| Nowadays, the study of structural health monitoring and damage identification for the long span bridges has become hot issues in civil engineering. Damages of different level appear on the bridge due to the traffic loads, wind load, earthquake and other environmental effects (temperature, humidity and strike, etc.) in operation, which will greatly threaten the safety of bridges and may cause great loss to human life and property. Some serious accidents such as the collapse of the bridge will cause great disaster to our country and society. Therefore, in order to secure the safety of the bridge structures and prevent the case of disaster, one need to identify and alert the damage of the bridge structures in time, which is of great significance in saving the economic loss and protecting the human life and property. A great number of researchers and engineers expect to make use of the characteristics of the bridge, especially its dynamic property to investigate the damage identification algorithm and apply the structural health monitoring system to detect and alert the damage by constructing the damage index which is sensitive to the damage of the bridge. The study is carried out under this background.In order to better study the structural health monitoring and damage identification of the cable-stayed bridge, a cable-stayed bridge model with the scale ratio1:150was built in the laboratory. Based on this physical model, three issues including finite element model updating method, wavelet-based transmissibility function and the structural health monitoring of the cable-stayed bridge on the basis of measuring the distributed strain are studied by carrying out the model test and numerical simulation. The main work and research scheme are stated as follows.First of all, the significance of structural health monitoring and damage identification for the bridge structures is stressed in the introduction part. Secondly, the development of structural health monitoring for the bridge structures in recent decades is briefly introduced. Then the concept, composition, history and application of the structural health monitoring system are also presented. According to the recent study on the damage identification methods for the cable-stayed bridge, different kinds of damage identification methods adopted in bridge engineering are presented, and their application fields and advantages as well as disadvantages are also evaluated. Finally, the outline and the idea of this research are provided at the end of the introduction based on the issues and problems occurred in the structural health monitoring and damage identification areas for the cable-stayed bridges.Based on the engineering background of Yellow River Shengli Bridge in Shandong province, a small scale cable-stayed bridge model with the scale ratio1:150was built according to the similarity theory and the model tests were carried out to obtain both of the static and dynamic experimental data. The goal and significance of the model test are explained at first, and then the application of the similarity theory in model test is introduced. The whole process of the design and fabrication of the model bridge is presented, including model material selection, derivation of physical quantities, design of the structural components as well as boundary simulation, etc. The dial test indicators are used to measure the displacement of the bridge deck, and self-made FBG optical sensors are used to monitor the cable force change during the model tests. Modal test was also performed to obtain the first three vertical bending frequencies and mode shapes. Finally, the study on the effects of bridge displacement and cable force change caused by the damage of the cable is carried out, and the influence of the cable force change on the bridge health status is discussed. This physical model was used as a baseline model for the study of cable-stayed bridge of structural health monitoring and damage identification.A two-step FE model updating strategy for long span cable-stayed bridge is proposed in the Chapter3. The various methods of FE model updating for cable-stayed bridge were reviewed in the first time, then a three dimensional cable-stayed bridge model was established using ANSYS software. Based on the experimental data from the model test, a multi-objective function was formulated including frequency, mode shape and displacement differences. The FE model was updated to improve the accuracy of estimating both of the static and dynamic responses. An optimization program was developed using first order and sub-problem method to update the FE model based on APDL (ANSYS Parametric Design Language). The two-step FE model updating strategy proposed in this study takes consideration of both static and dynamic responses of the bridge model, and is different from the conventional FE updating methods, which only take account of either dynamic or static responses of prototype. The frequency difference was first minimized by selecting the appropriate parameter with sensitivity analysis, which makes the computed frequency as close as the measures one. Then other parameters were selected to minimize the mode shape and displacement differences, which makes the static responses predicted by the FE model agrees with the experimental results after the second step of model updating. Meanwhile, the updated parameters were used to compare with the actual ones of the model bridge. The agreement of them verifies the effectiveness of the updating program. At last, using the updated FE model to predict the responses of the model bridge and prototype bridge to further validate the accuracy of the FE model.In order to search the damage index which is sensitive to the damage, a wavelet-based transmissibility function method is proposed to detect the damage. The concept of transmissibility function between the responses between the degrees of freedom (DOF) is introduced and its features and applications are also presented. The damage identification methods using the transmissibility function are summarized. Then, a new damage index using wavelet-based transmissibility is derived and constructed. A statistical method named outlier analysis is adopted in the study to detect the damage in the structure and the concrete damage detection procedure is also presented. Numerical simulation and experiment were also carried out to verify the effectiveness of the proposed method. Also, a comparison study was carried out by using the conventional transmissibility method. The results show that the damage detection method proposed in this study is very sensitive to the damage, which highlight the strength of the method to detect the slight damage in the early stage of the structure.Structural damage is a typical local phenomenon, which commonly leads to the abnormality in the local strain. Monitoring the structural damage in a distributed manner, as well as extracting the damage-sensitive feature, is an effective way to detect the damages in the long-span bridges. A damage detection method for the long span bridge is proposed by using the distributed optical sensor. The strain signals of the deck, which are measured by distributed optical fiber sensor, are decomposed into multiple levels using wavelet transform. The proposed method has alleviated the noise effects on the strain signals and averaging effect caused by the spatial resolution of BOTDA (Brillouin Optical Time Domain Analysis), thus determines the exact location of the singular point of the spatial signal accurately and conveniently. The features of the distributed optical sensor, the principle of BOTDA, and its application are presented. The damage index is constructed using wavelet transform to detect the singularity of the distributed signal of the bridge structure. Different types of the optical sensors were used in the study. Both of the numerical simulation and model tests were carried out on the cable-stayed bridge model to testify the effectiveness of the proposed method. The results show that the proposed damage detection method can detect multi damage locations of the bridge structure and it is also sensitive to the damage and robust to the environmental noise.Finally, the summary is given at the end of the research, and the prospect for the structural health monitoring and damage identification of the cable-stayed bridge is also proposed. |
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