National

National Stadium canopy has design flaw, architect says

Canopy material is flammable, not suitable for roof: Moriyama

by Masaaki Kameda

Staff Writer

The planned replacement for Tokyo’s National Stadium — already under fire for its massive size and cost — is now facing further criticism: safety.

The current design has a major flaw in its retractable canopy, says Tokyo architect Takashi Moriyama, a consultant on public buildings, who alleges that the designers twisted its description to get around building regulations.

“I think this is deceptive,” he said. He believes the canopy could be a threat to spectators or to people sheltering in the building in the event of a major disaster.

The stadium is currently designed to seat 80,000 spectators and is slated to be the central venue for the 2020 Olympic Games. The design, by British architect Zaha Hadid, was approved in late May by a panel of experts set up by the Japan Sport Council, or JSC, which will oversee the facility under the auspices of the Ministry of Education, Culture, Sports, Science and Technology.

Moriyama’s concern centers on the material intended for the retractable canopy. The roof is to be curved, so the only option is to use a flexible membranous material that can gather into folds, he said.

Moriyama said there are four kinds of possible membrane for roofs, known in the industry as types A, B and C. The fourth is permitted only for tents and barns.

Types A and B are rigid, nonflammable materials made of glass fiber. Type C is flexible but flammable and must be replaced about every 10 years because it is made of polyvinyl chloride, which decays, Moriyama said.

“The membrane roof of the Tokyo Dome stadium is Type A. Major buildings should use nonflammable Type A or B roofs,” he said.

“The Building Standards Act mandates that principal structural parts of buildings, such as roofs, floors, walls, posts and beams, be nonflammable,” he said, noting that in addition to hosting sports tournaments the new stadium is expected to become a designated public shelter in the event of a national emergency.

“These parts (of a building) could affect the lives of people when disaster strikes,” he said.

The design of the new stadium necessitates using a Type C membrane. Moriyama accuses the designers of twisting their description of it to get around regulations.

“It effectively functions as a roof, but since the design cannot use nonflammable material, it is designated instead as a ‘sound insulation device’ in the approved basic design,” he said.

Moriyama said Hadid was until recently known among peers as an “unbuilt architect” — whose designs and ideas were innovative but often posed problems in realization.

Moriyama noted that the situation has improved over the past decade due to advances in computer technology, which makes it possible to put designs through a detailed structural analysis, but nevertheless he says the JSC panel should have weighed her proposal carefully.

“When you try to put your ideas into action, you have to make some compromise in design; it’s inevitable,” he said. “Zaha would never compromise. . . . So the panel members should have examined if her design was really feasible, but they never did.”

Moreover, Moriyama said the “sound insulation device” will not meet the description, given that it will merely comprise a thin membrane.

“The basic design stated the device would be able to cut the noise by up to 15 to 20 decibels,” Moriyama said. “But with that level of noise cancellation, it can’t be called a ‘sound insulation device.’ ”

Moriyama said a typical rock concert produces noise at around 100 decibels, and thus the “insulation device” at the new stadium would barely muffle it, reducing it to only 85 decibels or so.

“That level of noise is equivalent to that on a street with heavy traffic,” he said. “Typically insulation must cut it by 40 to 50 decibels if it is to be called ‘sound insulation.’ ”

The JSC says the structure is a sound insulation device because it will not always cover the roof area.

“What we call a ‘sound insulation device’ is basically folded and stored, which makes the roof area always open,” Yoshitaka Takasaki, a division manager of the new National Stadium planning and construction bureau at the JSC. “It’s not what you call a roof, which covers the stadium all the time.”

Takasaki noted the new stadium will be open-roofed when it hosts sports events, including soccer and rugby, while the membrane will close to deaden the sound for concerts.

As for its likely poor performance, Takasaki said he understands there are no clear standards for the designation “sound insulation device.” He said the average sound level from concerts at the current National Stadium is around 80 decibels, and the cover could reduce that to around 60 decibels.

Takasaki added that the JSC has long been requiring concert organizers to keep the sound down at the current stadium and will ask them to do so at the new one.

There are no plans to review the stadium’s design.