Tensile Strength of Douglas-Fir Laminated-Veneer Lumber

Second-growth, 80-year-age-class coastal Douglas-fir No. 2 sawlogs were rotary peeled and clipped into lengths of 1/4-inch-thick veneer. All of the recovered material was dried, trimmed, and continuously assembled by butt-joining randomly selected veneers end to end in stair fashion six layers thick, which were incrementally hot-press glued with phenol-formaldehyde adhesive. The long, thick, 24-inch-wide planks formed were sawn into ninety-four 2 x 4’s, 92 inches long and tested to failure in tension. The ultimate strengths, which were statistically evaluated, indicated that all the laminated-veneer lumber from this limited sample could be assigned tensile working stress and modulus of elasticity at least equal to Select Structural grade. Strength-reducing characteristics of natural origin, such as knots and severe slope of grain, were dispersed throughout the volume of wood by laminating veneers which were randomly selected. This yielded a product having a small coefficient of variability (13.9 percent) compared with sawn lumber (37 percent). However, the test material originated from a localized growth site, whereas the sawn lumber was from a larger portion of the true population. Visual examination of weak specimens indicated that failures usually occurred at butt joints in association with grain deviations in adjacent laminations.

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