Separation of the contributions of aerodynamic and structural damping in vibrations of inclined cables

Large amplitude cable vibrations have occurred on several major cable-stayed bridges. Complex mechanisms including rain-wind excitation and possibly cable–deck interaction have been responsible, but the extremely low level of damping of bridge cables has been an important contributory factor. Measurements of cable damping have been undertaken on several cable-stayed bridges, but previously, the full contribution of aerodynamic damping, which could be a significant proportion of the total measured damping, has not been considered. To address this issue, theoretical expressions for the aerodynamic damping are proposed for the general case of an inclined cable and an arbitrary wind velocity, for both in-plane and out-of-plane vibrations. Damping estimates from cable vibration tests during construction of the Second Severn Crossing cable-stayed bridge, with varying wind velocity, have shown the expression for in-plane cable vibrations to give good estimates of the actual aerodynamic damping. The remaining contribution from structural damping is also determined and the effects of the injection of corrosion prevention wax into the cable sheaths, temperature, and interaction with other cables are considered.