Seismic fragility assessment of fire-damaged reinforced concrete frames using probabilistic analysis

An increase in temperature during a fire in concrete members causes physical and chemical changes and reduces compressive strength. Therefore, even if the structure remains stable after the fire, evaluating its seismic behavior is of great importance. Accordingly, this study aims to investigate the seismic fragility of reinforced concrete moment frames that have been damaged by fire and have not been repaired or strengthened, using probabilistic and sensitivity analysis methods. For this purpose, a seven-story reinforced concrete frame was first designed, and its thermo-mechanical properties were modeled in OpenSees software. Three fire scenarios lasting one, two, and three hours were applied to the structure. The heat transfer analysis of the beam and column sections damaged during the fire was carried out using Abaqus software. The incremental dynamic analysis and seismic fragility curves of the structure were obtained and compared using both deterministic and probabilistic approaches, considering uncertainties in material properties, gravity load, seismic load, and geometry. The results of these curves show that the probability that the structure will exceed the limit states (IO, LS, and CP) increases as the duration of thermal loading increases. Specifically, the probability of exceedance from the LS limit state at a spectral acceleration of Sa= 0.9g for the no-fire case and for frames exposed to one-, two-, and three-hour fires is 71%, 86%, 90%, and 97%, respectively. The results of the seismic sensitivity analysis indicate that reinforcement yield stress, concrete strength, and reinforcement cover are more seismically sensitive compared to other random variables.