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Beijing Dream

Just six months to go until the Beijing 2008 Olympics and it looks as if China's capital has come up trumps.
Ruby Kitching spoke to the engineers who helped construct the showpiece structures.

The run up to the 2008 Olympics is a very different affair to Athens in 2004. When NCE reported from Greece four months before the opening ceremony, the main stadium roof was still to be jacked into position and just a week before the start of the Games, test events were being cancelled because mechanical and electrical equipment was not ready.

Now, with six months to go until Beijing takes centre stage the Olympic stadium is structurally complete and fitting out has begun. And at the aquatics centre, a programme of test events just started.

These two structures are the most spectacular of the 2008 Games; the national stadium is dubbed the "Bird's Nest" because of its intricately arranged steel roof structure and the national swimming centre's plastic bubble cladding has earned itself the nickname of the "Water Cube".

"It's more than just about the business, it's about national pride," says National stadium project director Michael Kwok speculating on what has driven the Chinese nation to excel in completing its 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, together with China Architecture Design and Research Group (CADR Beijing) and Swiss architect Herzog & DeMeuron, makes up the stadium design team.

"People fought really hard to work on Olympic projects. Because most contractors are Chinese, they are doing it for their country and put more effort into it. They work day and night because they want to build things on time," he adds.

Kwok says that when he first set eyes on the Birds 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. "This produced a very compact design. It was very important from a structural point of view that spans were as short as possible – this impacts on cost, so it was a clear objective to minimise the spread of the stadium".

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 which cross the elliptical opening. They effectively form 12 portal frames which span the stadium.

The opening itself is framed by a 10m deep steel truss ring. This structure resists wind and seismic loads 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 secondary steelwork supports reduce the distance the cladding must span. "It also gives extra stiffness to the structure to dissipate energy during earthquakes: these elements are designed to reach their yield point, protecting the main members in the primary structure."

The steelwork was designed using the 3D software package, CATIA to define to millimetre accuracy the spatial arrangement of each steel element. These coordinates were then passed to the fabricator, the contractor and eventually the steel erectors to ensure the accuracy with which the structure was designed could be transferred onto site.

"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.

The steel elements which make up the roof are all box sections fabricated from flat plate and, on the outermost layer of steelwork, all measure 1.2m by 1.2m. "Where stresses are low, the plate thickness of the box sections is just 25mm. But this increases to 100mm where the stresses are highest. From the outside they all look the same," Kwok adds.

Like many spectacular stadiums this project has had its ups and downs. In Beijing, it was the scrapping of the retractable roof after the whole structure had been designed. During a design review with the client in 2004 the issue of cost savings came up due to the increase in the price of steel.

Says Kwok: "In terms of savings, removing the retractable roof reduced the weight of steel by 20%, but it affected the whole concept of the Bird's Nest," He explains that removing the burden of the retractable roof's weight, steel member sizes could be reduced and the central opening above the athletic field enlarged.

"We knew we had to make these cost savings, but we were nervous about how long it would take for the new design to be approved. We had to work very hard not to compromise the Bird's Nest image and still satisfy the client." The main changes were in redesigning the connections, rather than altering the overall shape. However, the process ate up nearly four months of the tight programme.

The stadium's success, he says was achieved chiefly because design changes were approved and agreed quickly.

"There was a really focused effort to move forward and create the least disruption while still improving the design. Much of this was driven by the government and the Olympic Authority who were prompt in giving their consents. They understood the pressure we were under and the last thing they wanted was to hold things up."

Deputy chief engineer for the stadium with CADR Zhong Fan adds that the success of the project is largely due to the "scientific attitude of the design team". He told NCE, "We did a lot of testing and scientific research, and a huge amount of calculations, reading a tremendous amount of manufacturer's literature and meeting with many specialists to design and build this complex building."

With such a unique structure, the onus was on the design team to make the structure as simple, safe and economic to construct. "What the National Stadium has done for China and the world, is to introduce a new type of structure," says Fan. "The London Olympics can learn from this and produce an even better Games, as each city does every four years."

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 being televised live.

The live broadcast was symbolic of the increased confidence the Chinese government and Olympic Committee has developed over the course of the construction period, says Kwok.

"To start with there was very little media coverage of the stadium and for a long time, our client didn't want to talk to journalists for fear of getting an adverse reaction. But the atmosphere changed when we finished the stadium and people started saying how much they liked it."

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 going 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 overdesigned with nearly 42,000t of steel structure to enclose 91,000 seats. But Fan says that steel was the only material which could achieve the high strength required in each component, with a relatively low mass. "For us, it was the most economical choice because there was no other choice."

Kwok is more pragmatic about these criticisms, "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.

National Swimming Centre

The National Swimming Centre, or Water Cube, as it is known thanks to its form and translucent appearance, opened for its first test event last month, on time, and reportedly meeting its £50.3M budget. Consultant Arup came up with the design with CSCEC, the national swimming centre design consortium.

The project was awarded to Arup in July 2003 and the following months up to December were spent in a design frenzy because the Mayor of Beijing had promised publicly that construction would start in 2003.

"That period was very hectic, but the winning design was based around a fundamental idea, and all we had to do was implement it," says Arup project leader Tristram Carfrae.

This idea was to design an "insulated greenhouse", maximising natural light, with the option to allow artificial lighting during televised events.

"We didn't want to use glass because with [swimming pool] tiles, they create a nasty acoustic environment. We opted for ETFE because it is acoustically transparent - sound passes through rather than being reflected," he says.

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. We had to do a lot of convincing [about using soap bubble theory], but once people agree, its 'off we go' with a lot of will."

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 is made up of 22,000 steel tubes, connected by nodes to create a polyhedron space frame. This supports the 4,000 ETFE cushions, which come in seven different sizes on the roof and 15 in the walls. The maximum span of the cushions is 9m. Computer and physical modelling was carried out to check the design.

"We built a structural model where we took an important part of the wall and applied gravity and horizontal loads. We saw that the structure worked very well compared to other space frames because it was better at resisting bending moments," says National Swimming Centre Design Consortium (Beijing) designer Xueyi Fu. Fu and the 100 strong design team researched and optimised the design. This took two years, Fu personally working 50 to 60 hours a week.

Of concern was how the ETFE cushions would cope with low external temperatures in the winter (causing the pressure in them to drop) and the extra weight from snow. "We found that the structure could withstand 17 times the normal snow load, so it's safe. But the ETFE is sensitive to temperature so in low temperatures, more pressure must be applied to inflate the cushions Đ the building managers must understand this."

The structure was built completely on site, in order to maximise the use of low cost labour.

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