Mode interaction in cold-formed steel members: state-of-art report - Part 2: Couplings involving global buckling

Extended keynote paper of Eurosteel 2021
This is Part 2 of a two-part paper providing a state-of-art report of the most recent findings concerning the behaviour, strength and Direct Strength Method (DSM) design of cold-formed steel (CFS) columns and beams affected by mode coupling phenomena not adequately covered by the current specifications for CFS members. This second paper covers interactions involving global buckling modes, namely local-distortional-global (L-D-G), distortional-global (D-G) and global-global (flexural-torsional/flexural - FT-F) interaction - note that local-global (L-G) interaction, already well mastered by the technical/scientific community, is not dealt with. Like Part 1 [1], this paper also addresses experimental tests, numerical simulations and DSM-based design approaches, intended to i) acquire in-depth knowledge on the non-linear behaviour (elastic and elastic-plastic), load-carrying capacity and failure mode nature of the members under consideration, and ii) make use of the above knowledge to develop, propose and assess the merits of efficient DSM-based design approaches to estimate their failure loads/moments. Taking into account the fundamental concepts and DSM design curves presented in Part 1 [1], the paper addresses separately each mode coupling phenomenon dealt with, for columns, and only D-G interaction for beams - recall that L-D interaction (columns and beams) was covered in Part 1 [1]. For columns undergoing L-D-G interaction and angle columns susceptible to FT-F interaction, the work reported includes experimental studies, numerical simulations and DSM-based design considerations and/or guidelines. For columns and beams experiencing D-G interaction and channel columns prone to FT-F coupling, only numerical results are reported - they reveal surprising behavioural features that will be very useful in planning future test campaigns and achieving efficient design approaches. Finally, the two-part paper closes with a few concluding remarks and a perspective about future developments in this field.