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The bridges in Taiwan are used to crossing riverbeds or valleys with different geologic conditions. However, their influences were overlooked when traditional analyses were performed. Under such conditions, the bottoms of the piers were set up to be fixed and thereafter behaviors of pile caps, piles, and foundation soils located underneath the piers cannot be studied. To investigate the effects of different geologic conditions on the results from modal analyses of a bridge, the authors of this paper adopted a complete model which includes all of the parts mentioned above. For the purpose of comparison, foundation soils of loose, median dense, dense conditions were selected for a cable-stayed bridge. Then, modal analyses were performed by adopting the finite element program SAP2000. Results from this paper indicate that the variations of the modal participation mass ratio versus modal number, the modal number for the largest modal participation mass ratio, the period corresponding to the largest modal participation mass ratio are quite different when foundation soils are changed from loose to dense.
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