Non-linear Damping Properties And Dynamicresponse Of FRP-confined Reinforced Concrete Columns

Externally wrapping fiber-reinforced polymer (FRP) composites has gotten increasingly wide applications in strengthening and retrofitting of existing concrete structures. However, as a new kind of composite material, the study for material damping of FRP-confined concrete is still in the initial stage. In the seismic analysis of FRP-confined reinforced concrete (RC)structures, the constant damping ratio of general RC structures, instead of a proper damping value, is usually adopted at present due to the complexity in mechanism and the difficulty in experiment, which, greatly affects the reliability of the dynamic analysis results. The proper damping model of FRP-confined RC structures has not yet been set up.This paper aims to explore into the non-linear damping characteristics of FRP-confined RC and damaged RC materials. Firstly, a series of tests are conducted to observe the damping and hysteretic energy dissipation of FRP-confined RC columns. Subsequently, hysteretic performance and hysteretic energy dissipation characteristics of FRP-confined RC columns are investigated considering initial damage and different confinement levels to establish the non-linear damping formulas. Finally, based on the proposed damping models, an improved method for dynamic analysis considering non-linear damping is developed to analyze the seismic response of FRP-confined RC and damaged RC columns. This study is helpful to the development and application of non-linear damping theory and anti-seismic analysis theory of FRP-confined RC structures. Main researching works of this dissertation are as following:(1) Test process is designed to study the hysteretic performance of FRP-confined RC and damaged RC columns, and carrying capacity decline and stiffness degenerate of FRP-confined concrete under cyclic loading. By using the modal identification method, the effects of different parameters on the natural frequencies and damping ratio of FRP-confined RC columns are tested and analyzed.(2) FRP-confined concrete constitution models considering initial damage and different confinement levels and Karsan-Jirsa unloading-reloading law are utilized to compute the hysteretic performance of FRP-confined RC and damaged RC columns subjected to compression and bending by the OpenSees program. The results of program are in good agreement with the experimental results.(3) Based on the study for the relation between material damping and stress of Lazan, the energy dissipation of FRP-confined RC and damaged RC columns are computed. Further, the effect of a few parameters on energy dissipation is analyzed, and the non-linear damping formulas of FRP-confined RC and damaged RC columns subjected to compression and bending are set up by non-linear regression with a commercial software SPSS.(4) Based on the proposed damping models, Finite Element Method is adopted to obtain the real material damping values and corresponding dynamic responses of FRP-confined RC and damaged RC columns iteratively in dynamic system with hysteretic damping, and the results are compared with the dynamic responses by constant material damping. Influencing law of parameters on the material damping and dynamic responses is analyzed quantitatively.(5) By introducing the the non-linear damping models into the structural dynamic equation, the responses of FRP-confined RC and damaged RC columns under the uniaxial and biaxial harmonic and earthquake loadings are calculated, the analytical results indicate that there are remarkable differences between the dynamic response based on non-linear damping and constant damping. In view of design, the seismic response level based on constant damping recommended by current code for seismic design of FRP-confined RC component may be significant underestimate. The effects of the concrete strength, longitudinal reinforcement ratio, initial damage level and FRP volume ratio on the damping and natural frequencies time history of FRP-confined RC columns are analyzed.