Estimation of damping ratio of cable-stayed bridges for seismic design

This paper proposes a method to evaluate the structural damping ratio of cable-stayed bridges, based on energy dissipation. As demonstrated by many field forced-excitation tests, the damping ratio of cable-stayed bridges varies from bridge to bridge. This is due to the fact that the energy mechanisms predominant in the bridges studied are different. Therefore an analytical approach capable of evaluating the damping ratio from structural components is desirable. By dividing a cable-stayed bridge into several substructures in which the energy dissipation mechanism can be regarded as the same, it is proposed for each substructure to evaluate the energy dissipation function, which relates the amount of energy dissipation, with either the strain energy or the displacement at specific points in the substructures. By evaluating the overall energy dissipation and strain energy in the whole bridge, the damping ratio for the desired mode shape can be obtained. The proposed method is applied to a cable-stayed bridge model. Dependence of the damping ratio on oscillation amplitude and cable type of the bridge model is predicted with reasonable accuracy by the proposed method.