Optimisation of cold-formed steel beams for best seismic performance in bolted moment connections

Typical thin-walled cold-formed steel (CFS) elements and connections used in portal frames are not generally suitable for moment-resisting frames in high seismic regions due to their low ductility and energy dissipation capacity and inherited susceptibility to local/global instabilities. This study aims to improve the seismic performance of CFS moment-resisting frames by developing a methodology to obtain more efficient bolted moment connections using optimised CFS beams with enhanced non-linear post-buckling behaviour. By taking into account material non-linearity and geometrical imperfection effects, the detailed Finite Element (FE) models of a typical CFS bolted moment connection is developed using ABAQUS software, and then validated based on experimental cyclic test results. Particle Swarm Optimisation (PSO) algorithm is linked to GMNIA ABAQUS FE analysis to optimise CFS bolted moment connections based on either energy dissipation capacity or ductility. To demonstrate the efficiency of the method, connections with five different beam cross-sectional shapes are optimised, and the results are compared with a standard CFS channel section used as a benchmark. The relative dimensions of the cross-sections, the inclination of the lip stiffeners, and the location of triangular intermediate stiffeners are selected as main design variables. To provide practical beam cross-sections, the plate slenderness limit values defined by Eurocode 3 (EC3) along with a range of manufacturing limitations are imposed as design constraints in the optimisation process. It is shown that, for a given plate width and thickness, the proposed optimisation framework results in a considerable (up to 195%) improvement in the energy dissipation capacity and ductility of the CFS bolted moment connections.