Seismic Fragility Curves for Horizontal and Vertical Ground Motion of Bridge Piers with the Macro-Element SSI for Shallow Footings

Abstract

This study investigates the combined effects of vertical and horizontal seismic excitations on bridge piers. It explicitly accounts for Soil–Structure Interaction (SSI) using a macro-element modeling approach. The ssilink1 macro-element simulates the interaction between the pier and the underlying shallow foundation soil. To assess the influence of foundation size on seismic response, a range of variations in the width of rectangular foundations is introduced (2.5 m, 3.5 m, and 4.5 m). A suite of 21 ground motion records, incorporating both vertical and horizontal components, assesses the bridge piers' dynamic performance. The seismic response is analyzed through Incremental Dynamic Analysis (IDA) using Seismostruct software. This approach allows for the generation of IDA curves, which represent the evolution of structural demand with increasing seismic intensity. Based on the maximum drift ratio, fragility curves are subsequently developed. These curves quantify the probability of exceeding predefined damage states. Three distinct performance levels are considered: Immediate Occupancy (IO), Life Safety (LS), and Collapse Prevention (CP). Each level is defined by specific drift limit thresholds. The fragility functions are constructed using Peak Ground Acceleration (PGA) as the intensity measure, assuming a lognormal distribution. This enables a probabilistic assessment of seismic vulnerability across the different foundation widths.