The vast canopy nearing completion at Greenwich is probably the most misunderstood and certainly the most ill-named structure in recent engineering history. Almost literally the 'opposite of a dome', to quote its designers, the Dome would be better described as the Millennium Umbrella. Domes are three-dimensional arches, compressive structures, massive, static and opaque. The Dome is flexible, translucent, lightweight and, by contrast to St Peter's in Rome or St Paul's in London, ephemeral and insubstantial.
As such the Dome resembles all other tensile fabric structures, right back to the fabled Black Tents of the last millennium (see box). The only real difference to its ancestors - and the only major conceptual advance - is not its size but the use of a network of cables to support the canopy. These cable nets are a comparatively recent development, one which opens all sorts of possibilities, as Dome structural engineer Buro Happold partner in charge Ian Liddell confirms.
'Because of its size the radial cable net at Greenwich was assembled on site before being lifted into position. But prefabricated cable nets made up of 12mm cables at 500mm centres were first used by German pioneer Frei Otto on the German Federal Pavilion for the Montreal Expo in 1968. Their advantage is that the mesh deforms under tension and adapts to the shape like a very flexible fabric.
'They also act as a safety net during construction and cut the risk from tears in the edge of the fabric. Large unsupported fabric panels are always at risk during erection.'
There had been other roofs held up by networks of cables before, of course, but these usually sat on conventional walls and were clad with materials other than fabric. The Federal Pavilion was structure reduced to the minimum - masts, net, cables, and a PVC-coated polyester fabric hung below the mesh to create a weatherproof envelope.
The relationship to the Black Tent is obvious. Earlier, non-cable net Otto structures resemble Black Tents even more closely, especially in their use of multiple masts and tensioned fabrics.
Otto's concepts represented a complete break with the accepted approach to tensile structures at the time. These were driven by the need to create long clear spans inside buildings and stadia and drew on the proven technology of suspension and cable stayed bridges. By contrast, the type of tented structure Otto was developing had no engineering tradition, no structural theory, and only the craft of tent making as a source of inspiration.
'The problem was not just structural analysis,' Liddell points out. 'Even working out the cutting patterns for the fabric was a major challenge. For his earlier projects, Otto had to build physical models of his designs and measure directly off those.
'A lot of tolerance had to be built in. But by the time of the Federal Pavilion he was beginning to use computers as well.'
It is the development of specialised software for tensile fabric structures that has been the real breakthrough, Liddell maintains. 'Most specialists developed their own programs - we have already rewritten ours twice for different computers and are rewriting them again. These allow us to carry out structural analysis and patterning - and to work out what shape the fabric will develop.'
Buro Happold founder Professor Ted Happold used to rail against those designers who tried to make tensioned fabrics conform to their pre-conceived shapes. 'Clothes design rather than structural design,' he called it. Liddell says much the same.
'You cannot impose your own ideas on tensile structures if you want them to be efficient. The real problem is finding engineers who understand this basic principle.'
Liddell's first experience of tented structures was nearly a quarter of a century ago, when Buro Happold designed a 2,500m2 temporary pavilion for BP in Aberdeen. This 'humped tent' was skinned in traditional cotton canvas, but by the time Buro Happold's 1977 Jubilee Pavilion at Battersea was complete, cotton/polyester blends were available.
New software was ready by the time the Bath-based practice tackled the Baltimore Pier 9 concert pavilion in 1981. Similar structures in the Middle East followed, but the real breakthrough, according to Liddell, was the trend-setting fabric roof on the Imagination building in London, finished in 1990
'This won a record 18 awards,' he reports. 'It was an enormous boost to the use of fabric as a permanent feature of high quality buildings.'
The client's brief, Liddell remembers, was that his new roof 'had to be smooth, with no sticking-out bits.' This oblique reference to the distinctive multiple masts of the classic Otto structures may have been no more than a desire for a fresh image for a landmark building. But there is no doubt that double curvature in tensile fabric canopies can be a mixed blessing, as the collapse of the Montreal dome two weeks ago illustrates (NCE news, last week).
Highly curved canopies require less prestress for stability, but the valleys that usually result can be magnets for drifting snow. As the snow melts, ponds develop and rapidly deepen as the fabric sags further. If swift action to drain these ponds is not taken, fabric rupture is likely.
Most vulnerable to meltwater ponding have been the ultimate tensile fabric structures, those supported not by masts and cables but by internal air pressure alone. 'Small, simple air-supported structures, such as those over tennis courts, have not had many problems from meltwater ponding,' says Liddell.
'But on large, complex structures made up of double-skinned cushions, snowdrifts 1.5m deep can form in the valleys, where the corners between cushions are very soft and vulnerable to ponding.'
The smooth semi-spherical surface of the Dome is unlikely to suffer from the problem that ripped the Montreal roof apart, he maintains. The potential for ponding - should east London ever experience a bitter winter again - is greatest near the centre, where the Teflon-coated glass fibre fabric is virtually flat. But here the radial cables are closest together, so fabric spans are small, and deflections under even the most unlikely snowfall will be minimal.
From the centre, meltwater and rainwater will have a clear, unobstructed run to the perimeter, where the cables are 15m apart but slopes are steep. As it goes, it should carry London grime away with it - thanks to the 'non-stick' Teflon coating. This particular type of material has been around for 27 years, but Liddell is not expecting any major new developments in fabric technology in the near future.
'The requirements for a cladding fabric are daunting,' he points out. 'It has to be UV-resistant, durable, fireproof and easy to join and repair on site. There are very few around with these properties and a decent track record.'
PVC-coated fabrics have improved, he says, but for larger structures the much stiffer glass fibre-based fabrics are more efficient. The shading canopies that are becoming common in the Middle East - and which could become more common in so-called temperate zones as the ozone layer shrinks - are likely to be more suited to PVC-coated fabrics.
Their flexibility will be crucial as many of these will be giant parasols, opened up in the summer, folded away for winter. Tensile fabric structures are unlikely to replace conventional buildings when full environmental control is needed throughout the year.
Butlins' new Skyline Pavilions (see box) will be heated and cooled to a greater degree than the Dome, but even the 'double-glazed' Skyline canopies are poor insulators compared to the best conventional construction.
But when the internal environment needs to be modified rather than controlled, where occupancy is part-time, where unit costs and speed of erection are paramount, then tensile fabric structures have an assured future.
Liddell points out that the Dome, whose unit costs compare favourably to even the cheapest of 'tin shed' warehouses, is big enough to house a full size athletics track and several football pitches at the same time.
'For the cost of a Premiership player nearing retirement, a football club could roof over its entire spectator areas with fabric,' he adds. How long it will be until Old Trafford sits beneath a giant tent is another question.