Gantry Cranes offer an economical solution to bridge cranes when the building is inadequate to support a bridge crane or the crane system is to be located outdoors. There are two kinds of gantry crane: truss girder gantry crane and box girder gantry crane. A box girder gantry crane is a gantry crane that utilizes a "box" configuration in fabricating the girder. This box girder design incorporates a four-sided box with a running surface plate for the hoist trolley attached to the bottom of the box. The advantage of the box girder is that it possesses greater loading capabilities and is able to span greater bridge distances. The crane is generally utilized in pairs with the hoisting mechanism operating on rails attached to the top of each box girder.