Box girder

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

Britannia_Bridge_-_circa_1852.jpg
The old Britannia Bridge with train track inside the box-girder tunnel.
Britannia_Bridge_wrought_iron_section.jpg
Section of the original tubular Britannia Bridge
Fairbairn_steam_crane_jib.jpg
The patent curved and tapered box girder jib of a Fairbairn steam crane

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.