Distortional failure of cold-formed steel beams under uniform bending: Behaviour, strength and DSM design

This work reports numerical results concerning cold-formed steel simply supported beams buckling and failing in distortional modes under uniform bending and exhibiting three cross-section shapes, namely (i) lipped channels bent about the major-axis, (ii) zed-sections under skew bending causing uniform flange compression (worst case), and (iii) hat-sections subjected to either major-axis or minor-axis bending (compressed lips in the latter case). Two end support conditions are considered, differing in the warping and local displacement/rotation restraints, which are either completely free or fully prevented. The beams analysed have several cross-section dimension ratios and lengths, in order to assess their influence on the distortional post-buckling behaviour and ultimate strength - particular attention is paid to the influence of (i) the flange-web and lip-flange width ratios, and (ii) the critical (distortional) half-wave number. In addition, the beams exhibit different yield stresses, making it possible to cover wide distortional slenderness ranges. After presenting and discussing the numerical elastic and elastic-plastic post-buckling results obtained, consisting of equilibrium paths, deformed configurations, plastic strain distributions, failure moments and collapse mechanisms, the paper shows that the currently codified Direct Strength Method (DSM) design curve fails to predict adequately the failure moments of some of the beams analysed and addresses the development/proposal of novel DSM strength curves, providing better quality predictions of all the numerical failure moments available.