Assessment of the seismic behavior of short-to-medium overall length girder bridges under asynchronous ground motions

Abstract

Proper seismic analysis and design of bridges are critical, especially in locations prone to high seismic activities where critical infrastructure is at a high risk of seismic damage leading to direct and indirect losses. The seismic performance of bridges has been widely investigated in the literature, but the effect of asynchronous ground motions for short-to-medium overall length girder bridges is a phenomenon that has not been adequately addressed by bridge codes and thus it warrants further research. For such bridges, the often-employed envelope response spectrum is not generally applicable for asynchronous ground motions as this is a multiple-support excitation problem where potential local demand concentrations are critical. This study investigates the effects of asynchronous ground motion on the seismic response of short-to-medium overall length girder bridges with reinforced concrete columns considering crustal, subcrustal and subduction earthquakes. The main source of spatial variability of ground motions considered was the variation in the local soil conditions at the foundation of 3-span (short) and 7-span (medium) overall length prototype girder bridges. Soil classes A (rocky soil) and E (softer soil) were considered to establish different combinations of soil distribution in the foundation and then compared to baseline models where site class C was applied to all the supports of the structure to study the effects of asynchronous ground motions. It was found that the variation of site class in the foundations for such structures could produce detrimental effects on the dynamic response of the structure. The presence of softer soil in most of the structure's foundations elongated the vibration period of the structure and resulted in higher displacement demands. Results also showed that critical demands are concentrated at locations where the soil conditions change, indicating increased sensitivity to seismic effects at certain locations, which would not be captured by the typical code-prescribed procedure in the presence of asynchronous ground motion effects.