Development of cold-formed steel moment-resisting frames using optimum beams in seismic applications

Cold-Formed steel (CFS) elements are increasingly being used as the main load-carrying members in modern construction, including in seismic regions. The main deficiency of CFS elements is their susceptibility to premature buckling due to their limited thickness, which results in less seismic energy dissipation and lower ductility compared to their conventional hot-rolled counterparts. However, the seismic characteristics of CFS elements can be increased by using more efficient cross-sectional shapes. This paper aims to improve the seismic performance and post-buckling behaviour of CFS lipped channel sections by optimising their geometry to achieve maximum energy dissipation capacity under cyclic loads. A novel optimisation framework is presented, using the Particle Swarm Optimisation (PSO) method, linked to detailed non-linear finite element models in ABAQUS. The relative dimensions of the cross-section and the inclination of the lip stiffeners are considered as the main design variables. All Eurocode 3 plate slenderness limit values and limits on the relative dimensions of the cross-sectional components, as well as a range of practical and manufacturing limitations, are considered as design constraints in the optimisation problem. It is shown that the proposed optimisation method can significantly increase the energy dissipation capacity of the CFS sections. Subsequently, the assessment of CFS beams connections is conducted by simulations of cyclically loaded moment resisting connections using an experimentally validated model. Finally, the efficiency of the optimised sections is investigated at the structural level by conducting non-linear cyclic analyses on a CFS moment resisting portal frame with CFS back-to-back lipped channel sections for the columns and the beam. The results indicate that using the optimised CFS sections can considerably improve the seismic performance of the CFS frames, leading to higher energy dissipation capacity and lower global damage under strong earthquakes.