Seismic Fragility Assessment of SMA-Actively Confined Bridge Piers for Retrofitting and Precast Construction

Reinforced concrete bridge piers in high seismic zones often experience steel congestion, particularly in new constructions with strict transverse reinforcement requirements in plastic hinge regions. Shape memory alloys (SMAs) have proven effective in retrofitting seismically deficient piers through active confinement, offering unique thermal prestressing capabilities, whereas other strengthening methods in plastic hinge regions have proven problematic. Precast concrete offers a unique advantage of sequentially applying steel and SMA confinement, which reduces steel congestion in plastic hinge regions without increasing the seismic demand. This paper evaluates the seismic vulnerability of precast and retrofitted piers actively confined with shape memory alloy spiral in the plastic hinge regions, explicitly considering the stress-strain response of actively confined concrete. A reference design, as per existing code provisions, is used as a target performance benchmark for the precast and the retrofitted pier designs. Incremental dynamic analysis (IDA) is performed to obtain the fragility curves for the suggested designs, and the required SMA wrapping length is proposed for both the precast and the retrofitted piers. The results indicate that the precast pier with an SMA wrapping length of half the section diameter and a 50% reduction in the transverse steel in the plastic hinge region can preserve the overall performance. Moreover, a longer SMA wrapping length equal to the section diameter is required for the retrofitted piers to achieve a similar performance level.