Modeling Structural Degradation of RC Bridge Columns Subjected to Earthquakes and Their Fragility Estimates

Abstract

Current seismic design of bridges is based on “one-time” performance of the bridges during the design seismic event. However, there might be a considerable probability of observing more than one damaging earthquakes in a bridge’s service life. Bridge components are known to accumulate seismic damage and deterioration in their structural properties. In such a scenario, design criteria that account for the cumulative seismic damage of bridges over time and performance objectives that span more than one seismic event are needed. This paper computes the probability of occurrence of more than one damaging earthquake during a bridge’s service life. Furthermore, it investigates the importance of accounting for cumulative seismic damage in seismic design. This paper develops a probabilistic model to compute the degraded deformation capacity of flexural reinforced concrete (RC) bridge columns as a function of cumulative low-cycle fatigue damage incurred in the past earthquakes. The model is developed for flexural RC columns because low-cycle fatigue is most significant for such columns. The proposed model accounts for the degradation in the ultimate curvature capacity and deformation capacity of RC columns associated to low-cycle fatigue. The proposed model is calibrated by using data from cyclic-load analyses of RC columns performed by using the finite element method (FEM). The FEM model accounts for the cracking and pinching of RC sections to properly simulate the degradation process. Finally, the proposed model is used to assess the fragilities of RC bridge columns conditioning on the values of low-cycle fatigue damage and deformation demand. It is found that in seismically active regions there is a significant probability of observing more than one damaging earthquake in a bridge’s service life. Therefore, accounting for the cumulative seismic damage in the seismic design is important. It is also found that the cumulative seismic damage can significantly affect the reliability of bridge columns. The developed models are useful for a seismic design that properly accounts for long-term safety and reliability of bridges.