Wednesday, 17 September 2014
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High-wire artist

Michel Virlogeux masterminded the construction of France’s Millau Viaduct and proved that structural engineers can collaborate with architects. Stuart Nathan reports.

It has been called one of the wonders of the modern world. Soaring impossibly high above the wide valley of the Tarn River in southern France, the Millau Viaduct is the world’s highest road bridge. Its deck is supported 885ft (270m) above the valley floor, frequently above cloud level, on seven concrete pylons, two of which are the highest in the world. It is also the longest cable-stayed bridge in the world, with almost 1.5 miles (2.5km) of roadway supported by the masts that sit atop the pylons.

To Michel Virlogeux, the structural engineer who masterminded the viaduct’s construction, Millau is as much a work of art as a piece of engineering. But that is how he views much of his work.

An impish Frenchman and a passionate advocate of rational design that blends harmoniously with its surroundings, Virlogeux has been designing bridges since 1974. He masterminded a number of landmarks in France, such as the Normandy Bridge linking Le Havre with Honfleur before tackling Millau. Elsewhere in Europe he designed the second Tagus crossing in Lisbon.

In the UK, however, Millau is more closely associated with the architect with whom Virlogeux and his engineering team collaborated, Sir Norman Foster. But Foster came on board the project relatively late, when Virlogeux’s concept had already been researched and finalised; it was the joint design, however, which won the competition to build the bridge.

Virlogeux insists Foster’s contribution cannot be downplayed. ‘It must be absolutely clear that Foster had an important role in the Millau Viaduct and it’s possible that for many reasons, including Foster’s reputation, that maybe this project wouldn’t have been built at all,’ he said on a recent visit to London to open an exhibition, which includes some of his designs. ‘It certainly wouldn’t have been anything like as beautiful.’

The story of the viaduct is a good example of how a collaboration between structural engineers and architects can work, he said. ‘I began work on the project in 1987, looking at the alignment of the roadway — the valley is so wide that the first problem was how to pass the motorway through it.’

There were two options — a low route, which would have entailed taking the road down into the valley, across one bridge, then back up and across another bridge that would have continued through a tunnel; and the high route, directly across the valley. ‘We had been reluctant to think of the latter, because of the height of the piers we would need to build, but when the problem was really posed, there was no reason not to build them.’

Virlogeux then began sketching to decide how the bridge might work in its surroundings. ‘I considered from the start that the site was very natural and very, very quiet. There’s practically no habitation there — the only building, even now, is the tourist centre, and that was originally the construction office. I thought it was necessary to build a bridge that was very calm, very soft in character. It had to be very slender, and for that reason the bridge had to be a cable-stayed.’

But the valley was so wide it had to be multiple pier cable-stayed, rather than the more familiar suspension bridge with two towers. ‘It’s a big problem to balance the live loads in a single span. And because Millau is very long, it has to adapt to large thermal variation — the expansion joints, for example, move by a metre.

‘The piers have to be flexible enough to accept this variation but, at the same time, they have to be extraordinarily rigid to support the unbalanced bending movement. This is why they are made of big box girders, which divide into two slim elements below the road, with the tower with the inverted V on top of that holding the cables.

‘None of that comes from the architectural design; it comes from the structure. But the architect translated that engineering structure into this wonderful and elegant design.’

Foster’s team came on board in 1993 and refined Virlogeux’s engineering concept into an elegant structure. They also helped ensure it was practical to build. ‘The Foster team worked with the engineers so that each new section of the pier could be built with industrial equipment. The piers are octagonal with four fixed elements in direction and dimension, and four elements that vary from segment to segment in width and orientation.’

These were built in situ, with self-climbing shuttering supporting the concrete as it was poured around the box girders, then moving up and adjusting to the new profile before fresh concrete was poured. ‘It’s very elegant, and totally the result of the close collaboration between engineer and architect,’ said Virlogeux.

Although his bridges are the essence of modernity, he eschews the use of computers. ‘I don’t use them. I always draw by hand. The rendering of computer drawing is still not very good — although it is better for architectural drawings — but even if it were to improve greatly, I would still hand-draw.’

Virlogeux’s drawings are derived from, and supported by, structural analysis calculations but he performs these on a simple hand calculator. ‘Afterwards, of course, people do use computers for the detailed design. But all my bridges start from a consideration of the aesthetics of the location and the design within it, and I always make different drawings with different options to see how they would look on site.’

What allows him to take this simple approach is an insistence that his designs are based firmly on the flow of structural forces. ‘I think engineers must design structures that are rational, logical, and work in such a way that they make elegant structures based on the flow of forces. The greatest elegance is to design something which is slender and beautiful and well integrated with the site. There are some structures you see where you just cannot understand it — there is no logic in the shapes.’

This tendency, he believes, stems from engineers who take on an architect’s role, and instantly forget all their engineering. ‘Then they make crazy things. The most crazy designs, in France, are made by architects who are officially engineers.’

So while Virlogeux agrees with Master of the Royal Designers for Industry Chris Wise (The Engineer, 14 January) that engineers often have no need to perform complex maths, he is adamant that a solid grounding in mathematics — or at least in structural mechanics — is an absolute must.

‘Structural mechanics is the basis of engineering. Yes, engineers must have more imagination, more creativity. they must go to concerts and exhibitions of paintings and engage in society and be inspired by all these things — but they must never lose mathematics and structural mechanics. If they lose that, they lose everything. They’ll become under-architects.’

And for all his respect for architects and their contribution to his projects, it is clear that, for Virlogeux, that would be a disastrous fate.

Virlogeux’s designs can be seen at the free exhibition ‘Spans: Viaducts, Bridges and Walkways’ at the Building Centre, Store St, London until 20 September

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