Aerodynamic interference between the cables of the suspension bridge hanger

The hangers of long-span suspension bridges are significantly prone to wind-induced vibrations due to their light mass, low frequency, and small structural damping. However, the underlying mechanism of the hanger vibration is not clearly clarified yet. To study the aerodynamic interference between the cables of the hanger, which is a possible mechanism for the hanger vibration, a series of wind tunnel tests were carried out to measure the mean aerodynamic drag and lift coefficients of a leeward cylinder. Then, the motion equations governing the vibration of leeward cable were derived based on the quasi-steady assumption. The numerical results show that large-amplitude vibrations of the leeward cable will occur in the region of 1 <= |Y| <= 3, where Y is a non-dimensional vertical coordinate normalized with the diameter of the cylinder. It appears that the stable trajectory of the leeward cable is ellipse, and trajectory is clockwise above the center line of the wake, whereas anti-clockwise below the center line of the wake. An important finding is that the frequency of the stable vibration of the leeward cable is slightly smaller than its natural frequency, which implies that a negative aerodynamic stiffness might arise. The time histories of the aerodynamic stiffness and damping forces on the leeward cable were identified from the numerical results. It seems that there is always a positive work done within a period by the aerodynamic stiffness force, whereas a negative work by the aerodynamic damping force. The response characteristics of the leeward cable of the hanger of suspension bridge obtained in this study are identical with those of the wake-induced flutter widely discussed for the power transmission line. This implies that wake-induced flutter theory could well illustrate the underlying mechanism of the aerodynamic interference effects on the hangers of a suspension bridge.