Simultaneous influence of spatially varying ground motions and soil support on a rockable bridge with unequal slenderness

Abstract Low-damage seismic design has been considered by allowing the footing to rock to avoid damage to bridges and thus the subsequent downtime when the bridges are unusable. Real-world examples of rockable bridges, e.g. the 315 m long South Rangitikei Viaduct with unequal pier slenderness in New Zealand, have demonstrated the effectiveness of a rockable bridge. It has survived more than ten thousand earthquakes. Although it is not uncommon for a bridge to have unequal pier slenderness, its influence on the dynamics of rockable bridges is still not well understood because in conventional analyses, a fixed base is normally assumed. This study investigated the simultaneous consequence of unequal pier slenderness, soil support and spatially varying excitations by conducting shake table experiments on a two-segment rockable bridge. The response of a fixed base bridge or a rockable bridge on a rigid base under uniform excitation was considered as a reference. Abutment was intentionally ignored to limit the number of influence factors, and the two bridge segments have the same fixed-base fundamental frequency. The insights derived from this study are: (1) The slenderer bridge segment with rocking has a lower dominant response frequency, (2) The propagating waves due to girder pounding interfere with the incoming seismic waves and thus alter the subsequent response, (3) Soil permanent deformation due to footing separation and re-contact alters the subsequent bridge overall behaviour, (4) The simultaneous effect of rocking, pounding, soil support and spatially varying excitation on the bridge response cannot be obtained by a superposition of the response due to individual influence factors. When the spatially varying excitation was not considered, the opening relative displacement and hence the girder unseating potential can be significantly underestimated.