Dynamic Response of Suspension Bridge with Floating Towers

Abstract

The Norwegian Public Road Administration has been investigating the possibilities ofcrossing the fjords on the west coast of Norway for the coastal highway E39 project.For the 5 km wide Bjørnafjorden a floating three span suspension bridge with two py-lons standing on tension leg platforms (TLP), is considered. A parametrized model of afloating multi-span TLP suspension bridge has been created, where the parameters havebeen given the planned geometry of the Bjørnafjorden bridge concept. A hydrodynamicanalysis conducted on a FE-model of a pontoon where added mass, damping, restoringforces and the wave transfer function were obtained. Buffeting theory and aerodynamicderivatives were used to obtain the buffeting forces, stiffness and damping terms. A gener-alized method was developed to couple system matrices and forces from wind and wavesin modal coordinates.Modal-, response- and motion induced instability analyses, all accounting for hydrody-namic added mass, damping and restoring forces and aerodynamic damping and stiffness,have been conducted in the frequency domain. The environmental effects resulted in sig-nificantly lower eigenfrequencies. A response analysis was carried out for combinations ofdifferent stationary parameters for wind and waves. It was evident that wind governed theresponse in transversal- and waves for the vertical and rotational directions for modes withhigher corresponding eigenfrequenciees. The most participating modes was the first modein transversal direction and the first mode in vertical direction, obtained from the modalanalysis, frequency response spectra and standard deviations. An aerodynamic motioninduced instability limit was found to de 88 m/s at 1.5217 rad/s