| Summary: | 碩士 === 國立中央大學 === 土木工程學系 === 107 === In the past, Fourier Transform (FT) was usually used to investigate structural health condition. It transforms signals from time domain functions into frequency domain functions. However, Fourier Transform expands the signals by using pre-determined and time-invariant bases. Therefore, it is only suitable for dealing linear and steady signals. Instantaneous properties cannot be obtained by this method. For analyzing nonlinear and unsteady signals such as earthquake waveforms, better method should be applied.
Hilbert-Huang Transform (HHT) is an effective algorithm to deal with time-frequency domain signals. It possesses two characteristics, posteriori base and adaptive base. Thus, it is suitable for dealing nonlinear and unsteady signals. Hilbert-Huang Transform expands the signals into energy distribution in both time domain and frequency domain, which makes it possible to interpret the properties of structural dynamic signals by introducing the concept of instantaneous frequency and determine the structural safety as well.
A recently developed analytical method called HHT SHM takes Hilbert-Huang Transform as its core, integrating other two numerical steps, time-frequency domain amplification function (T.F.AF) and modal temporal variation curve (MTVC). The method defines modal parameters which quantify the dynamic characteristics with statistical means.
This research utilizes a finite element software, ABAQUS, to establish steel structure models with different damping. Apply earthquake forces on the base of the model and obtain the acceleration responses from various floors. HHT SHM method is adopted for analysis to convert acceleration signals into time-frequency spectrum, and the modal vibration characteristics can be extracted from the spectrum. Finally, compare the analysis results from different models and study the influences of damping ratio on the modal parameters.
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