Definition - What does Girder mean?
Girders are main horizontal support members of structures, such as drill rigs, and support other structural members, like beams. They are also known as beams, but all beams are not girders. They usually have a cross-section like an "I", with two flanges and a web. They also come in other shapes such as box or "Z". Girders have the capacity to carry dynamic as well as rolling loads and transfer the load to the columns on which they rest.
Trenchlesspedia explains Girder
Plate and box girders are used in drilling rig decks. They are sometimes of long spans and take much greater loads than a beam, and support smaller beams. Girders are of different types, such as plate girder, gantry girder and box girder. Plate girders are made using three rectangular plates assembled and welded together. Gantry girders are used in cranes. Box girders are formed by using steel plates to form a rectangular box i.e. like twin plate girders.