INELASTIC BUCKLING OF STEEL BEAMS WITH CENTRAL TORSIONAL RESTRAINTS

It is well known that a central elastic torsional restraint restricts the lateral buckling shape of an elastic I-beam and increases the elas tic flexural-torsional buckling moment However, the effect of torsiona l restraints on inelastic buckling has not been studied, and it is not known whether the limiting stiffness for elastic buckling can be appl ied to beams that buckle inelastically. This paper develops a finite-e lement model for the inelastic nonlinear flexural-torsional analysis o f steel I-beams and uses it to investigate the effects of central elas tic torsional restraints on the inelastic flexural-torsional buckling of steel I-beams. It is found that a central elastic torsional restrai nt increases the inelastic strength of the beam, but that the increase in the inelastic strength decreases as the beam's modified slendernes s decreases, The limiting value of the stiffness of a central elastic torsional restraint at which the inelastic strength of the beam is equ al 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 modifi ed slenderness that buckles inelastically the limiting restraint stiff ness is much smaller than that for a beam that buckles elastically. Fo r slender beams the moment transferred by a central torsional restrain t of the limiting stiffness exceeds the strength design requirement fo r the restraint implied by Standards Australia's AS4100 as a result of excessive central twist rotation.