Effect of Size on Tensile Strength of Clear Douglas-Fir and Hem-Fir Dimension Lumber

Nominal 2- by 4-, 6-, 8-, and 10-inch clear, coast Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) and hem-fir (Tsuga heterophylla [Taf.] Sarg.) and (Abies spp.) dimension lumber was seasoned to 10 percent moisture content and tested in tension parallel to grain. Tensile stress of Douglas-fir decreased as width increased, and average values (.20 pieces in each sample) from the 4- and 10-inch widths were significantly different. A similar trend of reduced tensile stress with increased width of piece was obtained from hem-fir lumber, but no significant difference was shown among the four average values. Although mean values for modulus of elasticity in flatwise bending and specific gravity were significantly higher for Douglas-fir, average tensile stress for the two species was not significantly different.

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