Effect of the spatial variability of ground motions on the seismic response of reinforced concrete highway bridges

The effect of spatially variable ground motions on the seismic response of two reinforced concrete highway bridges, a two-span straight and a three-span skewed bridge, is examined in this study. Finite element models of the two bridges were created for dynamic time history analyses. The linear models were developed using ANSYS and the nonlinear models using DRAIN-3DX and OPENSEES. Three sets of ground motion time histories at different local soil conditions are selected for each bridge. For each set of time histories, three different types of excitations are considered: The first utilizes spatially variable ground motions (the SV case scenario) incorporating the effects of wave passage, loss of coherency and local soil conditions as input motions at the bridges' supports. The time history with the largest peak displacement (the WORST case scenario) and the one with the smallest peak displacement (the BEST case scenario) from the spatially variable ones are selected as identical input ground motions at all bridge supports. Parametric studies are conducted for the initial gap sizes of the nonlinear models of the two bridges. Detailed information about the seismic response of the two bridges is presented. DRAIN-3DX and OPENSEES produced the same trend for the seismic response of the two bridge models; however, some of the resulting response quantities can differ significantly. The damage behavior of the pier columns of each bridge is also investigated. The comparative analysis of the bridge models shows that the uniform ground motion input with the largest peak displacement (the WORST case scenario) cannot always provide conservative seismic demand for the bridge structures, and in many cases, it predicts lower response than that induced by spatially variable motions. The present results indicate that there is still difficulty in establishing a criterion to define identical input motions that would produce a comparable effect on the seismic response of bridge models as that of spatially variable ones. Therefore, spatially variable input motions need to be applied as excitations at the bridge supports for their seismic design and retrofit.