Inelastic lateral-distortional bucking of continuously restrained beam-columns

Overall buckling of steel I-section members is usually in a lateral-torsional mode, in which the Vlasov assumption that the cross-section does not change shape during buckling is assumed. While in many cases this assumption is vindicated, members whose compression flange is restrained only by the stiffness of the web may buckle in a lateral-distortional mode, in which the mode of buckling is usually associated with distortion of the web in the plane of the member cross-section. Lateral-distortional buckling is prevalent in the hogging region of continuous composite beams, in beams with partial end restraint, and in columns-and rafters in industrial portal frame buildings where the cladding combined with purlins or girts forms a diaphragm restraint to the tension flange. This paper presents a study of the inelastic distortional buckling of hot-rolled I-section beam-columns with continuous full translational but elastic torsional restraint applied to the tension flange. The method allows for the inclusion of the well-used 'simplified' distribution of residual stresses in the section. Whereas previous studies have indicated that elastic distortional buckling is important in these restraint cases, the extension in this paper to inelastic distortional buckling also indicates the importance of cross-sectional distortion on predicting the strength of the member to the overall buckling limit state.