Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Civil Engineering and Engineering Mechanics


A.C. Heidebrecht


A. Ghobarah


A study has been conducted on the seismic site response of alluvial valleys with limited width and the effects of spatially varying surface motions in these valleys on the response of a suspension bridge. The intent of this investigation is to extend the work carried out by seismologists on the two-dimensional seismic response of alluvial valleys to include the effects of stiffness variation with depth and non-linear behaviour of soils which were mainly considered by engineers in a one-dimensional perspective. One objective is to provide engineers with useful guidelines to predict the valley response at different sites. Another objective is to analyze the potential of soil conditions and subsurface topographic structure in alluvial valleys for producing significant variations in the bridge response to a multiple-support excitation. A simplified engineering model (frame model) has been developed to predict the non-linear seismic response of symmetrical valleys. The proposed model is a one-dimensional model which accounts for the limited horizontal extent of soil in a two-dimensional valley. The frame model provides valuable insight into the dynamic behaviour of the alluvial valley by identifying the vibration modes of significance and their variation in the horizontal and vertical directions. Response results from the frame model show good agreement with those results from the two-dimensional finite-element model of the valley. Effect of the key parameters governing the spatial variations of motions at different sites in the valley has been analyzed. This includes the effect of soil type, valley geometry and control motion characteristics in rock. It is found that the two-dimensional effect from the valley edges extends toward the centre up to a distance after which only one dimension, the depth, governs the response. The soil amplification varies from one soil type to another, depending on the stiffness of soil and the amount of damping existing during excitation. The magnitude of the surface spectral acceleration at a site depends on the proximity of the local-amplification period at site to the dominant period of the input rock record. Finally, the free-field surface motions in a hypothetical valley are applied to the Humber suspension bridge in U.K. It is concluded that the inclusion of the multiple-support excitation case in the analysis is significant when soil conditions and topography produce noticeable variations in the intensity of support motions in the period range of interest for the bridge response.

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