Capacity and design of cold-formed steel warping-restrained beam-column elements

In current design standards, cold-formed steel (CFS) beam-column elements are generally designed by considering fully warping free behaviour in their supports, which means the benefit of warping-restrained boundary conditions is neglected. In addition, while a non-linear relationship governs the interaction of axial compression and bending, simplified linear expressions are prescribed in design standards, which may lead to unreliable designs. This paper aims to investigate the efficiency of the well-known Direct Strength Method (DSM) as well as the methods proposed by previous researchers for CFS warping-restrained beam-column members. The results of experimentally validated warping-restraint Finite Element (FE) models, considering material nonlinearity and geometric imperfections, are used as a benchmark. A total of 270 CFS elements with various lengths, thicknesses and cross-sectional dimensions are considered under ten different load eccentricity levels. The results are then employed to investigate the effects of warping-restrained boundary conditions as well as code recommended interaction curves on the efficiency of the existing methods to estimate the strength of CFS beam-columns with warping restraint. The results indicate that the estimated capacity of CFS beam-columns is significantly affected (up to 55%) by the warping restraint effects and the errors associated with using the simple linear interaction curve, depending on the element length and thickness. While the influence of warping restraint is generally less than 6%, it is demonstrated that, on average, all existing design methods underestimate the capacity of CFS beam-columns by at least 20%, which highlights the need to develop more accurate design methods for practical design purposes.