Framework for fragility assessment of reinforced concrete portal frame subjected to elevated temperature

Despite realizing the importance of risk-based frameworks in fire engineering, current fire safety guidelines are largely prescriptive and over-conservative in nature, which do not consider the effect of uncertainties influencing the fire resistance of the structural systems. In this context, a probabilistic framework is presented to investigate vulnerability of a reinforced concrete (RC) portal frame subjected to fire. Here, a three-dimensional (3-D) nonlinear finite element (FE) model for the RC portal frame exposed to elevated temperature is developed, and transient thermo-mechanical analysis is carried out by duly incorporating temperature variation of thermal and mechanical properties of concrete and steel rebar. Probabilistic study is conducted by developing fragility curves based on limit state of deflection to quantify the vulnerability of the structural members exposed to fire loading. Parametric studies are conducted by varying the material properties of the RC portal frame to investigate the influence of uncertainties in the fire loading on its structural performance and fragility. The failure probability starts at a higher fire load density (450 MJ/m2) for the structure with higher concrete grade as compared to the relatively lower fire load density (200 MJ/m2) for lower grade. Therefore, the fire resistance of the RC structures has been recommended to be based on the strength of the member, rather than merely sizes of the members. The proposed stochastic analysis framework, thereby, provides a rational approach to improve the performance-based fire design of RC structures at member and structure levels by identifying the parameters affecting the structural fire resistance.