Foster and Partners鈥� Viaduc de Millau in southern France is the highest, longest cable-stayed bridge in the world, and it opens in December. We admire the view, talks to the engineer and meets some enthusiastic locals.
c鈥檈st magnifique!
For three years, frustrated motorists using the A75 motorway in France have been tantalised by progress on Europe鈥檚 largest civil engineering project. The drive from B茅ziers in the south starts out well enough, as you hurtle northwards through mountain scenery. Frustration sets in 19 km south of Millau as the motorway empties its traffic onto the equivalent of an A road and you slow to a grinding crawl 鈥� it can take up to three hours to get through the town and back on to the A75 north of town. Eventually, however, the road dips and rounds a corner to reveal a vast panorama: a huge 2.5 km wide valley slices deep between two elevated plateaux and a nearly completed motorway viaduct strides across the void.
This structure rouses a host of emotions. The first is amazement at the sheer arrogance of trying to bridge such a gap. But then you marvel at the brilliance and scale of the engineering. You also wonder how on earth permission was granted and how the contractors kept off protesters in one of the most beautiful parts of France. The 鈥渉ow did they get away with it鈥� feeling is tempered by the extraordinary elegance of what is after all a motorway bridge 鈥� this makes more sense once you realise architect Foster and Partners was involved in its design. Finally there is the anticipation in knowing that next year there won鈥檛 be infuriating traffic jams to plough through but the chance to fly across the valley on the Viaduc de Millau and on to Clermont-Ferrand. Above all else, however, rises one question: how did they do it?
The Viaduc de Millau鈥檚 vital statistics read like an entry in a book of records. It is the world鈥檚 longest multi-pier cable-stayed bridge, at just 40 m short of 2.5 km, and the highest at 343 m. Seven piers step delicately across the valley over the River Tarn to support the deck. These are not the usual Mr Blobby style motorway piers but are hexagonal in plan and curve gently towards their apex from the base. Each pier splits in two like a tuning fork 90 m short of the deck to make the structure appear even more refined. These piers are the highest in the world 鈥� E E to put this in perspective, if the Eiffel Tower was sitting in the River Tarn it would barely protrude above the deck. Pylons rise 90 m above the deck, elegantly mirroring the forked design. Cables fan out from the pylons to transfer loads from the deck down into the piers.
It has taken many years to get to this stage. A range of options were explored between 1987 and 1989, according to Michel Virlogeux, who was the head of bridge engineering at the government-run major roads division, the Service d鈥橢tudes Techniques des Routes et Autoroutes at the time. Suspension bridges in two different locations were rejected because of local opposition and unsuitable ground, and a bridge combined with a tunnel was rejected because of its complexity. 鈥淲e were practically in front of a wall,鈥� says Virlogeux. 鈥淭he director of road engineering said it is not possible to span one plateau to the other because of the length of the span and for aesthetic reasons.鈥� This got Virlogeux thinking. 鈥淎t this time I had the idea of the cable-stayed bridge.鈥�
This idea was adopted and eventually culminated in a competition in 1994 to select a winning design. The government realised it had to go to exceptional lengths to win the public over to the idea. 鈥淎 project from the administration would not have been accepted,鈥� says Virlogeux. 鈥淓verything that comes from the administration is to be fought against. The best way was to have a competition as the public accepts the decision of a jury.鈥�
The second strand to the strategy of winning over public opinion was to make the proposed viaduct more visually acceptable by insisting an architect be on each team. 鈥淚t was unusual that the project stipulated that all the competing teams had to have an architect in the team,鈥� says Norman Foster.
Foster says his design differed from the other entries in one important respect. 鈥淭hey had let a competition for a bridge over the River Tarn,鈥� he explains. 鈥淓very entry had made a response that focused on this very small river. These were large-span structures with symbolic arches over the river. My point was this was not a competition for a bridge over the River Tarn. We have not done a bridge over the River Tarn but the most economical crossing between two plateaux that also happens to cross the River Tarn. In a way we have taken a divergent, philosophical line. The highest part of the columns just happened to be where this tiny dribble of a river is.鈥�
The architect and engineers worked closely together on refining the design once the result of the competition was announced in 1996. The bridge is in many ways a perfect synthesis between architecture and engineering as aesthetic considerations combined successfully with engineering demands. An example of this is the forked piers. This bridge relies on very stiff piers and pylons combined with a flexible deck. The piers need to be very wide so they don鈥檛 bend towards an asymmetric load on the deck. But because the bridge is 2.5 km long, thermal expansion up to 600 mm is possible at either end. Because this movement is incompatible with the two wide piers at each end of the deck, the solution is to divide them into two so they can provide the necessary stiffness yet accommodate thermal movement.
Foster has turned this into an aesthetic virtue. All the piers share the same, forked design as it lightens their appearance, and much effort has gone into shaping the piers to make them as elegant as possible. They are multifaceted for added visual interest and also incorporate grooves that cast shadow lines that vary in width according to the time of day. The piers are also arranged on a slight curve giving the bridge a radius of 20 km. This is intended to add to the visual impact of driving across, as motorists can see the bridge unfolding before them.
Virlogeux says that when engineering demands made meeting aesthetic requirements difficult, Foster was very accommodating. Originally Foster wanted a triangular-shaped deck in section but tests showed that this wouldn鈥檛 work. 鈥淲e put it in a wind tunnel and it was a disaster,鈥� says Virlogeux. He says the deck behaved in a similar way to the deck on the Tacoma Narrows Bridge in Washington state, USA, which in 1940 started swaying and ultimately collapsed in high winds. Virlogeux wanted to use a trapezoidal deck that he had previously used on the cable-stayed Pont de Normandie. He says the architect could have insisted on the triangular deck as the politicians had stipulated nothing could be changed without the architect鈥檚 agreement. 鈥淚 met with Foster and explained that we had the same problem as Tacoma. He said 鈥榥o problem鈥� and changed the design in 10 minutes and we went back to a trapezoidal shape.鈥�
All the effort put into finely honing an aesthetically pleasing solution has paid off, as there have been virtually no protests either before or during construction. The A75 cuts through the traditionally isolated and poor region of the Massif Central, and was intended to open the area up. Virlogeux says people were very much in favour of the motorway because it was hoped it would bring economic benefits and halt rural depopulation. Also, Millau residents were sick and tired of the traffic jams that made getting into and around the town a nightmare. 鈥淭he population was very upset by the delays to the project 鈥� which doesn鈥檛 usually happen,鈥� Virlogeux explains. 鈥淲e have exactly the opposite situation in the French Alps where a proposed motorway has been completely blocked.鈥�
Indeed the construction of the Viaduc de Millau has become something of a celebratory experience for France. Eiffage, the contractor for the viaduct, built a visitor centre under the emerging structure and says 400,000 people have visited since work begun. The town has helpfully signposted several vantage points where the viaduct can be more fully appreciated and these are populated by hordes of snappers armed with digital cameras.
For the A75 motorist the bridge cannot open soon enough and Eiffage has kindly obliged. It is scheduled for completion on the 10 January 2005 but the contractor has managed to bring this forward to the week before Christmas. President Chirac will snip the ribbon to spare motorists seasonal gridlock and the chance to experience a new, tantalising view of Millau that, traffic permitting, will last just three minutes.
Project team
commissioning authority
The French Government
project owner and operator and main contractor
Compagnie Eiffage du Viaduc de Millau
architect
Foster and Partners
project manager
Setec, SNCF
civil engineers
Eiffage TP, EEG-Simecsoc
structural steel engineer
Greisch
concrete contractor
Eiffage TP
steelwork contractor
Eiffel
What the locals think of the viaduct
Viviane de Sousa, camp site owner
Although the traffic jams will go, people are worried that businesses will go, too. Now two out of three people support the construction of the viaduct. Five years ago it was half and half; as people have seen the project develop they have become more accepting of it, and the fact that it will get rid of the traffic and is physically beautiful does make it more acceptable. At the moment there is a buzz in Millau because of the visitors and the workers who have bought houses and business to the town. The question is, what will things be like in five years鈥� time?
Monsieur Perez, estate agent
I have always been in favour of the viaduct, as it is a good thing for Millau. Everybody in Millau is tired of the traffic jams throughout July and August. The project has brought lots of tourism to Millau; many people have come to see the viaduct being built. Everybody in France has heard of the project, and wherever I go I take a photo to show people how good it is as I am proud that the viaduct is here. It is a shame that a French architect did not win the contest, but Mr Foster has created a magnificent viaduct. I think the fact that it is so beautiful means more people have accepted the project. House prices have been going up in the area, although that is true in all of France. Local people are concerned about what will happen after it opens, but I think that many people will still stop off at Millau to admire the viaduct.
Herv茅 Bougnol, hotelier
I think that the viaduct is a good thing for Millau because there is a big increase in tourism at the moment and I have noticed we are getting busier. Something had to happen, as everyone has become fed up with what happens every summer - the whole of the town is brought to a standstill by the traffic jams. I do not think that in the long run Millau will suffer. people will still come, as there are lots of things happening around here. The Viaduct is a national project, and the other two options, a route far to the west or a tunnel under the mountains, would have left Millau out of the picture. I think that tourism will increase after the Viaduct opens as tourists were put off in the past by the traffic jams; they were so tired of trying to get through, they didn鈥檛 want to stop.
The Viaduct is handsome, which is good as nobody locally would have put up with a design that was not going to look good. The competition was a good idea 鈥� it was fair and the outcome needed to be good for the people of Millau.
Clement Forin, stationer
Of course something needed to change as the town was brought to a standstill every summer. The architect produced a fine design and the viaduct looks very good. At the moment there are lots of visitors to the town, but I don鈥檛 know if the viaduct will be good for Millau in the long run as I think everyone will just head for the coast without stopping. However, at least the traffic jams will stop.
How the Viaduc de Millau was constructed
The French contractor Eiffage won the concession from the French government to build and operate the 鈧�320m structure for the next 75 years, using the money from tolls to fund its construction. Marc Legrand, the director-general of Compagnie Eiffage du Viaduc de Millau, the Eiffage subsidiary set up to build and operate the viaduct, says, 鈥淲e have worked on large projects including the TGV and the Pont de Normandie but never on a project of this scale before.鈥� Eiffage had to respect the overall design and had to ensure the viaduct would last at least 120 years, but was free to select a suitable construction method.
The seven piers were constructed simultaneously on substantial foundations that will be buried once the viaduct is completed. Their construction was tricky because of the complex shape and because of the giddying heights involved. Once the foundations were finished, specially made self-climbing formwork was brought in to build the piers in 4 m high lifts, with one completed every three days. 鈥淓very lift has different geometry,鈥� says Vincent Bonnefous, Eiffage鈥檚 director of works. 鈥淭he formwork consists of a series of panels that can be changed for ones of different sizes to suit the geometry. In some places a panel would be used only once.鈥� The piers are hollow inside and have wall thicknesses ranging from 600 mm to 1200 mm.
Once the point where the piers split was reached a completely new set of formwork was brought in to construct the two forks together. 鈥淭his was the difficult part,鈥� says Bonnefous. 鈥淏ecause of the dense steel reinforcement there was very little space and it was difficult to get it to work. We were taking four to five days per pour but we managed to get this down to two days once we understood how it worked. It was very difficult but we did it.鈥� Indeed, Eiffage had to make some design changes because of the densely packed reinforcement. 鈥淭here was a lot of steel at the top of the piers because of the high loads,鈥� explains Legrand. 鈥淣orman Foster accepted some slight changes where the piers met the deck but we respected the overall design.鈥�
The piers are solid for the last 18 m except for voids needed to accept 100 m long cables to pre-stress the piers. These prevent cracking and are intended to make the structure more durable.
The construction phase with the biggest 鈥渨ow factor鈥� was the steel deck. This was assembled on the ground at each end of the bridge and progressively pushed out over the void from each side until the two halves met at the highest point of the valley. Unsurprisingly this is the biggest bridge launch ever. 鈥淲e decided not to use concrete for safety reasons as it would鈥檝e meant having men work at heights of 245 m,鈥� says Legrand. 鈥淎nother benefit is that steel is lighter, so the number of stay cables is reduced from 25 pairs to 11 and the deck is 4.2 m deep instead of 4.6 m. This gives a cleaner result.鈥�
Extensive temporary works were needed for the launch. Because the deck is pushed out over the valley without its final supporting pylons and stay cables, each 342 m long section of deck between two piers had to be propped in the middle. Seven giant steel towers provided this support. These are essentially oversized crane support towers. However the highest section of the bridge couldn鈥檛 be supported in this manner because it spans the section of the valley directly above the river. The solution was to erect the pylon and stay cables on the leading section of each half of the deck only to provide support for this final section. Because the river is on the north side of the valley, 1743 m of deck was launched from the south side, and 717 m from the north.
The deck was fabricated in sections by Eiffage steelwork subsidiary Eiffel at its two factories. These sections were brought to site and welded together on a production line at each end of the bridge to form a complete, painted deck section with the balustrade pre-fitted. After three weeks the 171 m long sections, enough to span between pier and temporary support, were finished.
A special weather forecast was made to ensure average windspeeds did not exceed 35 km/h over the next four days prior to launching the deck over the void.
鈥淭he biggest problem was how to slide the deck over the piers. You couldn鈥檛 push too hard horizontally on the piers as this would damage them,鈥� says Jean Pierre Gerner, Eiffel鈥檚 director of works responsible for the deck. The answer came in the form of 64 devices called translators positioned on top of the piers and the temporary supports. Devised by US specialist Enerpac, these are an arrangement of computer-controlled, synchronised hydraulic transfer jacks that lift the deck slightly, push it along by 600 mm, drop it again, then repeat the process. This moved the deck along at a rate of 10 m/h so it took two days for the deck to move the full 171 m to the next support. The biggest concern was that the deck would become stuck between its supports during the launching operation. 鈥淭he worst-case scenario was that the deck would get stuck 5 m before it reached the support,鈥� says Gerner. 鈥淲e had an emergency system to make it safe if this happened but it never got stuck.鈥�
On 28 May 2004, the two halves of the deck met 270 m above the Tarn 鈥渨ith a difference of only 10 mm,鈥� according to Legrand.
After welding the two halves of the deck together, the next stage was to lift the steel pylons into place and attach the cables to permanently support the deck. Currently the temporary supports are being dismantled and clear plastic aerofoils are being attached to the balustrade to deflect the winds that whistle up the valley while allowing drivers a view over the sides. The road surface is being completed and lighting is being installed inside the split piers to subtly illuminate the bridge - it has no other lighting, in order to minimise its impact on the landscape. Six kilometres north of the viaduct, the finishing touches are being made to a high-tech toll collection and command centre ready for the opening in December.
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