Effect of Foundation Flexibility on Seismic Energy Dissipation in RC Bridges Subjected to Fling-step Ground Motions

Bridges are critical components of transportation networks, particularly in the aftermath of seismic events when their continued operation is essential for emergency response and recovery. The severity of seismic impact on a bridge depends largely on the characteristics of ground motion and the structure’s proximity to the fault. While bridges located in far-field regions may experience moderate effects, those situated in near-fault (NF) zones—typically within 15 km of a fault rupture—are highly susceptible to severe damage due to intense ground motions. NF ground motions (NF-GMs) are distinguished by high-energy velocity pulses, long-period pulse-like waveforms, and high peak values that pose significant demands on structures. Although several studies have investigated the seismic vulnerability of bridges under NF-GMs, most assume fixed-base conditions and neglect the influence of soil flexibility. The role of soil-structure interaction (SSI) in altering the seismic performance of reinforced concrete (RC) bridges under such demands remains inadequately addressed. This study performs a comparative vulnerability assessment of a simply supported multi-span RC bridge subjected to NF-GMs with forward directivity and fling step (FS) effects. Incremental Dynamic Analysis (IDA) is carried out, and fragility curves were developed to quantify the probability of damage across two configurations: a fixed-base model and a flexible-base model incorporating SSI via a pile group foundation. The results indicate that the flexible-base model experiences higher displacement demands but reduced base shear, while the fixed-base model exhibits greater stiffness and force transmission. These findings underscore the need to incorporate SSI in seismic vulnerability assessments of bridges in NF regions.