Fjord crossings

Researchers mounting Lidar equipment on the Lysefjord bridge

Fjord Crossings Load and Response Modelling: 

The Norwegian government has adopted an ambitious plan for a ferry-free highway along the 1100 km coastal route South-West in Norway, between Kristiansand and Trondheim. Over a twenty years long period, eight new and challenging fjord crossings are planned for construction.

The extreme width (up to 5 km) and depth (up to 1 km) of several fjords to be crossed (e.g. Bjørnafjorden) call for new technological solutions, such as floating bridges, submerged bridges, suspension bridges on a floating supports. An example is the floating bridge concept over Bjørnafjorden.

The design partly builds on the expertise developed in offshore engineering, in relation to hydrodynamic loading, anchoring systems etc. The UiS’s expertise in hydrodynamics, numerical flow modelling, aerodynamics, marine operation etc. is very relevant to these projects, to arrive at a safe and economic design. The UiS will continue to perform research directed to the following aspect s of the offshore bridge design:

Improved assessment of the design environmental conditions

Together with its research partners from the Norwegian Center for Offshore Wind Energy (NORCOWE) and the Technical University of Denmark, UiS has been pioneering applications of the remote wind sensing to fjord crossing.

The UiS team directed two ground breaking measurement campaigns with lidars, starting from an existing bridge (Lysefjord), and then continuing in Bjørnafjord, where a floating bridge is being considered for construction. This has led to other lidar measurement campaigns where UiS’s expertise is used to develop and analyse wind measurements above the sea surface.

The significance of such the data is that they concern the very location of a future crossing, without the distorting effect of the local terrain which is unavoidable in the long-term measurements from masts on land.UiS has also performed a statistical analysis of the wave measurements in the Sulafjord, with emphasis of the wave propagation from offshore into the fjord.

The UiS will continue to perform research on environmental conditions, considering e.g. uncertainties in the lidar measurements, joint probabilities of wind and wave conditions etc.

Validation and improvement of the buffeting wind load model

One of the remaining issues in the modelling of the buffeting loads on the bridge girders is the relationship between the spatio-temporal characteristics of the incoming turbulence and the associated characteristics of the fluctuating forces (drag, lift and overturning moment) along the bridge deck.

Wind tunnel studies suggest that the influence of the small-scale turbulence is attenuated over the bridge deck width (e.g. from 10 to 30 m), but that the uniform bridge girder “obstacle”, more than several hundred meters long, may introduce a higher span-wise correlation of the forces compared to the correlation in the incoming wind. The validity of these findings is hampered by the disproportionally high-frequency turbulence, due to scaling limitations in wind tunnels.

UiS is currently planning a measurement campaign from an existing bridge to fill-in this gap, and acquire a unique set of full-scale observations of the wind gust interaction with a bridge-deck girder. The proposed measurements of turbulence around the bridge deck will be integrated with the ongoing long-term observations of wind conditions and the wind-induced response launched at Lysefjord bridge in November 2013.

Coupled time-domain analysis considering wind, waves and ocean current

Similarly to challenges regarding the load and response modelling of offshore wind turbines, bridges on floating supports require an integrated assessment of wind, wave and current loads.

UiS has developed an in-house software for this purpose, which combines the simulation of the turbulence wind field and the associates wind loading, with hydrodynamic loads on a tension leg platform type of bridge foundation, using a finite element model in Abaqus as the basis. The tool, which allows one to study the relative importance of the various environmental factors for the design, will be further developed and adapted to the specific bridge designs.

Bridge cable vibrations

Long-span bridges are normally designed as cable-supported structures, in which cables in different arrangements support bridge deck and transmit the dead, service and environmental loads to supports and foundations. While cables are essential for safe operation of a bridge, they are extremely sensitive to various types of dynamic excitation and thus susceptible to fatigue and other types of damage.

The wind-induced vibrations of cables include phenomena such as rain-wind vibrations of stay cables, dry cable galloping, parametric excitation, tower wake excitation of hangers, vortex-induced vibrations, ice- wind vibrations etc. Improved understanding of various types of the vibrations is vital for their effective suppression, by establishing necessary levels of damping to prevent the excessive response.

UiS is involved in the joint international research focusing on the aerodynamic countermeasures to limit the galloping cable vibrations. A novel cable surface design has been proposed and will be further optimized.