河川橋梁在台灣相當普遍，因此穩定橋梁結構對交通上具有相當的重要性。當颱風來的時候，橋梁因大水的沖刷而造成基樁裸露，因此橋梁沖刷對橋樑結構的穩定性有相當的影響。然而，由於基礎通常在水面以下特別是在洪水來的時候，基礎之裸露情形通常很難直接的觀測到，為了克服此問題，本論文利用橋梁之自然頻率去探測其基礎之沖刷深度。首先利用有限元素法在電腦上建立土壤-橋梁互制之模型，模擬橋梁在不同的基礎沖刷深度下其自然頻率的變化情形，並且利用現地量測到的頻率跟模型模擬出來的頻率做比較。現地量測到的資料則利用隨機遞減法、亞伯拉罕時域分析法以及快速傅立葉轉換法識別量測到的橋樑自然頻率。最後，對台南市十座河川橋梁進行量測和分析，探討量測到的橋梁自然頻率是否穩定。

River-crossing bridges are considerably common in Taiwan, so the stability of the bridge structure has much important for the traffic. When the Typhoon invaded Taiwan, some bridge foundations were exposed due to the flood scour. Therefore, the bridge scour would have much influence on the stability of the bridge structure. However, the pile exposure cannot be always observed directly, because piles are often under water, especially on the condition of flood. In order to overcome the issue, the thesis uses the bridge natural frequency to estimate the depth of pile exposure. First, the model with soil-bridge interaction can be established using the finite element method on the computer, and then the model is used to simulate the variance of the bridge natural frequency from the different depth of pile exposure. Then, the bridge natural frequency measured from the field experiment is compared with the natural frequency from the numerical simulation. The random decrement method, Ibrahim time domain method, and Fast Fourier Transform (FFT) method are used to identify the bridge natural frequency from the experimental data. Finally, the field experimental data of ten river-crossing bridges near the Tainan city are used to investigate the stability of the bridge natural frequency.

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