A box girder bridge is a bridge in which the beams are made up of box shaped girders. The box girders consist of concrete, steel or a combination of both. Most of the modern elevated structures are built on the basis of the box girder bridge. Most of us have travel on the box girder bridges regularly. The most common examples are the flyovers and the structures built for light rail transport. They can sustain heavy weight and hence used in the construction of flyovers. These are used to construct crossways for pedestrians. Box girders are used on cable styled bridges and other forms but generally they are a form of Beam Bridges. The support beams of the girders are tightly coupled together to form a hollow box. This hollow box can have the shape of a rectangle, tightly coupled triangles or trapezium. The box girder bridges offer a stronger support for bridges constructed with an arch. The most famous Bay Bridge in California, connecting San Francisco and Oakland is a classic example of the Box girder bridge.
The box girder bridges are made of prefabricated steel which are manufactured in the factory and assembled onsite. The girders can also be made up (EDIT : these are still being made so I think it should be are made of than can also be made of)of high performance, prestressed, reinforced concrete or a mixture of both concrete and steel. Both the girders and bridges can be manufactured elsewhere and installed at a different place as they are prefabricated. That is the beauty of this construction. The bridge can be installed in its position by incremental launching. Huge cranes are used to place the new segments onto the completed portions of the bridge until the whole structure is completely assembled.
Box girder bridges have a number of key advantages when compared to the I- beam girders. Box girders offer better resistance to torsion. This is beneficial especially for a curved bridge. Larger girders and stronger flanges can be used as this allows for a longer span between the support posts that hold the bridge. The huge hollow boxes can be used to place the Water lines, telephone cables and other utility lines.
Box girders are more expensive to fabricate and difficult to maintain because of the need of a confined space inside the box. There were major disasters that occurred when the West Gate Bridge of Australia and Cleddau Bridge of UK collapsed. But these disasters gave rise to a new design in the box girder bridges. Bridges being constructed these days are being designed keeping in mind the occurrences of earthquakes as well. They are designed to withstand even high magnitude earthquake.
One of the major threats is the corrosion of steel cables inside the bridge. Recently in 2009, an inspection was conducted on the Cline Avenue Bridge over the Indiana harbor and the ship canal. This inspection revealed a serious corrosion of the steel cables and the steel within the girders of the box due to water seepage. It was so badly corroded that there was no solution to this and the bridge had to be closed down permanently. Therefore these bridges have to be checked regularly and serviced to prevent corrosion. Using high grade steel and corrosion free metals or alloys in the construction of box girder bridges can help in preventing the bridges from getting corroded.