The objective of this Master Thesis is to design and compare three kind of functional bridges, continuous over two spans, with the same length. The first one will be a footbridge 4m width, the second one will be a Road bridge 12m width and the third one will be a Railway bridge 10,10/ 14,00m width where a system of track on slab is used The decks, in the three cases, must be designed in prestressed concrete and the prestressing layout, in each bridge, will be of two types: the first one will be internal and the second one will be extradosed.