以結構振動訊號診斷橋梁受沖刷之安全性研究

Implementation of a Vibration-Based Bridge Health Monitoring System on Scour Issue

蔡鎮安(Chen-An Tsai)

指導教授 ： 張國鎮

國立臺灣大學/工學院/土木工程學研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.00648

摘要

台灣地形破碎，跨河橋梁眾多，加上地處熱帶、亞熱帶氣候交界，受經常性颱風侵襲，降雨強度大，且本島地理山高谷深，河川坡陡流急，因此跨河橋梁基礎普遍存在沖刷問題。無論國內外，橋梁損傷主因之一皆指向基礎沖刷，而橋梁基礎埋置深度是影響橋梁結構穩定性之重要指標，是目前計算基礎安全性不可或缺的參數，故發展出一套可即時掌握橋梁基礎剩餘埋置深度之方法，進而可準確的推算橋梁安全係數，供相關橋梁權責管理單位進行災前應變乃為當務之急。
本研究提出以橋梁微振動訊號作為識別橋梁埋置深度之方法，首先透過靜力試驗簡化沖刷問題，得知埋置深度是掌控結構主要頻率之主控因子，且其關係可以二次曲線表示，並以有限元素模型驗證實驗趨勢，而後以短時傅立葉轉換作為即時結構頻率擷取工具，再以埋置深度與頻率之關係式將頻率轉換成埋置深度，結合環境因子資料收集，輔以碎形理論作為結構穩定性判別，最後以埋置深度計算安全係數，以碎形理論計算安全指標，判定橋梁安全性。
為證實本研究方法之可行性與精確性，於實驗室進行縮尺橋梁沖刷試驗，藉由縮尺橋墩內之攝影機紀錄即時沖刷深度，驗證以微振訊號推算出之剩餘埋置深度。經一系列試驗結果可證實此研究方法可有效預測埋置深度之變化，環境因子之監測有助於釐清因沖刷以外造成橋梁頻率變化之因素。此方法對橋梁結構健康診斷有相當助益，可在災害發生前提出預警，減少生命財產之損失。

關鍵字

橋梁沖刷 ； 結構健康診斷 ； 短時傅立葉轉換 ； 碎形理論 ； 結構穩定性

並列摘要

Bridges are prone to suffer from multiple hazards such as earthquake, wind, or floods for the special structural characteristic. To guarantee the stability of bridge structure, how to precisely evaluate the scour depth of bridge foundation has become an important issue recently as most of the unexpected damage or collapse of bridges are caused by hydraulic issue. In this paper, a vibration-based bridge health monitoring system considering the response of superstructure only is proposed to rapidly evaluate the embedded depth of bridge pier. To clarify the complex fluid-solid coupling phenomenon, the effect of embedded depth and water level was first verified through a series of static experiment. A finite element model with confinement simulated by soil spring was then established to illustrate the relationship between the fundamental frequency and the embedded depth. With the proposed algorithm, the health condition of the bridge can be inferred by processing the ambient vibration response of the superstructure. To implement the proposed algorithm, a SHM prototype system monitoring the environmental factors such as temperature, water level, and inclination was developed to support on-line processing. The performance of the proposed system was verified by a series of dynamic bridge scour experiment conducted in laboratory flume and compared with the reading from water-proof camera. The result has shown that by using the proposed vibration-based bridge health monitoring system, the embedded depth of bridge pier during complex scour process can be reliably reflected.

並列關鍵字

Bridge scour ； Structure health monitoring ； Short-time Fourier Transform ； Fractal theory ； Structure stability

參考文獻

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