Development of fragility curves for concrete three-pier bridges under near- and far-fault earthquakes-Case study

This study investigates the seismic vulnerability of a case of three-column reinforced concrete bridge piers subjected to near-fault (NF) and far-fault (FF) ground motions by developing comprehensive fragility curves under varying axial load ratios. Incremental Dynamic Analyses (IDA) were conducted on a representative bridge pier model considering two axial load levels (Load Factor LF = 0.05 and LF = 0.2) to assess structural responses across different seismic intensity levels. Ground motion records were classified based on their proximity to the fault, with distinct spectral characteristics used to capture the influence of pulse-like near-fault effects versus more broadband far-fault excitations.
Five damage limit states were defined for the bridge pier, including Buckling of Longitudinal Reinforcement (BL), Compression Failure of Unconfined Concrete (CF-U-NC), Compression Failure of Confined Concrete (CF-C-NC), Fracture of Longitudinal Reinforcement (FL), and Low-Cycle Fatigue (LCF) of longitudinal reinforcement. IDA curves were generated, and for each limit state, seismic fragility functions representing the probability of exceeding each damage state under increasing spectral acceleration levels were generated.
The results indicate that under lower axial load (LF = 0.05), the bridge pier exhibits greater spectral acceleration thresholds for damage initiation when subjected to far-fault ground motions compared to near-fault events. On average, spectral acceleration demands in FF records were 17% to 37% higher than those in NF records for various damage states. Additionally, median drift ratios were generally lower under FF motions, except in the case of FL, where the NF drift was slightly lower.
For higher axial load conditions (LF = 0.2), the differences between NF and FF scenarios became more pronounced. The spectral acceleration required to reach certain damage states in FF motions was up to 31% higher compared to NF motions. Drift values also reflected this trend, with NF ground motions generally producing higher deformation demands, particularly in brittle and fatigue-prone failure modes. This shows that under increased axial loads, the vulnerability of piers to NF excitations escalates significantly.
The fragility curves further confirmed this trend. At a spectral acceleration level of 0.5g and LF = 0.05, the probability of exceeding the BL state under FF ground motions was 43.5% lower than under NF motions. In more critical damage states such as CF-U-NC and LCF, this reduction reached up to 65.2% and 58.3%, respectively. At higher spectral acceleration levels (1.0g to 2.0g), this pattern of reduced exceedance probability under FF ground motions remained consistent across all damage states. Similar trends were observed at LF = 0.2, where the influence of NF ground motions in increasing fragility was more significant, especially for brittle and fatigue-related failure modes.