Vibration of inclined sag cables with oil dampers in cable-stayed bridges

This paper presents both free and forced vibration analyses of inclined cables, including the effects of cable sag, equipped with oil dampers in cable-stayed bridges by using a hybrid method recently developed by the writers. The hybrid method readily takes into account cable sag and inclination, damper stiffness and direction, and other parameters. The obtained results confirm that the installation of an oil damper to a stayed cable near the support can increase cable modal damping ratio and thus reduce cable vibration in most cases if the parameters of the damper are properly selected. However, it is found that if the sag parameter λ2(the ratio of the elastic-to-catenary stiffness) of an inclined cable falls in a certain range around one of the frequency avoidances of inclined sag cables, the cable modal damping ratios provided by the damper in the corresponding symmetric or antisymmetric or two hybrid modes can be significantly reduced. To facilitate the design of oil dampers for inclined sag cables in cable-stayed bridges, two sets of graphs are provided with respect to the achievable maximum modal damping ratio, the optimum damper size, and their relations with cable sag and inclination. One typical example is presented to demonstrate how to use these graphs.