] because of their superior strength.

The product is manufactured as a 12-by-12-inch (300 mm × 300 mm) or 12-by-18-inch (300 mm × 460 mm) billet in a rectangular cross-section, which is then typically sawn and trimmed to smaller cross-sectional sizes. The beams are continuously formed, so the length of the beam is limited only to the maximum length that can be handled and transported. Typical widths are 3 1⁄2, 5 1⁄4 or 7 inches (89, 133 or 178 mm); typical depths are 9 1⁄2, 11 7⁄8, 14, 16 and 18 inches (240, 300, 360, 410 and 460 mm). Typically the beams are made to a maximum length of 60 feet (18 m).[3]

  • PSL grain from the side
  • A beam of PSL installed in an apartment
  • An apartment with an exposed PSL beam and column

References
  1. 2012 International Building Code. Country Club Hills, Ill: ICC. 2011. p. 36. ISBN 1609830407.
  2. Stark, Nicole M.; Cai, Zhiyong; Carll, Charlie G., eds. (2010). Wood Handbook. Madison, WI: Forest Products Laboratory. pp. 11–21.
  3. Kam-Biron, Michelle; Koch, Lori (Oct 2014). "The ABC's of Traditional and Engineered Wood Products". Structure Magazine.
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