A Modified Approach Towards Estimating the Lateral Torsional Buckling Effective Length

The lateral torsional buckling (LTB) resistance equations for beams in design specifications require the calculation of an effective length. In lieu of using rigorous analyses, design specifications allow the calculation of an elastic effective length factor (K) for beams. The most commonly used method in calculating K for LTB resistance of a critical beam span was proposed by Nethercot & Trahair, which makes use of alignment charts for braced columns, while accounting for the restraint provided by its adjoining segments. The authors find that this approach results in values of K that are larger or smaller than the true K for certain bracing and loading conditions. A K estimate that is larger than the true solution adversely effects the formulation of the design LTB curve, wherein a larger beam capacity is ascribed to a given effective length. A K estimate that is smaller than the true solution may result in unconservative design solutions. This paper discusses some conditions which are commonly encountered in design and experimental test setups, where unfavourable K estimates are made. The examples presented include cases such as multiple equal laterally unbraced lengths under a uniform moment, beams where the critical segment is an end segment, and cases where the critical segment is bounded by unsymmetric restraining conditions at its two braced ends. A modified approach to estimate K is presented for such cases, while also considering singly-symmetric cross-sections. The paper is limited to elastic buckling of compact (Class I) I-sections. 

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