Torsional rigidity of rectangular wood composite materials

The torsional rigidity of wood members is necessary for predicting lateral torsional buckling of laterally unsupported beams, and is useful for estimating the stiffness of two-way floor systems and the natural frequency for wood floors. Current estimations of torsional rigidity of composite wood materials are based upon elastic constant ratios of solid wood. Recently published work has found differences in the elastic constant ratios of solid wood versus structural composite lumber (SCL) materials. These differences in elastic properties may indicate differences in torsional rigidity. Rectangular sections of solid-sawn lumber and various SCL materials were tested to determine values of torsional rigidity. Torsional rigidity of solid-sawn lumber was significantly different (p < 0.05) from laminated veneer lumber, while direct comparisons of parallel strand lumber and laminated strand lumber to solid-sawn lumber were not possible due to dimensional differences of test sections. Predictions of torsional rigidity based upon isotropic and orthotropic elasticity and shear moduli derived from bending tests were compared to the experimental results for each material. The solid-sawn lumber torsional rigidity was predicted best by the isotropic elasticity assumptions, while the parallel strand lumber and laminated strand lumber torsional rigidity values were predicted best by the orthotropic elasticity assumptions. The laminated veneer lumber torsional rigidity was predicted well by isotropic elasticity assumptions if shear moduli values derived from torsional testing were used. Torsional rigidity values for both solid-sawn lumber and SCL materials were not predicted well using an E:G ratio of 16:1 and isotropic elasticity assumptions.