The gantry crane is so named because it is similar to a bridge in appearance. Its structure is similar to an overhead crane except the support is equipped with two footing seats with mobile wheel sets built on both sides. The footing seats can travel along ground railways. Some footing seats are equipped with rubber tires that make the crane capable of turning around like a vehicle. Gantry cranes are mostly placed outdoor, and the distance between the footing-seat pairs is normally 15m, with some large cranes having a distance up to 100m. Trolleys equipped on bridge railways are categorized as crab trolleys or rope trolleys.