Sustainability and cost-effectiveness of steel truss pedestrian bridges - A case study focusing on truss optimisation, deck systems and materials

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Examensarbete för masterexamen
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2019
Författare
Tjernlund, Josefin
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ABSTRACT The interest to design more durable steel bridges has increased in the last few decades. This is mainly to minimize extra costs associated with steel corrosion that requires cumbersome preventive measures, such as painting and airtightness of welded closed-sections, and extensive repair and maintenance work. For the case of pedestrian bridges, it is fairly common to utilize steel trusses made with standard hollow profiles in carbon steel as primary load-bearing elements of the structure, thus posing a high risk for corrosion. Such risks, and consequently extra costs, could be significantly reduced by implementing stainless steel instead of carbon steel. Moreover, the deck could also be replaced by more lightweight and durable materials such as fiber reinforced polymer (FRP) deck systems. Both stainless steel and FRP would lower the amount of reparation work which decreases the future costs over the service life of a bridge. The initial cost could, however, be higher than for the conventional alternatives. This dilemma is investigated through a case study comparing five different truss bridge solutions made in either carbon steel, stainless steel or FRP, showing the effect on material-, cost- and environmental parameters when optimising the bridge designs. The aim is to compare and evaluate the outcome when using these new approaches, with the main objective to show differences gathered from the case study. The results show that the initial costs increase as the corrosion steel is replaced with stainless steel. This upgrade of material cost more than twice as much, while FRP as a substitute to the steel deck has similar investment price. But for the total cost over a 50-year long service life, these implementations are more cost efficient for this specific bridge. The overall cause for this outcome is mainly the expensive corrosion paint that’s required to be added to the carbon steel periodically. However, the carbon dioxide emissions are increased when looking at the production of stainless steel, but decreased for FRP due to the much lower weight compared to an ordinary steel deck. Also the investigated truss optimisation were shown to contribute to the material efficiency of the bridge with a truss configuration of different profiles between chords and diagonals. It is also shown that together with the new materials and design concepts, it is possible to achieve lighter, more durable and less costly pedestrian bridges.
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pedestrian bridge, steel, truss, deck system, optimisation, stainless steel, FRP, life cycle cost, analysis, life cycle assessment
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