More efficient design of CFS strap-braced frames under vertical and seismic loading

Strap-braced stud walls are one of the primary lateral-force resisting systems for conventional cold-formed steel (CFS) buildings. While CFS wall-panels in general exhibit a satisfactory seismic performance, previous studies showed that they may experience a premature brittle failure in the presence of vertical loading. However, current design codes do not make any provisions for calculating the lateral load capacity and ductility of strap-braced stud wall frames under vertical loading. This study aims to develop, for the first time, a practical design methodology for seismic design of CFS strap-braced stud wall frames under such conditions. To this end, a comprehensive parametric study was carried out using experimentally validated detailed numerical models in ABAQUS, accounting for material nonlinearities, initial geometric imperfections, and secondary moments due to P-Delta effects. Parameters of the investigations were the strap thickness and the intensity of the vertical loading. Design formulae were derived, and a preliminary design methodology was proposed for predicting the lateral load capacity and ductility of single strap-braced stud walls under a range of vertical loading ratios. The efficiency of the proposed method compared to Eurocode 8 design was then demonstrated for a 6-storey CFS frame, which highlighted the importance of considering the effects of vertical loads in the seismic design of these systems. It was demonstrated that ignoring those factors can lead to a brittle lateral response even if all other code requirements are satisfied, while the proposed design methodology was shown to be efficient to reach the target lateral load and ductility capacities.