Structural fire safety has traditionally focused on buildings and has paid little attention to bridges. This paper aims to develop knowledge on the behavior of steel bridges under fire. It focuses on the study of a simply supported bridge of 12.20 m (40 ft.) span length designed by the Federal Highway Administration of the United States of America. The cross section of the bridge has five steel girders supporting a reinforced concrete slab, the slab not being structurally connected to the girders. A 3D model of the bridge has been built with the commercial software LUSAS using solid elements. The model is submitted to a hydrocarbon design fire that could be caused, for example, by the crash or the overturning of a gasoline tanker. The bridge response to such a fire was studied by varying the following parameters: magnitude of gravity load (dead and live) and the axial restraint (fixed or free). Results show that the times to failure of the bridge are very small (less than 10 minutes in all the cases analyzed) which gives almost no time for the firemen to reach the bridge in case of a fire event. Therefore some protective measures should be undertaken in these kinds of bridges if they are in a fire risk situation and its fire safety wants to be ensured. Furthermore, horizontal displacements of the bridge are big enough to require the consideration of the interaction between the bridge deck and the adjacent span or abutment.