Box girder

Originally constructed of wrought iron joined by riveting, they are now made of rolled or welded steel, aluminium extrusions or prestressed concrete.

The theoretical basis of the box girder was largely the work of the engineer Sir William Fairbairn, with the aid of the mathematician Eaton Hodgkinson, around 1830.

(The preserved Britannia Bridge section shows that both top and bottom flanges were of cellular construction, but (according to Fairbairn) the cellular construction of the bottom flange was adopted, not because of the nature of the forces it had to withstand,[2]: 206  but because of their magnitude and the consequent "practical difficulties which would have been encountered, had it been attempted to achieve the requisite sectional area in a solid mass")[2]: 183 In some ways this isn't a "cellular girder" as such (compared to a spaceframe or geodesic construction) as the cells don't share loads from the entire girder, but merely act to stiffen one plate in isolation.

Robert Stephenson engaged both him and Hodgkinson as consultants to assist with his Britannia and Conwy bridges, both of which contained the railway track within a large tubular girder.

That led to serious concerns over the continued use of box girders and extensive studies of their safety, which involved an early use of computer modelling, and was a spur to the development of finite element analysis in civil engineering.