Equivalent uniform moment factors for lateral–torsional buckling of steel members

Abstract

Steelwork design standards consider lateral–torsional buckling as one of the ultimate limit states that must be checked for steel members in bending. The buckling resistance assessment is usually based on buckling curves and requires the computation of the elastic critical moment, which is strongly dependent on both the bending moment distribution and restrictions at end supports. This paper focuses on the equivalent uniform moment factor (EUMF) which is used to compute the elastic critical moment. A review of EUMF values given by modern steelwork standards and their comparison with recent results presented in the literature shows that whilst codes may lead to very conservative values for simply supported beams, non-conservative values are obtained in the case of support types designed to restrict lateral bending and warping. In order to clarify situations where EUMF values proposed by modern codes appear contradictory with recent computational results, the paper presents a significant set of EUMF values obtained using both finite difference and finite element techniques. Particular attention has been given to instances where lateral bending and warping are prevented at beam supports since very few results from these cases have been published. A major advantage of codes, such as the American AISC LRFD and the British BS 5950-1, is that they provide closed-form expressions to compute the EUMF for any moment distribution. Unfortunately, these closed-form expressions do not take into account changes in the EUMF due to end support restrictions. This paper presents a general closed-form expression that not only delivers similar advantages but also improves the results given by those codes for some loading cases.