Hysteretic behavior of UHPC hollow piers and seismic fragility analysis of corresponding bridge system

Compared with conventional normal-concrete (NC) piers, ultrahigh-performance concrete (UHPC) hollow piers may offer a lightweight, high-strength alternative. However, their seismic performance has not been systematically investigated. In this study, quasi-static tests were conducted on four hollow pier specimens, including one NC and three UHPC piers, with material type, stirrup ratio, and wall thickness as key parameters. Test results demonstrate that UHPC hollow piers exhibit superior yield and peak strength, ductility, energy dissipation capacity, and initial stiffness. Numerical models were developed in OpenSees to perform parametric analyses, indicating that reduced stirrup ratio mainly degrades ductility, whereas reduced wall thickness primarily decreases stiffness. Bridge-level fragility analyses further suggested that replacing conventional RC piers with UHPC hollow piers can reduce pier damage probability, while potentially increasing seismic demand on other bridge components. Overall, these findings support the seismic application of UHPC hollow piers.