Consider Santiago Calatrava's Trinity Bridge in Salford, UK. At first sight it appears little more than an unusually complex cable stayed footbridge across the River Irwell, with the steeply raked single pylon that features on so many of its Spanish superstar designer's projects. Yes, skateboarders and the weather have left its brilliant white fiish in need of urgent cosmetic attention, but its overall visual impact is stunning, a credit to the art and craft of bridge engineering.
Look closer, however, and the picture becomes less rosy and more ambiguous. At the ends of the decks are sturdy tie downs, stressed to 200t. Their purpose?
To tension the cat's cradle of cables that run between pylon and decks. Most of these cables are structurally redundant.
Their function is purely decorative. With most of its structure actually over land rather than water, the bridge is more of a piece of sculpture than a crossing. Architecture has driven the design, not engineering.
To those brought up in engineering's Puritan tradition, such an approach is anathema.
Recently, Millau viaduct engineer Michel Virlogeux told NCEI: 'Engineers must be modest.
Attention grabbing designs are an easy way to make a reputation. I don't think art can come from provocation. The greatest art comes from making things very simple but elegant and perfectly adapted - or you soon have cracks, accidents, problems.' Others agree, pointing to the signifi antly higher maintenance input needed on many extreme structures, and the risks involved at every stage from design to construction. London's 'wobbly' Millennium Bridge is frequently cited as an example of what can happen when architectural pressure for something completely different is allowed to dominate sound engineering practice. But many others disagree.
'The public loves these special bridges and they can be a very powerful aid to regeneration, ' insists Whitbybird bridge team director Des Mair. 'Most clients are well informed on the need for good detail design, and as long as the adopting authorities are aware of the need for adequate maintenance there should be no durability problems.' Buro Happold head of bridges Davood Liaghat admits to disliking what he dubs 'contrived structures', but he still appreciates the challenge of 'more gymnastic' bridges. 'Yes, as structures get lighter, concrete grades get higher, cables become ever thinner, you do get longevity issues, and dynamic behaviour is crucial, ' he says.
'But a combination of good engineering and modern protective systems should keep maintenance costs at normal levels.' On extreme structures Buro Happold now issues the client with a much more comprehensive maintenance document than those which are standard for more conventional structures. However, Techniker director Matthew Wells believes there is a 'third way' to iconic status - one that should avoid excessive maintenance costs.
'Following the Calatrava route isn't necessarily the best option, ' he says. 'Iconic structures don't have to be overweight or over engineered. I believe there is a moral duty not to waste money on infrastructure projects. Efficient structures that give value for money don't have to be dull.' Dartford's QEII Bridge and the Second Severn Crossing will never achieve iconic status, however effi cient and cost effective they may be. The Millau viaduct, where engineering came first, already has. So has the 'Blinking Eye' Gateshead Bridge, although some say it is overweight and expensive to maintain.
Despite its perceived drawbacks, the Gateshead structure has brought huge benefits to the area. 'The contribution it's made to the image of the area and the inward investment it's attracted are priceless, ' Wells points out.
'That's what clients are looking for, the so-called Bilbao effect - the same impact as the Guggenheim museum had there.' And in the search for this effect, clients are increasingly adopting the design competition route, which is pushing designers towards ever more fanciful structures.
Gifford technical director Peter Curran, the engineer behind the Gateshead bridge, believes such an approach needs handling with care. 'There are two real problems with design competitions, ' he says.
'First, they are very expensive to enter, because if you don't put together a well-prepared presentation you aren't going to win.
And then there's the danger that the client will be seduced by the sort of stunning imagery it's possible to produce these days, and ignore potential problems with maintenance and logistics.' Perhaps the last word should be left to Salford City Council.
Lead member for planning Councillor Derek Antrobus says: '[Trinity Bridge] is a complex structure that was the first of its kind in the country and which is now much imitated. From our point of view, it has paid for itself many times over in terms of the millions of pounds of new investment it has drawn into our city'.