Effects of Vertical Ground Motions on Axial–Shear Coupled Responses of Bridge Piers Considering Vertical Pulses

Abstract This study investigates the seismic performance of RC bridge piers subjected to combined horizontal and vertical ground motions. Three categories of ground motions were selected from the NGA-West2 database: (1) both horizontal and vertical components contain velocity pulses; (2) horizontal pulse and vertical nonpulse motions; and (3) both components are nonpulse motions. A detailed finite-element model of a multispan continuous bridge with double-column and single-column piers was developed in OpenSees (version 3.3.0), using fiber-based beam-column elements and shear springs governed by the Pinching Limit State Material model to simulate the axial–shear interaction and degradation of the piers. Representative ground motion pairs from each group were analyzed to evaluate the axial-force and lateral-displacement responses. Two quantitative parameters, the axial-force amplification factor and the axial–lateral frequency ratio, were defined to assess the influence of vertical ground motions from both amplitude and frequency perspectives. The results show that vertical peak ground acceleration significantly amplifies the axial-force response of bridge piers, particularly under vertical pulse-like excitations, and that the frequency ratio between axial force and lateral displacement is higher when both horizontal and vertical components are considered. These findings highlight that neglecting vertical ground motions in seismic design and performance evaluation may lead to underestimation of structural seismic demand.