Elastic critical buckling of t-sections subjected to axial or end moment loading

Abstract

It is common practice in the industry to use universal column and beam sections for the primary structural members when designing a steel frame structure. However, flanged cruciform sections (FCS) offer various advantages over other hot-rolled column sections, especially when utilised in moment-resisting frames. Research on FCS buckling behaviour, particularly for columns under end-moment loading, is limited, and codes such as EN1993-1-1 (2005) provide no specific design guidance. This study investigates the elastic critical buckling behaviour of the T-section undergoing compression in an FCS by introducing longitudinal web restraints to the free edge of the T-section, to simulate the web edge restraints in the FCS section. A parametric study for the elastic critical eigenvalue buckling stress was conducted using LUSAS (2024) for two different load combinations, an axial compression stress and a compression stress distribution for uniform sagging end moment bending, as well as five different web restraining support conditions. Four different cross-sections with a constant flange slenderness ratio (cf/tf) and varying the web slenderness ratios (cw/tw) in the range of 35.5 – 361.4 were analysed. The member lengths for each model were varied from 1m to 8m. A convergence study was carried out to determine the accuracy of the chosen finite element mesh, and a comparison between theoretical and FEA elastic critical buckling stress was conducted. A good correlation was observed between the theoretical and FEA predictions. The FEA results showed that the torsional mode of buckling is the critical mode of buckling. For shorter lengths, the distortional buckling mode was observed, and this mode of buckling tended to converge to the torsional mode in the models with a higher web slenderness ratio. The distortional mode of buckling also indicated that it is more sensitive to the bending stiffness of the web than the torsional mode of buckling. Ultimately, this study provides data for the future development of design equations that may be able to predict the elastic critical buckling load for members in compression and subjected to uniform sagging moment, thus facilitating a method for the design of FCS as well as any other compound section that consists of radiating T-sections joined together at the free end of the web.