Influence of ground motion duration on seismic behavior of RC bridge piers: The role of low-cycle fatigue damage of reinforcing bars

Abstract

This paper assesses the influence of ground motion duration on seismic behavior of reinforced concrete (RC) bridge piers using spectrally equivalent short- and long-duration ground motions. Special attention is paid to the role of low-cycle fatigue damage of reinforcing bars. To this end, an experimentally validated nonlinear fiber-based beam-column element considering buckling and low-cycle fatigue of longitudinal bars is adopted for nonlinear dynamic analyses and fragility modeling. An advanced material level damage index is used to account for both the maximum response (strain damage) and cumulative damage (low-cycle fatigue damage of longitudinal bars). The dynamic analysis results indicate that the strain damage is the major failure mechanism for RC bridge piers under short-duration ground motions. Under long-duration ground motions, by contrast, the strain damage is prominent at the early excitation stage, while the low-cycle fatigue damage may dominate the late excitation stage. In particular, the role of low-cycle fatigue is more prominent for RC bridges against long-duration ground motions with significant durations beyond 50 s and peak ground velocities over 30 cm/s. The fragility analysis demonstrates that, ignoring the low-cycle fatigue damage of longitudinal bars will obviously underestimate the seismic fragility of RC bridge piers under long-duration ground motion, and the influences is more significant at more severe damage states. Therefore, the low-cycle fatigue damage of reinforcing bars is one of the main failure modes of RC bridge piers under long-duration ground motion, and should be considered in the seismic design,