Moment capacity of back-to-back cold-formed steel channel bolted joints: an experimentally validated design approach

Cold-formed steel (CFS) bolted joints formed with back-to-back channel sections are widely used in portal frame construction due to their efficiency and ease of fabrication. However, the discrete force transfer between individual channels produces non-uniform stress distributions within the joint region, leading to localised stress concentrations, shear lag effects, bimoment-induced stresses due to restrained warping, and premature web buckling. These effects can reduce the moment capacity of typical joints by up to 40% compared with the nominal capacity of the connected members; however, this reduction is not explicitly addressed in current CFS design standards, including AISI S100, AS/NZS 4600, and EN 1993–1–3, which treat member and joint capacities independently. To address this gap, this paper presents a comprehensive experimental and parametric investigation to quantify the capacity reduction in CFS bolted joints and provide a reliable design method for practical applications. A detailed finite element (FE) model was first developed and validated against experimental results from CFS portal frame joint tests. Subsequently, an extensive parametric study comprising over 3400 FE simulations was performed to capture the influence of cross-sectional non-dimensional slenderness, as well as a wide range of geometric, material, and joint parameters. Based on these findings, a simplified design equation is proposed to enable reliable prediction of the reduced moment capacity of back-to-back channel bolted joints. The application of the proposed design equation is then verified against both numerical and experimental results of CFS frames, demonstrating excellent accuracy, with predictions within 1–3% of the actual capacities on average and coefficients of variation ranging from 0.06 to 0.09.