Study On First-Order Modal Damping Identifycation Method For Bridge Under Service And Its Influencing Factors

Different methods can be used to evaluate the health status and service life of bridges that have been in service for many years,such as static analysis and dynamic analysis of bridges.In dynamic analysis,the stiffness matrix and mass matrix in the equation of motion can be determined by the geometric structure of the structure,and the calculation method is mature and reliable.However,the damping coefficient of the structure is difficult to identify accurately,and empirical damping coefficient is often used,which has a certain degree of human subjective interference,which will lead to inaccurate dynamic response analysis of the structure.In order to accurately identify the modal damping ratio of structures,a new method for identifying the first-order modal damping ratio of structures is proposed in this paper.The main research contents include:The integration of beam element global stiffness matrix,mass matrix and Rayleigh damping matrix is introduced;The state space theory is derived for numerical simulation and structural dynamic response analysis.On the basis of the motion equation,the identification model of the first-order modal damping ratio of the structure based on the amplitude extraction of the free attenuation response is derived and established.When using this model to identify the damping ratio,first intercept a section of the acceleration free attenuation time history of the structural dynamic response,logarithmically process the envelope curve equation of the truncated attenuation section amplitude fitting,and then use the least square method to estimate the slope to obtain the first-order modal damping ratio of the structure.The feasibility of identification model of first-order modal damping ratio is studied by numerical simulation method.In the analysis of simply supported beams,the global stiffness matrix and mass matrix of the structure in the global coordinate system are established.The damping ratio is set,and Rayleigh damping is used to calculate the damping matrix of simply supported beams under different damping ratios.The global stiffness matrix,mass matrix and damping matrix are substituted into the state space model after being processed by the boundary conditions,and the output acceleration response is under the random excitation.The output response is used to obtain the identified first-order modal damping ratio through the damping ratio identification model established in this paper.The identified damping ratio is compared with the set damping ratio and the relative error is analyzed to verify the reliability and accuracy of the identified first-order modal damping ratio model.At the same time,the identification accuracy of the first order natural frequency is discussed,the difference of the identification damping ratio of different nodes is analyzed,and the influence of different excitation positions on the identification method of damping ratio is explored.The applicability and feasibility of the damping ratio identification method are verified through the analysis of a complex truss example.An example of steel arch bridge is analyzed.Firstly,the acceleration signals obtained from the test are collected,and then the acceleration signals that can reliably identify the first order natural frequency of the structure are screened out by using the theoretical vibration mode.Substitute the filtered acceleration signal into the damping ratio identification model to obtain the identification damping ratio,and analyze the relative error of the identification damping ratio.In order to judge whether the relative error of the identification damping ratio at different nodes of the steel arch bridge is reasonable,the identification damping ratio calculated at different nodes is substituted into the state space theory respectively,the random excitation is input,and the mutual error of the structural dynamic response results simulated with different node damping ratios is analyzed and compared with the results simulated with empirical damping ratios.In order to make the analysis results not lose generality,El Centro seismic wave is used to replace the random excitation to analyze the dynamic response error of the structure,and the results obtained are consistent with those obtained by using the random excitation,which indicates that the error of damping ratio for identification of different nodes is acceptable,and further explains the applicability and feasibility of this method.Finally,a summary of the thesis and a look ahead to subsequent research.