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Royal Albert Bridge, Saltash

Saltash Bridge is a one off. When it was built in 1857 it was an amazingly advanced structure. It is still a terribly clever piece of structural thinking, and it is a mystery why the principle has not been used since.

Saltash is, in effect, a suspension bridge where the deck is hung from chains. It is a rare type, using what is called a 'closed system'; that is, the forces are all contained within the structure rather than directed into or tied back to the ground at either end.

The bridge combines three classical engineering forms - the compression arch, the tension chain and the beam deck, working together using the bowstring principle. Outward thrust of the main arch is counteracted by tension chains with struts holding the two elements apart.

Nowadays, we would use steel rather than iron and join elements by welding instead of with rivets.

Improvements in the quality of materials mean the main arches would be more slender.

But methods used to build Saltash Bridge would not differ greatly today. The main spans, each 138.7m long, were fully prefabricated on shore, then floated on the river on pontoons and progressively jacked up the piers to their final position 30.5m above water level. Each complete span weighed in excess of 1,000t.

Interestingly, the tubular main arches are oval in cross-section.

This shape provides extra lateral stiffness for wind loads and also allows smoother wind flow. Brunel certainly had none of today's sophisticated structural analysis tools - but he probably had some intuitive sense that the oval was right.

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