Site effects on the seismic response of a suspension bridge

The free-field surface motions in a hypothetical sedimentary valley are calculated and applied to the supports of the Humber suspension bridge in the UK. The purpose is to analyse the dynamic response of the bridge to spatially varying.longitudinal excitations at its supports. The support motions differ in amplitude and frequency due to variations in soil patterns of amplification at different valley sites. Two soil categories with soft and stiff profiles are used. Five cases of excitation are considered by moving the bridge supports to different locations in the valley. The SAP IV computer program is modified to perform a multiple-support dynamic analysis on the bridge using mode superposition. It is concluded that the inclusion of the multiple-support excitation case in the analysis is significant, when soil conditions and topography cause noticeable variations in the intensity of support motions in the period range of interest for the bridge response. Soil type, site topography and the location of the bridge supports in a valley must be considered for more realistic evaluation of the ground motions affecting the bridge.