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There are a number of constructive methods to construct the bridge superestructure. Girders can be launch or cast in situ. In the case of launching girdgers, we have 2 methods: Underslung and Overhead. Underslung Method For long spans, an alternative is cantilever segmental construction. Beam is started in the pier and built up as cantilever in two ways. In this way, cantilevers must be symmetrical. Segements can be cast in situ or precast and erect it on site. Another type is the incremental launching. A factory is built at the beggining of the bridge where the girder is constructed and launched as a cantilever. It is not required any prop since at the tip of the deck is placed a beam that gives support to the deck that comes behind. Incremental Launching References: http://www.nbmcw.com/articles/bridges/2586-launching-systems-for-segmental-bridges.html