Influence of stiffener configurations on the post-fire flexural capacity of mild steel based cold-formed steel beams

Purpose Because of its many benefits over conventional steel kinds, cold-formed steel (CFS) members – which are reputed by their thin profiles – are becoming more common in the building industry. The performance of CFS composed of mild steel (MS) under very high temperatures, especially about its flexural behavior, has, however, received comparatively little attention. Design/methodology/approach The flexural behavior of CFS beams made of MS under high temperature and after being cooled down to room temperature with both air and water is carefully examined in this work. A detailed analysis is conducted of the effects of temperature loading and the efficiency of the two cooling methods. The direct strength method (DSM) and ABAQUS finite element (FE) modeling findings are compared and evaluated with the experimental data. Additionally, a parametric analysis is conducted on the beams to assess the effects of not providing a stiffener versus using a horizontal stiffener. Findings Among the experimental sections, the maximum load was observed for the reference specimen at 90.26 kN. The specimen that was heated for 90 min and cooled with water had the least reported load, which was 35.5 kN. For the beam that was heated for 60 min then cooled with water, there is a noticeable drop in load. The load capacity of the sections that were heated for 60 min and cooled with air and water differed by 41.9%. Interestingly, specimens that were heated for 90 min showed a notable reduction in stiffness. Sections that have been heated for 90 min exhibit an abrupt reduction in stiffness. Additionally, the stiffness of sections heated for 60 and 90 min and cooled with air differs by 100%. The mode of failure for beam sections with vertical and horizontal stiffeners was distortional buckling (DB), with some instances of local buckling observed in members with vertical stiffeners. Conversely, beam sections without any stiffener experienced lateral torsional buckling as the dominant failure mode. Compared to the reference section, the loads obtained through parametric analysis were lower by 45–85% for horizontally stiffened beams and by 68.5–123.5% for beams without stiffeners. The maximum load reduction of nearly 60% was recorded for beams exposed to 90 min of heating followed by water cooling. These quantified effects provide clear insights into the role of heating duration, cooling method and stiffener configuration on post-fire flexural behavior. Originality/value This study provides a comprehensive evaluation of the post-fire flexural behavior of CFS beams made of MS, including the effects of both air- and water-cooling methods. Unlike traditional fire performance studies, this work examines residual strength characteristics after cooling, reflecting real-world post-fire assessment needs. The paper also contributes new insights into the influence of stiffener configuration on failure modes under elevated temperature exposure.