Girder bridges

Girder bridge

Two different girder bridges. The top is a plate girder bridge, while the bottom is a concrete girder bridge.
Ancestor	Beam bridge
Related	Trestle, truss bridge, moon bridge
Descendant	Box girder bridge, Plate girder bridge
Carries	Pedestrians, automobiles, trucks, light rail, heavy rail
Span range	Short, Medium
Material	Iron, steel, concrete
Movable	No
Design effort	low
Falsework required	No

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 (abutments and piers), the superstructure (girder, truss, or arch), 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 often 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 typically used to refer to a steel beam. In a beam or girder bridge, the beams themselves are the primary support for the deck, and are responsible for transferring the load down to the foundation. 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 directly result in a deeper beam. In truss and arch-style bridges, the girders are still the main support for the deck, but the load is transferred through the truss or arch to the foundation. These designs allow bridges to span larger distances without requiring the depth of the beam to increase beyond what is practical - however, with the inclusion of a truss or arch the bridge is no longer a true girder bridge.

All bridges consist of two main parts: the substructure, and the superstructure. The superstructure is everything from the bearing 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 several parts:

The substructure is made of multiple parts as well: