Collapse mechanisms and fragility curves based on Lumped Damage Mechanics for RC frames subjected to earthquakes

Reinforced concrete structures under earthquakes present nonlinear behavior with cracking evolution and plastic strains that can lead to failure due to large displacements with development of collapse mechanisms. This manuscript presents a methodology to apply Lumped Damage Mechanics (LDM) in a Performance-Based Earthquake Engineering (PBEE) framework, to evaluate the seismic vulnerability of reinforced concrete frames. The lumped damage model represents the evolution of damage and plasticity lumped into local inelastic hinges at the nodes of the elements. This paper proposes a novel procedure to identify and characterize global collapse mechanisms from the internal variables of damage using system reliability theory. In this way, the internal variables of damage are used as engineering demand parameters (EDPs) to calculate the fragility curves of the structure, which are also compared with usual interstory drift fragilities. To evaluate the seismic vulnerability, incremental dynamic analyses are conducted. The main results demonstrate efficiency of LDM in a PBEE framework, and that the internal variables of damage are objective indicators of collapse for RC frames. Results show how to identify the global failure mechanisms that are more likely to appear for each frame. For regular frames, high correlation is observed between the local hinges forming each global collapse mechanism. This allows collapse mechanisms to be readily characterized by the strongest link in a parallel system.