Structural Modal Parameter Identification Under Seismic Excitation And Validation Of Shake Table Test

Earthquake can release huge energy in a short time, which is easy to cause severe damage to structures. Researching the changes of structural parameters under seismic excitation can not only provide guidance to seismic design of structures, but also assess the structure performance after earthquake. Earthquake is a natural excitation. Therefore, using seismic input and structural response under seismic excitation to identify modal parameters is a natural and convenient method. If the seismic excitation and structural response are given, then the modal parameters can be identified directly by the seismic record (Input-output). However, for most structures, it is difficult to get the seismic data whereas the response can be measured easier. In this condition, the identification is based on the seismic response (Output-only). The main purpose of this thesis is to research two kinds of modal parameter identification methods discussed above under seismic excitation. Comparison and validation are made through a shake table test of a bridge model. The research has significant theoretical meaning and application value in engineering.The main work and conclusions are:1. If the seismic excitation can be measured, then the Fourier transform can be used to get the Frequency Response Function in frequency domain and Impulse Response Function in time domain. The modal parameters can be identified through the relationship between the two functions and parameters. Five methods are discussed carefully: Component analysis, Weighted least squares iterative method, Rational fraction polynomial method, ITD and time domain complex exponential method. All algorithms are realized by programming.2. Four methods:Peak-Picking, FDD, PolyMAX and SSI are introduced carefully, which are based on output-only data. The data from numerical simulation validates that even though seismic excitation cannot fulfill the assumption of white noise input, the results of these four methods still have higher accuracy.3. A shake table test of a reduced scale bridge model is conducted. Input-output and Output-only methods are used respectively to identify modal parameters. The results turn out to be consistent, which validates the effectiveness of Output-only methods.4. ITD and time domain complex exponential method increase the system order and cause false modes which need modal assurance criteria (MAC) to exclude them. Even though Peak-Picking is simple and fast, the results of damp ratio and mode shape are not accurate. PolyMAX and SSI both need assume system order to calculate the stabilization diagram, which need more calculation time. In engineering, the final judgment should be based on the results of these methods synthetically.