Coupled Effects of Fire and Cyclic Loading on the Seismic Resilience of Steel Moment Resisting Frames

Steel structures are significant in modern construction due to their high strength, durability and rapid construction. However, one of the primary vulnerabilities of steel is its performance under elevated temperatures. Therefore, degradation of properties can lead to large deformations, instability or even collapse of steel members during post-earthquake fire scenarios. Hence, understanding the thermo-mechanical behaviour of steel and evaluating the structural response under sequential earthquake and fire loading is critical for ensuring structural resilience. Both experimental and numerical approaches can be used to investigate structural behaviour. However, numerical analysis is often preferred due to the high cost and time consumption of experimental studies. A numerical model is developed in this study to assess the seismic performance of a 4-bay, 2-story hollow steel moment-resisting frame under the combined effects of fire and cyclic loading. A coupled Computational Fluid Dynamics (CFD) and Finite Element Method (FEM) simulation was used to analyse the fire development, thermal response and structural behaviour. Non-uniform fire scenarios were generated using PyroSim which is a graphical interface for the Fire Dynamics Simulator (FDS), while ABAQUS was employed for finite element analysis. Thermal data obtained from CFD simulations were mapped to the FE model using the FDS2FEM tool through the NSET-BNDF method enabling unidirectional coupling. Subsequent thermal and structural analyses were sequentially performed, with structural performance evaluated under lateral cyclic loading to represent seismic effects. The accuracy of the numerical model was verified through validation, and the parametric study was performed to assess the impacts of fire location and beam flexural stiffness on the seismic performance. The results indicated that fires located at columns adjacent to the cyclic loading point caused the most significant reductions in strength and ductility, with reductions of 9.8% and 15.09%, respectively. In contrast, fires directly at the cyclic loading point led to the smallest improvement in energy absorption capacity, measured at 10.01%. Additionally, increasing the flexural stiffness of the middle beam enhanced both strength and energy absorption capacity, while resulting in a slight reduction in ductility.