Preferred solution, K12 – Appendix I Fatigue analyses
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This report describes the work performed in consideration of fatigue capacity of the selected concept (K12) in the concept development work of a floating bridge over Bjørnafjorden. Fatigue calculations have been performed for selected details in the bridge girder, the connection between columns and bridge girder/pontoons, mooring chains and stay cables. Large parts of the assessed structures have an acceptable fatigue utilization. However, some details are found to have insufficient fatigue life and need additional measures. Most notably these are the details in the bridge girder deck which are subjected to local traffic loads. A measure that has been applied during the current project phase is to increase the plate thickness in the deck to 16mm along the entire length of the bridge to increase fatigue robustness. Still, additional measures are needed in order to get acceptable fatigue lives. For future fatigue work it is proposed to developed a traffic load model based on historical/forecasted traffic data for the actual bridge location. This is expected to give a less conservative load model which will improve fatigue life. A sensitivity study has been performed on the traffic load model to identify the load reduction required to achieve acceptable fatigue lives. Several of the typical details in the deck currently have a calculated fatigue life of around 30 years. The required load model reduction to achieve acceptable fatigue life for these details is to use the medium range traffic distribution and reduce axle loads to 75% of the full FLM4 axle loads. Most of the girder deck details are expected to get sufficient fatigue life with a moderate reduction in the traffic load model. However, for the cut-out detail in the transverse frames around the longitudinal trapezoidal stiffeners additional measures are required. For this detail further design optimization remains to get a fatigue friendly design. Another measure that has been proposed is to utilize the asphalt stiffness in the local FE-analyses, which is believed to give reduced stresses due to a more realistic transfer of loads onto the steel deck. Another detail which currently have insufficient fatigue life is the connection between bridge girder and columns in the high part of the floating bridge. Here, insufficient fatigue life is found at axis 3 and 4 in the vicinity of the cast pieces at the top corners of the columns. At Axis 3 the calculated fatigue life is 44 years and 47 years for the girder side and column side of the corner respectively. At Axis 4 the calculated fatigue life is 89 years (column side). The recommended measure to achieve acceptable fatigue life for these locations is to increase structural dimensions locally. For future fatigue work it is also recommended that this connection is assessed by a more refined calculation method.