Dynamic modeling and analysis of hanger cables with spacers and dampers for vibration mitigation

Many suspension bridges are designed with hangers composed of pairs of cables. The hanger cables are featured by low fundamental frequencies and extremely low inherent damping, and thus subjected to various wind-induced vibrations, typically including wake-induced and vortex-induced vibrations. Spacers are required to connect the cables for suppressing wake-induced vibrations, which nevertheless provide nearly no additional damping. Damping devices are also required to suppress other types of vibrations, e.g., vortex-induced vibrations. A combination of spacers and dampers for vibration mitigation of hanger cables leads to a complex dynamic system. This paper focuses on modeling dynamic behaviors of such a system. A generalized and dimensionless numerical model is first established. Parametric analysis is then conducted to investigate the influences of spacers and dampers on system frequency and damping. It is found that equally distributed spacers with a dimensionless stiffness coefficient larger than 102 can be considered as rigid and further increasing stiffness cannot increase the system frequencies but leads to increased number of localized modes. A combination of near-anchorage dampers and viscoelastic spacers can provide adequate damping to both in-phase and out-of-phase cable vibrations. Variation in tension, mass and length ratios of the two cables is not effective in improving damping provided by viscoelastic spacers to in-phase vibrations.