A continuous superstructure for the Sognefjord bridge

Abstract

For the E39 highway in Norway a project is underway to replace the ferry crossing in the Sognefjord with a fixed crossing. Previous thesis projects have resulted in a design for a 4500 m long buoyancy bridge which consists of 22 concrete pontoons that carry a steel truss superstructure. To reduce lateral movements the pontoons are fixed to a submerged anchoring cable system. In the middle of the bridge a 400 m wide, 70 m high ship fairway is created. In previous bridge designs the superstructure consisted of separate girders for every span which were connected to the pontoons through hinges. The purpose of this research was to investigate the structural feasibility of creating a continuous superstructure without internal hinges for the Sognefjord bridge. After making a design for a continuous CHS steel truss superstructure, behaviour of the whole Sognefjord bridge with the new superstructure was researched for different load combinations. It was found that maximum lateral displacement of the bridge is 27 m, while maximum longitudinal displacement is 46 m. These were deemed acceptable values. A research was conducted into which parameters influence bridge behaviour the most. It was found that of the bridge structure, rotational stiffness of the pontoons influences bridge deformations the most. A two times higher rotational stiffness of the pontoons leads to a maximum reduction in lateral bridge displacements of 44%. The stiffness of the superstructure was found to have only minor effect on bridge behaviour. Internal loads in the superstructure were found to be mainly determined by displacements of the top of the pontoons, upon which the superstructure rests. Internal loads in individual truss members under a ULS storm situation were investigated. Member stress levels under a ULS storm are very diverse in value, with a maximum peak member stress of 590 N/mm², resulting in a unity check for stability of 1.36. Under reduced bridge deformations from double rotational stiffness of the pontoons, member stress in de superstructure on average drops by half. Peak member stress in the superstructure under a ULS storm with double pontoon rotational stiffness is 293 N/mm², resulting in a unity check for stability of 0.67. Preliminary investigations into bridge dynamics and ship collision were performed. Vortex-induced vibrations of structural elements, as well as pontoon displacements and shockwave effects under ship impact are challenges that require more investigation. The results of this research suggest that creating a continuous bridge girder without internal hinges for the Sognefjord buoyancy bridge is structurally feasible. This would require doubling the rotational stiffness of the pontoons, which is expected to come with large material costs. More in-depth research into other load situations is recommended.