Seismic Design and Fragility of Self-Centering Bridge Bent With Replaceable Stretch Length Anchors

Seismic performance assessment of a modified two-column bridge bent of an existing bridge is presented. The columns of the existing bridge are posttensioned with high strength steel bars. The modified bridge bent combines self-centering, from unbonded posttensioned (PT) bars with low initial posttensioning stress in the columns, with energy dissipation by replaceable external stretch length anchors (SLAs) attached at the column ends. SLAs are replaceable anchors that dissipate hysteretic energy through tension-only action without buckling or any significant reduction of column self-centering ability. An analysis method for evaluating the nonlinear static pushover of the modified bridge bent is described and validated with experiments. A fiber-based numerical model of the bridge bent is developed in OpenSees. The OpenSees model is used to perform cyclic and time history analyses of the three configurations with PT bars and SLAs and develop fragility functions for the maximum likelihood estimation (MLE) of damage parameters based on the drift ratio. The fragility functions show that the modified Riverdale Bridge bent with SLAs can withstand a spectral acceleration of 3.3 x g for both near-field (NF) and far-field (FF) earthquakes while maintaining excellent self-centering performance. The presence of replaceable SLAs contributes to the redundancy and functionality of the lateral force resisting system. The analysis method and fragility functions for the maximum drift ratio and the maximum residual drift can be used in performance-based seismic design of self-centering bridge bents with SLAs.