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Millennium marquee

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The simple idea of a flat cable structure for the Dome has won Buro Happold's team a prestigious engineering award

That the designers of the structure for the Millennium Dome should win a prestigious engineering award this year seems appropriate. Even more so since the Royal Academy of Engineering MacRobert Award has only been previously awarded to one other construction project.

Buro Happold's Millennium Dome highlights the role of innovation in construction and the value of design. Like many innovations the idea is simple and comes from observation of how marquees work. Buro Happold has created a tension structure without the inefficiencies of double-curvatures normally associated with that form. 'The neat bit' says senior partner Ian Liddell, 'is having the tension cables and stretching the fabric in between.'

The first of the annual MacRobert awards for innovation in engineering was awarded thirty years ago to Freeman, Fox & Partners for the Severn Bridge.Last year, the winner was a breath-activated inhaler for asthmatic drugs, and the Millennium Dome project won against vastly different engineering projects: a gyroscope no bigger than a penny, cheap enough to be used for car braking systems; a free-standing carbon-fibre sailing rig with the mast and boom rigidly fixed to allow the whole unit to swivel, and a digital terrestrial television.

The team which won this year's medals and a cash prize included Ian Liddell, Paul Westbury, Dawood Pandor and Gary Dagger. Liddell, who led the team, is one of the founding partners of Buro Happold and one of the world's leading experts in tension structures. He is familiar with some of the world's most renowned structures.As a young engineer just out of Cambridge employed by Ove Arup and Partners, he worked on the roof of the Sydney Opera House.After a further two years as a post-graduate student at Imperial College, London, studying prestressed-concrete and shell structures, he was engineer for a number of industrial concrete structures including the Drax power-station chimneys. Rejoining Ove Arup in the 1970s, he moved into lightweight tension and fabric structures to become one of the world's experts in the technology. In 1996 he was elected Fellow of the Royal Academy of Engineering.

Paul Westbury, group manager of the special structures group at Buro Happold, was responsible for the development of the engineering for the Dome. Dawood Pandor was responsible for the detailed design of the steel masts and cable-nets, and Gary Dagger provided technical CAD support.

The surface of the Dome, which geometrically is the cap of a sphere rather than an hemisphere, is held in place by a network of cables supported by twelve 100m-high, open-lattice masts.Each mast supports three radial cables on either side, and points towards the centre of the sphere. Spacing of the radial cables is maintained by five rings of circumferential cables. The 72 radial cables are connected together at the centre by a cable ring in tension, and held down into the ground at the perimeter by ground anchors and a perimeter concrete compression ring. Between the 72 radial cables, tensioned PTFE-coated glass-fibre fabric is used as the cladding the giant umbrella covering some 80,000m 2on the Greenwich peninsular.

Generally, tension structures are anticlastic, doubly curved surfaces where the down loads are taken by one set of cables, or by weave in the fabric, while the up load from wind is taken by another set. On the Dome, straight cables take the loads in both directions. Both the tensioned cables and the cladding carry the loads by deflection accompanied by some increase in tension.Whilst the traditional shape is effective at resisting loads, there is a limit to the size of structure that can be built with it and each segment of fabric must be heavily engineered, separately modelled and then cut by computer. The efficient design of the Dome enabled replication of the segments.

The notion of using a flat cable structure was developed from observing marquee-type tents which rely on flat fabric stretched out by guy ropes. Whether they were loaded upwards or downwards, these flat fabric structures produced tensions in the fabric that are in the same direction.On this principle, much larger structures can be built using straight cables. It is then possible to slide the fabric into a groove in an aluminium extrusion. Before the Dome, Buro Happold had already designed two structures using this principle: a tennis hall at Eastleigh, Hampshire and the large audience tent for a religious community, RSSB, in Bedfordshire which covered some 20,000m 2.The design for the Dome owed much to Buro Happold's experience with the audience tent. Although the concept of a large flat fabric roof is simple there are dangers associated with the deflections, particularly ponding caused by snow or heavy rain.To avoid this, the circumferential cables had to be lifted off the fabric to stop them acting as dams. This was achieved by raising the circumferential cables above the surface with rigid wishbones and connecting them to the nodes with criss-cross cables. Because cables rotate at node points, they were all cut to pre-stretched dead lengths connecting between each node - allowing movement and rotation of the constituent parts of the cable net without danger of fatigue.

The design of this node was critical.

At the outset of the project, when Gary Withers of Imagination and Mike Davies of Richard Rogers Partnership came up with the idea of one simple covering to shelter a collection of exhibitions on the site at Greenwich, there was no doubt in Liddell's mind about just what the structure would be. The concept design for a 400m-diameter cable structure with two rings of masts was faxed back to RRP the following morning. Following some revision, the diameter was reduced to 320m.

One ring of masts was moved towards the outside and the outer ring of masts was dropped altogether.

'You can make it bigger, that's the interesting thing, ' said Liddell. He cited a Buro Happold feasibility study in 1980 to cover a 14ha city in the Arctic with an airsupported structure. There, snow loading would have been a problem, but Liddell reckons that a flat cable structure might prove to be the answer in such instances. It might also be useful in hot desert climates where shading structures are required.

The Teflon-coated glass-fibre would reflect a lot of the solar radiation. The value of such structures lies in the ability to enclose large volumes without the interruptions of columns. This makes them ideal for sports stadia.The temperature inside the Dome during the summer is 2-3degrees higher than outside, but direct solar radiation is eliminated by the fabric.

If used in a sports environment it would be relatively simple to cool the incoming air for the bottom 3m.

Liddell believes that it is only a matter of time before such a single-volume structure is built to cover an area the size of a town and to cope with extreme climates.

For the moment however, the engineers can study the performance of what could be described in more than just metaphorical terms as a giant enclosing shelter.

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