This division of labour - engineers for the structure, architects for applied form, laid on like piecrust - goes back a long way. The Wearmouth Bridge of 1796 - an early example of an iron road bridge, considered a wonder in its day - was promoted by the politician and entrepreneur Rowland Burdon, and designed by his friend the architect Sir John Soane with the engineer Thomas Wilson. Soane, the most technically adept classicist of his period, was as much the project manager as the designer: he brought in Wilson to refine existing iron-bridge ideas by Tom Paine and others, and built it. Most bridges are still essentially designed by engineers, and only then is the form aestheticised to some extent by architects. Such grand affairs as Lord (Norman) Foster's Grand Viaduc du Millau, not far from Cahors, falls into this category: the engineering parameters were set before architects were invited to compete for the design. But there was a real architectural competition, and the architectural input was considerable. More usual, however, is the case of the William Natcher cable-stay bridge carrying Route 231 over the Ohio River between Kentucky and Indiana. This, one of the larger cable-stay bridges in the USA, was designed by Vijay Chandra and others of the engineering firm Parsons Brinckerhoff. Chandra's description of the genesis of the project, given at a conference in 1999, is instructive. The shape of the towers, he remarked, was selected on the basis of economy, functionality, constructibility, inspectability, ease of maintenance, torsional stability, cable connectivity and so forth. Once all that was agreed, the bridge was handed over to the company's in-house architects. They gave the towers' sides a slight taper and sculpted the exposed faces at the tower tops to give them greater definition. That, it seems, was it. Pure cosmetics, lightly applied.

Architects' design input varies from zero to 100 per cent at the concept stage, with zero being much more common. Later on in the process, the architectural design contribution can however still be crucial. When French architect Alan Spielmann was commissioned for a new viaduct near Clermont Ferrand - within sight of Eiffel's iconic Garabit rail bridge of a century before - the engineering solution was, once again, an a priori decision. But it took Spielmann a further year to refine the solution - in particular, reducing its visual squatness. Other bridge architects, such as Ronald Yee who works predominantly in the Far East, are involved sooner. Yee is a staunch proponent of the classical "golden section" as a proportioning system for bridges, and uses it both vertically and horizontally on his Ting Kau cable-stay bridge and Tsing Ma suspension bridge, both in Hong Kong. At Tsing Ma, even the aerofoil deck sections were golden-proportioned. Yee also uses a version of another classical device, entasis, shaping his cable-stay pylons to counteract the optical illusion of bulging caused by the splay of cables.