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projects under way in Europe and the USA, it鈥檚 the price the architect鈥檚 Tokyo-based practice pays for being on the other side of the world. One of these projects is a design for the World Trade Centre site in New York, which is 14 hours behind Japan. As one of the principal architects in the THINK consortium 鈥� which last week was chosen as one of the two finalists in the competition 鈥� Ban has contributed to plans for what, at 507 m, would be the world鈥檚 tallest building. Back in Japan, he has recently completed a project that also breaks new ground, but on the more modest scale of one storey.

The Atsushi Imai Memorial Gymnasium was designed for a hospital in the city of Odate in the mountainous Akita region

of northern Japan. Named after the hospital鈥檚 founder, it was commissioned to give patients a place to exercise in winter, when the region receives an average of 2-3 m of snow. Ban decided to use an oval dome as the gymnasium鈥檚 roof, but this gave rise to the question of designing it to support the weight of all that snow. Faced with this challenge, most architects and structural engineers would no doubt have looked to a sturdy combination of steel and concrete. Ban chose plywood.

The choice will come as no surprise to those who know Ban鈥檚 work. His reputation is largely based on his use of unconventional materials; or, to put it another way, he is the man who has built everything from houses to churches out of paper tubes. At the Expo 2000 in Hanover, he built the world鈥檚 largest paper structure as the Japanese pavilion 鈥� a 72 m long undulating tunnel. But Ban is no purveyor of gimmicks; he is a pragmatist of the highest order. These tubes are a cheap, standardised by-product of the paper industry, and they are surprisingly strong. Ban used them to create elegant and sustainable architecture.

Plywood 鈥� or laminated strand lumber, which is the type Ban used 鈥� is certainly more conventional as a building material than paper tubes, but its use as the only structural material in a dome 28 m long and 20 m wide is no less startling. Ban chose it because it was lighter and more flexible than timber and could be used to make longer beams. But it was also much cheaper: 鈥淚 got a good deal from the supplier,鈥� he says.

His practice had already built a daycare centre for the hospital, but this was pulled down to make way for the gymnasium and rebuilt elsewhere on the site. The daycare centre had also used bent plywood, but on a much smaller scale and using a simpler design. Ban wanted to develop the idea for the gymnasium and, in the same empirical way he experimented with paper tubes in the mid-1990s, set about testing different structural systems.

As Ban has learned in the past, introducing a new material means overcoming the inveterate distrust of planning authorities. Ban still smarts at the memory of German building control adding unnecessary timber supports to his pavilion in Hanover. This time his design went uncompromised, but it meant taking it to a special hearing and submitting a huge amount of data, along with one full-scale arch beam for testing. 鈥淚n Japan,鈥� he says, 鈥渆ven if there are no previous examples, as long as you can prove that a structure works, they will allow it.鈥�

What gives the dome its strength is the system of arched

trusses, which run the length of the structure and are

sandwiched in between pairs of shorter, lateral arches. The trusses are joined to each other from edge to edge at near right angles to create a concertina-effect dome that uses diagonal forces to spread the weight. Ban says it was simpler to build than he expected: 鈥淒esigning it was the most difficult part 鈥� that, and getting approval.鈥�

It looks more complex than most of the architect鈥檚 other buildings, but in many ways it is a typical Ban solution: a lightweight structure that is sophisticated but not high-tech, and which maximises the potential of prefabricated materials. As with the paper tubes, the plywood is not coated or disguised 鈥� instead, it becomes the predominant feature. For Ban, the aesthetic and the structural methods are one and the same.

On the outside, however, the dome wears a striped skin of polycarbonate and steel. This double-glazed polycarbonate lets natural light into the gymnasium through evenly spaced gaps in the trusswork. The simple abstraction of the dome鈥檚 exterior belies its zig-zag underside. In fact, from outside it scarcely looks like a building at all, more like a giant cocoon or a modernist woodlouse. With the main body of the gym sunk underground, only the dome and the two canopied entrances poke through the surface. The effect is striking, but Ban was just being sensitive to the surroundings. 鈥淭he hospital is only two storeys and the houses are low, so I didn鈥檛 want to make a huge volume in the area,鈥� he says.

The result is an exterior that, Tardis-like, gives no hint of the cavernous space within. This is something of a departure for Ban, who has always been influenced by Mies van der Rohe鈥檚 ideas of fluid space. In the mid-1990s, he built two houses playing on Mies鈥� curtain-walled Farnsworth House: the Wall-less House and the Curtain Wall House (which literally used curtains for walls). But the gymnasium presents more of a paradox. The high, luminous ceiling and the perforated screen around the sports court make for an expansive, osmotic sense of space, but one that is nevertheless contained. In the pool area, only thin slivers of daylight give a sense of the outside world.

Is Ban moving beyond paper architecture? It is a question that

he would probably find fatuous. 鈥淚n all my projects,鈥� he says, 鈥淚 develop an idea and then move on to the next step, so that the gymnasium was a development of the daycare centre. These are just steps in a continuous process.鈥�

Ban says he has tried not to be one of those architects who just designs prestige projects and extravagant homes for the rich. His international reputation is built largely on small-scale projects such as this one. Ironically, his designs for houses and refugee shelters have been so distinctive that he is now one of the potential designers of the world鈥檚 most prestigious 鈥� and tallest 鈥� building.