The moment-gradient factor in lateral–torsional buckling on inelastic castellated beams

Abstract

It is well known that if the moment in an I-beam is not constant throughout, the lateral–torsional buckling moment is greater than the same moment in pure bending. In other words, the value of moment-gradient factor ( C b) is always greater than unity. However, the effect of discontinuities in the cross section on the moment-gradient factor has not been studied, and it is not known whether the C b factor given by AISC in Load and Resistance Factor Design Specification for Structural Steel Buildings can be applied to castellated beams. This paper develops a three-dimensional (3-D) finite-element model using ANSYS [User’s manual, version 5.4, ANSYS Inc, Houston, PA, 1998] for the inelastic nonlinear flexural–torsional analysis of castellated beams and uses it to investigate the effects of slenderness on the moment-gradient factor of simply supported castellated beams. It is found that the C b factors given by AISC-LRFD are not accurate for the cases considered in this paper, in which castellated beam buckles inelastically. Therefore, instead of using a constant C b factor for any of the cases, alternative equations for evaluating the C b factor for each case in terms of the beam’s modified slenderness is proposed.