Life cycle assessment of a single span bridge : Study of the Bryngeån Bridge and four steel girder alternatives

Sammanfattning

When designing a railway bridge, it is important to consider not only the initial construction cost but also the costs and environmental impacts that occur during the entire service life. These include inspection, maintenance, repair and eventual replacement. By discounting all future cash flows to present value, a life-cycle cost (LCC) analysis can be carried out. In parallel, life-cycle assessment (LCA) makes it possible to evaluate the environmental impact associated with the production of the materials, construction, maintenance and end-of-life of the bridge.The purpose of this thesis is to compare and identify the most effective girder configuration for a simply supported railway bridge, from both an economic and an environmental perspective. A reference solution based on the existing design was studied together with four alternative steel girder cross-sections: a double I-girder, a double box girder, a semi box girder and an inclined box girder.The LCC analysis was first performed for a base case with conventional structural steel, a span length of 48 m and a discount rate of 4 %. The model includes investment cost, inspection and maintenance activities, repair and replacement over the design service life. The results for this base case show that the inclined box girder alternative has the lowest LCC. In order to test the robustness of these conclusions, several sensitivity analyses were carried out: (i) replacing conventional steel with stainless steel in the main girders, (ii) varying the span length to 32 m and 60 m, and (iii) varying the discount rate between 2 % and 7 %.The life-cycle assessment confirms that the environmental impact is dominated by the amount of structural steel. Alternatives with lower steel mass systematically exhibit lower emissions, whereas the contributions from welding and coating are relatively small for all span lengths considered. Consequently, the double I-girder shows the lowest environmental impact, followed by the double box girder, while the inclined box girder has the highest impact due to its larger steel consumption. When the LCA results are converted to monetary units and combined with the LCC, the double I-girder becomes the preferred solution.Finally, it should be noted that the numerical results depend on the specific assumptions adopted in this thesis, such as unit prices, discount rate, service life, maintenance strategy and background LCA data. Therefore, the values and rankings presented here should be adjusted if the methodology is applied to other bridge types, locations or boundary conditions.