SITE EFFECTS ON THE SEISMIC RESPONSE OF A SUSPENSION BRIDGE

The free-field surface motions in a hypothetical sedimentary valley ar e calculated and applied to the supports of the Humber suspension brid ge in the UK. The purpose is to analyse the dynamic response of the br idge to spatially varying longitudinal excitations at its supports. Th e 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 excita tion are considered by moving the bridge supports to different locatio ns in the valley. The SAP IV computer program is modified to perform a multiple-support dynamic analysis on the bridge using mode superposit ion. It is concluded that the inclusion of the multiple-support excita tion case in the analysis is significant, when soil conditions and top ography cause noticeable variations in the intensity of support motion s in the period range of interest for the bridge response. Soil type, site topography and the location of the bridge supports in a valley mu st be considered for more realistic evaluation of the ground motions a ffecting the bridge.