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Bubble wrap

Cover Story

Inspiration for the 2008 Olympic swimming centre was initially elusive, confesses Arup Australia principal Tristram Carfrae. Arup, Australian architect PTW and China State Construction & Engineering Corporation spent the first four weeks of a 12 week competition bid period chucking paper into the bin.

'Then BOCOG revealed the winning design for the main stadium, and that galvanised us, ' Carfrae says.

'We knew that what we did for the swimming centre had to sit well with the bird's nest on the other side of the street, and to mesh with the Olympic site master plan it would have to be square sided - a cube.

'We were really keen to wrap the cube in a seamless fabric that went up the walls, over the roof and down the other side.'

Having set out formal criteria, Carfrae's team next set about fulfilling technical criteria.

'How do you most efficiently heat a swimming pool?' he asks.

'Put it in an insulated greenhouse.

The basic philosophy is that we are trying to trap heat.'

Creating 3.5m thick walls and a 7m deep roof clad inside and out by triple layer ETFE cushions would provide a thick thermal buffer between internal and external environments. This serves the double purpose of trapping solar heat - 30% of the building's requirement, equivalent to covering the roof in photovoltaic cells, Carfrae calculates - and of protecting structural steel from the aggressive swimming pool atmosphere.

The next big question was what to use to fill the wall and roof space.

Carfrae says his attention was captured by a hugely magnified photograph of cells in what he assumed was a natural, organic material, but later deduced was polyurethane foam. 'That led to some intensive exploration of what geometry we could use.' Carfrae was looking for a pattern that would lend itself to the standardisation essential for easy construction, provide maximum structural efficiency, yet give the impression of organic irregularity.

He was after a close approximation of soap bubbles.

A century ago Lord Kelvin produced what he believed to be a geometry of unbeatable space filling efficiency, consisting of octohedrons with their corners cut off. 'It worked well but didn't look organic.' However, in the early 1990s Irish professors Weaire and Phelan bettered Kelvin by 2% with a design of pentagons interspersed with occasional hexagons. It looked more natural, and Carfrae set about disguising repetition in the structure by rotating adjoining blocks of cells by arbitrary amounts.

'I met Denis Weaire and he told me that if you take a regular pattern in three dimensions, rotate it by a random amount and take a two dimensional slice you'll get a random pattern, ' Carfrae says. 'To me that was the magic moment.'

What Carfrae got was a form that provided both structure and insulation, and gave the illusion of infinite variability: The edges of his bubbles would be described in tubular steel and the junctions between bubbles by fabricated spherical connecting nodes. Yet the whole structure could be created using just four different length elements.

Structurally Carfrae's froth of bubbles will behave as a giant moment frame, 177m long and wide by 31m high. It is more than capable of absorbing 1:100 year event seismic energy, snow and wind loading, he says. Yet it will be extremely light, at 100kg/m 2.Bubbles will average 7m across the face, with the largest measuring 9m, meaning there is a relatively low structure to space ratio.

And each of the 22,000 tubular steel elements has been optimised by running the structure through stress analysis software. They range in diameter from 168mm to 610mm, with wall thicknesses of 4mm to 40mm, depending on how highly loaded they will be.

Fabrication of the £60M structure will be carried out using conventional CAD-CNC machine technology. Though a contractor has yet to be appointed, Carfrae expects tubes will be welded around nodes into spiky jack-like units. These will then be bolted into the structure on site, with connections disguised by facing plates.

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