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UBC Theses and Dissertations

Shear strength of Canadian softwood structural lumber Yee, Hon Wing


An experimental study has been conducted to evaluate the longitudinal shear strength of Canadian softwood structural lumber using a two span five point bending test procedure. Nominal 38mm x 185mm Douglas-Fir, nominal 38mm x 185mm and nominal 38mm x 285mm Hem-Fir and Spruce-Pine-Fir boards have been considered. Flat-wise and edge-wise modulus of elasticity tests have been conducted for each specimens prior to the destructive shear tests. A two span five point bending test procedure has been chosen because of its ability to produce a relatively high percentage of longitudinal shear failures. Approximately 40% of the failures can be attributed to shear failures in the nominal 38 mm x 185 mm and 30% in the nominal 38 mm x 285 mm specimens. Two test configurations have been considered: test span to specimen depth ratios of 6:1 and 5:1. American Society for Testing and Materials (ASTM) shear block tests have also been conducted to evaluate the shear strength of small clear specimens. Based on the ASTM shear block test and the edge-wise modulus of elasticity results, a linear elastic finite element analysis coupled with Weibull weakest link theory has been used to predict the shear failure loads at different levels of failure probability. Good agreement between predicted and measured failure loads at different probability of failure levels for the different sizes and spans of each species have been observed. Predictions from the finite element and size effect analysis procedures have been compared to an empirical approach for predicting longitudinal shear resistance in sawn lumber and glued laminated beams proposed by the US Forest Products Laboratory. The US proposed design equation tends to overestimate the longitudinal shear resistance of the full size beam results obtained from this study. In lower grades of dimension lumber, shear failures do not govern because the bending to shear strength ratio is usually significantly lower as compared to the Select Structural grade material. Therefore in the lower quality dimension lumber, bending failure mode would most likely dominate.

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