Review: Interactive behaviour, failure and DSM design of cold-formed steel members prone to distortional buckling

The aim of this work is to provide an overview of the current status of an extensive ongoing investigation on cold-formed steel columns and beams affected by mode coupling phenomena involving distortional buckling, namely local-distortional (L-D), local-distortional-global (L-D-G) and distortional-global (D-G) interaction. The investigation comprises experimental tests, numerical simulations and design proposals, intended to (i) acquire in-depth knowledge on the post-buckling behaviour (elastic and elastic-plastic), ultimate strength and failure mode nature of the members under consideration, and (ii) take advantage of the above knowledge to develop, calibrate and validate efficient Direct Strength Method (DSM)-based design approaches to predict their ultimate strength. Initially, column results are used to illustrate and help grasp some fundamental concepts, namely the characterisation of the (i) three above mode coupling phenomena, (ii) different sources of mode interaction that may cause ultimate strength erosion, (iii) global post-buckling behaviour and (iv) the most detrimental initial geometrical imperfections. Then, the paper addresses separately each mode coupling phenomenon, for columns, and only L-D interaction for beams. For columns undergoing L-D and L-D-G interaction, the research activity reported concerns experimental investigations, numerical (shell finite element) simulations and the development and/or assessment of DSM-based design approaches and/or guidelines. The picture is different for columns experiencing D-G interaction and beams affected by L-D interaction, due to the lack of available experimental results − nevertheless, even if only numerical results are reported, they enable unveiling rather interesting (and unexpected) behavioural features concerning the real nature of these mode coupling phenomena.