The moment-gradient factor in lateral-torsional buckling on wide flange steel sections

The equations for evaluating the moment-gradient factor (Cb) given by AISC in Load and Resistance Factor Design Specification for Structural Steel Buildings are empirical equations; the Cb equation in the 1986 edition of the specification is valid for a linear varying moment diagram between lateral supports, whereas the Cb equation in the 1994 edition is valid for any moment diagram. There is a possibility that for some moment diagrams, the equations give incorrect results. Therefore, the feasibility of both equations are investigated and compared with a Cb factor which is a ratio between the critical moment in a varying moment diagram situation computed using a finite difference solution of the governing differential equation and the critical moment in a uniform moment situation. The investigation is performed for a few loading cases. From the results, it is shown that the equations for evaluating the Cb factor in both editions of the AISC Specifications are not accurate for some of the cases considered in this paper. Therefore, instead of using one single equation for any moment diagram, alternative equations for evaluating the C-b factor for each loading case is proposed. (C) 2002 Elsevier Science Ltd. All rights reserved.