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.