The effect of moment gradient on the buckling moment resistance of I-beams can be taken into account by the use of an equivalent moment-gradient factor Cb. The Cb factors given by the previous researchers have been derived from elastic lateral-torsional buckling (LTB) analysis of compact I-beams. However, according to the available literature it is not known whether these factors can be applied to the LTB analysis of inelastic slender-web plate girders (SWPGs). The aim of this paper is to investigate the nonlinear LTB analysis of unstiffened SWPGs subjected to central loading. To this end, a 3D finite-element model using ABAQUS is developed for the inelastic nonlinear flexural-torsional analysis of SWPGs and used to evaluate the unbraced length and load-height effects (located at center, top flange and bottom flange) on their moment-gradient factor. It was found that the Cb factor given by AISC in Specification for structural steel buildings is generally unconservative for the elastic and inelastic SWPGs under the considered point load cases. Also, it was observed that the moment resistance versus unbraced length (M-L) relationships for the elastic SWPGs are not significantly affected by the applied load height.
