Many very large bridges with high piers and long spans are under rapid construction in mountainous regions especially in Western China. However, the current seismic design methods in China are based on a code which only applies to bridges with span up to 150 m. To evaluate the risk of the inapplicable design method and the influence of spatially variable ground motions (SVGM) on the seismic response of very large bridges, a high-pier, long-span, continuous RC frame bridge is numerically studied. This study considers whether multiple support excitation can be simplified into specific uniform excitation cases while guaranteeing the conservative seismic demands for this bridge. Non-stationary SVGM on both bedrock and the surface of multiple soil layers are simulated including wave passage effects, coherency effects and site amplification effects. The nonlinear dynamic finite element model of the bridge is analysed for two groups of earthquake motions, namely group 1 bedrock and group 2 - ground surface excitations. Each group contains three different excitations, i.e. i) multiple support excitation ii) the largest and iii) the smallest accelerations from the SVGM. The relative displacements, internal force responses and ultimate damage modes are obtained and compared. For this bridge the uniform ground motion input with the largest accelerations provides conservative seismic demands for most structural components when the site amplification effect is not considered (group 1). However, for the ground surface motions, where site amplification needs to be taken into account (group 2), in several cases the uniform ground motion with the largest accelerations results in lower response than that predicted when considering SVGM. The present results indicate that only when the bridges are located on ideal simple topography where site effects have little influence, the uniform excitation with the largest accelerations taken from the SVGM may be an alternative input for seismic analysis. However, for bridges on complex terrain, where site effects can significantly amplify the ground motions at the bedrock, SVGM need to be applied as input for the seismic analysis. As spatial variability of input motion is not a mandatory requirement in the Chinese bridges design code, these results suggest that the existing design code for very large bridges should be modified accordingly.
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