In its simplest form, the structure consists of an arch restrained on either side by a conical web of cables – like a bicycle wheel sawn in half through the hub. Each structural member carries its optimum share of the total load, which gives the whole form great strength and lightness. "There simply cannot be any less structure," explains Halford. He claims that if the domes at the Eden Centre had been constructed using this system instead of space frames, they would have been about one-third of the weight.
The result is that the structure can be used to span huge areas, which opens up opportunities for enclosures that, until now, would not have been cost effective.
Working on the scheme with Halford is Angus Brown of Fraser Brown Mackenna Architects. He says: "The structure could be used to cover sports pitches or theme parks, to form aircraft hangars or even to cover whole cities with one clear span arch."
Another of the structure's selling points is that it can be erected easily. According to Halford, it should be possible to put it up with nothing more than a winch. "It is easy to set out and it needs no cranes or scaffolding – so nobody has to work above ground." And, because it is easy to erect, it is easy to take down.
The idea for the structure first came to Halford five years ago, when his practice was putting together a proposal for a millennium dome. The scheme was for an enclosure one mile across. Halford says he remembers the moment when he looked at the dome proposal and thought: "Crikey, if you were to rearrange certain elements, it would become a theoretically efficient structure," – one that uses the minimum structure.
The largest element of the structure is the main arch. Surprisingly, this is not constructed from a single element. Instead, it is formed from a series of articulated segments connected by universal joints so that before it is erected, the assembly hangs together like beads on a necklace. The joints allow the arch sections to "slightly rearrange themselves" so as to share the load evenly, thereby reaching a state of equilibrium and ensuring that any bending forces are not transferred from section to section. The joints also allow the arch to be broken down into smaller sections for transportation.
The arch is restrained by two fans of cables that spread out from two anchor points, one on each side of it. For the structure to work, an imaginary line joining the anchor points must pass beneath the arch. Each of the cables stretches from the anchor to the centre section of each of the universal joints linking the articulated arch sections.
The flexibility of the system means that arches and anchor points can be combined in an almost endless variety of ways to match the shape of enclosure to the applications.
To prove that his idea would actually stand up, Halford wrote a software program to analyse the structure. Having satisfied himself that it would work theoretically, he set about constructing a 7.5 m wide, 9.5 m long, 3 m high two-arched prototype in his back garden to demonstrate his innovation to others.
Despite its room-sized dimensions, the prototype's entire structural components "could be held in one hand", he says.
Halford tested loading by getting his colleagues to hang from the arch to assess its stiffness, and the prototype's deflections were checked against the predictions of the software (the box on the right explains how he put it up). To his amazement, the structure was rigid. "It's just remarkable, it just goes rigid and doesn't move – it's much more rigid than conventional structures," he says.
The structure's eventual use as an enclosure will dictate how it is clad. ETFE – the cladding material used at the Eden Centre – is the team's favoured material for sports training grounds because of its transparency and relative lightness. ETFE is relatively expensive, however. Other cladding options include PTFE, which was used for the Millennium Dome. Brown says it would even be possible to use sheet metal roofing.
To market the system and offer a complete turnkey solution to clients, including the design of accommodation within the dome, Peter Dann and Fraser Brown Mackenna have formed a consortium with ETFE specialist Vector Special Projects and Daniel Ptacek, the project manager for the roof on the Millennium Dome and Dynamic Earth in Edinburgh.
The design has been patented and is being marketed as Xanadome. The consortium is now looking for its first customer. Halford says he has had a few promising enquiries, although nothing definite as yet. But with the sheer number of potential applications the enclosure permits, it is likely that his phone will be ringing a lot more soon.
Erecting the prototype in the garden
For more information, contact Angus Brown on 020-7251 0543.