Stress-related Damping Model And Its Applications To Dynamic Analysis Of Beam Bridges

Damping has great influence on dynamic response of bridge. Numerous studies indicate that damping value is a variable during vibration process of bridge; however, most existing damping models cannot reflect this variation. The uncertainty of damping parameter severely restricts the solving accuracy of dynamic response.On the basis of material damping experiment of concrete, a damping identification method and relevant calculation formula are proposed in this paper. Based on experimental data, the Kelvin model is proposed and employed for the energy dissipation calculation of concrete.In addition, full-scale data on the damping ratios of highway bridges in China have been collected and analyzed, a calculation method of stress-related damping is proposed for the bridges. The dynamic responses of vehicle-bridge and earthquake are calculated. Main researching works of this dissertation are as follows:(1) A uniaxial hysteretic experiment including the influence of dynamic stress amplitude, frequency and stress level is conducted to study the stress-related damping characteristic of concrete. By use of the experimental results, the plastic energy dissipation and damping energy dissipation are separated, showing that the damping energy dissipation of concrete increases with the increase of stress amplitude, and the relationship can be represented by a power function. Moreover, the damping energy dissipation of concrete is not sensitive to loading frequency and stress level.(2) The relationship between dynamic energy dissipation and model parameters are derived for four common viscoelastic models. A formula for calculating viscosity coefficient of Kelvin model is proposed and validated by experimental results. The calculating results show that the viscosity coefficient of concrete increases with increase of stress amplitude. The proposed viscosity coefficient being independent of frequency can well simulate the frequency insensitivity of the damping energy dissipation of concrete. Finally, the validity of the formula is verified by the comparison with others test results.(3) Full-scale data on the natural frequencies and the damping ratios of114highway bridges in China, and the dynamic characteristics monitoring data of four bridges under ambient excitation, have been collected and analyzed. The data indicate that the damping ratios of different bridge types have large difference, and for the same bridge type, the damping ratios are scattered. With the increase in service ages, the damping ratios of the bridges decrease and tend toward stable after certain years; maintenance and reinforcement do not considerably affect the damping ratios. The test results of damping ratios under vehicle excitation are much larger than that of ambient excitation. The proposed damping ratios of different bridge types are given on the basis of damping ratio data.(4) A calculation method of stress-related damping is proposed for bridges, and a vehicle-bridge coupled programming is developed to calculate the dynamic response. A helpful conclusion is obtained by comparing the test results; it shows that the stress-related damping is capable of presenting the variation of damping during the vibration process of bridge, and fitting the reality better. In comparison with the traditional constant damping ratio (5%) and Rayleigh damping, the stress-related damping is also much closer to the measured data.(5) A calculate method is proposed for reinforcement concrete column to separate the plastic energy dissipation and the damping energy dissipation in quasi-static test. Correctness and validation of the method is testified by the comparison of measured data and the calculation result. Using stress-related damping theory, the dynamic response of reinforcement concrete column is calculated and compared with the quasi-static test result. Comparing with measured test, the result calculated by the stress-related damping is more accurate than that by constant damping ratio (5%), especially in small-moderate earthquake. Taking continuous rigid frame bridge as an instance, the stress-related damping ratio of a bridge is calculated by the method of response spectrum. The calculating results show that the damping ratio increases with the increase of earthquake intensity.