Quasi-static tests and seismic fragility analysis of RC bridge piers with high-strength steel bars and high-strength/ultra-high performance concrete

The use of high-strength steel bars (HSSB), high-strength concrete (HSC) and ultra-high performance concrete (UHPC) in bridge piers can obviously improve bearing capacity and save material consumption. However, the use of high strength materials may detrimentally affect the seismic performance of reinforced concrete (RC) piers. This paper tries to study the seismic performance of RC piers with HSSB and HSC/UHPC by quasi-static tests and seismic fragility analysis. Three RC piers, solid pier with normal strength steel bars (NSB) and normal strength concrete (NSC), solid pier with HSSB and HSC, and hollow pier with HSSB and UHPC, were designed and tested under quasi-static loading. The failure mode, hysteretic performance, stiffness degradation and residual displacement of three specimens were compared and analyzed. A fiber-based beam-column finite element model (FEM) was developed to simulate the nonlinear response of RC piers with HSSB and HSC/UHPC. On this basis, the seismic fragility analysis was performed to study the effects of high-strength materials on the seismic performance of RC piers. Research results show that, compared with NSC piers, HSC piers reinforced with a small amount of HSSB still exhibited slightly larger lateral bearing capacity but lower deformability due to the higher brittleness of HSC. The UHPC hollow pier presented obviously larger deformation capacity, and more slow decline speed of lateral stiffness than NSC and HSC solid piers. Meanwhile, the matching use of UHPC and HSSB in hollow pier can improve the elastic deformation, and then reduce residual displacement of RC piers. The proposed fiber-based finite element model (FEM) can simulate the nonlinear response of RC piers with different high-strength materials with reasonable accuracy. Fragility results demonstrate that, HSC pier presents lower seismic fragility than NSC pier at different damage states except for collapse due to lower ductility of HSC pier. The UHPC hollow pier with HSSB has lower seismic fragility than NSC and HSC solid piers at the same seismic intensity. Overall, the hollow pier with HSSB and UHPC can maintain good seismic performance while greatly saving material consumption.