Mega Floating Concrete Bridges

Abstract

Introduction This graduation project has been initiated to research the technical feasibility of floating bridges. The project has been done in co-operation with the FDN engineering company and Delft technical university. A design is made of a continuous pontoon floating bridge, which connects a mainland to an island. Floating bridges can be constructed where conventional bridges are impractical (under the conditions that are described in section 1.4.2). The buoyancy forces support the bridge in the vertical direction and the mooring system in the horizontal direction. The project contains also a survey of the most known floating bridges in the world. Problem definition Which design limitations and structural parameters can ensure stability of the continuous pontoon floating bridge under wind and wave load? Research The objective of this thesis is to understand the response of the continuous pontoon floating bridges under wind-wave load and traffic load that is described in the euro code which enables us to optimize the structural design procedure. That can be accomplished by studying the influence of the pontoon dimensions and the pontoon connector rotational rigidity on the global stability of the bridge. Determination the principles of response reduction and converting the displacement into internal forces are a relevant part of this thesis as well as the mitigating of the local environmental loads to be redistributed along larger parts of the bridge. The scope of the graduation project is very wide. It contains an overview of floating bridges; its hydrodynamic behaviour will be defined and due to the limited period of time will be not analyzed. The hydrostatic behaviour and analysis of continuous pontoon floating bridges will be researched as a multi-body slender structure with flexible connections. Also the conceptual design of the pontoons and the connections and the shape effects were researched. Detailed design calculations for the case study are included in this thesis. Results Environmental loads are the main loading on the floating bridge. Because of the random form of the sea wave forces and the wind force, it is difficult to expect the precise value and direction of loading on the bridge. The environmental loads twist the bridge and excite it in the horizontal and in the vertical direction. When it is possible to construct a sliding pile mooring system to introduce the wind and wave load in the horizontal direction, the floating bridge will have a satisfactory stability. The efficiency of the mooring cable is lower than the sliding pile due to the relatively large compliance range. That is valid also for the vertical displacement; the bridge response will be introduced by the bridge flexural rigidity, the bridge mass, the water spring, the water damping and the pontoon connector stiffness when discrete pontoons are used. The linear theory is applied to determine the sea wave load. The hydro-static and the hydro-dynamic analysis of a multi-body slender structure consisting of rigidly or flexibly connected elements will be made.. The design procedure of the mooring cables of the offshore structure is applied to design the mooring system. Conclusions and recommendations The connection stiffness of the pontoon has a large influence on the bridge response. The ratio connection stiffness over pontoon stiffness is very important for the bridge structural behaviour. When this ratio is smaller than 10-8 the bridge behaves as rigid bodies connected by hinges. When this ratio is larger than 10-3 the bridge behaves as a continuous elastic beam. The largest deflections, moments, and shear forces occur in the ends of the bridge. Additional supports, masses, or dampers at these ends can reduce these moments and forces strongly. Embedment steel crossheads are used to introduce the large mooring line forces without damaging the concrete walls of the pontoons. Further optimization of the pontoon length is recommended. Fatigue of the prestressing steel and the concrete are shown to be critical and need to be further investigated.