Behaviour of Unrestrained Steel I-Section Beams in Case of Fire

Steel beam sections have mainly two failure modes: local buckling and lateral-torsional buckling, and interaction between these modes affects the critical load capacity of beams. Past case studies on structures such as World Trade Centre Tower 7 and large-scale fire experiments showed the vulnerability of steel buildings under elevated temperatures. In the case of the high-temperature gradient, floor beams suffer the local as well as global buckling collapse. Global failure at floor level leads to a progressive collapse of the steel structure buildings. In case of fire loading web of steel, section undergoes instability at the initial stage of fire loading. It leads to faster degradation of shear capacity and flexural capacity. This paper studies the inelastic lateral-torsional buckling behaviour, and moment capacities of the steel I-section beams. Beams are analysed under varying moment loadings with simply supported boundary conditions at elevated temperatures. A parametric study is presented to investigate the effect of loading cases and elevated temperature on the buckling capacity of the beam members. In this study, a total of 216 linear and nonlinear numerical model analysis are conducted using the finite element method (FEM) analysis with the help of a robust FEM-based ABAQUS software. It is observed from the numerical study that at elevated temperature the buckling capacity changes significantly compared to current design rules in Eurocode 3. It is also seen that the loading patterns have a vital effect on the buckling capacity of the beam.