Distortional Failure and DSM Design of Cold-Formed Steel Lipped Channel Beams under Non-Uniform Bending

摘要

Recently, Martins et al. (2017a) reported an in-depth numerical investigation that providing solid evidence that the current Direct Strength Method (DSM) beam distortional strength curve overestimates the ultimate strength of cold-formed steel simply supported beams, thus leading to unsafe designs - the beams analyzed were uniformly bent (mostly about the major-axis). Moreover, the above authors used the distortional failure moment data obtained to propose new DSM beam distortional design curves, differing visibly from the current one for moderate-to-high slenderness - these curves were already successfully employed in the context of the design of uniformly bent beams undergoing local-distortional interaction (Martins et al. 2017b). The work reported in this paper, which may be viewed as a continuation of the aforementioned investigation, concerns the distortional post-buckling behavior and DSM design of simply supported cold-formed steel beams buckling and failing in distortional modes under non-uniform bending due to unequal end moments - five bending moment diagrams (including uniform bending, for comparison purposes) are considered. As in previous studies, two beam simply supported end conditions are considered, differing in the end cross-section warping and local displacements/rotations, which may be either completely free or fully prevented. The beams analyzed buckle and fail in modes exhibiting various half-wave numbers. After acquiring in-depth insight on how the bending moment diagram influences the beam distortional buckling and post-buckling behaviors, an extensive numerical (shell finite element) parametric study is carried out in order to gather significant distortional failure moment data concerning lipped channel beams with various cross-section dimension ratios and yield stresses (to enable covering wide distortional slenderness ranges). These failure moments, together with additional values collected from the literature, are then employed to assess the merits of the available DSM beam distortional strength curves in predicting them, and also to propose slight adjustments/modifications aimed at achieving DSM design curves able to provide accurate failure moment predictions that take into account the variation of the bending moment diagram.