Box girder bridge

A box girder bridge is a bridge in which 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.

Development of steel box girders

The box girder bridge was a popular choice during the roadbuilding expansion of the 1960s and many new bridge projects were in progress simultaneously. A serious blow to this use was a sequence of three serious disasters, when new bridges collapsed in 1970 (West Gate Bridge and Cleddau Bridge) and 1971 (South Bridge (Koblenz)). Fifty-one people were killed in these failures, leading in the UK to the formation of the Merrison Committee and considerable investment in new research into steel box girder behaviour.

Box girder

A box or tubular girder is a girder that forms an enclosed tube with multiple walls, rather than an I or H-beam. Originally constructed of riveted wrought iron, they are now found in rolled or welded steel, aluminium extrusions or prestressed concrete.

Compared to an I-beam, the advantage of a box girder is that it better resists torsion. Having multiple vertical webs, it can also carry more load than an I beam of equal height (although it will use more material than a taller I beam of equivalent capacity).

The distinction in naming between a box girder and a tubular girder is imprecise. Generally the term box girder is used, especially if it is rectangular in section. Where the girder carries its "content" inside the box, such as the Britannia Bridge, it is termed a tubular girder. Tubular girder is also used if the girder is round or oval in cross-section, such as the Royal Albert Bridge.

Where a large box girder contains more than two walls, i.e. with multiple boxes, it is referred to as a cellular girder.

Girder bridge

A girder bridge, in general, is a bridge that utilizes 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.