Engineering excellence

Multi-award-winning French bridge designer Michel Virlogeux was the keynote speaker at the Austroads Bridge conference, held in Auckland in June. He had a simple message: Bridge design is the realm of the engineer and should not be given to architects.   BY MARY SEARLE

“Bridge architecture, bridge aesthetics and bridge elegance is the problem of engineers. We must not leave this major part of our job to architects,” says Michel Virlogeux, who developed the preliminary design for the Millau Viaduct (pictured right).

“I always work with an architect, but the architect is not selecting the structure. I always wear the trousers. When I am working on a building I do not try to take the trousers, because when you are working on a building, the work of the architect is to organise the living life – where the light must be in a flat or how to organise a public centre like a cinema. To design a bridge is to master forces, and this is not the job of the architect.”

This does not leave architects out of the bridge design game altogether, however, Virlogeux himself frequently works with architects, saying they can bring a lot to a project.

“I’ve been designing bridges for 35 years now and I can foresee very well which type of structure is most adapted to the site. I have a very good idea of proportions – where to place the piers and so on – I don’t need help for that. But then later, when we have a good idea of what we can do, I consider that I am not able to refine the shapes – to play with light and shadow. And there the architect is much better than I am.

“If I take the example of the Millau Viaduct. Foster, after looking at the models we had first prepared, he wanted to reduce the bridge to the essence – just the straight line, which was also my idea from the beginning. But he pushed to take out the small piers and the side span that we had in the beginning. It made the technical problem more difficult, but the bridge was reduced to just the straight line and the seven identical towers. Myself also, I wanted identical towers, but I could have left, probably, the small piers at the end.”

Virlogeux says Foster also wanted the viaduct to give the impression that it penetrates the hill. “This made the abutments much more complicated. But really, he improved the design of the bridge very much and the bridge is much simpler in the end. This doesn’t mean it was easier, in fact it was much more complicated to do it, but it’s much simpler, because the shapes are very, very, very simple, and people feel that.”

Virlogeux told Contractor the trouble begins when architects try to do the job of the engineer: “To organise the flow of forces with 10,000 tonnes everywhere, it’s not the problem of an architect, it’s the problem of an engineer. And the engineer masters the way to erect, the construction techniques, the flow of forces, the passage of forces, how to connect elements. Not the architect. So it’s absolutely crazy to give bridge designs to architects.”

He gives the example of the Alamillo Bridge in Seville, Spain (pictured right), designed by award-winning architect Santiago Calatrava.

“It’s a very expensive structure because it has absolutely no structural logic. You have just one single span, cable-stayed from a tower that is inclined backwards but there is no backstay at all, so that means that the tower has to take all the effect of the load – the weight takes a very small part of it – and you have, of course, below that, an enormous foundation to balance the load. This is a structure which is exactly what I think we cannot do. This costs very much. This is completely illogical. And even if Calatrava is very famous, I am very much against that.

“I will take another example, the [nearby] Barqueta Pedestrian Bridge (pictured right) – this is a logical structure. There is an arch and this arch is carrying the load through a compressive element, which is the arch itself. As the arch is just a unique member, it has to divide at the beginning to let people pass and walk on the bridge. And so the compressive force that is in the arch is divided into the two incline legs, which extend the arch downwards. We reach the support, and I must be clear, the compressive force coming from the strut is balanced by the tensile force in the deck. Then as the deck has to horizontally incline elements, the horizontal transverse action of the compressive force of this strut is balanced by the other one through the cross beam at this level. Finally, we have the vertical load coming down on the pillar that is below.

“This structure is absolutely logical.”

So why is bridge design being given to architects instead of engineers?

“The situation that I consider critical comes from the fact that engineers have not done their job properly. Too many have designed things that are not beautiful. Also because they are too concerned with codes – yes, we need codes, but we must not be the slaves of codes,” says Virlogeux.

“They are too concerned about cost and many details sometimes. The engineer must first be more sensible to elegance and beauty. And must have a much broader culture – they must go to museums, be interested in paintings and art, and must appreciate proportion, it’s extremely important. Harmony, unity, continuity.

“If we go back to the 18th Century, a series of drawings from the archives of the École de Ponts et Chaussées show engineers were directly and very much concerned by elegance, architecture and even decoration,” he says.

“But we lost very much this aspect of the formation of engineers, and engineers now have lost very much of their influence on construction because they have been able to build very ugly structures.

“We have seen a presentation during this conference of a bridge with approach spans and a main span. There is no unity between the two. There is no continuity in the shapes. There can be no elegance in such a structure.

“When you design that way, you’re killing yourself, and, of course, if engineers produce bridges like this it can be expected that the public says we must call for architects. So the problem this time is that architects will make something absolutely crazy, which is a completely different way of making things wrong.”

This problem is compounded by the decisions of bridge authorities to build ‘iconic’ bridges.

“I think that a decision to build an ‘iconic’ bridge is a poor decision,” says Virlogeux. “When you have to build a bridge somewhere you have to make a bridge that is fitted to the need and the site of the landscape.  And if you are successful it will be a good bridge. Maybe it will be iconic, maybe not.

“But really this tendency to make things that are ‘signature’ or ‘iconic’ is really the desire to show off, which is absolutely stupid,” he told Contractor.

“Some architects explain that they are designing pieces of art. No. You don’t decide that what you do is a piece of art. Future generations decide what is a piece of art.”

However another big problem engineers have, says Virlogeux, is that it is extremely difficult to convince the politicians and the public at the beginning of the benefits of a simple bridge.

“As we are trying to make things simple, the drawing is very poor when compared to drawings of a ‘signature’ or an ‘iconic’ bridge – things with pylons this way and the other. But when you make something simple, although it may not be impressive at the beginning, at the end, people feel it and see it.”

And therein lies the answer – sandwiched between the lowest-cost and the ‘iconic’ options.

“I think it is necessary to take a medium road, closer to reasonable cost. We can design elegant bridges with rather reasonable costs. This is not the case for the Millau Viaduct, because it has very high piers, and so it’s clear that this is not the cheapest solution we selected. But between the cheapest solution and Millau there is probably 10-15 percent cost increase – not three times the expense. And when you make a crazy design it could be three, four, five times the cost, because when you design outside of the normal flow of forces, the cost can be anything.”

Virlogeux gives a simple example of this, from a particular project he is revising. He says the architect has designed the tower legs of the cable-stayed bridge to be of varying shapes all the way up. As a result, during construction, it would be necessary to change the shutter every four metres: “The cost will be absolutely enormous and the aesthetical advantage is zero”.

“You can make very beautiful things, and sometimes you are obliged to have variable sections, but you can organise the variations in such a way that you still can built it with equipment that is of a reasonable cost.

“The cost to take functional to elegant could be perhaps as low as one percent of the cost. This is what we can do very easily.” 

Next: Five of Virlogeux's bridges