Book Description

The torsion test was evaluated as a method for determining the shear strength of full-size structural lumber. The evaluation involved an experimental length study, an experimental depth study, and a finite element study. The length study consisted of fifty nominal 2x4 specimens, ten specimens for each length, and ten American Society for Testing and Materials (ASTM) shear blocks. One 14 foot long board yielded one specimen for each length: (a) 21.0", (b) 28.5", (c) 32.0", (d) 35.5", (e) 39.0", and (f) an ASTM D143-94 shear block. The statistical analysis revealed no evidence that the length affected the shear strength. The depth study consisted of fifty specimens, ten specimens for each depth: (a) 2x4, (b) 2x6, (c) 2x8, (d) 2x10, and (e) 2x12. In addition, fifty ASTM shear blocks, one block for each specimen, were tested. The statistical study did not reveal convincing evidence of a depth effect on shear strength, even after accounting for specific gravity and shear span as covariates. Failure modes for the torsion samples involved a longitudinal shear crack at the mid-point of the longest side, which propagated toward the ends of the specimen and through the cross section perpendicular to the growth rings. The finite element model revealed that uniform shear stress occurs within the shear span, which begins and ends a distance of approximately two times the depth plus the grip distance away from each end of the member. In addition, torsion theory verified that the experimental shear failure plane that occurs within the shear span is parallel to the grain and the shear slippage is also parallel to the grain, similar to the known shear failure in specimens subjected to bending loads. Based on the results of this study, the torsion test is the best practical method to determine the pure shear strength of full-size structural lumber, because the test yields 100% shear failures and the specimen is in a state of pure shear stress.