The Application Of Modal Experimental Analysis Methods In The Bridge Structure Based On Ambient Vibration

The security and perdurability of bridge are directly related to people's life and wealth, therefore a predicting and warning and timely maintaining system based on identifying the modal parameters and inspecting damages of the bridge structures is developed which can help to eliminate hidden troubles and avoid the occurrences of catastrophic accidents. Modal experiment is a very important part in the finished experiment, health monitor of the bridge structure. The modal parameter identification is the final goal of modal experiment analysis, which provides evidence for the analysis of dynamical behavior and health monitor of bridge structure and so on. Thus modal parameter identification is main content of modal analysis of bridge.The tradition's modal experiment is need to proceeds to arouse to flap, so for many bridge constructions it makes experiment difficult to complete sometimes. However, the modal experiment method based on ambient vibration can solve the problem easily, it is a new modal experiment method that only need response signals for modal analysis, so it is adopted extensively in the engineering. Dynamic testing of the bridge structure under ambient vibration excitation has many advantages, such as no excitation equipment needed, no interruption of structural service conditions and less test time, which is more close to the real working conditions of the bridge structure. As only output is measured and real input remains unknown in terms of ambient vibration testing, the modal parameter identification therefore bases structural output-only data. The output-only modal parameter identification is different from traditional one based on input and output data and is now very active research topic in the system identification of engineering structures.In this thesis, at the condition of the outside excitation unknown, the state-of-the-art studies are carried out on the application of natural excitation technique and eigensystem realization algorithm(NExT/ERA) in the field of modal analysis under ambient vibration aimed at the bridge structure. It also studies power spectrum density peak method which is classical in engineering, and it can be used as a supplement.At first, in this article, by looking into plenty of articles at home and abroad for reference, it gives an overview of ambient vibration modal analysis. It expounds the great importance of ambient vibration modal analysis to engineering, introduce the development course of the methods in this field as well as its current status.Secondly, it introduces power spectrum density peak, a frequence domain modal analysis under ambient vibration method that is simple and shortcut in application. As a classical and practical method in ambient vibration experimental modal analysis, it replaces the frequency response function with power spectrum and achieve its mode shape from transmissibility between the response signals and reference signal. The paper deduces the theory of this method in details on the base of the traditional peak-picking method. It shows up the flow of modal parameter identification dealed with the way, and writes out the program in MATLAB language,and sums up the strongpoints and shortcomings of the way by the results of a numeric simulation of a simple beam. The mian conclusions included are validated:(1) suited for easily separate mode and low damp; (2) subjective peak-picking; (3) work flexibility figure is got,not mode shape; (4) reliability of damp identification is not good; (5) simple and fast identification.Thirdly, NExT/ERA identification method of a time domain united algorithm is the major in this article. The theoretical background of the method is discussed was dead against discrete state space mathematic model of structural vibration, and MATLAB program is worked out. The emulational response of a simple beam numerical model is achieved by operation of random white noises which is similar to ambient excitation. The test point number and the intensity level of white noises are considered in the chapter, and later verifies the effectiveness of natural excitation technique and eigensystem realization algorithm(NExT/ERA) and verifies the accuracy and anti-interference by simulating results.Finally, a three-dimensional finite element model of a prestressed reinforced concrete continuous girder bridge is established by SAP2000 in the thesis. It is member element, boundary condition and material behavior that is taken into account, so the model is accorded with the real fact badly. The finite element time-history anlysis is carried on based on the model, thereinto, simulated the ambient vibration on the model by simulating the moving automobile loads, next the response signals is measured. Next we process the response dates of experiment with power spectrum density peak method and NExT/ERA identification method respectively successfully identify the three-dimensional vibration modes of bridge, and comparison the two results with the theory result by finite element method to verify the two methods and bear out the superiority of NExT/ERA identification method compared to power spectrum density method.