SEISMIC PERFORMANCE ASSESSMENT OF A BASE ISOLATED REINFORCED CONCRETE BRIDGE

Abstract

Bridges in seismically active regions require rigorous assessment to ensure resilience. This paper presents a comprehensive structural evaluation of a 1.26-km‑long base‑isolated reinforced concrete bridge in the Philippines, designed to accommodate increasing traffic demands while maintaining integrity under seismic loading. A three-dimensional global finite-element model was developed, incorporating nonlinear high-damping rubber bearings with lead cores (HDR-LRB). Nonlinear time-history analyses (NLTHA) were performed for Level I (100-year) and Level II (1,000-year) earthquakes. Results demonstrate that the isolation system successfully elongates the fundamental period from 0.68s to 1.36s, effectively mitigating substructure force demands. Under Level II events, maximum isolator shear strains (γeq) remained below the 200% allowable linear limit. Total combined shear strain (γt), accounting for compression and rotation effects, reached a maximum of 237.7%, well within the 550% breaking elongation limit. Evaluation of pier response using the Caltrans displacement ductility approach yielded demands of 0.91 (longitudinal) and 0.38 (transverse) relative to idealized yield points of 56 mm and 45 mm, respectively. These values indicate that the substructure remains essentially elastic under design-level events, providing significant safety margins against the Caltrans allowable displacement ductility limit (μD ≤ 4.0). The study demonstrates that systematic performance‑based evaluation ensures code compliance and achieves desired displacement‑based performance objectives for critical bridges in moderate‑to‑high seismic environments.