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[[Image:Britannia Bridge wrought iron section.jpg|thumb|Section of the original tubular [[Britannia Bridge]]]]
[[Image:Fairbairn steam crane jib.jpg|thumb|right|The patent curved and tapered box girder jib of a [[Fairbairn steam crane]]]]
A '''box''' or '''tubular girder''' is a [[girder]] that forms an enclosed tube with multiple walls, as opposed to an [[i-beam|{{ibeam}}-]] or [[H-beam|{{hbeam}}-beam]]. Originally constructed of [[rivet]]ed [[wrought iron]], they are now made of rolled or welded steel, aluminium [[extrusion]]s or [[prestressed concrete]].
Compared to an [[i-beam|{{ibeam}}-beam]], the advantage of a box girder is that it better resists [[Torsion (mechanics)|torsion]]. Having multiple vertical webs, it can also carry more load than an {{nowrap|{{ibeam}}-beam}} of equal height (although it will use more material than a taller {{ibeam}}-beam of equivalent capacity).
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