Efficiency of an External Damper in Two-Cable Hybrid Systems

External dampers are commonly used on site to control excessive cable vibrations on cable-stayed bridges. The optimum damper size and the corresponding maximum attainable damping ratio of a damped cable can be predicted using existing design tools, such as the universal damping estimation curve. However, because an external damper is typically installed very close to the cable anchorage on the bridge deck, its effectiveness in suppressing vibrations of relatively long cables would be limited. The hybrid system solution can overcome this limitation by interconnecting a damped cable with its neighboring ones with transverse crossties to enhance its in-plane stiffness. However, when a single damped cable becomes a part of a hybrid system, its bending stiffness and sag would be altered, thus having an impact on the optimum damper size and the maximum achievable damping ratio. The current study evaluated how the configuration change of a damped cable from an isolated one to be a part of a hybrid system would affect its damping property. After a brief review of an analytical model of two-cable hybrid systems, a parametric study to investigate how variation in the crosstie stiffness and the spacing between the crosstie and the damper would influence the damping property of a hybrid system is presented. Damping estimation equations for predicting the optimum damper size and the corresponding maximum attainable damping ratio of two-cable hybrid systems are presented. The validity and applicability of the proposed damping estimation equations were examined using 12 symmetric and 12 asymmetric two-cable hybrid systems, the system parameters of which were taken as the same as those on real cable-stayed bridges. Results show that the proposed damping estimation equations can provide satisfactory prediction of the optimum damper size and the corresponding maximum achievable damping ratio for two-cable hybrid systems having different system properties and layouts. Numerical examples are presented to illustrate the application of the proposed damping estimation equations in the practical design of hybrid systems. (c) 2017 American Society of Civil Engineers.