Alternative approach for lateral torsional buckling of unrestrained beams in fire

Abstract

The chapter discusses an alternative approach for lateral torsional buckling of unrestrained beams in fire. A beam subjected to flexure of a stronger axis may fail below its plastic moment capacity unless it is a short-span beam or it has sufficient lateral restraints. This limit state of structural usefulness where the deformation of a beam changes from predominantly in-plane deflection to a combination of lateral deflection and twisting, while the load capacity remains constant, before dropping off due to large deflections and yielding is called lateral torsional buckling. A laterally unrestrained long-span beam will fail by elastic lateral torsional buckling at a load much lower than its full plastic moment capacity. For a beam of intermediate span, failure may occur by inelastic lateral torsional buckling. The chapter proposes an alternative approach based on the existing BS5950 method at ambient temperature. The equation incorporates the material degradation factor at elevated temperature and stress-strain non-linearity factor. The non-linearity factor is introduced inside the Perry imperfection factor to consider the highly non-linear stress-strain relationship at elevated temperatures. This alternative approach matches finite element analysis (FEA) trend well and gives quite accurate predictions of test results by Vila Real et al. Applying this new approach, the over-conservatism of the EC3 approach for temperature below 500°C can be reduced. There is also continuity with the BS5950 equation at the ambient temperature, thus providing continuity in design between the ambient and the elevated temperatures.