Long Span Buoyancy Bridge with Submerged Cable Anchoring

The purpose of this MSc thesis is to find a feasible concept for a circa 4500 m long buoyancy bridge, which is located at the Sognefjord in Norway. The concept should be structurally and aesthetically competitive. In contrast to bridges on shore with fixed supports, a buoyancy bridge is supported on floating pontoons. As result, loads will cause the buoyancy bridge to displace, and displacements will in turn cause internal forces in the structure. For these reasons, the environmental loads on buoyancy bridges are usually minimized by placing the bridge girder as low as possible above the water level (small wind load) and by using small spans (small bending moment and shear force on the bridge girder). In this study, the limits within civil engineering will be tested by trying to find new possibilities for a buoyancy bridge, which consists of 20 spans of 200 m and a large central main span of 465 m. Furthermore, the bridge deck will elevate up to 80 m above water level. This span and elevation are required at the 1000 m deep Sognefjord to create a large fairway clearance. A buoyancy bridge with these properties is unprecedented. For the buoyancy bridge concept in this study, a whole new approach has been made. 22 long, slim cylindrical shaped pontoons are used, which provide upward buoyancy forces and restoring moments to limit the rotations of the structure. The slim shape of the pontoons will lead to smaller water loads. The radii and lengths of the pontoons vary respectively from 12 to 20 m and from 115 to 202 m. For common buoyancy bridges, the relative position of the pontoons is maintained by the superstructure. However in this case, the dimensions of the cylindrical pontoons are so large, that a superstructure with plausible dimensions will not be able to restrain the movements of the massive pontoons. Therefore, an anchoring system, consisting of 2 main cables with diameters of 1200 mm and 44 cables of 350 mm, has been designed to maintain the relative positions of the pontoons as much as possible. From the top view, the anchoring system looks like two mirrored horizontal suspension systems, which restrain the displacements in the direction parallel to the fjord. The displacement due to the maximum combined wind and water load is approximately 6 m for the circa 4500 m long bridge. Separate lattice bridge girders with a width and height of respectively 24 and 25 m are designed, which have hinged like supports, except in the plane transversal to the superstructure. In this plane, the rotation of the bridge girder around its longitudinal axis is coupled to the rotation of the pontoons, and therefore limited by the restoring moment of the pontoon. Furthermore, the torsional rigidity of the lattice girder varies along its length. This way, a light-weight and flexible bridge girder is possible, which is capable of following the rotations instead of trying to restrain them. For the piers, a form study has been done. The concept gives rise to a lot of new possibilities, but it also has limitations. The results of this study are only valid when the recommended erection method is used. Different erection methods will induce different forces into the structure. This can affect the capacity and the displacements of the structure. Therefore, the structural design and the erection design should be defined together. This study provides the first steps to the design of the buoyancy bridge. Much more investigation is needed before the proposed concept can be deemed reliable. The global main structure is considered in this study, but no detailed calculations are done. Designs of several important parts, i.e. the connections, the supports, the piers, etc. should be done in next studies. Also, second order effects, eccentricity, dynamic effects, fatigue, impact loads and more should be investigated. Although the design only have a concept value, this study shows that a structurally and aesthetically competitive buoyancy bridge for the Sognefjord is feasible and it is recommended to conduct further investigations on this promising buoyancy bridge concept.