Reassessment of the Lateral Torsional Buckling Resistance of Rolled I-Section Members: Moment Gradient Tests

Abstract

The lateral torsional buckling (LTB) equations in some current standards are based on unified equations that are a fit to a large body of experimental test data. However, these curves tend to overestimate the capacities from finite-element (FE) simulations based on typical idealized end conditions, residual stresses, and geometric imperfections. Uniform moment tests conducted on various types of cross sections have led to proposed modifications of the current specification for LTB equations. This paper investigates and recommends, by means of FE test simulations, the use of the modified equations with additional improvements for moment gradient cases. The emphasis of the studies is on rolled I-section members. Specifically, the paper demonstrates an inelastic Cb effect that can be particularly significant for beams in which the maximum moment occurs within the unbraced length rather than at a braced point. A new way of applying the LTB modification factor, Cb, in the inelastic LTB region for these cases is recommended for ordinary application of current standard resistance equations. Also, it is shown that this effect can also be captured seamlessly via an explicit inelastic buckling analysis using stiffness-reduction factors (SRFs) derived from the design resistance curves.