The Great Belt is a bridge and tunnel link in Denmark. The tunnel part in the link constitutes the eastern part of the railway traffic. As in other building constructions, the tunnel has emergency exits that are sealed by fire-rated doors. The doors that are in the tunnel today have reached the end of their lifetime and are expected to be replaced soon. The Danish state has previously engaged Semcon and KISAB, into a project of exchanging the previous fire doors in the Øresund tunnel. The change includes a whole new design, construction and manufacture of the new doors. Everything is done based on today's diverse quality and safety standards. The project in Great Belt is still in a tender stage. However, Semcon and KISAB have good expectations of being able to get the project. Semcon's role in this project has been to develop different conceptual ideas on possible solutions that meet customer requirements and observed customer needs. The purpose of this thesis is to help Semcon to develop additional conceptual solutions that can be presented to the customers. The solutions generally mean that a new fire door design is developed and tailored to the requirements and needs of the Danish state, KISAB and Semcon. This thesis was based on two different working methods to work towards a result. The methods went hand in hand and complemented each other. The one method's contribution was most in the structuring of this product development project. Design thinking is a problemsolving method consisting of four phases dividing the product development project. The designer starts from scratch with minimal knowledge in the first phase and ends with completed and verified concepts. This method is iterative between its phases but provides a good structure and validation of overall results. The second method consisted of purely design work in the form of 3D modelling, solid mechanic calculations and finite element analysis. Based on collected data from theories and working methods, a result could be produced. The result of this project was developed concept ideas for various subproblems. These subproblems were raised and delimited earlier in the project's issue. The concept ideas produced responded to sealing solutions, locking mechanics, self-closing functionality, a durable frame structure, dimensioning and selection of various material profiles. Based on the collected results, conclusions could be drawn for each sub-problem. Some conclusions could be verified and other could be partially verified by mechanical engineers at Semcon and KISAB. The parts that were partially verified, were investigated and the uncertainties that were identified were suggested as a continued future work.

Keywords: Construction, Design, Concept, Thesis.