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Dome Design

Cover story

Conceived, designed and built in record time, handed over on schedule and to budget, the Millennium Dome is a fitting climax to a century of civil and structural engineering achievement. Dave Parker looks back at the crucial design phase.

Some structures have an essential rightness, a purity of form and function that sets them apart from the ordinary. Much was expected from the architects and engineers responsible for creating the Millennium Experience on an abandoned brownfield site in east London. A principal requirement was to deliver a landmark structure, one as instantly recognisable as that other supposedly temporary creation the Eiffel Tower.

This they have achieved, beyond all question, and at a bargain price. Never before has so much space been enclosed so quickly and so efficiently. Engineer and architect, working as equal partners, have delivered the 20th Century's answer to the Great Exhibition of 1851 - and set a benchmark against which all similar structures in the next century will be measured.

Neither dome nor tent, the structure which eventually took shape at Greenwich was a response to essentially simple parameters set by the site, the design brief, and, above all, the iron deadline of 31 January 1999. If a conventional dome is an arch rotated through 360degrees, this Dome is a three-dimensional cable stayed bridge, its 12 giant masts supporting the world's largest fabric roof. Beneath it is a space big enough to hold the Eiffel Tower lying on its side, second in enclosed volume only to the enormous assembly building at Boeing Aircraft's Seattle works.

It was architect Richard Rogers Partnership project partner Mike Davies - the 'man in red' - who first came up with the idea of putting the whole Millennium Experience under a single 'umbrella'. An April 1996 visit to the bleak, windswept, Greenwich peninsula convinced Davies that visitor comfort had to be high on the list of priorities. After all, the event had to attract millions of visitors during the winter months if it was to meet its financial targets.

At that time the basic concept was 12 separate pavilions grouped around a circular central arena. This circular theme fitted neatly into the rounded 'nose' of the Greenwich peninsular, but without any idea of what would eventually be inside the pavilions it was difficult for architects and engineers alike to begin detailed design work, let alone apply for planning permission.

Creating an all-enveloping weatherproof umbrella over the entire exhibition space solved most of these problems at a stroke. Design of what eventually came to be known as Zones could proceed virtually independently of the main structure - nor did the Zones themselves need to be weatherproof. And it promised serious financial benefits as well.

'Cladding costs on a series of separate pavilions were beginning to look astronomical, compared to the volume of space enclosed,' explains Buro Happold special structures group director Paul Westbury. 'At this time we were still planning a temporary venue, so the numbers just didn't add up.'

In May 1996 discussions between Davies and the Buro Happold team led by project partner Ian Liddell produced the first recognisable sketches of what was inevitably dubbed the Dome by the non-technical press. At first Davies thought, literally, of a giant umbrella structure with the roof supported by radial ribs spanning between the ground and a cluster of masts around the central arena. 'These ribs would have been huge steel trusses, very difficult to design, and very expensive,' Liddell says.

'Instead, I told the architect we would do a cable net structure for him, one that would be twice as big as anything ever done before.'

Buro Happold built its reputation on innovative tensile fabric structures like the Diplomatic Club in Riyadh and the Jeddah sports stadium. It had also developed its own software, Tensyl, to analyse tensile fabric structures. However, what took shape on Liddell's drawing board bore only a vague resemblance to earlier cablenet structures.

A giant tensioned spider's web of radial and transverse cables would hang from 100m tall steel masts. Individual panels of PVC-coated polyester would be stretched between the cables to form the weatherproof envelope, quite unlike the classic tent form, in which the fabric is an essential part of the load bearing structure. Radial cables would run down to ground level, their inward forces resisted by a perimeter ring beam. There would be none of the dramatic double curvatures in the roof fabric so characteristic of earlier cablenet and fabric structures. The Dome roof, however curved it might appear from a distance, would be made up entirely of straight lines and flat panels.

Liddell says early designs had an outside diameter of 400m and two rings of masts, 12 close to the centre around the planned arena and 24 further out towards the perimeter. Cables spanned about 25m between nodes, with each node connected by hanger cables or tie-downs to the top and bottom of a mast. By August this had shrunk to a 320m diameter structure with a single ring of much taller masts sitting on 10m high steel pyramids.

'This gave a much clearer floor and moved the tie-down cables to well above the height of most zones,' Liddell explains.

Flying struts increased headroom around the perimeter. Later, the edge detail was refined to minimise the 'circus tent' effect by running only 24 of the 72 radial cables direct to massive ground anchorages. A 90mm diameter 'scalloped' boundary cable was added between the anchor blocks, picking up the tensile loads from the four intermediate radial cables and transferring them to the ground anchors and ring beam. Says Westbury: 'This brought the roof down to the ground at the anchor blocks, giving emphasis to the larger door openings that were now possible'

At the centre the radial cables hook onto a three-dimensional 'hub' made up of a 30m diameter ring of 12 cables clamped together, into which a cable truss was inserted to support the central cladding system. Originally the rim of the hub had been designed as a steel fabrication but concerns over safety led to this being changed to a ring of cables. Six cables could break before the safety of the Dome would be compromised, a continuation of the fail-safe philosophy that dictated the choice of twin cables for all important sections of the cablenet.

In service, the biggest threat to the integrity of tensile fabric structures has been meltwater ponding. Snow falling on the fabric eventually melts and drains into any low areas, depressing them even further. As more snow and rain falls the pond becomes bigger, deeper and heavier, until at last the fabric fails. Buro Happold was well aware of this risk, and took elaborate precautions against it.

'In fact the shape of the roof is inherently resistant to ponding,' Liddell says. 'The radial design means the fabric panels are much stiffer at the centre, where the roof is virtually flat, than at the perimeter. We had to ensure there were no transverse 'hard spots ' behind which meltwater and rain could build up.'

Hence the design of the four circumferential 'cable trusses', which allow fabric to pass through rather than over them. Closer to the centre, the fabric is held at a steeper slope than the cable net proper by thousands of aluminium spacers clipped onto the radial cables.

Evocative of the famous 1951 Festival of Britain Skylon they may be, but the driving force behind the design of the 12 main masts was structural efficiency, not aesthetics. The designers had to balance resistance to the bending loads induced during the raising operation against speed and simplicity of construction. There was also the desirability of keeping the dimensions of the individual sections to be joined together on site small enough to obviate the need for a police escort during transport.

'We eventually went for a simple Vierendeel design made up of straight sections, with big box section hoops and shear forces taken by the welds between the hoops and the main tubes,' Westbury reports. 'This kept overall weight of each mast down to a reasonable 95t. Maximum diameter is only 4m, which eases transportation.'

Mast fabricator Watson Steel later fine tuned the design still further, reducing some section sizes and substituting a standard pot-type bridge bearing for the original jack designed to go between mast and pyramid support. This sat well with what Liddell calls the 'Crystal Palace philosophy - proven technology, lots of repetition.'

Overall, the Dome's masts, cablenet and fabric were calculated to weigh in at 2,200t The 2.2Mm3 of air inside the Dome weighs around 2,600t. Unit cost is 'less than an out-of-town warehouse' - at least half the cost of conventional exhibition pavilions. And the building envelope would take only 15 months to construct, from first pile to completion. Not bad for a landmark structure, even if it was originally designed to last only two years.

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