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Going for Gold

Just months to go until the Beijing Olympics and China's capital city looks as if it has come up trumps – NCE Insite spoke to the engineers in China who helped make it happen.
The most spectacular of Beijing's Olympic venues include the national stadium, dubbed the "Bird's Nest" because of its intricately arranged steel roof structure, and the national swimming centre which has earned the nickname the "Water Cube" because it is covered in plastic bubble cladding.
"It's more than just about the business, it's about national pride," says National stadium project director Michael Kwok describing what has driven the Chinese to excel in completing their Olympic sporting venues in good time for the start of the Games in August.

Kwok is director of consultant Arup's Hong Kong and China office. Arup and China Architecture Design & Research Group (CADR Beijing) and Swiss architect Herzog & DeMeuron, make up the stadium design team.

Kwok says that when he first set eyes on the Bird's Nest scheme, he knew it would be a winner because it was so different, but recalls that his initial reaction of, "Wow" was swiftly followed by, "well how do we design this?"

He explains that the stadium was designed "inside out" and that his first priority was to make sure that spectators were as close as possible to the action, with clear sight lines. The other challenge was to establish whether the twisted and skewed steel components could be fabricated and erected in China to the accuracy required. "It's such a random, chaotic-looking structure, but for it to work, we needed to find some logic to it."

Kwok explains that the roof consists of a primary steelwork structure of 24 trussed columns spaced regularly around the stadium perimeter and connected at the top by horizontal trusses. The opening is framed by a 10m deep steel truss ring. This resists wind and earthquake shudders and is "quite regular " he assures, while the secondary structure which sits on top of it, is what gives the stadium its "chaotic" feel.

The steelwork was designed using 3D computer modelling

"I think the stadium is simpler than it looks. People think that there's no order to it, that it's a random collection of elements – but when you look at each layer, there is a clear geometry. Each quarter of the roof is actually identical, "says Kwok.

Erecting the roof involved building it first on temporary towers until the whole structure was complete. It was then ceremoniously de-propped over three days, with the final day's activity televised live.
Kwok hopes that the lasting legacy of the stadium will be more than just as a sporting venue. "It shows the role an engineer can play in defining what is possible in architecture. It also shows that China, and Beijing, want to innovate and have no fear in challenging existing systems and gong beyond them. For China, it shows its ability to construct some of the most difficult buildings in the world, and shows off our determination to succeed."

One of the main criticisms of the stadium is that it is vastly over-designed with nearly 42,000t of steel structure to enclose just 91,000 seats.

"If the stadium was just for sport, then it wouldn't be cost effective. But the Bird's Nest is more than just that. It is a temple for the people, a place to visit and photograph, a reason to stay in Beijing for an extra day, " says Kwok.

Next door to the main stadium is the National Swimming Centre, or Water Cube, as it is fondly known thanks to its form and translucent appearance. Consultant Arup came up with the design with the CSCEC, national swimming centre design consortium (Beijing).

The idea was to design an "insulated green house", maximising the use of natural light. We opted for ETFE because it is acoustically transparent – sound passes through rather than being reflected," says Arup project leader Tristram Carfrae.

After seeing the curvaceous design of the neighbouring national stadium, a complementary cubic form was chosen for the aquatic centre and so the plastic bubble box was born. But it was still up to Carfrae to work out what the best structural frame would be.

"The theoretical geometry of an infinite number of soap bubbles," was the solution, having the advantage of being both "fantastic" visually and structurally sound.
"Fortunately, China is a country in love with technology and wants to move forward."

The Water Cube structure is a steel framed box measuring 177m by 177m by 31m high and clad on the outside and inside with inflated ETFE cushions.

The structure was built completely on site, to maximise the use of low cost labour – 2,000 people were involved in the construction – rather than producing elements off site and craning them into position. The construction sequence was computer modelled to check that stresses would stay within design limits and also to help explain to the client and subcontractors how it would be built.

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