Elastic Buckling of Columns in Cold‐Rolled Aluminium Built‐Up Section Portal Frames

Abstract This determination of effective length factors for computing the elastic buckling load of compression members, subjected to flexural, torsional, and flexural‐torsional buckling in cold‐formed section portal frames, is challenging due to the semi‐rigid nature of their connections and the lack of proper guidance on how to calculate these factors. This study addresses these challenges by determining the elastic buckling load (Noc) and the elastic lateral‐torsional buckling moment (Mo) for columns in cold‐rolled aluminium portal frame systems under various buckling modes. The analysis begins with the calculation of effective lengths for buckling about different axes and for twisting, idealizing end restraint conditions and neglecting intermediate torsional restraints. The energy method is subsequently employed to account for the influence of knee braces acting as intermediate discrete elastic torsional restraints, which significantly enhance the column's buckling capacity beyond the predictions from conventional design codes. Ultimately, this research presents a comprehensive framework for assessing buckling capacities in portal frame systems, addressing limitations of traditional methods and proposing improved strategies for incorporating system effects into design practice. The findings aim to guide more accurate assessments of buckling loads and moments in practical engineering applications.