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Up in the air: At 343 meters, the Millau viaduct is the highest bridge in the world.
Nail-biting build: Norman Foster (now Lord Foster), the architect of the bridge, says he felt physically sick during the build.
Inside the thrilling construction of the Millau viaduct
Soaring across the scenic landscape, it’s indisputably one of the most beautiful bridges in the world.
Often swathed by mist, so that it feels like crossing through clouds, it is so famous that it has its own visitor center, and people plan trips to the area solely to drive across it. The bridge can even be easily seen from space.
The Golden Gate? No. This is the Millau Viaduct, a perfect example of where engineering meets art. Cantilevered high over the Tarn gorge in southern France, and yawning 2,460 meters (8,070 feet) in length, the Millau Viaduct is the world’s tallest bridge, with a structural height of 336.4 meters (1,104 feet).
But not even those impressive statistics do it justice.
Unlike other famous bridges, which usually connect two points of similar altitude, the Millau Viaduct effectively becomes the opposite of a rollercoaster, plying a flat course across the valley, as the land ripples up and down underneath it.
The seven piers range from 78 meters to 245 meters (256-804 feet) in height, each calculated to the millimeter to make a perfectly smooth experience for drivers soaring across the Tarn. There’s a 342m (1,122ft) span between each pairing – large enough for the Eiffel Tower to slot in the gap. The piers are coupled with seven steel pylons, each 87 meters (285 feet) high, with 11 cable stays fanning out on either side. This all helps keep the “deck” – the road surface, which is around 14 feet thick and weighs 36,000 tons, or the equivalent of 5,100 African elephants – steady.
At the same time as being a work of absolute precision, it’s also beautiful. The Gorges du Tarn area is a protected landscape, yet instead of spoiling the view, the Millau Viaduct enhances it.
It’s a “wonder of the modern world” and an “engineering marvel,” says David Knight, director of design and engineering at Cake Industries and specialist adviser to the Institution of Civil Engineers.
“It’s that perfect interplay of architecture and engineering that means that everybody who sees it thinks it’s spectacular.”
Those living in the valley below look up with wonder; those driving across it – this road, the A75 from Clermont-Ferrand to Béziers, is one of the main north-south routes in France – see the gentle curve arcing across the landscape as they approach. “It gives everyone who uses it a sense of awe,” says Knight.
No wonder that for many, driving across the viaduct is something you travel to do, not something you do while traveling.
So how did this wonder of the modern world come to be built in the middle of France? Why did it take two decades to plan, before opening to traffic in December 2004? And how did it effectively change the map of Europe?
A bridge too far?
The answer to all those questions is geography. The Massif Central is a vast area of highlands cut by deep valleys and gorges, roughly located in the middle part of the bottom half of France. Sprawling across about 15% of the country, and bordered by the Alps to the east, it’s one of the obstacles anyone traveling from north to south of the country – or from northern Europe to Spain – must pass.
So important was this viaduct – but also so difficult – that it was two decades in the planning, according to Michel Virlogeux, the engineer who led the design team – and who first started work on it in September 1987.
“The first problem was not what bridge to build, but where the motorway would pass,” he says.
At the time the Massif Central was remote, despite its central location. There was a single-track railway line, and the roads “weren’t very good,” he says. “The central part of France couldn’t develop due to lack of transport.”
So in the 1980s, the French government decided to upgrade the road network, with then-president Valery Giscard d’Estaing deciding on a freeway. One of the aims was to unclog the notoriously choked road around Millau, where the road descended into the valley and crossed the Tarn river in the city center. Every day there were tailbacks of around 20 kilometers (12.5 miles) either side of the town.
“Going through Millau used to be a traffic blackspot for tourists,” says Emmanuelle Gazel, current mayor of Millau. “There were lots of traffic jams. There were kilometers and kilometers of tailbacks. It gave a very bad image of our area… in terms of pollution it was terrible. And locals took a long time getting from one point to another.”
In the words of Lord Norman Foster, who became the architect of the bridge, the area was “a valley of extreme beauty which had become one of France’s worst bottlenecks.”
The decision to build a bridge around Millau was taken in September 1986, says Virlogeux, who at the time was head of the large bridges division of the French administration. There was just one problem: the geography of the area meant there was no obvious solution. “We started looking where was possible, but many options were bad, and it took almost three years to find a solution,” he says.
One idea was to route the freeway east of Millau, keeping the road on the plateaus, with two suspension bridges to cross the valleys on either side. But that wouldn’t have allowed a connection with Millau – “the only big city between Clermont-Ferrand and Béziers,” says Virlogeux – which needed the economic boost.
So they called in the experts: geologists, geo technologists, road engineers and Virlogeux, who had already designed the Pont de Normandie – the 7,032-foot bridge spanning the river Seine in the northern region of Normandy.
The team’s first idea was to run west of Millau, bringing the road lower in altitude down into the valley, across a bridge at a lower level and up again to the plateau and then a tunnel. They were in the stages of planning when the team’s road engineer, Jacques Soubeyran, had a lightbulb moment.
“He asked, ‘Why are you going into the valley?’ and it was a big shock,” remembers Virlogeux. “The motorway was passing 300 meters above the river. I hadn’t even considered the possibility of passing at a high level. Immediately, I said we were being stupid. We started working on the idea of passing plateau to plateau.”
After just eight days they had detailed drawings of the rippling ground levels, as well as a possible altitude for a freeway snaking across it.
The importance of elegance
They knew where they wanted the viaduct – but what should it look like?
Virlogeux immediately knew that the best option would be a cable-stayed bridge. “Cable is the most efficient structure to carry a load, and you can have a very slender deck so it’s much better to look at,” he says.
Slenderness was important. There was already controversy about the idea of running a bridge through such a famous landscape. To avoid ruining the landscape, it had to “look very quiet.”
Getting the go-ahead took some years. The French government started a competition for the design of the bridge, and in 1996 the commission was won by a group led by Virlogeux as engineer (who had left his previous job a year earlier) and the UK’s Norman Foster – now Lord Foster – as architect. Foster calls their plan to span the valley, rather than the river, a “philosophical concept” that distinguished them from other competitors.
But with the local community up in arms at the idea of their area of natural beauty being spoiled, they face what Foster calls a “design challenge… to create something that would enhance the landscape, sit gently on the floor of the valley – to be the most delicate and light intervention.” Virlogeux says it had to be “pure and simple.”
Yet this precious landscape, which had to be protected aesthetically, was extremely difficult to work around.
“The wind forces at this level are huge and the columns have to accommodate the enormous expansion and contraction of the deck,” says Foster. And we’re not just talking a gentle bounce. The 2,460-meter (8,070-foot) bridge can expand or contract by 50 centimeters (1.6 feet) depending on the weather. Their solution was to add extension joints.
Luckily, while the old adage is that architects and engineers should be at loggerheads, and while you might imagine that two titans of architecture and engineering might clash, Foster and Virlogeux have nothing but praise for each other. Virlogeux says it was a “very easy” working relationship, while for Foster it was a “meeting of minds.” The team had twice-monthly meetings in London while working on the design. “He would ask me, ‘Why do you want this and not that?’ and after that he’d take a decision in five minutes,” says Virlogeux. “Once there was a major controversy about the shape of the deck. He asked me what I proposed, if I was sure it would work. Then he said, ‘OK – architecture must not go against scientific needs.’”
Above the road, the sturdy columns “split” into two more flexible arms, making an artistic statement out of an engineering necessity.
The same goes for the curve of the road, which gently arcs across the valley. It’s not just beautiful; it ensures that there’s no visual overlap – and therefore confusion – for drivers at such a great height. Meanwhile the piers become slimmer as they rise towards the roadway, more or less halving from 24 meters wide at the bottom to 11 meters at the top.
Their design for a cable-stayed bridge with seven elegant piers marching across the landscape and what Foster calls “the snake of a road, improbably thin like a razor blade,” has stood the test of time.
The anxious build
If the design wasn’t challenging enough, then came the construction which started in October 2001. The project cost a cool 400 million euros ($437 million) and was financed by Eiffage, a private construction company which still has the concession for the bridge today. There were 290,000 tons of steel and concrete used to build it, and around 600 builders working on it.
“The huge challenge is what happens when you build it,” says Knight. “As you put the weight in different locations, it moves in different directions. There are different materials interacting with each other – this is as difficult as engineering gets.”
Foster calls the assembly of the deck “a true challenge.”
“It was serenely and slowly launched simultaneously from both sides over the temporary structural supports, meeting in the middle with millimeter precision.”
Virlogeux remembers the “critical wind situation” which risked damaging the structure during the build. Each “launching” operation – during which the deck was installed from both sides – would take up to three days, so they had to monitor the five-day forecast before starting out, to avoid causing damage before the deck reached the next pier.
It was only as the viaduct was put into place that the team could see if their design had worked – from an aesthetic point of view as much as an engineering one.
Every detail had been considered for its potential effect on the landscape, as well as whether it could resist the forces at that altitude.
Foster says that the first time he went to see it, “I was anxious to the point of almost being physically ill.” He had “agonized” over the color of the 154 cable stays – if they were light, they’d blend in with the sky but stand out against the landscape. He went with white – “but the agony was that I wouldn’t know if it was the right decision until it was built – and then it would be too late to change it.”
Luckily for him, the white worked. “I was almost sick with apprehension, but I remember arriving in a car and the bridge gradually coming into view and finally realizing that it was, after all, the right decision.”
Virlogeux was more sanguine. For him, the greatest challenge was overcoming local opposition to be able to build. Getting the contract signed, he says, was the most stressful part. Signing it was “the moment I knew we would build it.”
President Jacques Chirac came to open the bridge and shake hands with the construction workers. Two days later, Virlogeux drove across it on his way back to Paris.
Source: https://edition.cnn.com/travel/millau-viaduct-tallest-bridge-construction/index.html