Factors governing onset of local instabilities in fire exposed steel beams

• A model for evaluating fire resistance of beams, with local buckling, is developed.
• Effect of temperature-induced local buckling on capacity of beams is identified.
• Effect of critical factors on temperature-induced local buckling is quantified.
• Sensitivity of fire exposed beams to web local buckling is demonstrated.

This paper presents critical factors that influence the onset of local buckling in steel beams when exposed to fire conditions. A three-dimensional nonlinear finite element model, capable of accounting for critical factors that influence local instability in fire exposed steel beams is developed. This model is applied to investigate the effect of beam-slab interaction, strength properties (Grade) of steel, and presence of fire insulation on the onset of local instability, and resulting capacity degradation in fire exposed steel beams. Results from numerical simulations are utilized to evaluate failure of beams under different limit states including flexure, shear, sectional instability and deflection. These results infer that web instability can occur at early stages of fire loading, leading to faster degradation of shear capacity and premature failure of steel beams before attaining flexural capacity. Also, results from the analysis indicate that the contribution of concrete slab to shear capacity can counterbalance the adverse effect of web local instability to a certain degree. Overall, neglecting the effect of fire-induced web local instability can lead to unconservative design in steel beams or girder subjected to high shear loading and/or local instabilities.