Out-of-plane stability design of stainless steel I-section beam-columns under moment gradients

To address the limitations of existing design specifications for laterally unrestrained stainless steel I-section beam-columns, new design rules are proposed in this study. A comprehensive parametric study was conducted using finite element modelling to investigate the member behaviour and generate benchmark member resistances, covering Class 1–3 cross-sections and considering a range of material grades, member geometries and combined loading conditions. It was observed that the numerically derived compression-bending interaction factor for out-of-plane buckling checks decreases with more pronounced bending moment gradients, indicating their beneficial influence on member resistances. Thus, calibrated against the numerical results, a new formulation of the interaction factor is proposed, covering combined compression and uniform or non-uniform bending and ensuring alignment between member buckling and cross-section resistance checks. The new proposals are shown to provide accurate and consistent member resistance predictions for all load cases, and can be applied with the partial safety factor of 1.1 as specified in EN 1993–1–4:2025. These proposals have been included in the new version of the European structural stainless steel design standard EN 1993–1–4:2025. • Out-of-plane stability and design of stainless steel beam-columns under combined compression and bending are investigated. • A comprehensive parametric study is conducted to generate benchmark member resistances and investigate member behaviour. • Interaction factors for the out-of-plane buckling check in the design of stainless steel beam-columns are proposed. • The new proposals are shown to provide accurate and consistent member resistance predictions for all load cases. • Proposals are suitable for use with the partial safety factor of γ M1 = 1.1 and have been included in EN 1993–1–4:2025.