What is a Girder Bridge?

M. Haskins

Girder is a term used in construction to refer to a supporting, horizontal beam that can be made from a variety of construction materials such as stainless steel, concrete, or a combination of these materials. A girder bridge is a basic, common type of bridge where the bridge deck is built on top of such supporting beams, that have in turn been placed on piers and abutments that support the span of the bridge. The types of beams used for girder bridges are usually either I-beam girders, so called because their shape is reminiscent of a capital Roman letter I, or box girder beams that are made of steel or concrete and shaped like an open box. Girder bridges are most commonly used for straight bridges that are 33-650 feet (10-200 m) long, such as light rail bridges, pedestrian overpasses, or highway fly-overs. The longest girder bridge in the world is 2,300 feet (700 m) long and located in Brazil.

Girder bridges are usually built with I-beams.
Girder bridges are usually built with I-beams.

There are four types of girder bridges, classified depending on the construction material and type of girders used. A rolled steel girder bridge is built using I-beams made from prefabricated steel, while a plate girder bridge is constructed by welding flat pieces of steel together on-site to make the I-beams. Concrete girder bridges are constructed using concrete I-beam girders that can be made from various kinds of reinforced concrete, including pre-stressed concrete and post-tensioned concrete. A box girder bridge can be made from either steel or concrete, and uses box girders to support the bridge deck.

Cantilever truss bridges are designed to hold heavy loads, such as constant automobile traffic.
Cantilever truss bridges are designed to hold heavy loads, such as constant automobile traffic.

Whether I-beam girders or box girders are used to construct a girder bridge depends on various factors. It is easier and cheaper to build and maintain a girder bridge using I-beam girders. However, these girders do not always offer sufficient structural strength and stability if the bridge is very long or the bridge span is curved, because they are sensitive to the twisting forces, or torque, such a span is subject to. Box girders are preferred for such bridges. There have been concerns raised of corrosion of box girders, especially if rain water seeps into the open space inside the girders.

Girder bridges belong to a category of bridges called beam bridges. This category of bridges includes girder bridges, truss bridges and trestle bridges. Beam bridges can be constructed by using a wide variety of materials including stone, timber, steel, iron, and concrete. An example of a basic type of beam bridge is a log or slab of stone laid across a creek.

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Discussion Comments


@Telsyst - You wouldn't get very far in a city like Pittsburgh or New York without the bridges they have. There is a reason that a lot of the stuff in Pittsburgh is called three rivers.

The problem with a lot of rivers, like in Pittsburgh and New York, is the lane closings. There is construction all the time in Pittsburgh. I think it is something I don't mind seeing too much though, better safe than sorry. If I was in the city daily I might have a different point of view.


Bridge building is something that is always going to be done. There are so many reasons for bridges, bridges over roads, bridges over train tracks even bridges over other bridges. We are always going to need people to design new bridges and build them.

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