Research On Modal Analysis And Damge Detection Of Grid Structure Based On Ambient Vibration

The inspection and testing of the grid structure in service in time to discover the damage that may arise disaster, early warning, assessment of structure safety has become a necessary requirement for future projects, but also the development of a civil engineering discipline important areas, has important theoretical significance and application value.Surrounding the safety assessment of grid structure, from the point of modal analysis and damage detection of the grid structure based on ambient vibration,the article carried out the three core issues:(1) The method research to obtain the true modal parameters of grid structure based on ambient vibration;(2) The theory research of grid structure damage detection according to the structural modal parameters;(3) The research of the safety assessment system of grid structure in service.There are many modal analysis ways based on ambient vibration such as the random subspace method, the peak picking method. This article takes into account the merits of modal analysis methods, using a variety of modal analysis method to analyze the collected ambient excitation-based dynamic test data to identify the modal parameter,and form a set of structural modal parameters obtained process.This paper has established the structural damage detction method based on modal strain energy method.The method considering characteristics of grid structure which reference model of this method is undamaged grid structure model or the finite element model, as long as testing out the state of in-service modal parameters of the grid structure to calculate the grid structure of the unit modal strain energy, then ,can determine the reliability of the grid structure of the state. At the same time, given the grid structure damage identification numerical example, to construct a different grid structure damage cases, through the development of procedures that can automatically identify the grid structure of the damage.This paper has also established the finite element model updating damage detection method of grid structure adopting direct optimization algorithms based on dynamic test results. According to the characteristics grid structure,the theory which select the appropriate damage identification parameters to achieve the same damage identification of the grid structure considering characteristics of grid structure. To verify the model updating method based on direct damage identification method , a numerical example of truss structure is adopted to verify the validity .To execute the dynamic testing of experimental model which has 32 bars and 13 node of pyramid grid structure and a pyramid grid structure in service which has 1258 bars and 341 node,verify the effectivness of modal analysis and damage detection theory of grid structure under ambient excitation .Modal test analysis showed that modal analysis methods under ambient excitation, can obtain more effective modal parameters ,especially for large-scale grid structure, the effect is more pronounced. Based on ambient vibration modal analysis method, the peak picking method and the random subspace method can both obtain more effective modal parameters .The results compared with the finite element analysis results shows that the modal parameters obtained by the peak picking method, is relatively stable and credible to the low modal parameters, while the modal parameters obtained by stochastic subspace method is relatively stable and credible to the high modal parameters.Experimental verification of damage detection showed that the grid structure damage detection method based on modal strain energy and based on the finite element model updating, can achieve the target of damage detection of grid structure,if the environmental impact of various external effects on the structure is of the average. For details of the specific injury and damage parts, the means of deeper detection and identification of further testing is necessary.Propose and establish safety assessment system of grid structure in service, which is composed of the apparent quality inspection, measurement methods based on static testing and modal parameters based on dynamic testing ,which indexes is the basic grid size, deflection and modal parameter, and realize the safety assessment.of grid structure in service. Combined with the implementation of the safety assessment of the actural grid structure, to verify the effectiveness of the system, and prepare for further study of the life assessment , prediction of grid structure in service.