Underrunning or underhung cranes typically run on the lower flange of the runway beams. Underrunning cranes can be floor supported or ceiling hung. For floor supported cranes, footings might be required depending on crane capacity and span. A major advantage to underrunning cranes is that they can be supported by ceiling structures if crane loads are considered during new building design or the expansion of existing buildings. In some cases, existing ceiling structures can be properly reinforced to allow for the installation of underrunning cranes. Double girders underrunning cranes can be designed for higher capacities and special applications, but are typically more expensive to design and install. Single girder underrunning cranes are the most common and are used for lighter applications where top running cranes are not ideal. One major disadvantage to underrunning cranes is decreased hook height since the crane runway, the bridge and hoist all hang below the crane support structure and the structure must be installed below overhead obstructions such as sprinkler systems, gas lines, lighting, etc.