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 pre-stressed 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. 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.