Top running cranes run on top of a fixed track, typically crane rail, placed on top of runway beams. The support structure for the runway beams can be floor supported or incorporated into the building support structure during new building design or the expansion of existing buildings. The advantage to incorporating the crane load into the building design structure is that additional footings will not be required if a crane is installed at a later date. Top running cranes can be single girder or double girder, structural steel or box girders depending on the span and capacity required. Top running double girder cranes offer more hook height since the hoist trolley runs on top of the bridge beams.