Torsional rigidity of wood composite I-joists

The torsional rigidity of I-joists is useful in determining the lateral torsional buckling of unsupported beams, the stiffness of two-way floor systems, and the natural frequency for wood floors. The torsional rigidity of two I-joist materials, one manufactured with laminated veneer lumber (LVL) flanges and the other with laminated strand lumber (LSL) flanges, was measured. There were no significant differences in the measured torsional rigidity of the two I-joist materials. The measured torsional rigidity terms were compared with predictions of torsional rigidity based upon the distinct cross-sectional dimensions and previously measured material elastic constants. A finite element model was used to predict the torsional rigidity of the I-joist section and to examine the effect of isotropic and orthotropic assumptions. An isotropic torsional rigidity prediction using G₁₂ values of the web material and G₁₃ values of the flange materials provided agreement with the measured 95% confidence intervals for both I-joist materials. Prediction of torsional rigidity was heavily influenced by the planar shear moduli in the larger cross-sectional dimension. The use of an assumed E:G ratio of 16:1 for all wood materials overpredicted the torsional rigidity values by 30% compared to more refined predictions.