Experimental Evaluation of the Torsion Test for Determining Shear Strength of Structural Lumber

Abstract The torsion test was evaluated as a method for determining shear strength of full-size, structural lumber by length and depth studies. The length study consisted of fifty 38 by 89 mm specimens, ten each of lengths 0.5 m, 0.7 m, 0.8 m, 0.9 m, and 1.0 m, and ten shear blocks. The statistical analysis showed no evidence that length affected shear strength. The depth study consisted of 50 specimens, ten each of sizes 38 by 89 mm, 38 by 140 mm, 38 by 184 mm, 38 by 235 mm, and 38 by 286 mm, and 50 shear blocks. The statistical analysis did not show convincing evidence of a depth effect on shear strength, even after accounting for specific gravity and shear span as covariates. Failure modes were similar to shear failures observed in bending tests, and involved a longitudinal shear crack at the wide face, which propagated toward the ends of the specimens and through the cross section perpendicular to growth rings. Based on results of this study, the torsion test is the best practical method for determining pure shear strength of full-size, structural lumber.