Lateral bracing force of IPE-240 beams at ultimate load

Almost traditionally the design load in an intermediate lateral restraint against lateral torsional buckling of beams has been taken as a percentage of the maximum value of the factored force in the compression flange. National design codes have suggested values of around 2%. This has mainly been backed up by theoretical elastic analyses. Inelastic analysis on braced columns have yielded higher bracing forces. To investigate the influence of inelastic behaviour of beams subject to lateral torsional buckling on the forces in a single lateral brace, geometrical and material non-linear analyses including imperfections (GMNIA) were performed on IPE240 beams with different lengths, support conditions and types of loading. The location of the non-rigid lateral brace is varied along the length of the beam and along the height of the section. The stiffness of the lateral restraints was determined in such a way that the elastic critical moment is 95% of the elastic critical moment with rigid lateral restraints. This will keep the reduction in buckling load to a maximum of 5%. It was found that the forces in the restraints can be larger than design forces suggested by building codes.