Modal Parameter Identificatin And Natural Vibration Characteristics Calculation With The Fem Of Gravity Dam Based On Seismic Response Data

The natural frequency and mode shape are collectively referred as the natural vibration characteristics,which is not only the key to determine the dynamic response of gravity dam with the mode superposition method but also an important index evaluating seismic performance of structures.The so-called modal parameters are the natural frequency,damping ratio and mode shapes of the structure,the measured earthquake acceleration record is the dynamic response of gravity dam in working condition,and the natural vibration characteristics of the water-gravity dam-foundation system could be truly refected by the modal parameters identified with the seismic records.At the same time,because of the influence of foundation boundary conditions,the natural vibration characteristics of the conventional finite element model are of the low credibility and influenced greatly by the elastic modulus of dam body,the foundation range,the foundation quality and other parameters.Therefore,it is significant to study on the natural vibration characteristics calculation with the FEM of the gravity dam-foundation system on the basis of the modal parameters identification results using the acceleration vibration response signal.The main work and conclusions of this paper are as follows:1)The vital significance of natural frequency and vibration mode in the dynamic analysis of concrete gravity dam and several common methods for calculating the natural vibration characteristics of water-gravity dam-foundation system are introduced in the paper.At the same time,a summary about the modal parameters identification methods only using the vibration response signals in hydraulic structures is made and the problems of the natural vibration characteristics calculation are presented.2)According to the characteristics of seismic acceleration record,the modal parameter identification methods of gravity dam are studied from the two aspects of frequency domain and time domain.At first,the way of identifying the structural mode coefficients is presented by the improved power spectrum peak method with the cross power spectral density function.Then,the applicability of the above methods to the acceleration vibration response is explored,and time domain ITD method is improved by using the total least square method in order to improve the accuracy of modal parameters.At last,the MATLAB program is compiled and its reliability is verified by numerial examples after formulas are deduced in detail.It is realized that the modal parameters of civil structures are identified without the input excitation signal.3)Based on the features of the natural vibration characteristics calculation,the fundamental theory of the vibration equation without damping,the Visco elastic artificial boundary simulated by the Goodman element without thickness,the reservoir water finite element method and the direct filtering method of solving the natural vibration characteristics are introduced at length,while the finite element model of water-gravity dam-foundation system is established in the paper.The 3D finite element program D.F calculating the natural vibration characteristics of gravity dam is compiled by FORTRAN and the influence of reservoir water and foundation on the natural frequency of gravity dam is discussed by the practical engineering application.4)The 19th block dam at Shuikou Hydropower Station is chosen as research object,the natural vibration characteristics calculation of the finite element model of gravity dam is studied by the natural frequencies and the mode shapes with the spectral analysis,the power spectrum peak method,the random decrement technique and the time domain ITD method.Without considering the foundation quality,the calculated natural frequency of the 3D foundation model considering the lateral elastic foundation is in accordance with the peak point on the power spectrum and mode shapes are consistent with the modal parameter identification results,which indicates that the seismic behavior of gravity dam under earthquake could be reflected in the finite element model.In addition,the conclusion that the quality of dam concrete is better than the design value could be drawn by the variation of natural frequency with the elastic modulus of dam body.