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Thirty- year wonder

COVER STORY; On 24 November Prince Philip will present a gold medal and a cheque for £50,000 to a team from Millennium Dome structural engineer Buro Happold. Dave Parker reports on the prestigious MacRobert Award for engineering innovation - and why the D

Had Ian Liddell not been a Fellow of the Royal Academy of Engineering, Buro Happold may never have thought of entering the Millennium Dome for the MacRobert Award.

Despite its £50,000 prize, and the gold medal, the award has a low profile in the construction industry. Few projects are entered, and the last one to win was way back in 1969. Happold project partner Liddell says this is largely because the one-off nature of most major projects discourages rapid innovation.

'There are no second chances, so innovation usually has to proceed in small, careful steps. Even if we as designers want to try something really radical, we have to remember we're working for the client, not for ourselves.'

Hence the award-winning tensile engineering of the Dome. However dramatic and mould-breaking it may appear, says Happold special structures group director Paul Westbury, none of the technology on display at Greenwich was developed specially for the project.

The elegant curves of roof and masts are made up entirely of straight lines, the Teflon-coated glassfibre fabric used for the roof has a long track record, the circular planform not only fits neatly on to the Greenwich peninsular but offers the maximum structural efficiency.

Using straight cables measuring no more than 25m node to node to make up the main cable net was the key decision. The result was a fully determinate structure supporting a roof made up of separately tensioned panels - a smooth roof which contained none of the deep gutters typical of earlier large tension fabric structures.

Eliminating these Achilles' heels - where snow melt tends to pond and rip the fabric - is one benefit. The simplification of design, manufacture and erection that follows was the real secret of the Dome's success.

'It's the same principle as the Crystal Palace,' explains Liddell. 'Simple, proven technology with lots of repetition to cover the maximum space in the minimum time.'

This technology has provided the New Millennium Experience Company with a covered space of 80,000m2 at a unit cost less than that for a typical edge of town DIY warehouse. However it was the potential for even larger, even more efficient structures that really impressed the judges (see box). Liddell says it would be relatively easy to build a structure with the existing technology that would be twice as big - 15ha or more.

'This would be big enough to cover a small town and protect it against extremes of weather - in the Arctic or in a desert area, for example,' he points out.

Although the fabric used at Greenwich would make an excellent sunshade - the anti-condensation lining used on the Dome could be omitted - a different approach would be needed in the Arctic. Much more light transmission would be essential. ETFE foils offer up to 95% translucency, but have several drawbacks to balance against other advantages.

Westbury explains: 'They are very light, which is very important in ultra long span structures where the biggest problem is always self-weight.

'But they have a very low yield strength which means they can't be used as tension membranes. A secondary cable grid would be needed.'

He adds that introducing new membranes without a track record is very difficult. 'Tensile fabric structures are so new that any serious problem on a high-profile job - even the fabric getting dirty too soon -- could make them unacceptable to clients.'

Future 'Domes' need not be circular in planform, Liddell points out, and a second row of masts could be added if necessary. He adds: 'A circle is the most efficient shape structurally, as this allows you to lead all the main cables down to a circular concrete ring beam, which minimises anchorage costs. But any smooth, regular geometry would work.'

The four award winners (see box) will receive £5,000 each, the balance will go to fund Buro Happold's long-term promotion of civil engineering as an exciting and rewarding career. 'Youngsters are fascinated by technology,' Westbury points out. 'But this interest seems to tail off at sixth-form level.

'We need to make sure that the brightest people are excited by civil engineering - and this award should help.'

For those who would like to learn more about how the Dome was designed and built, a special exhibition at London's Science Museum opens on 25 November.

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