Implications of bidirectional interaction on nonlinear seismic response of steel piers

Abstract

The seismic performance assessment of bridges by nonlinear time history analysis using unidirectional spectrum-matched accelerograms is generally adopted in Japan's specifications. However, the simultaneous action of two horizontal components is known to induce a potentially higher inelastic response in steel piers relative to that under unidirectional excitation. In this background, a rising concern within research communities is the bidirectional effect on the risk of bridge failure or damage under a given seismic intensity during the service life of a bridge. In this study, the difference in the risk of steel pier damage/failure between unidirectional ground motion and bidirectional ground motion is assessed using seismic fragility. Fragility curves are constructed by subjecting a single steel pier with various geometric designs to a large number of earthquake ground motions using the incremental dynamic analysis methodology. The study indicates that the damage state of collapse is not changed by bidirectional excitations, whereas the use of bidirectional excitations in less damaging states reduces the bridge seismic performance and induces higher fragility. The response difference is quantified by introducing an equifragility factor. This factor is calculated as the ratio of intensity of the unidirectional input to the bidirectional input, which causes the same fragility caused by the unidirectional counterpart. The constant characteristic value of the equifragility factor provides useful information for considering the bidirectional effect in seismic design.