A girder bridge, in general, is a bridge built of girders placed on bridge abutments and foundation piers. In turn, a bridge deck is built on top of the girders in order to carry traffic. There are several different subtypes of girder bridges:
- A rolled steel girder bridge is made of I beams that are rolled into that shape at a steel mill. These are useful for spans between 10 meters and 30 meters (33 feet to 100 feet). Rolled steel girders are practically available with a web height of up to one meter (3 feet).
- A plate girder bridge is made out of (mostly) flat steel sections that are later welded or otherwise fabricated into an I beam shape. Plate girders can have a greater height than rolled steel girders. Plate girder spans can be used for spans between 10 meters and more than 100 meters (33 feet to more than 330 feet). The web (vertical section) of a plate girder can be taller than that of a rolled steel girder, providing greater strength than a rolled steel girder. The thickness of the top and bottom flanges of a plate girder does not have to be constant; the thickness can be changed (typically at a field splice) to save on material costs. Stiffeners are occasionally welded between the compression flange and the web to increase the strength of the girder.
- A concrete girder bridge is made of concrete girders, again in an I beam shape. The concrete girders can be either prestressed cast concrete or post-tensioned girders. Concrete girder bridges are best for spans between 10 meters and 50 meters (33 feet to 164 feet). Prestressed, precast concrete girders are readily available.
- A box girder bridge is built from girders in a rectangular box shape instead of an I beam shape.
An I beam bridge is simple to design and build, and works well for straight spans. However, if the bridge needs to be curved, the beams are subject to twisting forces (torque). This can be alleviated by building several shorter, straight spans with a curved bridge deck, or by using box girders. Building metal box girders is more difficult, though, because the welding of the inner corners between the flanges and the webs has to be done either by a robot or a human, depending on who can fit inside.