| Summary: | 碩士 === 國立成功大學 === 土木工程學系碩博士班 === 100 === Most bridges in Taiwan are built across the river, so the stability of the bridge foundation is an important issue. As the bridge is burst, it may cause the damage for the human life, the interruption of the traffic, and the economic loss. When the typhoon invades, the bridge foundation may be scoured and cause the bridge collapse. Therefore, the bridge foundation scour depth can be applied to judge whether the bridge are in danger or not. However, it is difficult to measure the bridge foundation scour depth directly because most bridge foundations in Taiwan are under water, especially when the floods come. To overcome the problem, this thesis provides the bridge natural frequency to estimate the foundation scour depth. In the beginning, the bridge models for the various scour depths with the different soil strengths and the different style of the pile foundations are created, and the finite element method will be applied to determine the bridge natural frequency with the soil-bridge interaction problem. Then, the relationship between the bridge natural frequency and the foundation scour depth can be obtained to estimate the pile exposure level. Moreover, a general Gaussian function is applied to fit the curve of the frequency data. The formula of the curve fitting obtained in this study is acceptable to find the relationship between the bridge natural frequency and the foundation scour depth. A field experiment was performed in this thesis. By measuring the frequencies of the Sih-tsau Bridge, the random decrement method, Fast Fourier Transform method, and the Ibrahim time domain method are applied to identify the true natural frequency of the bridge.
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