Optimized interaction equations for more efficient design of CFS channels under combined compression and biaxial bending

This study aims to develop optimized interaction equations for more efficient design of cold-formed steel (CFS) lipped channel beam-columns under combined compressive load and biaxial bending, aligned with the direct strength method (DSM). A comprehensive data set was first generated using detailed experimentally validated finite element (FE) models of over 500 CFS sections subjected to combined compression and uniaxial and biaxial bending moments while the effects of initial geometric imperfections and material nonlinearity were included. The compiled data set consisted of a range of key design variables, including cross-sectional geometry and element length, as well as combinations of compression and bending moments caused by various eccentricity levels in terms of direction and value. The results were subsequently utilized to evaluate the efficiency of the simplified interaction formula prescribed by the Australian/New Zealand Standard (AS/NZS-4600) and American Iron and Steel Institute (AISI-S100), as well as the extended DSM, in predicting the capacity of CFS lipped channel beam-column elements. It was demonstrated that, on average, using existing interaction equations may lead to a 32% error in the capacity predictions of CFS beam-column members. Following a reliability analysis, a new interaction expression was developed with optimized parameters using DSM nominal pure strength values. For the first time, different exponent parameters were proposed for minor- and major-axes bending, which resulted in a considerable improvement in the accuracy of the beam-column strength predictions compared to the existing methods.