Torsional buckling at the limit state of collapse in braced multi-storey i-section steel column design

Abstract

Following full-scale tests the column design method of the Joint Committee on Fully-Rigid Multi-Storey Steel Frames (1971) was superseded by a new variable-stiffness design method. The controls for torsional-buckling were, however, retained in their original form and their derivation is now published for the first time. Since they were based on limiting stresses, a revision is required to put torsional buckling on the correct basis of vanishing stiffness at collapse. Torsional buckling results in a gradual reduction of permitted slenderness, due to axial load and plastic bending, which affects warping and beam restraint, all of which the designer can determine graphically. Worked examples illustrate how this method can supplement the revised draft code. New light is thrown on controversies concerning the governing equations in torsion.