Critical temperature evaluation for laterally unrestrain steel I-beams in case of fire

Fire resistance may be determined by simple calculation models applied to individual members and should be based on safe results. This safety can be verified on three distinct domains, such as, temperature domain, load bearing domain and time domain. The temperature domain was chosen to show how a beam element might achieve the design limit state by lateral instability, for a specified degree of utilisation. A set of uncouple numerical simulations using ANSYS software will be presented, regarding that mechanical deformation does not produce temperature change in the structural material (no explicit creep is considered in the mechanical model). The thermal deformation, due to temperature increase, is responsible for the introduction of mechanical stresses, which means that first is necessary to define the fire and than, for each mechanical load increment, update deformation field, under an iterative procedure (mechanical response model). The numerical model is based on geometric and material nonlinearities. The numerical tests use shell finite elements, with initial out-of-straightness and material behaviour in accordance to the Eurocode 3. The beam element will be tested with two forks supports and with a concentrated mid span dead load Q, with increasing temperature, based on 800 [ºC/h] heating rate. A set of results for different beam lengths will be presented and demonstrate that all critical temperatures are greater than the specified value according to Eurocode 3. For critical temperature validation, a set of experimental full-scale tests will be presented