Numerical analysis of non-restrained long-span steel beams at high temperatures due to fire

Steel structures are being increasingly used because of their excellent structural efficiency and flexibility in construction. However, in the case of fire they suffer a great reduction of yield stress and Young’s modulus under the effect of high temperatures. For an economic fire safety design of unrestrained solid steel beams under lateral torsional buckling, it is necessary to include geometrical imperfections and evaluate their fire resistance for the appropriate load level condition. The main objective of this paper is to investigate the mechanical behavior of solid unrestrained steel I-beams under uniform temperature increase when subjected to fire, simulated by the standard ISO 834. ANSYS FE models are produced to study a series of IPE600 beams with initial imperfection effects created from the first eigenmode analysis with the aim of estimating the temperature at which the failure occurs. The numerical results include lateral as well as midspan vertical displacements, under uniformly distributed mechanical load and uniform temperature increase. The analysis also estimates the critical load and critical temperatures in steel beam cross sections. Comparison of results producing critical temperatures and bending resistance in steel sections has been made with the analytical ones from the Eurocode 3 part 1–2 and design guides.