Research On Modal Analysis And Application Of Long-Span Bridges Based On Ambient Excitation

Modal analysis is a research method of structural dynamic characteristics appeared recently, and it is an application of systematic identification method in the field of vibration domain. Modal analysis and FE analysis are stanchions of structural dynamics. Along with the rapid development of construction of long-span bridges, more and more attentions are paid to the evaluation and health monitoring of bridges to assure the safe operation of the bridge. Experimental modal analysis has significant senses in many aspects, such as the design scheme verification, the security condition evaluation and health monitoring in service stage and the establishment of baseline finite element model. Vibration of bridge structure is needed in the traditional method of modal test and it makes the test hard to be completed. The method of modal test based on the ambient excitation is a method which can carry out the modal analysis under the condition that only response signal is surveyed, so it is more in line with the features of long-span bridge actual use, and it is applied extensively in the engineering.This thesis take Dongping Bridge as the research background, begins with the basic theories of vibration testing and modal analysis, and deeply research on modal test and identification of modal parameters based on ambient excitation, finite element model updating of long-span bridges are carried out, the main contents included as follows:(1) Conversion of coordinates and modal superposition theory of multi-degree- of-freedom system vibration modal analysis are discussed in this thesis, and the expression of frequency response function of damping ratio model is deduced, in addition, the FFT and power spectral density and its main characters are discussed systematically.(2) Two kinds of modal parameter identification methods based on ambient excitation studied in this paper: cross-power spectrum method and stochastic subspace identification method. Cross-power spectrum method is a classical and practical method in ambient vibration experimental modal analysis. It proved that cross-power spectrum method is much the same as peak-picking method based on frequency response function by deducing the theory of this method in this thesis. Meanwhile, the process of identify modal parameters by using the method were discussed derived in detail. Stochastic subspace identification method is a more advanced time domain method of structural modal parameter identification under the conditions of ambient excitation. In allusion to space mathematic model of structure vibration discrete state, algorithm process of stochastic subspace identification method is deduced gradually in this thesis and the corresponding theoretical explanation is given.(3) A cost savings method of long-span bridges modal test under the ambient excitation—mobile measurement is given in this thesis, modal experiment of Dongping bridge proved that ideal modal frequencies, mode and damping ratio in modal test of long-span bridges by using mobile measurement can obtained.(4) A hierarchical modified FE model technology combined dynamic and static is proposed in this thesis, which is based on the theory of the modified finite element model. Initial finite element model in service stage and established the reference FE model of bridge health monitor and security evaluation are modified in this thesis, which is based on the experiment data of the dynamic and static test of Dongping Bridge. This method can use the test data of the structure static-dynamic fully and obtain the better modal correction effect compared to the modify method based on single dynamic or static. It could reduce optimization scale, times of iteration and obtain more satisfied correction results compared with one-time modified FE model technology combined dynamic and static.