Applicability of North American standards for lateral torsional buckling of welded I-beams

Abstract

Design specifications in North America, unlike their European counterpart, use same equations for design of rolled and welded shape beams for lateral torsional buckling (LTB). A recent study by MacPhedran and Grondin (2011) has shown that the current Canadian design equations might overestimate the capacity of welded wide flange (WWF) beams, which are welded I-shapes, requiring further investigation. This paper evaluates the performance of the current design equations for LTB capacities of welded I-shape beams. Nonlinear Finite Element (FE) analysis is performed for simply supported WWF beams subjected to constant moment, linear and nonlinear moment gradient. Three types of linear moment gradients are investigated: end moment ratios of 0.5, 0.0, and − 1.0. Also, two types of transverse loadings are considered: a concentrated load at mid-span and uniformly distributed load along the length, and applied at the top flange, shear center, and at the bottom flange. In total, 416 FE models are analyzed and it is observed that for constant moment loading both CSA and AISC overestimate the LTB capacity of welded I-shape beams by as much as 37%. FE analysis also shows that current CSA strength curve overestimates significantly the strength of WWF beams by as much as 40% for end moment ratio of 0.5. For transverse loading, current CSA strength curve overestimates the capacity significantly for top flange loading and underestimates for bottom flange loading. Also, Eurocode is found to be conservative for all cases and the proposed equation by MacPhedran and Grondin (2011) provides better predictions of LTB strengths of WWF shapes than the current CSA approach.