Utvikle en finite element modell av Søsterbekk bru nummer 2 og utføre strukturelle og modale finite element analyser på modellen

Developing a finite element model of the Søsterbekk bridge number 2 and performing structural and modal finite element analyses on the model.

Ofotbanen is one of the most important railway lines in Norway, where huge quantities of iron ore are transported from Sweden for shipment in Narvik, in addition, large parts of Nordland and Troms and Finnmark are supplied with goods transported on Ofotbanen, the fishing industry also depends on Ofotbanen for rapid dispatch of fish. Considering the geopolitical situation, as well as Sweden's and Finland's membership in NATO, the route has also gained more importance for military logistics and movement in the north. Total traffic load on Ofotbanen in 2023 was over 33 million gross tonnes.

Søsterbekk bridge 2 is a post tensioned concrete bridge positioned at km 35,989 on Ofotbanen in Narvik. The bridge has two spans, one pillar and a horizontal curvature of 350 metres radius. The vertical curvature is 0, and there is an 15,5 ‰ incline towards east. The bridge has fixed support in one land vessel and simple support in one land vessel. The bridge is originally constructed in 1985 and put into operation in 1987. It was constructed to sustain 25 tonne axle load trains. Today the upper limit for the trains trafficking Ofotbanen is 31 tonnes axle load. The traffic on Ofotbanen is also increased since construction. This puts a lot more strain and wear cycles on the bridge compared to what it was constructed for.

Building new concrete bridges on Søsterbekk will be very expensive and will entail large greenhouse gas emissions and constitute major negative environmental interventions that will affect nature, cabin neighbors and reindeer husbandry for a long time. At times, the work may also cause traffic challenges.

Because of this Bane NOR needs to acquire an insight into the structural health of the bridge. In June and July of 2025, the bridge will be instrumented with accelerometers, inclinometers, strain gauges, laser and cameras to acquire data from the bridge. This data will be used to monitor the structural health of the bridge and to adapt a finite element model to the real-world responses and movements of the bridge, so that the model will be a true digital twin of the bridge.

There are plans to use this data and digital twin to calculate remaining lifetime of the bridge and to see if there are any possibilities for strengthening the bridge to sustain axle loads of 31 tonnes or higher in the future. The allowed axle load on Ofotbanen is planned to be increased first to 32,5 tonnes and later to 35 tonnes.

The task for the thesis of the candidate is to make a finite element model of the bridge, perform static structural analyses for relevant loads modal analyses to acquire eigenfrequencies and mode shapes of the bridge. The challenge in the modelling of the bridge is to model it as accurate as possible including the horizontal curvature. The selection of a proper element technology is of high importance. There will be a need to do research to find an adequate mesh refinement of the model and the correct supports in the analysis software. The results of the static structural and modal analyses will have to be presented in fine detail, with relevant comparison and validations of the results.

Bane NOR will provide technical drawings of the bridge.

Supervisor: Patrick Norheim Berg, MSc Track engineer at Bane NOR.