Many old moveable bridges have exceeded their technical lifetime. Replacement of the foundations is expensive and often unnecessary if the new bridge superstructure is lighter than the old one. With stricter design guidelines being in effect nowadays, a conventional orthotropic steel deck design will end up being heavier. A lightweight fiber reinforced polymer deck supported by steel girders is proposed as a solution. In this thesis the potential mass reduction resulting from a lightweight FRP deck is analyzed. Analyses are initially performed under the assumption that realizing composite action will generate the best results. However, a finite element model of a bolted connection between an FRP deck and steel girders indicates that the strength of such a connection is too low to efficiently realize composite action. An alternate solution where the deck is draped over the main girders with a thin FRP layer is considered and outperforms the composite action design. The design with FRP but without composite action is compared with a standard orthotropic steel design. The FRP design weighs order of magnitude 10% less than its orthotropic counterpart. However, it is still significantly heavier than the original component in need of replacing.