許多土木結構健康監測與系統識別的研究利用微振量測來進行，不過因為缺乏輸入訊號而只能得到結構的輸出反應，所以必須開發只需利用輸出訊號的特殊系統識別技巧，即現地量測模態分析。然而微振量測所能得到的結構輸出訊號並非自由振動反應，因此目的在從系統隨機振動反應取得自由衰減反應的隨機遞減法。近年針對鋼纜之振態參數識別，提出一個稱為多重隨機遞減法的改良技巧，就是將隨機遞減法一次次重複應用，使集中的外力完全濾除，後續利用快速傅立葉轉換(Fast Fourier Transform)來分析出自然振動頻率、模態振型及模態阻尼比，近年來微振現地量測模態分析多用於房屋結構上，因次本文將套用於現地橋梁(碧潭人行吊橋、新長虹大橋、太魯閣大橋及新豐平大橋)上做使用，實行現地量測前，也會利用簡支梁來進行隨機遞減法及快速傅立葉轉換之分析。;Many studies on health monitoring and system identification of civil structures use ambient vibration measurement. However, due to the lack of input signal, only the output response of the structure can be obtained. It is necessary to develop special system identification techniques that only use the output signal. That is ambient vibration measurement model analysis. The structural output signal obtained by the ambient vibration measurement is not the free vibration response, so the purpose is obtain the random decrease method of the free decay response from the random vibration response of the system. In recent years, an improved technique called the multiple random decreasing method is proposed for the identification of the vibration parameters of the cable. It is to repeat the application of the random decreasing method again and again, so thoroughly filter out concentrated external forces. Subsequent use of Fast Fourier Transform to analyze the natural frequency, mode shape and damping ratio. In recent years, the modal analysis of ambient vibration measurement is used in building structures. Therefore, this article will be applied to the existing bridges. Before performing the on-site measurement, the simply supported beam will also be used for the analysis of the random decreasing method and the Fast Fourier Transform.
