Study On Subway-Induced Environmental Vibration And Floating Floor Isolation

With many more subways having been completed or being planed to be constructed in recent years, on the one hand, the subway trains are running closer and closer to the residential or industrial areas, and some even go exactly under the up-ground buildings; and on the other hand, the limit on vibration level is becoming increasingly strict due to the improvement of people's living standard and the enhancement of delicate instruments' sensitivity. As a result, the problem of environmental vibration induced by underground trains has become more serious than ever before. Therefore, the further research on the subway-induced environmental vibration has important theoretical and practical application values. In this dissertation, some theoretical and numerical techniques are developed to study several aspects of the subway-induced vibration problem, benefiting from the design information provided by the Architectural Design & Research Institute of Tongji University and the measurement data performed at the site. The main contents of this study are as follows:1. The influencing factors of which vibration impacts on environment include the vibration intensity, direction and frequency. The method of calculating acceleration RMS (root mean square) values in one-third octave frequency, by the recorded data of acceleration time history, is presented. And with this method, subway-induced environmental vibrations can be quantitatively evaluated, according to the national standard of evaluating vibration level in frequency domain.2. The forces acting at rails are the combination of the quasistatic forces caused by the successive axle load, and the dynamic forces mainly originated by the unevenness of rails and of carriages' wheels. The accelerations of the source of environmental vibrations caused by underground trains are calculated, and the comparison of these calculated accelerations with the measured ones is made. The accelerations of the vibrational source significantly result from the dynamic forces, and the ones caused by the quasistatic forces can be neglected.3. Although Fourier spectral analysis provides a general method for examining signals' frequency property, there are some crucial restrictions of the Fourier spectral analysis: the system must be linear; and the signals must be strictly stationary. However, the subway-induced ground vibration signals are not strictly-defined linear stationary signals, because of the complexity of the mechanism of the subway-induced vibration source and transmitting media, the diversification of train's loads and speeds, and the interaction of background vibrations. The frequency spectrum properties of the measured subway-induced ground vibrations are analyzed based on the orthogonal Hilbert-Hung Transform which is ideal for nonlinear and non-stationary signal analysis. The spectral properties of the subway-induced ground vibrations eliminating background ones are displayed by the investigation of the marginal Hilbert spectra of the data.4. The complex beat phenomena occur in soil or other complicated structures subjected to the excitation with wide range of frequencies. Based on the thorough examination of the damping ratio-frequency relation, the Caughey damping model, of calculating the damping of the system excited by vibrations with wide range of frequencies, is built. According to the Caughey damping model, the dynamic response analysis of soil under the action of subway-induced vibrations is made, based on the dynamic finite element method, and the results are greater than those obtained by the Rayleigh damping method.5. The comparison is made, of the dynamic responses analyzed by two-dimension and three-dimension model of the same frame structure building under the subway-induced vibration excitation, and there is little difference between the two results calculated with the two models. Therefore, Two-dimensional finite element model is developed, using the results cf a ground vibration measurement campaign purposely performed at the site as input excitations. Based on the Caughey and Rayleigh damping model, the vibration levels in the structure are calculated respectively, to multi-support, traveling wave and uniform excitation, and the comparison of the vibration levels is made. The quantitative evaluation of the vibration levels is made based on the Chinese national standard on limit of vibration, and the effects of traveling wave and damping are investigated.6. An analytical model of the single degree of freedom system of a floating floor is established. The characteristics of the spring vibration isolation are presented based on the calculation of the analytical model. The responses of the floating floor subjected to subway-induced vibrations are respectively analyzed with the Caughey and Rayleigh damping model, and the isolation efficiency of the floating floor is quantitatively evaluated.