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AW: How does that interweaving of metal one sees in the Bird's Nest translate into structure?

SB: The brief was it had to have 91,000 seats and couldn't be a certain height above the ground. In a nutshell that was it.

There are three types of roofs for stadiums. There is the cable-supported structure, which requires mass and height. That wouldn't have been appropriate. The second type is a cantilever. You'd wind up with a huge bowl. That's a very inefficient form of structure. And there is a spanning beam.

We went for something that a first-year engineering student could do. We said we'd have a beam that would span the roof opening. Instead of going across the roof, the shortest distance was the inner curve.

Then we've got 24 columns that split into a V-shape on the outside. When the column gets to the fillets where the wall meets the roof, one beam goes off left and one right. Forty-eight beams crisscross the roof from 24 columns. If you stare at the building for long enough, you can see it all.

In most buildings the columns and mullions are differently sized. It's obvious to your eye that the big, heavy ones carry the load and the little ones hold the windows on. On the stadium they're all the same size, so your eye can't distinguish between the columns that hold the roof up and the ones that frame the windows. Make them all the same size, but only as big as they need to be. It looks far more complicated than it is.

Staircases run through the building to make the secondary structure. So you impose another fairly regular pattern. They go over the roof and are attached to the same opening, going around the building. You put a pattern on top of a pattern, and it looks irregular. It looks discordant but it's not, kind of like two crisscrossing triangles.

AW: Architects often lament value-engineering budget cuts to their design. Does the stadium suffer for having the retractable roof that was originally planned for the Bird's Nest taken away due to budget cuts?

SB: But cutting the roof back reduced the amount of construction cost quite significantly.

The original brief for the competition had a roof for good reason: to deal with the high rainfall they get this time of year for the Olympics. The opening and closing ceremonies couldn't be interrupted by the weather or windblown dust. And we designed to that. We strictly followed the competition rules.

However, I wouldn't say the stadium is any the worse for the value engineering that took place. It wasn't dictated to us. It was acceptable to everybody. There were rumors in the press that there was a safety reason, but it was for cost reasons.

AW: In wearing its structure on the outside like this, the Bird's Nest reminds me of Richard Rogers and Renzo Piano's Pompidou Center in Paris, or Rogers's Lloyds of London headquarters. Do you think the Bird's Nest structural makeup has any strong precedents?

SB: I've thought about precedents but I can't think of when this has been done visually. I think Daniel Libeskind is the closest. On the Imperial War Museum, we talked about the stair going on forever in perspective. I really didn't think that visual trick would work, but it does. The Jewish Museum has a lot of visual tricks, too. The Bird's Nest is a visual trick, after all.

AW: What did you learn from on the Bird's Nest that you can take on to other projects?

SB: Quite a lot, actually. One is fairly early on in the process we realized we were dealing with a building we couldn't really draw. We had conversations with our Chinese partners to say, "How are we going to present it?" They said, "We'll get an army of CAD drawers." But you can't change one piece of steel on this project without affecting 5,000 others. How will we know what's affected?

We realized the only way was a model, and so we modeled it in 3D really early. Arup did the 3D for the competition. We set the geometry of the building in space, and that continued. So we learned a lot about 3D modeling and writing script and geometrical software and analysis-design software.

But equally, I think the Chinese construction industry also trained a lot of people and as a result now has a huge legacy opportunity to do projects of this scale and type. I'm currently working on a twisty, curvy tower in the Middle East. And presto: our Chinese contractor for the Bird's Nest has appeared on this project with all the knowledge and skill they got from this.

AW: Herzog & de Meuron have said that the Chinese government could sometimes be difficult to work with, but that the Chinese authorities also had more power than governments in the West to get things done quickly. How would you rate the set of conditions in Beijing that gave way to the Bird's Nest?

SB: In China there's always discussion on fees. In the West they're usually higher, and so that's always going to be an issue for something that requires an international team. But because of Arup's long history in China we knew the fee levels were appropriate and palatable for a public sector organization. So we knew we had to work within that by using the lower costs of our Chinese design partners when we could. That in itself is not always easy.

With the stadium, the good thing from a design point of view is there's probably nobody who's done that scale of stadium there in a generation. The client knew they needed international expertise. That's a good start. The other special thing about the stadium was it had to be done. If we were doing an opera house or a commercial tower or a library or whatever, they don't have to be ready for the games.

From the top of Chinese political leadership down they were going to do this. They could have found us not compliant with Chinese code and made us go through the infinite loop of proving the unprovable. But they always found a way to keep it moving forward. There was always an attitude of, "There must be a way." It's the Olympic spirit: achieving what appears to be impossible. That was present right through the process. When you're doing something unusual, somebody obstinate and difficult can make it a chore. On the stadium there was a can-do attitude.

AW: Even though all Olympic stadiums receive attention when the games are being played, it seems like the Bird's Nest has people more excited than usual, and the chance to endure as an important work of architecture and engineering.

SB: I think it's pretty incredible. When I was flying over [to Beijing] recently, there was a documentary on the plane about the National Stadium. This one was from the perspective of the welders on site and other people working on it. What struck me was the incredible pride. The welders were so proud they'd worked on the National Stadium.

When we were designing it, people in our office were so excited to work on it. I've been in this industry 30 years now, and I've never been on such a project where people were so enthusiastic from the get-go.

AW: What has really changed for you about structural engineering practice over three decades?   >>>

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SUBSCRIPTION SAMPLE Close up, it is hard to perceive any regularity in the structure of the Bird's Nest. Photo: Ben McMillan Extra Large Image During construction, the stadium's 24 perimeter columns were easier to identify. Photo: Courtesy Arup Extra Large Image Each steel column location bears an equal share of the superstructure's weight, which totals 45,000 metric tons (49,600 tons). Photo: Courtesy Arup Extra Large Image The primary steel structure of Beijing National Stadium includes 24 curved double columns spaced regularly around the building perimeter. Image: Arup Extra Large Image Secondary and tertiary curved structural elements overlay the primary structural grid. Image: Arup Extra Large Image Because of the National Stadium's unusual structure, Arup opted to model the project in 3D from a very early stage. Image: Arup Extra Large Image Each column was fabricated on the ground and set in place, with nodes ready to receive other members of the superstructure. Photo: Courtesy Arup Extra Large Image The concrete interior structure of Beijing National Stadium was in place before work began on the steel exterior. Photo: Courtesy Arup Extra Large Image   Click on thumbnail images to view full-size pictures.