結構系統的動態特性可藉由其模態參數來描述，包含了自然頻率、阻尼比及模態振形，而一般模態參數識別法通常需同時利用激勵及響應資料來識別模態參數。許多工程結構在環境振動作用下，僅能獲得其響應資料，因此無需激勵信號的量測而直接由響應資料識別模態參數，是為本文重點。本文針對隨機遞減法樣本擷取標準提出改良方法，使得在有限的時域訊號內，可得到更多訊號樣本數；並考慮激勵信號為零均值之非定常過程，其響應信號經改良隨機遞減法處理後與脈衝響應或自由振動衰減響應有相同的數學形式，進而利用Ibrahim時域模態參數識別法進行模態參數的識別。由數值模擬結果顯示，在非定常環境振動情況下，本文所提出之分析方法可得良好的模態參數識別結果。

Dynamical features of a structural system can be characterized by its 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. In many cases, only output measurements are available for structures under ambient vibration conditions. How to use the output data to identify the modal parameters is the key point of this thesis. The objective of this thesis is to modify the sampling method of the random decrement technique in order to obtain more samples from limited time-domain data. If the input can be modeled as a zero-mean non-stationary process, it is shown that the modified random decrement vibration signatures of the response of a linear structure are in the same mathematical form as free vibration of the structure. Furthermore, the Ibrahim time-domain method is employed as the modal identification scheme to extract modal parameters from vibration data. Through numerical simulation, the effectiveness of the proposed method of modal parameter identification from non-stationary ambient vibration data is demonstrated.

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