The effective width method (EWM) for calculating the ultimate strength of cold-formed steel (CFS) members assumes simply supported boundary conditions along the plate edges. Unstiffened plates, simply supported along three edges and unsupported along one longitudinal edge buckle in a single half-wave equal to plate length when subjected to uniform compression. It has been observed from the finite element analysis results that the simply supported unstiffened plates with large length to width ratios (L/ b≥ 10) the deformation pattern changes from a single half-wave to a combination of three half-waves corresponding to the ultimate load. One of the possible reasons for this is the nearly same elastic buckling stresses in the first few modes leading to the interaction of higher buckling modes. At larger L/b ratios the interaction of the third buckling mode with fundamental mode was observed causing the reduction in the ultimate strength. A similar reduction in the ultimate strength due to buckling mode interactions are also found in CFS plain equal angle sections, as such sections composed of only unstiffened plates which buckle simultaneously. In sections like plain channels, where additional rotational restraint along the supported longitudinal edge is present, such mode interactions are found to not influence the strength of unstiffened elements in sections. © 2021, The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.