Seismic Fragility Assessment of Balulang-Macasandig Bridge in Cagayan De Oro City with Consideration of Soil-Structure Interaction

Regions located along the Pacific Ring of Fire, including the Philippines, are particularly susceptible to frequent and potentially damaging seismic events. Historical records have shown that seismic events in the Philippines have caused significant damage to infrastructure, including bridges, which are highly vulnerable to large deformations and displacement-induced failures. This study evaluates the seismic performance of the Balulang-Macasandig bridge in Cagayan de Oro City, through nonlinear static pushover analysis, the ATC-40 Capacity Spectrum Method, and probabilistic fragility assessment, incorporating both fixed-base and soil-structure interaction (SSI) conditions. The inclusion of SSI increased ultimate base shear in the X-direction by approximately 43% and 26%, while reducing it in the Y-direction by about 12.6%. Displacement demand increased with seismic intensity. At about 1g, moderate damage states became common and severe damage states began appearing at about 2g. SSI consistently resulted in higher displacement demands and earlier exceedance of damage thresholds. Fragility curves exhibited a leftward shift under SSI, indicating higher probabilities of damage at lower peak ground accelerations (PGA). Median PGA values in the X-direction ranged from approximately 0.32g for slight damage to 3g for collapse, while the Y-direction only reached moderate damage within the analyzed range of 0.84g-2.21g, reflecting greater transverse resistance. Lognormal dispersion values ranged from 0.85-1.59 and 0.93-1.05 in the X and Y-direction, respectively, indicating consistent variability in structural response. These findings emphasize the need to incorporate soil-structure interaction and site-specific geotechnical characterization in seismic assessment to ensure more accurate and reliable evaluation of bridge performance.