Post-earthquake fire analysis of a low-rise steel frame in OpenSees using a hybrid model

Given the significant structural damage and increased potential for loss of life that can occur during a post-earthquake fire (PEF), it is essential to adopt a multi-hazard analysis approach for buildings rather than evaluating each hazard independently. This study examines the PEF response of steel wide-flange sections by integrating strength and stiffness deterioration into a distributed plasticity model in OpenSees. Although the distributed plasticity model has been extended to include temperature-dependent properties, it lacks the capability to account for strength and stiffness deterioration due to local buckling. To address this limitation, this study incorporates a low-cycle fatigue model into the PEF analysis to account for deterioration during an earthquake, which serves as the initial condition for the fire. The low-cycle fatigue model parameters are based on previous research and tuned with the well-known modified Ibarra-Medina-Krawinkler model. Evaluations confirm that the distributed plasticity model incorporating low-cycle fatigue material effectively captures deterioration in the analysis. As a case study, the behavior of a 4-story steel moment-resisting frame is evaluated under seven ground motions and varying PEF intensities. A hybrid modeling approach, combining concentrated and distributed plasticity, is adopted to efficiently simulate PEF effects. The interstory drift ratio, rotation, and bending moment capacity are assessed against their respective limits to determine if they exceed collapse prevention performance level values. The results indicate that earthquake-induced damage reduces the fire resistance of this particular building, with beams being found to be more vulnerable than columns, which can be attributed to seismic design requirements. Collectively, this study contributes to the development of a comprehensive PEF analysis framework by integrating low-cycle fatigue as a critical component.