Simplified modeling of bridge response on soft soil to nonuniform seismic excitation

Simple modeling is developed for the response of bridges on soft soil to spatially nonuniform seismic excitation. The bridges are modeled as single-and multispan elastic frames founded on a homogeneous soil layer over rigid rock. Three different excitation types are considered: (1) general harmonic excitation with different amplitude and phase at each support; (2) oblique harmonic SH waves traveling within the soil layer ("wave-passage effect"); and (3) vertically propagating SH waves in conjunction with different soil conditions at the supports ("site-response effect"). Response of a single-span symmetric bridge is obtained by decomposing the excitation into symmetric and antisymmetric components. The analysis is then extended to a multispan integral bridge subjected to translational and rotational input. Harmonic and transient responses are considered and analytical expressions for various response parameters are obtained. Soil-structure interaction (SSI) effects are studied by considering long, drilled shaft foundations. To compare the effects of uniform and nonuniform input, pertinent response ratios are introduced. It is shown that when the amplitude of ground motion varies among the supports, defining a reference uniform excitation is not straightforward.