Operational and Classical Modal Analysis to Damage Detection in Steel Structure

Ghasemi Khademi, Ali (2017) Operational and Classical Modal Analysis to Damage Detection in Steel Structure. Masters thesis, University of Mohaghegh Ardabili.

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Abstract

Experimental modal analysis is a process for identifying the dynamic properties of structures, including vibrating modes, by applying their dynamic response to vibrations, which is a useful and accurate method for assessing damage in structures and based on non-destructive health tests. The use of experimental modal analysis methods is usually associated with fundamental problems. Because most of the structures that surround our surroundings are under various environmental loads such as wind, sound waves and noise. Therefore, the nature of the loads loaded into these structures is not exactly clear, and in most cases it is impossible to measure them. In order to solve these problems, researchers in recent years have provided new methods for modal analysis, based on measurements, only responses. These methods are called operational modal analysis. In these methods, the structure is stimulated by environmental loads. In this research, operational modal analysis and experimental modal analysis are used to troubleshoot the steel frame. To achieve this objective, the three-story three-dimensional frame model was initiated by impact load and earthquake record. To extract the modal information from frequency response functions (FRF), methods of a degree of freedom (Pick Peaking, circular fitting, inverse FRF) and several degrees of freedom (fractional polynomial) were used. The results of the analysis show that the impact load versus the earthquake record has a better performance in determining the vibrational structure of the structure, and all methods of a degree of freedom and several degrees of freedom provide acceptable results for determining the modal parameters. Secondly, an environmental modal analysis method is used. In this case, the frame was propelled by wind and noise perturbation load and the methods of frequency domain and subspace randomization analysis, which are in the frequency domain and time domain method respectively, were used to approximate the modal parameters. Results show better performance of the frequency domain analysis method than the random subspace method in determining the inherent characteristics of the system. In the last step, applying hypothetical damage to the structure and using the results of empirical and environmental modal analysis methods, the damage to the structure was initially investigated using changes in the frequency response functions and the McAcet criteria and, finally, the location of the damage to the 3D frame using the change method Moody's shape is identified. Key words: Operational modal analysis, Experimental modal analysis, Steel 3D frame, Frequency response functions, Mac criterion

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