Strongback (girder)

A strongback is a beam or girder which acts as a secondary support member to existing structure. A strongback in a staircase is usually ordinary two-by dimensional lumber attached to the staircase stringers to stiffen the assembly. In shipbuilding, a strongback is oriented lengthwise along a ship to brace across several frames to keep the frames square and level. 
Some rockets like the Antares (rocket), the Falcon 9 and the Falcon Heavy use a strongback to restrain the rocket prior to launch. This structure reclines several degrees away from the rocket to clear the launch, either at the moment of launch or in some case a few minutes before. 