Research On Damage Detection Of Time-varying Bridge Structures Based On Wavelet Theory

The safety redundancy and reliability of practical bridge structures are usually lower than that of other structures due to theirs exposed working environment,complex load excitation and structural type.With the past 30 years of reforming and opening up,the bridge construction in China has become the forefront of the world in terms of overall scale,scientific and technological level.However,according to statistical analysis more than a quarter of the bridges have the problems of structural defects,variety of damage levels and hidden damages of functional failure,especially the important bridges are badly in need of safety monitoring,assessment,maintenance and reinforcement.In order to ensure the safety,integrity,applicability and durability of structures,it is quite necessary and urgent to use effective methods to evaluate safety and detect damage of bridge structures in service.To address these issues,this study combines the characteristics of bridge structures and their service environment,and accurately identifying the instantaneous characteristic parameters of time-varying structures based on time-varying and nonlinear structural response signal processing technologies.After that,some new damage indexes are proposed and their accuracy and effectiveness are verified via several numerical examples and an experimental test.The purpose of this dissertation is to make some progress on the damage identification methods of time-varying bridge structures,and to provide the basis and foundation for better study on failure mechanism,health monitoring and safety assessment of large bridges,which also has great theoretical significance and practical values in civil engineering.The main contents and conclusions are drawn as follows:1.The displacement responses are analytically decomposed into a series of intrinsic mode functions(IMF)by using the analytic modal decomposition theorem(AMD).After performing continuous wavelet transform on the first order intrinsic mode function(IMF1),energy ratio(ER),the maximum change ratio of auto power spectrum(MCRAPS),and the multi-scale displacement curvature's Teager energy operator(DC-TEO)are established to identify the damage locations of these structures;2.Based on the results of damage localizations,three new time-varying indices,including wavelet energy changing rate(WCR),the improved wavelet energy changing rate(IWCR)and the normalized displacement curvature of ith IMF(NDCI),are defined to identify the time-varying damage and predict the damage evolution process of the structures;3.Numerical examples of a three-story shear structure,a time-varying simply supported steel bridge and a three-span continuous beam are simulated to verify the effectiveness and accuracy of the damage indices mentioned above;4.A steel simply supported beam test with abrupt stiffness reduction and linear stiffness degradation under moving load is designed.Three rectangular steel blocksat different positions are cut from the steel bridge deck to simulate the time-varying damage reduction,while a trolley travelling through the bridge at a specific speed is used to simulate the moving load.By analyzing the test data,the effectiveness of the proposed indices is verified with a comparison of theoretical results.