Modal Parameter Identification Method In The Structural Condition Monitoring Study

Engineering Structure, especially some large-scale civil engineering structures, such as the large-span bridges, ultra-high-type building, large dams and offshore platforms, because of the effect of the environmental erosion, material aging and fatigue effect during their long service period, it is inevitable that the damage of the system will accumulate and the resistance will decay, in extreme cases, sudden catastrophic accidents will happen. It is quite necessary to monitor these structures's condition and assess their health status timely before the disaster coming. The basic principle of Structure Condition Monitoring based on the modal parameter identification is that modal parameters (natural frequencies, damping ratios and mode shapes) are the function of the structure's physical characteristics (mass, damping and stiffness), and they are also the parameters of the structural dynamic characteristics. Change in structural characteristics due to damage subsequently affects the dynamic characteristics of structures. Therefore, it is an effective way to monitor the structure's condition by observing the changes of its modal parameters which can be measured by the structural dynamic testing. Additionally, it has been widely used in civil engineering in recent years.Comparing with the traditional modal parameter identification methods, the modal parameter identification methods under environmental excitations always use the traffic, wind, wave, vibration of the earth, or the combinations of them as the excitation of structure, and only use the structure's vibration response data to extract modal parameters. It is not only an economic and time-saving way but also quite fit the practical engineering application. Firstly, the thesis discusses the theory of some commonly used vibration signal pre-process methods, and presents the implementation and application of these methods in practical engineering application. Secondly, the research is focused on theory, algorithm and implementation of three popular modal parameter identification methods, the Least-Square Complex Exponential method, the Complex Mode Indicator Function method and the Stochastic Subspace Identification method, which are under the environmental excitations. At the same time, a simulating example is used to verify the algorithms respectively. Thirdly, a new modal parameter identification method by using independent component analysis is proposed. The main idea of this method is to interpret the normal coordinates of a dynamic system as virtual independent sources. Under this certain assumption, the traditional modal analysis problem is converted to the blind source separation problem. Finally, through the simulating example and modal parameter identification of laboratory-made simply supported beam, the experimental results show that this method can solve the modal parameter identification problem of the linear system properly.In conclusion, the thesis summarizes the main work of the research, and the future direction of research and development are discussed.