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Engineer-turned-architect Hole and architect Gordon Wilson have designed a tropical greenhouse, the Brazilian Rainforest Pavilion, that eschews the self-conscious trickery of most Expo architecture. The 拢10m structure is a simple delight. From outside, it is a translucent bulging oval that peers out of the ground like a giant bug's eye. But inside, it plunges 10 m below the surface to house a miniature Brazilian rainforest. Although not part of the Expo 鈥� it is located in the city, rather than on the main Expo site 鈥� it has become known locally as the "Brazilian pavilion". Volkswagen, the building's main sponsor, is encouraging Expo visitors to stop off at the rainforest on their way to or from the exhibition. Hole says the car maker, whose head office is in nearby Wolfsburg, expects the building to be filled with visitors during the Expo.

While the exterior provides a strikingly alien image, it is only inside that the ingenuity of the design becomes apparent. Beyond the rainforest tree canopy there is almost nothing to get in the way of the daylight that floods in. Hole says this was made possible by cladding the roof in ETFE 鈥� a tough, see-through plastic whose most useful quality is its weight, or lack of it

The 1000 m² surface of the roof is clad in strips of double-thickness ETFE foil. During installation, these are run up secondary steel supports, like sails. Once in place, air is pumped between the two layers, inflating them and forcing the edges of the foil to lock into seams on the steelwork.

Unlike conventional roofs, where the problem is how to support the weight, the main challenge with the ETFE roof is how to stop it flying away, says Hole 鈥� who now works for sports and leisure architect KSS but originally worked on the project for Tarmac Professional Services. Because the roof is so light, it can be supported by slight primary structural supports, adds Albert Taylor of structural engineer Adams Kara Taylor, which worked with Hole on the project. In fact, the roof is suspended from only three arches made from 457 mm diameter hollow steel tubes. The tubes are grounded at the corners of an equilateral triangle and arc sharply to almost meet at its centre, which is also the centre of the roof. The resulting structure looks sturdy from the outside but is barely noticeable from the inside. And, according to Taylor, ETFE structures weigh only 4% of those made from glass, which allow it to be used in huge 20 m spans.

ETFE looks like it might puncture easily, but Hole says the roof is tough enough to walk on and that the material, which is similar to Teflon, is resilient. ETFE also appears to be less transparent than glass, but Hole 鈥� an expert on conservatory design, having worked on the Princess of Wales building at Kew 鈥� refutes this. "More than 20% of light is lost in glass," he says. "With ETFE, the figure is 5%.

"We had the same question from the client when we proposed ETFE, so we carried out light tests. The light sensors used couldn't tell the difference between light intensity before and after it had passed through the ETFE." And anyone who has recently walked under the glass canopies of Canary Wharf Station on the Jubilee Line Extension in London will know that glass picks up grime quickly. This is not a problem with ETFE, says Hole. He explains that, unlike ETFE, glass has a microscopically rough surface that allows grime to grip to it. By contrast, particles cannot get a grip on ETFE. "They settle on it, but can be washed off easily," he says.