Experimental Study On Flutter And Buffeting Responses Of Xia-Zhang Cross-Sea Bridge

Determination of aeroelastic and aerodynamic parameters through wind tunnel investigation is an essential component of the aerodynamic modeling of bridges. Due to the fact that cable-stayed bridges being constructed and planned with very large span and more flexibility for future, the aerodynamic mechanisms have become even more complex than before. Recently, developed flutter and buffeting analyses have significantly improved the capabilities of the existing analytical methods. However, the availability of all the aerodynamic and aeroelastic parameters and other relevant descriptors is important for the investigation of complex modal interactions and aeroelastic coupling expected in such long and flexible bridges.The thesis describes the equipment used in the present study, including the facilities, the model and its motion-driving system, the computer data acquisition system, and all relevant instrumentation. The experimental methods employed for the extraction of aerodynamic and aeroelastic parameters are also explained. All the experiments were conducted in the China Aerodynamics Research and Development Center in China. Brief descriptions of the wind Tunnel and bridge models used in this study are given.One of the major components of this study is the development of an instrumented 2-DOF suspension system that allows the movement of the section model in two degrees of freedom (vertical and torsional), simultaneously, and enables the displacement time histories of these motions to be recorded. The design of the suspension system and the followed procedures are described in detail. From these time histories, and using the free vibration identification method, all the eight flutter derivatives of a bridge section models can be simultaneously extracted. There are three models:?The recommended layout model of the North Branch Bridge.?The comparing layout 1 model of the North Branch Bridge.?The comparing layout 2 model of the North Branch Bridge. Four parameters that significantly affect the flutter derivatives and critical wind speed were selected and studied. These are: the wind speed, the wind attack angle, the wind flow field, and the railings. Based upon the experimental results, and using the multiple linear regression method, three formulas were proposed for the estimation of the flutter derivatives as a function of the reduced wind speed and wind attack angle? The full aeroelastic models are used for more important and large bridges, so they were used to simulate Xia-Zhang Bridge. The objective of using full models is to determine the flutter critical wind speed and buffeting responses of the bridge under erection (unique tower, single cantilever, and double cantilevers) and service stages. The main parameters that significantly affect the flutter wind speed and buffeting responses, which were selected and studied herein, are the wind speed, the wind attack angle, the wind skew angle, the wind flow field, and the railings. Experiments were conducted in both smooth and turbulent flows. The experimental set-ups used to measure the mean wind velocity and its fluctuations (longitudinal and vertical) are described. For the study of buffeting responses, turbulence was generated by placing spires and roughness elements of the model. Details of the spires, their location in the wind tunnel and properties of the turbulence generated are presented. The procedures to obtain aerodynamic parameters used in the buffeting study are described, and the obtained results presented. Based upon the experimental results, and using the multiple linear regression method, formulas were proposed for the estimation of the RMS of the displacements as a function of the wind speed, wind angle?and wind attack angle?for three full models and in all the testing conditions. In this Study, active control system using active mass dampers (AMD) system are proposed to mitigate the vibration of the pylon, the single cantilever and double cantilever stages, to enhance the performance of the bridge and under strong wind gusts. The adaptabilities and the possibilities of applying of the AMD to the bridge under construction stages are proposed to increase the critical wind speed of the bridge.In two-dimensional flutters, Selberg's formula is the most famous and cited in literatures. It is an empirical formulation that was used for estimating the bridge flutter onset speeds for sectional models. It clearly points at the effect of structural and aerodynamic characteristics on bridge flutter performance, which helps to better understand how and where the structure may be tailored for better flutter performance. The critical flutter speeds obtained using Selberg's formula section models compared with the experimental critical flutter wind speeds were found to be in exceptionally reasonable agreement and therefore, are acceptable. However, one should not rely absolutely on prediction results based solely on an empirical formula.In the entire bridge model and the unique tower model conditions, no obvious flutter or buffeting excitation was detected. For both the uniform and turbulent flow fields, in the Maximum single and double cantilever conditions, the structure vibrating acceleration remained very small, even when the testing wind speed was much higher than the critical flutter wind speed. Furthermore, no flutter or static collapse was detected for all the tested conditions. In the turbulent flow field, there were very obvious buffeting phenomena, especially in the maximum double cantilever condition, when the attack angle was -3°and the obtained testing wind speed higher than 36.7m/s. The displacements in the vertical direction of the end of the cantilever, and displacements in the direction along the bridge at the top of the tower were quite big. In the unique tower condition, no galloping oscillation of the unique tower was detected when the inflow wind speed was below 159m/s and the vibrating acceleration is very small. In the turbulent flow field, the vibrations in the direction across the bridge at the top of the tower are also very big; attention should be paid to these phenomena. No flutter or static collapsing was detected in all the testing conditions.In the section models tests, the railing has great influence on the flutter derivatives results; while the turbulence has little influence. The values of (H₁^* ) and (A₂^* ) of the models decrease with increase in the reduced wind speed. Due to the fact that the wind speed corresponding to the negative aerodynamic damping is small and far less than the structural damping, the structure instability would not happen. According to all the obtained results, the bridge's critical wind speed is higher than the testing wind speed.Based on the experimental results, the full aeroelastic models and the sectional models have very good aero-stability. Therefore, the Xia-Zhang Bridge has excellent wind resistant performance for flutter and buffeting responses investigations.