Performance of RC beam-column assemblies during and after elevated temperature to resist progressive collapse

In practice, fire is one of the accidental loads that can result in the progressive collapse of buildings but the commonly used alternate load path (ALP) approach often ignores the impact of fire. Therefore, numerical models based on the finite element software ABAQUS are employed to investigate the performance of reinforced con-crete (RC) beam-column assemblies during and after high temperature to resist progressive collapse. The ther-mal-mechanical sequential coupling method is used for analysis. The effects of fire durations, boundary conditions (middle or penultimate column removal) and side column subject to fire are studied. Previous test results demonstrated that compressive arch action (CAA) and catenary action (CA) are developed in sequence to prevent progressive collapse for the assemblies at ambient temperature. However, the numerical results show that the in-fire assemblies could not develop CA owing to severe degradation of rebar elongation and thus, the load resistance of in-fire assemblies controlled by CAA capacity. Further increasing fire duration to 90 mins, the CAA capacity of the assemblies decreases over 70% compared with that at ambient temperature. Since the rebar can recover most of its mechanical properties after cooling down, the assembly after fire (i.e., exposed to fire then cooled down) can also successively develop flexural action (FA), compressive arch action (CAA) and catenary action (CA) to resist progressive collapse, which is similar to that of intact RC frame at ambient temperature. Under the penultimate column removal scenario, the exterior side column of the post-fire specimen suffers large eccentric compression failure, which causes the highest collapse risk.