Study On The Chaos Phenomena In Bridge Wind-Induced Vibration

With the constant social and economic development, the bridge engineering will be a higher level that has larger span, lighter structure, smaller stiffness and lesser frequency. Then the bridge is becoming more sensitive to wind. Wind will be considered one of the main control factors in the cable-stayed or suspension bridge design. The wind-induced vibration mainly includes galloping, flutter, vortex vibration andbuffet. As divergent vibration, flutter and galloping both are huge devastative. Vortex vibration and buffet are also serious threats to the durability and feasibility of bridge structure. Because those four vibration forms all are fluid-solid coupled vibration that has a complicated mechanism, it is difficult to use linear theory to explain them. Therefore, it is significative to try to use nonlinear theory to explain it. As one of nonlinear theory, the chaos theory has already been popularly applied to the engineering. So this paper applied the chaos theory to research on the wind-induced vibration’s math model and wind tunnel test. The main researches of this paper as follows.Firstly, this paper provided the wind-induced vibration’s math model that is a nonlinear dynamic system in four dimensions. Then though analyzing the change of stable limit cycle caused by dimensionless wind speed, this paper provided the double periodic bifurcation of this system. In addition, the butterfly effect of this system was studied. The small difference of initial condition causes the huge difference of result.Secondly, this paper presented two bridge’s wind tunnel tests with different typical cross sections. One is open section and another bridge is closed section. Then the Lyapunov exponent MATLAB program was developed based on chaos theory. The Lyapunov exponents of the acceleration time series of different section’s wind tunnel tests was Calculated. This paper provided the relationship between the Lyapunov exponent and wind speed. The results indicate the Lyapunov exponents of the acceleration time series all are positive when the vortex vibration or the flutter occurred. That’s to say, the vortex vibration and the flutter both are chaos phenomenon. In addition, the Lyapunov exponent has relevance to the structural damping ratio.Thirdly, this paper presented several main factors’ influence on the Lyapunov exponent. Then the comparative analysis revealed that the structural damping ratio has a obviously and correlative influence on it; The mass has nothing influence on it; The wind attack angel has influence on it, but the mechanism is not clear.