A box girder bridge is just one of the many styles of bridges. It's actually a common style of bridge that has it's defining feature in the beams of the bridge. The support beams of box girders are fastened together to form a hollow box shape. Box girders are sometimes called a tubular girders. That hollow box may have the shape of a rectangle, trapezoid or multiple isosceles triangles butted up against each other in alternating directions (e.g., tip facing up, tip facing down, tip facing up). Box girders offer stronger construction, especially for bridges that must have a curve to them. They are commonly used in freeway overpasses. The Oakland/San Francisco Bay Bridge is a box girder bridge.
The girders of the bridge are usually made from prefabricated steel. This means that the girders are created in a factory, and then brought to the location of the bridge for assembly. Girders can also be made of concrete — prestressed, high performance, or reinforced — or a mix of concrete and steel.
In addition to the girders, the box girder bridge itself can be prefabricated and then moved to another location. This is one of the great benefits of this style of bridge. It can also be cast using supports that are removed later, or the bridge can be completed in sections. Cranes must be used to move the bridge or its components into place. Having the ability to move the prefabricated bridge to its final location makes it ideal for areas of heavy traffic that can only be closed for a limited time.
While people tend to think of bridges as being over water, box girders are often found over highways. Many people may have driven across a box girder bridge in the form of a highway overpass and never even noticed it. It can also support heavier weights like light rail transport, or lighter traffic such as pedestrians.
A box girder bridge is strong. One of its advantages is a high resistance to torsion, which is the twisting of an object due to torque, a helpful feature if the bridge is curved. Moreover, larger girders and stronger flanges can be used, which allows for a longer span between support posts that hold the bridge. There are some drawbacks, however, to this type of structure. The design can be expensive to build, and difficult to maintain, for example.
The West Gate Bridge in Melbourne Australia is a box girder bridge. A section of it collapsed in 1970; out of that tragic accident, which resulted in the deaths of 35 construction workers, came greatly improved designs. Some box girder bridges are even being designed to withstand powerful earthquakes.