結構系統的動態特性可藉由其自然頻率、阻尼比及模態振形加以描述，而一般模態參數識別法須同時利用激勵及響應資料來識別模態參數。本文考慮當線性結構系統在環境振動下，假設激勵信號為非定常過程，以時間序列模型模擬，並轉換至離散狀態空間模式以得到狀態系統矩陣，透過此系統矩陣求得結構系統之自然頻率、阻尼比與模態振形。許多工程結構在環境振動作用下，僅能獲得其響應資料，將響應訊號經隨機遞減法處理後，隨機遞減訊號與脈衝響應或自由振動衰減響應有相同之數學形式，進而利用亞伯拉罕時域模態參數識別法計算出結構系統之自然頻率、阻尼比與模態振形。最後經由數值模擬，比較時間序列分析法與Ibrahim時域模態參數識別法識別結果，並且驗證本文所探討的方法之可行性。

　Dynamical systems can be characterized by their modal parameters, which include natural frequencies, damping ratios and mode shapes. Identification of system characteristics is usually accomplished using both input and output data from the structural system. It can be shown that the non-stationary input signals modeled as an equivalent state-space model derived from measured input and output data is transformed from the time series model. The structural modal parameters can be estimated accurately using the equivalent state-space model form the time series models. In addition, the natural frequencies, damping ratios and mode shapes of structural system can be evaluated directly by the state system matrix. In many cases, however, only output measurements are possible for structures under ambient conditions. To used the Random Decrement Technique which auto- and cross-random decrement vibration signatures of the response of a linear structure are in the same mathematical form as free vibration of the structure. The objective of this thesis is to develop a modal parameter identification method by using time domain identification techniques with the response randomdec signatures treated as free vibration data. By means of being compared Time Series Method with Ibrahim Time Domain method, we confirm the applicability of the modal parameter identification method proposed through numerical simulation.

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