A plate girder is a type of beam built from plates of steel that are either bolted or welded together. The purpose of the plate girder is to build a beam that is larger than anything that can be built by a steel mill or factory. This type of girder is usually used to make certain types of bridges, and the girders themselves are very often in the shape of an I-beam. The size and shape of the girder allows builders to construct bridges that are much longer and heavier duty than bridges constructed with other designs.
The most common application for a plate girder is the construction of railroad bridges. Plate girder bridges are necessary for railroad use because the engines and cars that pass over the bridge will be exceptionally heavy, which means the bridge will need to be heavy duty enough to support the weight. The plates are welded or bolted together in a specific configuration for optimal strength and reliability, and while these spans are not likely to be exceptionally long, they will be durable enough to accommodate railroad use. I-beam girders are less likely to be used on this design in favor of box girders, which feature four walls to enclose the beam.
Along the length of the plate girder beam, vertical pieces of steel called ribs are welded in place to add structural rigidity. These ribs help the girders resist buckling or other damage from weight or movement of the bridge under load. The spacing of the ribs can vary according to the length of the span and purpose of the structure, though it is likely that numerous ribs will be present on a single span. Additional supports may be welded in areas of the bridge in which excess forces are likely to be experienced. These may not be regularly spaced as the other ribs will be.
In some cases, the plate girder bridge may be built in conjunction with other materials such as concrete. This composite bridge is strong and may accommodate difficult building situations. The deck of the bridge can be wood or concrete, or it may simply be the railroad ties only. If the bridge will span a long length, or if the bridge goes over a waterway, concrete piers may support the weight of the bridge and the girders themselves will be fastened to the concrete for added strength and rigidity.