Q&A: Design Values For Site-Made Beams

A.The performance of a site-made beam depends on how the beam is constructed. Plywood and lumber box beams or I-beam headers perform very differently from ganged timbers spaced with plywood laminations. If you make an effort to closely follow established design guidelines for glue-nailed, built-up beams, you can count on the plywood to increase the capacity of the beam. But if you are simply nailing a 1/2-inch layer of plywood between two 2x12s to get a flush-fitting beam, you can only claim the design values for the solid-sawn timbers.

The strength of a composite plywood and lumber beam relies primarily on the shear strength of the glue lines in the plywood and in the laminated joint. This is not always controlled on site as closely as it ought to be. In addition to the allowable stress limits of the wood itself, such factors as the size and spacing of the fasteners, the type and amount of glue, the temperature at which the glue cures, the placement of plywood joints, and the grade and condition of the plywood play a role in the strength of the beam. The National Association of Home Builders Beam Series (15th and M Sts. NW, Washington, D.C. 20005; 800/368-5242) describes this fabrication process for built-up beams, and provides span tables for different beam configurations.