A box girder bridge is a bridge where the main beams comprise girders in the shape of a hollow box. The box girder normally comprises either prestressed concrete, structural steel, or a composite of steel and reinforced concrete. The box is typically rectangular or trapezoidal in cross-section. Box girder bridges are commonly used for highway flyovers and for modern elevated structures of light rail transport. Although normally the box girder bridge is a form of beam bridge, box girders may also be used on cable-stayed bridges and other forms.

Advantages and disadvantages

Compared to I-beam girders, box girders have a number of key advantages and disadvantages:

• Better resistance to torsion, which is particularly of benefit if the bridge deck is curved in plan
• Larger girders can be constructed, because the presence of two webs allows wider and hence stronger flanges to be used. This in turn allows longer spans
• More expensive to fabricate
• More difficult to maintain, because of the need for access to a confined space inside the box

Construction

If made of concrete, box girder bridges may be cast in place using falsework supports, removed after completion, or in sections if a segmental bridge. Box girders may also be prefabricated in a fabrication yard, then transported and emplaced using cranes.

For steel box girders, the girders are normally fabricated off site and lifted into place by crane, with sections connected by bolting or welding. If a composite concrete bridge deck is used, it is often cast in-place using temporary falsework supported off the steel girder.

Either form of bridge may also be installed using the technique of incremental launching.

Development of steel box girders

The key events in the development of the steel box girder bridge were three serious disasters, when new bridges collapsed in 1970 (West Gate Bridge and Cleddau Bridge) and 1971 (Koblenz Bridge). 51 people were killed in these failures, leading to the formation of the Merrison Committee and considerable investment in new research into steel box girder behaviour.