Box girders are a particular form of plate girder, with two webs joined by common top and bottom flanges. Boxes may be used either singly or in multiples. They may be straight or curved in plan and be of constant or variable depth. If using a concrete top flange then the box girder is called "open top". Box girders with a steel top flange (often called an ‘orthotropic deck’) will give a minimum weight solution, and so are generally used on long span bridges.
Steel box girder bridges have great torsional rigidity, which makes them ideal for bridges that are highly curved in plan or where construction by launching is required. In addition, they are widely used as the deck elements of cable-stayed and suspension bridges, where the torsional stiffness of a box is important for the dynamics of such long span bridges, an example of this is the M11 bridge at Stansted Airport.
M11 bridge at Stansead Airport, UK
The Nene Bridge is a 5-span structure comprising four ‘closed’ steel box girders, 155m long acting compositely with the deck slab. The trapezoidal girders are curved both vertically and horizontally.
Nene bridge, Peterborough