Influence of Welded Joints Modelling on the Structural Response of Steel Frames under Fire Situation

Abstract Steel frames with compact cross‐sections usually fail describing a plastic failure mechanism, redistributing internal forces. In order to fully form a plastic failure mechanism, the designed joints must be able to withstand the increase in stresses resulting from the redistribution of internal forces and to provide enough stiffness to prevent large second order effects. According to current European codes, full‐strength joints can be modelled as perfectly rigid joints and for structures with Class 1 cross‐sections carrying out a plastic analysis is allowed. Both of these statements have been widely studied at room temperature but whether these assumptions are valid under fire situation needs to be assessed yet. With this goal in mind, this paper presents a numerical study comparing the response of steel frames for which the welded joints between members are accurately modelled with the results for frames with perfectly rigid joints, both at room temperature and at elevated temperatures. The obtained results show that modelling full‐strength joints as perfectly rigid joints for numerical purposes can also be done under fire situation without major differences in the structural response. Moreover, all the analysed Class 1 frames could form a fully plastic mechanism under fire situation.