A girder bridge, in general, is a bridge that uses girders as the means of supporting the deck. A bridge consists of three parts, the foundation, the superstructure, and the deck, a girder bridge is very likely the most commonly built and utilized bridge in the world. Its basic design, in the most simplified form, can be compared to a log ranging from one side to the other across a river or creek, in modern girder steel bridges, the two most common shapes are plate girders and box-girders. The term girder is used interchangeably with beam in reference to bridge design. However, some authors define beam bridges slightly differently from girder bridges, a beam may be made of concrete or steel - many shorter bridges, especially in rural areas where they may be exposed to overtopping and corrosion, will utilize concrete box beams. The term girder is used to refer to a steel beam. In a beam or girder bridge, the beams themselves are the support for the deck. Material type, shape, and weight all affect how much weight a beam can hold, due to the properties of inertia, the height of a girder is the most significant factor to affect its load capacity. Longer spans, more traffic, or wider spacing of the beams will all result in a deeper beam. In truss and arch-style bridges, the girders are still the main support for the deck, all bridges consist of two main parts, the substructure, and the superstructure. The superstructure is everything from the pads, up - it is what supports the loads and is the most visible part of the bridge. The substructure is the foundation, what transfers the loads from the superstructure to the ground, both parts must work together to create a strong, long-lasting bridge. The superstructure consists of parts, The deck is the roadway or walkway surface. In roadway applications it is usually a reinforced concrete slab. The deck includes any road lanes, medians, sidewalks, parapets or railings, the supporting structure consists of the steel or concrete system supporting the deck. This includes the girders themselves, diaphragms or cross-braces, and the truss or arch system, in a girder bridge this would include only the girders and the bracing system. The girders are the primary support, while the bracing system both allows the girders to act together as a unit, and prevents the beams from toppling. The job of the pads is to allow the superstructure to move somewhat independently of the substructure
Image: Girder Bridge 2
A concrete girder bridge pier during construction prior to installation of the bridge deck and parapets, consisting of multiple angled pylons for support (bottom), a horizontal concrete cap (center), and girders (top) with temporary wood bracing
The stubs at the eastern end of the Dunn Memorial Bridge give a good cross section of girder bridge construction.