Inelastic buckling of steel beams with central torsional restraints

Abstract

It is well known that a central elastic torsional restraint restricts the lateral buckling shape of an elastic I-beam and increases the elastic flexural-torsional buckling moment. However, the effect of torsional restraints on inelastic buckling has not been studied, and it is not known whether the limiting stiffness for elastic buckling can be applied to beams that buckle inelastically. This paper develops a finite-element model for the inelastic nonlinear flexural-torsional analysis of steel I-beams and uses it to investigate the effects of central elastic torsional restraints on the inelastic flexural-torsional buckling of steel I-beams. It is found that a central elastic torsional restraint increases the inelastic strength of the beam, but that the increase in the inelastic strength decreases as the beam's modified slenderness decreases. The limiting value of the stiffness of a central elastic torsional restraint at which the inelastic strength of the beam is equal to that of the corresponding beam with a rigid restraint is related to the modified slenderness of the beam. For a beam with a low modified slenderness that buckles inelastically the limiting restraint stiffness is much smaller than that for a beam that buckles elastically. For slender beams the moment transferred by a central torsional restraint of the limiting stiffness exceeds the strength design requirement for the restraint implied by Standards Australia's AS4100 as a result of excessive central twist rotation.