Incremental Dynamic Analysis of Bridge Column Seismic Vulnerability Considering Soil-Structure Interaction Effects

Abstract

This study evaluates the influence of Soil–Structure Interaction (SSI) on the seismic vulnerability of a Department of Public Works and Highways–derived reinforced-concrete bridge column subjected to near-fault ground motions. Nonlinear fiber-based models were developed for fixed-base, stiff-soil, and soft-soil support conditions. Ten horizontal records from spatially separated stations during the 1995 Kobe earthquake were applied through Incremental Dynamic Analysis using Peak Ground Acceleration as the intensity measure and maximum column drift ratio as the damage measure. Lognormal fragility curves were developed for drift limits of 0.5%, 0.7%, 1.5%, 2.5%, and 5.0%. Foundation flexibility increased the first lateral period from 0.2183 s for the fixed-base model to 0.3486 s and 0.4416 s for the stiff- and soft-soil models, respectively. At the 0.5% drift limit, the median PGA capacities were 0.3435 g, 0.3998 g, and 0.3704 g for the fixed-base, stiff-soil, and soft-soil conditions. At the 5.0% limit, the corresponding medians were 1.6240 g, 1.6745 g, and 1.5125 g. The results show that SSI effects are damage-state-dependent: soft-soil flexibility delayed lower drift-limit exceedance but increased extensive-damage and collapse vulnerability. The findings provide a controlled component-level assessment of foundation-flexibility effects for a Philippine bridge-column prototype. The fragilities remain conditional on the Kobe record set, PGA-based scaling, and linearized foundation springs.