Theory And Method Of Modal Identification Of Large Engineering Structure

The modal identification for engineering structures can be widely applied in civil engineering, and the identified modal parameters is used to estimate structural damages and determine future responses of the structure. Hence, this method can be used in health monitoring of bridges and active control of huge architectures. The measured input and output data of structures are used to estimate the modal parameters for the traditional identification methods, but it is diffculty to exert artificial excition on huge engineering structures or measure the exterior excition of huge engineering structures. In the later of 1990s, it is widely interested of ambient vibration method to identify the structural modal parameters according to the measured output data. There are many traditional identification methods, such as the time series analysis, ITD method, ERA method and stochastic subspace method. System identification originates from the control engineering and is utilized to estimate the model parameters of discrete models by the discrete data and transform model parameters to structural modal parameters. In fact, the discussed structural vibration models are continuous models. Based on the parameters identification methods of continuous models, this paper puts forward the basic theory and method to estimate directly structural modal parameters by discrete output data. Firstly, this paper investigated roundly the basic theory of the ambient vibration method, analyzed systematically the main methods of ambient vibration identification, and discussed their advantages and disadvantages. Then we established the finite element model of structural dynamics and test model and analyzed their mutual relation, improved MUSIC method and correlation function method in the harmonic identification, and gave the subspace method and correlation function method based on continuous model, which is multi-harmonic retrieve technique. And then, this paper also puts forward least-squares method to estimate directly structural physical and modal parameters; this approach replaced the derivative operator in the continuous time model with an equivalent approximation, and estimated the physical parameters of structures by the least-squares method. And then, we discussed the approximation methods of derivative and their convergence. Finally, this paper researched the modal parameters of a frame structure by the simulation and test. The results demonstrated that subspace method, correlation function method and the least-squares method are effective and stable respectively. The influence of noise on identification precision, the effect factor of the computing precision and the reason of missing estimation some modes were analyzed and discussed. This paper improved the system of the modal identification for engineering structures. The research results show that those methods are of simplicity and effectiveness, and they can be used in health monitoring and vibration control for bridges and architectures.