Effect of Damping on the Dynamic Responses of a Floating Bridge in Wind and Waves

Abstract

The E39 Coastal Highway Route Project is being developed to reduce the travel time between Trondheim and Kristiansand in Norway and several ferry connections are planned to be replaced by floating bridges or submerged floating tunnels. In this study, a curved floating bridge design for crossing the Bjørnafjorden from COWI is considered. The design of the bridge is a 5 km long curved structure, supported by 19 floating pontoons and a tower with suspension cables close to one end of the bridge. Since it is a curved bridge design, no mooring system is considered. The long and slender bridge will be exposed to severe environmental conditions of waves, swell and wind in the fjord. The complex resonant responses with different natural periods and eigenmodes excited by the environmental loads and the resulting large bridge girder structural responses are the main concerns for design. Damping plays an important role in reducing the resonant responses in such conditions. In this paper, various damping effects (including potential and viscous damping of the pontoons, structural damping of the bridge girder, aerodynamic damping of the bridge) on the global dynamic responses of the complete structure are investigated numerically using a coupled numerical model. The 100-year environmental conditions are considered and numerical simulations using SIMA are performed. The viscous damping is modelled by using the 𝐾�𝐶� number dependent drag coefficient and applied to the time-domain simulations. It is found that the viscous effect on the given pontoon design is not significant in the vertical motion due to small heave responses. Instead, the structural damping of the bridge girder and the potential damping of the pontoons are more important. On the other hand, the viscous effect is noticeable in the horizontal motion responses and reduces the cross-sectional loads on the bridge girder. Moreover, the bridge girder weak axis bending moment about the local y-axis is a governing design parameter, which is mainly induced by the static effect due to the difference between the distributed buoyancy of the pontoons and the distributed weight of the bridge girder. As a result, prior to improving damping effects on the dynamic responses of the bridge, it is necessary to increase the number of pontoons to reduce the weak axis bending moment.