Spatial variation of seismic ground motions for bridge response analysis in complex topographic conditions

Abstract

The spatial variation of seismic ground motions plays a significant role in the safety of lifelines, such as bridges. Of particular importance is the case when the bridge is supported at variable ground conditions, as, e.g., rock outcrop for the abutments and sedimentary material for the piers. Spatial variability models for such sites currently do not exist. In order to circumvent this lack of model availability, it is, generally, assumed that the response spectra of the motions at the bridge supports reflect the appropriate ground types, and that the coherency models developed at uniform sites can also be used at sites with irregular subsurface topography. However, spatial variability models at uniform sites may not correctly reflect the variation in the seismic motions at sites with irregular topography.This study analyzes data recorded at a unique dense array in the Parkway Valley, Wainuiomata, New Zealand, a small alluvial valley surrounded by greywacke outcrops. This dense array consisted of stations on both the sediment basin and the surrounding rock, with station separation distances pertinent for earthquake engineering applications. Correlation patterns in the seismic ground motions depending on the local ground conditions, the station location and the wave types controlling different time windows are discussed. It is noted that the seismic excitations within the valley have a significantly longer duration than the seismic ground motions recorded at the rock stations due to the formation of surface waves. Interestingly, motions at the rock and soil stations indicate some correlation in the data for the shear-wave window. On the other hand, the correlation of the motions in the valley with those at the rock stations during the surface-wave window appears to be similar to the correlation of noise. Furthermore, data recorded at the stations close the edge of the valley are affected by the valley/rock boundary, whereas data recorded in the middle of the valley appear to be more coherent. Significant variability is also observed in the frequency content of the motions depending on their location and the time window analyzed. A significant outcome of the study is that the valley characteristics also affect the power spectral densities of the motions in the surrounding rock. The analysis sets the basis for the investigation and modeling of the spatial coherency at sites with irregular subsurface topography.%%%%M.S., Civil Engineering – Drexel University, 2010