Concept Study and Analysis of a Floating Bridge
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- Institutt for marin teknikk 
A floating bridge is a structure carrying traffic across a body of water and whose supports floatson the surface. The history of the floating bridge goes as far back as around 2000 BCE, whentheir use was mainly military. Today, several large floating pontoon bridges help relieve traffic inand out of large cities and populated areas. In Norway, the National Public Road Administration(NPRA) have made plans to build floating bridges across two large fjords to replace the ferriesthat currently ship traffic across. One of these fjords is Bjørnafjorden, for which the NPRA havecome up with a few different concept solutions for crossing. One of these concepts was chosenfor this study to look closer at static and dynamic responses. The model of the concept bridge was created in ANSYS 17.1, and several different analyses wereperformed; modal, static, regular wave and irregular sea state for three storm conditions. Themain aim was to determine the dynamic response of the bridge in waves, to ensure traveller ssafety and comfort even during certain storm conditions. Criteria included limitations to maximumaccelerations in y- and z-direction, and maximum rotations about the x-axis. The modal analysis showed that some vertical and horizontal eigenfrequencies of the bridgemay coincide with environmental loads and could potentially be of concern. The static analyseswith environmental and traffic loads showed that the bridge would remain structurally safe formaximum traffic and winds and currents with a 100-year return period. Of some concern werethe responses to the regular wave analyses, as the results showed little coherency and were largerthan expected. The reason for this was somewhat unclear, and the responses to regular wavesshould be studied further. The results from the irregular sea states were more consistent andshowed that the bridge would be safe for traffic during storms with a 1-year return period. Formore severe storms with return periods of 10 and 100 years the extreme responses exceeded thecriteria limits, and would therefore not be deemed safe for traffic. This was not considered to beof concern, however, as the NPRA for safety reasons close bridges when wind speeds exceed 25m/s, which they are likely to do during these storms.