Structural damping estimation for long-span bridges

Existing analysis techniques for the prediction of response of longspan bridges to wind and earthquake loads are generally based on a modal synthesis in which the structure is assumed to respond in one or several independent modes. In such analyses, damping generally cannot be estimated with the same approaches as those used for the mass and stiffness, and it is usually added to the modal equations after decomposition as a "known" percentage of critical damping. In the case of bridges, in particular, the total damping, of which part is mechanical or structural, is influenced substantially by the presence of a wind flow around the structure. The total damping is the sum of the mechanical and this aerodynamic damping. This paper presents the results of a study that attempts to separate and estimate the mechanical and aerodynamic damping of a prototype bridge structure using full-scale ambient vibration and wind measurements, finite element analyses of the bridge structure, and a wind tunnel model of the bridge deck section. The values obtained are compared with those traditionally used in design.