Three-Dimensional Motions of Unbonded Post-Tensioned Reinforced Concrete Bridge Piers Under Bi-Directional Earthquake Excitation

Abstract

The study of unbonded post-tensioned (PT) bridge piers continues to gain momentum, as they can produce self-centering lateral force behavior with limited structural damage; however, little attention has been given to the dynamic responses of these controlled rocking systems. This paper presents an experimental study to investigate the motion pattern and assess the seismic performance of unbonded post-tensioned reinforced concrete (PRC) rocking bridge piers under uni- and bi-directional earthquake excitation. A conventional PRC pier and two improved versions with end segments enhanced, respectively, by a steel tube and ultra-high performance concrete (UHPC) were investigated through shaking table tests. The experimental outcomes highlighted the potential limitations of the two enhanced strategies in terms of damage-tolerance and rotation-dominance. Tests also revealed that the bi-directional displacement of PRC piers can be estimated from the individual uniaxial responses using the square root of the sum of the squares (SRSS) rule. In contrast, the force responses are overestimated by SRSS. Moreover, it has been observed that PRC piers are characterized by a three-dimensional (3D) wobbling motion with a contact region around the circular base instead of the sudden point impact during in-plane rocking. An analytical model is presented for the 3D rigid motion of PRC piers subjected to bi-directional earthquakes, which accounts for variations in the contact region at the pier-to-footing interface and relevant energy loss. The model's predicted responses aligned closely with the rotation-induced results derived from experimental data, provided the interface's contact properties were properly calibrated. The findings and results provide significant insights into the seismic response of PRC rocking piers, as well as further refinement of damage-tolerant solutions.