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Sydney Lenssen Millau's marvel spans art and design

By any measure, the Millau viaduct will be breath-taking and another monument to the art of cable stayed bridges. The 160M structure will have six continuous main spans of 342m, and with two side spans of 204m, that makes a length of 2.46km. The road deck will glide up to 0.25km high over the Tarn Gorge when the A75 Clermont-Ferrand to Montpellier motorway is completed.

Try another measure: the Eiffel Tower would not quite fit vertically underneath the deepest span, but nor would it reach the top of the cable tower. Longer than the Champs- Elysees, the viaduct climbs 160m from north to south as it crosses the valley.

The most exciting feature for bridge engineers is the fully continuous deck with its seven sets of fanned cable stays. The top 90m of each of the seven column piers are identical, split like a tuning fork and then fixed integrally into the composite deck. Horizontal deck movements will be taken by flexure, and calculations show that over the shortest end columns, the 90m height will need to absorb 550mm of movement.

Movements below deck will not have much effect on the cable stays above, according to Virlogeux, although there will be some flexure in the longitudinal A-frame tower.

There has inevitably been a battle with objectors wanting to move the road far to the west of its line. Equally inevitably much of the A75 has been built already, both to the north and south, leaving a 30km gap in which the Millau Viaduct is key.

In 1987, the French highway authority and SETRA started investigating routes to take the road down 300m into the valley bottom and a low level bridge. The limestone cliffs and bad soils made these options more costly than going straight across at high level. The decision was made in 1992.

By that time the ministry was under pressure to allow architects and private engineers to design more, and in response an ideas competition was held, with no restrictions on bridge type or material. Sir Norman Foster's practice submitted two designs, a spindly metallic arch and a treatment of a cable stayed structure.

The French highway authority selected five separate teams to develop a different type or number of spans. Foster was invited to join a French team tasked with six spans, led by Sogelerg and including Europe Etude Gecti and Serf. By then, Michel Virlogeux had left the Ministry and joined them, pulling out halfway to avoid fear of unfairness.

In July 1996, the Sogerlerg team won, and Virlogeux pays genuine tribute to architects Tim Quick, Ken Shuttleworth and Sir Norman himself: 'Without their understanding of structure, their ability to simplify to the essential, it is likely that this solution would not have won.'

The jury and the Ministry chose Sogelerg, and the detailed final design has been submitted with more input from Foster and Virlogeux. SETRA is expected to give approval before Christmas. Dates have also been set in January 1999 for the final public inquiry, a formality to deal with tolling questions.

The birth of a masterpiece is seldom smooth. Even now, with a design that promises an elegant record-breaking solution, destined to attract thousands of tourists back to the Tarn Gorge and the historic town of Millau, it is not certain that this design will be built.

Next spring the Ministry will invite finance,construct,operate bids, and since the viaduct can never pay for itself, competing concessionaires will bid on the basis of capital subsidy they want and their toll proposals. Although they will be strongly encouraged to adopt the approved design, a joker in the pack could still choose another design and start again.

Virlogeux bides his time. He travels the world giving papers and attending technical committees. Patience is a commodity just as vital to bridge- builders as engineering skills.

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