A project is under way to replace the ferry crossings in Norway's highway E39 with fixed links, such as highway bridges or tunnels. This thesis research is on the crossing located at the Sognefjord, the widest and deepest of the straight crossings in the E39 highway. Previous thesis projects at IV-Consult have yielded a design for a floating bridge supported on twenty-two pontoons. The bridge is moored using a sub-sea cable system. The bridge design reaches a height of 70 m at its 465 m wide mid-span and is dimensioned on the basis of static calculations of the structural elements.
The goal of this thesis research is to calculate the dynamic response of the bridge system to environmental loads and to determine if the current bridge design is sufficient in relation to this response.
To reach this goal, several models have been developed for the structural elements that compose the bridge; the continuous bridge deck girder, the pylons supporting this girder, the floating pontoons supporting these and the sub-sea cable mooring system fixing the structure in place.
First a structural model for the bridge structure is developed with special attention being placed on the sub-sea mooring system. For these cables an internal design is made and a calculation method is developed to model and determine the internal hysteretic damping in the cables due to inter-wire friction. Second, a mechanical model describing the linear dynamic response of the pontoons for small rotations has been developed. The pontoons themselves are modelled as rigid bodies. Third, the bridge deck girder is modelled as an equivalent Euler-Bernoulli beam.
Finally, a load model is developed for the wave and current loads at the bridge location. Diffraction theory is used to calculate wave loads on the large pontoons and the current loading is identified and modelled according to prevailing design codes. Six critical wave load cases are formulated.
Models of the bridge structure are built using the SACS and Scia Engineer software packages. A non-linear solver is written in Python to implement the cable model for static calculation, utilizing Scia Engineer's non-linear solver. Verification calculations of SACS software results are performed.
The steady state response of the bridge structure is calculated using SACS software for the six critical wave load cases formulated. In conjunction with this analysis, the cable damping is calculated according to the cable model. The bridge deck motion and cable fatigue damage are evaluated and are found to be well within design limits, leading to the conclusion that wave loading will not lead to critical failure in the bridge design.
An analysis of vortex induced vibrations of the bridge system caused by cross-flow loading of the bridge pontoons is performed. The analysis is performed using Ansys Fluent in conjunction with the SACS Dynamic Response module to model the fluid-structure coupling. A large sensitivity to vortex induced vibrations is found for the bridge system, several potential solutions to this problem are presented and recommendations are made for further research into this phenomenon for the bridge design.
A verification calculation of the Fluent-SACS model introduced in this thesis is performed using a coupled wake oscillator model. The verification is based on only the cross-flow motion of a single pontoon in the bridge system and yields comparable results in terms of load and displacement amplitudes for both models.