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Pushing the envelope

Bridges - Millau viaduct

This month contractors in Millau, France, hope to complete the deck launch for the world's highest cable stay viaduct. Andrew Bolton reports.

Two hundred and seventy dizzying metres above the river Tarn, west of Millau in south west France, a mobile crane is at work. It is slowly lifting out sections of a temporary guide nose jutting from the northern end of the steel deck of what will be the world's tallest cable stayed viaduct.

As soon as the northern nose has been dismantled, the crane will be shifted to repeat the operation on a section of deck that has been advancing from the southern abutment.

Removing these two steel structures will clear the way for the final stage of a massive jacking operation, which has seen just under 2.5km of deck, weighing around 30,000t, slide slowly out across the Tarn Valley over the last 18 months.

This will result in the southern part of the deck being inched out to join its northern counterpart in mid air above the river.

When complete, the $320M Millau viaduct will form part of the new A75 Paris to Barcelona motorway (NCEI May 2003).

French contractor Eiffage is building it as part of a 75 year, toll financed concession.

Viaduct design is by UK architect Norman Foster with French engineers Sogelerg, EEG and SERF. Eiffage subsidiary Eiffel is responsible for steelwork.

At its highest point above the valley floor, the viaduct's steel cable stayed towers will stand 343m above the river. In all the structure comprises seven slender concrete pylons topped by steel cable stay towers, each with a fan of 22 cables, supporting a 2.46km steel deck.

Right now, the deck is some months away from resembling the sleek linear structure shown in Foster's drawings and computer simulations. It droops visibly between the seven concrete piers and seven temporary steel piers. 'It is sagging under its own weight, ' says Eiffel head of steel erection Jean-Pierre Gerner. This will be corrected when all the towers are installed and cable stays tensioned, but sag of around 600mm is allowed for at the moment as construction traffic provides relatively low loadings compared with those anticipated when the two lane dual carriageway viaduct opens.

At its southernmost end the deck slopes steeply upwards.

This is because fabrication work is taking place with deck sections resting on the trace of the approach road. As the final section of deck is pushed out, its trailing end will finish up resting on a temporary support tower, above its final resting place on the abutment. It will then be lowered into place as the support is removed.

At 342m, so great is the distance between the viaduct's permanent piers that the deck has had to be temporarily supported on seven colossal, custom-built steel towers. These will remain in place until all the cable stays are in position and fully tensioned. They are self climbing structures varying in height from 87.5m to what is claimed to be a world record breaking 163.7m, close to the river in the lowest part of the Tarn Valley.

Height considerations played a part in the decision to launch most of the deck from the south, leaving just 708m to be pushed out from the north.

The viaduct is at is highest - 245m - as it crosses the Tarn, two spans from the northern abutment. The deck sections will meet directly above the river. To prevent the ends drooping into space, Eiffage has to provide intermediate support between the two permanent piers. But Gerner says that building a 245m high temporary pier would have pushed the technology for such structures to its limits.

Meanwhile, even if it had been possible to construct, the temporary pier would have to have been founded in the river, raising the risk of foundation scour if the river flooded.

Eiffage has instead decided to do without a temporary structure beneath the deck and support the leading ends of the two deck sections from their steel cable stay towers which were installed on the sections before the start of the launch process.

The two towers are fitted with 12 of their 22 cables which, once the towers are finally in place above their permanent piers, will provide the restraint against bending needed to join and weld the two deck sections.

Work has progressed well since the deck launch began in September 2002 and Eiffage says it is running a month ahead of schedule, meaning the viaduct is expected to open this December.

Over this summer the remaining cables will be installed on the first two towers, while the remaining five cable stay towers will be erected insitu.

Tower sections have already arrived on site and are being assembled.

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