Unsteady Theoretical Analysis on the Wake-Induced Vibration of Suspension Bridge Hangers

The hangers on long-span suspension bridges are significantly prone to wind-induced vibrations due to their light mass, low frequency, and small structural damping. These vibrations are known to cause fatigue problems and impair the serviceability and safety of structures. The possibility of wake-induced vibration of the hangers is theoretically studied in this paper. First, a series of wind tunnel tests for two smooth circular cylinders were carried out to identify the eight aerodynamic derivatives of the leeward cylinder. Second, the motion equations of the leeward cable were established on the base of the unsteady and quasi-steady theories by using the data measured from the wind tunnel tests. These equations are numerically solved by adopting the Runge-Kutta method. Finally, a series of numerical simulations based on the structural parameters of a hanger on the Xihoumen Bridge were conducted to study the mechanism of the wake-induced vibration of the leeward cable of the hanger. The results show that obvious oscillations of the leeward cable take place in the spatial region of 5.2 X 5.6 and 1.1 Y 2.1, and the vibration frequency of cable is slightly smaller than its natural frequency. A positive work within a period is always done by the aerodynamic stiffness force if large-amplitude vibration of wake-induced vibration takes place. It appears that the aerodynamic stiffness force is the key factor to evoke the wake-induced vibration of the leeward cable. This agrees well with the feature of the wake-galloping vibration observed from the power transmission lines. Moreover, it seems that the quasi-steady theoretical model could reflect the main characteristics of wake-induced vibration just as the unsteady theoretical model. However, the oscillation amplitude obtained from the quasi-steady theoretical model is slightly smaller than that from the unsteady theoretical model.