Several bridge concepts for crossing deep and wide fjords along E39 at the west coast of Norway have been developed the last years. One of the most challenging fjord crossing is suspected to be the crossing of Sulafjord, 3 to 5 km wide, 400 m deep and with presence of relatively large swell waves.

A suspended floating bridge concept is a marine slender flexible structure with large volume elements as floating support. The hydrodynamic actions on the floaters is an additional excitation compared to a traditional suspended bridge with fixed piles. In order to assess the effects of this excitation, it is important to consider the whole system and accurate hydrodynamic methods.

While the superstructure type — a suspended bridge — is set, the type of floating foundation remains open. From the offshore experience, it is seen that different types of floaters are used for moored platforms, and these floaters have significantly different characteristics in particular with regards to wave response and stability.

The design requirements for an offshore platform differ greatly from those of a suspended floating bridge crossing a fjord. For a floating bridge, the payload requirements are not the most challenging, while it is more difficult to limit the tilting and dynamic excitation of the tower (mounted on the floaters). The bridge beam is suspended at the top of the towers and will respond to any excitation due to motions of the tower tops.

A global numerical model of the bridge to simulate nonlinear dynamic response due to regular and irregular waves is built. The numerical model of the bridge is simplified from a structural point of view. However, the dynamical properties and eigenmodes are verified against a more detailed structural model. Together with a 50-year long continuous time-series of wind, wind waves and swells a study of the bridge operability and extreme responses for different floater concepts is conducted.

Normally the design phase should aim at avoiding any natural periods to fall within the wave frequency domain. This seems difficult for the proposed 3-span floating suspension bridge, instead solutions to minimize the excitation from waves for wave periods around the given bridge eigenperiods are sought.

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