Measurement and prediction of lateral torsional buckling loads of composite wood materials: I-joist sections

Previous studies have shown a difference in the behavior of solid-sawn lumber and structural composite lumber (SCL) materials in terms of the elastic constant ratios and in the lateral torsional buckling behavior. The increased use of wood composite I-joists, many with SCL materials as flange stock, for longer span applications gives increased importance to the lateral torsional buckling determination for these materials. The current load resistance factor design (LRFD) equations for I-joist lateral buckling assume that the compression flange acts as a column restrained in the direction of the web. An unbraced cantilever beam test method was used to determine the critical buckling load (CBL) of two I-joist materials with SCL flanges. These measured CBLs were then compared to predicted CBLs using current LRFD design equations and elastic beam buckling theory. The current LRFD design equation CBLs were, on average, at least 77 percent less than the measured CBLs, while the elastic beam buckling CBL predictions were on average 28 percent less than the measured CBLs. The low CBL predictions using current LRFD equations demonstrate that assumption of lateral buckling behavior resembling the buckling of the compression flange supported in the direction of the web is not a satisfactory model. An elastic buckling solution incorporating the off-axis bending and torsional rigidity of the composite cross section of the I-joist provided a more reasonable CBL prediction.