Research On Damage Detection Of Reinforced Concrete Frame Structures Based On Modal Parameters

Many new problems appeared in practical application of structural damage diagnosis based on the vibration parameters. This thesis focused on the two aspects: the uncertainty analysis of the concrete structural modal testing and the applications of the local parameters to diagnosis damage, which are more sensitive to the damage. The main contents include:1. A four-story reinforced concrete frame structural model with scale of 1:3 was built in the laboratory and impulse excitation modal tests was conducted. The errors appeared during the tests and modal parameters identification were analyzed and the over-all modal parameters were given.2. Based on the structural frequency response functions, using the prior information, the Bayesian estimation of modal parameters was carried, and the maximal posterior probabilistic estimations of the structural mode parameters were acquired. By the asymptotic expansions method based on Laplace's method, the margin distribution can be approximated as Gaussian distribution centered at the maximal posterior estimation value, then the confidence intervals of the modal parameters can be calculated.From the results of modal parameters estimation of the concrete frame model, it was shown that the estimation would be affected significantly by the prior information when lack of enough measurement data, and with the updating of prior information and increasing of measurement, the estimation was convergent gradually. Among these modal parameters, the uncertainty degree of the natural frequency is very small. Comparing with the displacement mode shape, the variations of each station of strain mode shapes were quite different. The damp ratios are more affected by the types of the test systems.3. With the condensated story model considering the rotating of joints of frame, and using the different modal test information, the elastic modulus of each story of the concrete frame model was identified by the sensitivity analysis method. Monte-Carlo simulation was performed to analyze the variability of the structural physical parameters caused by the uncertainty of the modal parameters. The results showed good agreement with actual value.4. By the analysis of the strain modes, it was proposed that the strain modes could be regarded as bending moment modes of frame structure, which is a kind ofgeneral structural internal force mode. Damage detection of joint of shear steel frame structural model, and of changing stiffness of the concrete column was conducted in the laboratory. The relative changing ratio of the strain mode before and after the damage, and damage statistical indicator were used to diagnose the damages. The result showed that the first order strain mode could indicate the locations of the damage exactly and steadily. Moreover, relying on the local character of the strain mode, the degree of the local damage can be identified by local model updating method.5. High order modes of the frame structure represent the forms of member's local bending. Impulse excitation and sine stable sweeping tests of local bending modes of the frame columns were conducted. The analysis and the experiment results showed that for the regular chain structure, there exists mode-dense-region of high order local modes, the mode number of each dense region is the same as the number of the same type of structural members, and there exists static and dynamic coupling between the sub-structure and the whole structure. Based on the principle of maximal kinetic energy, the test results of the frame column local modes were selected, and the numerical simulations and tests indicated that the local mode representing the maximal kinetic energy would be more effective in member damage diagnosis.