The art of bridging the gap

It comes as no surprise that Michel Virlogeux prefers bridges to tunnels.

The French engineer is celebrated for his bridges and viaducts, which include the breathtaking and record-breaking Millau Viaduct in the south of his homeland.

Virlogeux was in Auckland this week as keynote speaker at a conference of bridge builders organised by Austroads, an association of Australian and New Zealand road and traffic authorities.

With debate ongoing about whether Auckland should address its harbour traffic problem with a tunnel (Transit NZ is pursuing a feasibility study for a tunnel with a tentative cost of $3 billion), a second bridge or a single, bigger bridge, seeking Virlogeux's views was an opportunity too good to miss.

"Why! A bridge, of course," was the inevitable answer from the man who has been designing bridges for 35 years.

With specific lighting, ventilation and safety issues, tunnels are much more expensive to operate and - being out of sight - do not offer an opportunity to enhance the landscape as a bridge does.

Following the banning, two years ago, of trucks from the outside clip-on lanes to ease pressure on the bridge structure, architects Jasmax released a conceptual design for a replacement bridge featuring a giant angled pylon supporting a splay of cables in the shape of a sail.

The idea was to spark debate about crossing options and some such as Richard Simpson, who at the time chaired Auckland City Council's transport committee favoured a replacement bridge that was "a world class piece of architecture".

Jasmax's design took its inspiration from the Puente del Alamillo in Seville, by architect, engineer and sculptor Santiago Calatrava, and adapted it to symbolise Auckland's sailing heritage.

Virlogeux has reservations. He points out he has neither the details nor the time to examine the plans but says his immediate reaction is that the design is "not very logical".

Good bridge design must first address the flow of forces (how you drive the forces down into the foundations) and he was unconvinced a design with cable stays only on one side of an inclined tower, which rose out of the sea rather than being anchored on solid land, met that first and foremost criteria. "It is unbalanced. It is not what I would recommend."

Demands have changed from when he began designing, a time when designs were constrained by budget and technical capability. "The public, much more today, want something interesting, whereas 35 years ago they wanted only to get across the river."

"It is asking too much and it is wrong" he says. "I often say that one signature bridge is a signature bridge. When four bridges are signature bridges, it is just disorder."

The risk is that the desire for an iconic structure gets in the way of fulfilling the purpose of a bridge. "A building is a bit different," he says. A bridge carries enormous loads. Each of the two legs of the Bridge of Normandy, for example, carries 20,000 tonnes.

"The forces are enormous. A bridge is not something that you can design like it is just for fun. The loads are so important."

That doesn't mean a functional bridge cannot be a thing of beauty. "For me the art of designing bridges is to organise the flow of forces elegantly."

Virlogeux says the principles of the world's first known engineer, Vitruvius, still apply 2000 years on. "Utilitas, fermitas and venustas. Utility and function, stability and durability, and beauty. In that order. It all begins with utilitas. It is the first treatise of civil engineering."

Current trends are either to build a bridge at the lowest cost or to construct a "signature bridge". "Both are not correct, in my opinion," he says. "You must try to develop good designs, elegant designs but control the cost at the same time. Especially now in this big crisis, spending huge amounts of money for crazy designs is not very reasonable."

Beauty, says Virlogeux, is possible on a budget, though not the lowest budget. Elegance comes from the illusion of simplicity. Illusion, because the appearance of simplicity is almost always the result of enormous graft.

The cable-stayed Millau Viaduct - on which Virlogeux collaborated with British architect Norman Foster - is a perfect example. The piers seem to rise out of the ground.

"Foster wanted the bridge to give the impression that it penetrates straight into the hill. It was rather difficult technically to solve the problems associated with that but the concept of pure continuity was so strong for us that we worked to achieve that."

More than 40 models were made of the piers to arrive at the perfect shape. The piers become more slender as they rise - from 25m wide at the base to 11m at the top - meaning each section has different dimensions. "It was not something that came easily."

He prefers bridge design to be driven by engineers rather than architects, but engineers with an eye for elegance. Architects are more prone to flamboyant statements, engineers better understand the forces, he says. "The first step is coming up with a design that is well-fitted to the site. After that, you have to design a bridge that has unity of its components, continuity of the shapes and shapes which express a flow of the forces."

His collaboration with Foster, whom the Frenchman describes as one of the best architects of our time, was such a success because they complemented each others skills and most importantly shared a similar idea.

"We worked to reduce the bridge to the pure essence ... The greatest art comes from making things very simple, but very elegant and perfectly adapted."

The bridge is a straight line across a valley supported by seven towers, some taller than others but all the same shape, helping create the sense of unity and proportion.

"Personally I know that I am not able to [produce] very refined detail sheets but I know much better than architects which are the proper proportions, which are the good lines, which structure is best adapted to a site. That, I know very well."

Millau is the tallest vehicular bridge in the world, with one mast reaching 343m slightly taller than the Eiffel Tower and only 38m shorter than the Empire State Building. It cost about €400 million ($892 million) to build and was paid for by tolls. The speed limit for traffic using the four-year-old bridge has been cut from 130 km/h to 110 km/h because motorists slowing to take photographs endangered other drivers.

The Normandy Bridge - the Pont de Normandie over the Seine - is another iconic Virlogeux structure. But the quiet, though ebullient, 62-year-old explains that is a by-product rather than the goal itself. "For me we must not set out to make a signature. The goal is to make something which is proper which is well adapted to the site which is efficient and which has nice shapes."

Engineers, compared to architects are unsung heroes, perhaps by the nature of their work. Technical drawings of a pier or abutment doesn't make a pretty and accessible picture for the public, says Virlogeux who adds that engineers don't tend to have strong links with politicians or media.

"Often we don't express ourselves well and we are not very good at selling ideas. We are obliged to work on the details because you must solve all of the problems [before construction]. It is why design must be developed step by step."

"People really understand [the engineer's work] only when they see the structure coming out of the ground."

For example, the Normandy Bridge was subject during planning to major controversies between technicians, particularly regarding wind effects. This failed to penetrate public consciousness but as the support towers rose from the ground public interest soared.

When it opened in 1995, the bridge set records for the longest cable-stayed bridge in the world and for the longest distance between piers for any cable-stayed bridge. It was swamped when opened to the public to walk across.

Shortly before Virlogeux arrived in Auckland, walkers and cyclists breached a police cordon and crossed the bridge in protest of the refusal to add a bike and pedestrian lane. They were ushered by police into the centre lanes causing long delays to motorists. This was done for safety reasons after a bounce effect on the clip-ons was noticed during the last hikoi.

After the hikoi, Virlogeux was contacted by one of the protestors about the bounce. He points out that without detailed information he cannot comment specifically, but notes that "large crowds of pedestrians can be aggressive for a structure".

Flex and sway have been problems on pedestrian bridges, such as the Millennium Bridge, dubbed The Wobbly Bridge by Londoners, and a bridge in France where one of the dignitaries there to open it fell over due to the flex.