The equation for evaluating the lateral-torsional buckling modification factor for nonuniform moment diagram when both ends of the unsupported segments of a steel beam are braced (C_(b)) given by AISC (2005) in Specification for Structural Steel Buildings is an empirical equation. The AISC Specification gives one single C_(b) equation for any moment diagram. There is a possibility that for some moment diagrams, the equation gives inaccurate results. Therefore, the accuracy of the equation is investigated and compared with C_(b) factor which is a ratio between critical moment in a varying moment diagram situation computed using a nonlinear finite element analysis and critical moment in a uniform moment situation. The finite element analysis includes nonlinear geometry and uses shell elements. The investigation is performed for few loading cases. From the analysis results, it is shown that the equation for evaluating the C_(b) factor in the AISC Specification is inaccurate for some 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. The equations agree very well with previous study using finite difference approach.
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