Covered in glory

The citizens of Toyota City in central Japan can thank their city's failure to host a World Cup match for the unique air-inflated retractable roof gracing their stadium. Originally, 15 were to be built in Japan to host the tournament, but the decision to co-host it with Korea meant only 10 were needed – and Toyota's was one of those that were dumped.

Japan's superstar architect Kisho Kurokawa had already designed the new stadium before the news came through that it was not going to be used. His plans were saved, however, when the people of Toyota decided to press ahead with the stadium as part of their celebrations to mark the city's 50th anniversary as a municipality.

Furthermore, in a last-minute change to the plans, the city decided that such a stadium would be more versatile if it incorporated a retractable roof – which would not have been considered had its primary purpose been as a World Cup venue. Kurokawa and Arup Japan, the project engineer, had the task of coming up with a retractable roof that would not destroy the beauty of the stadium's curved roofline or mean it had to be entirely redesigned.

The team was incredibly lucky: the shape of the planned stadium meant a retractable roof could be incorporated relatively easily. The stadium is circular in plan and has two separate semicircular permanent roofs covering the east and west stands. Each roof section is suspended from cable stays attached to two giant masts. The roofs curve upwards in the middle so that the maximum number of spectators can be packed into the high east and west stands. The lower north and south ends allow the wind and sun to keep the grass healthy when the roof is open.

Giant trusses run along the full length of the edge of the two permanent roof sections the full length of the stadium. They are bow-shaped to accommodate the height increase towards the centre of the roof. "We were lucky that the two roof lines were parallel," says Mitsu Kanada, structural engineer at Arup Japan. "If the original design had had to be radically changed, the architect wouldn't have agreed to it."

The retractable part of the roof fills the rectangular hole between the two trusses. It works like the sunroof on an old car; a fabric roof with metal supporting ribs slides back from the south end towards the north side where it sits folded up. This maximises the amount of sun reaching the grass from the south.

Fourteen sliding steel ribs bridge the gap between the two bow-shaped trusses. Between each rib there is a giant PVCu-coated polyester pillow that is inflated as the roof unfolds, keeping the rain out.

The original solution for the retractable roof employed a single-ply fabric membrane to cover a linked folding mechanism. The main problem with this was that the whole roof was one homogeneous mechanical unit, yet each of the 14 ribs had an independently moving motor. Keeping all these motors synchronised was found to be too difficult.

The solution came in the form of inflated plastic bags. This idea dispensed with the complicated folding mechanism and, most importantly, allowed a degree of movement between each of the 14 ribs as they opened and closed the roof. "It was cheaper and easier to control," says Kanada. "The architect also really liked the shape of the inflatable pillows."

The roof opens and closes by moving the 14 ribs along a toothed rack running along the main bow-shaped trusses. Motors at the ends of each rib drive pinion wheels to move them along the rack. A giant spring is incorporated in the drive mechanism to absorb any movement between the permanent roof and the folding part. "We were worried about the moving ribs connecting the two permanent halves of the roof," says Kanada. "We had to make it flexible in case of earthquakes and high winds."

The roof takes 50 minutes to close. The first rib moves 70% of its full extent, and one by one the other 13 follow until the whole roof is 70% closed. At this point the ribs all move in unison until the roof is fully shut. The pillows are inflated by two fans in each truss and pressure progressively builds up as the roof moves. Even when the roof is fully open, the folded pillows are partially inflated to resist wind forces.

The stadium was finished last July, but Kanada admits the whole project was a huge challenge. As for the people of Toyota City, they may have failed to get the World Cup but, as Kanada says: "They think it's the best stadium in the world."