The people's palace

It looks like the nest of a mythical bird, made from randomly interwoven twigs. Its huge bizarre form can also be perceived as a giant doughnut, a oversized tureen or an aberrant geological formation such as Ayers Rock in Australia. In fact, it is the design produced by Herzog & de Meuron and Arup for the 110,000-seater main stadium for the 2008 Olympic Games in Beijing. And it's plain to see that it has revolutionised the way these structures are conceived.

There's no undue modesty when Jacques Herzog describes the building-to-be. "It is the most visible icon of architecture," he says, referring not only to the building's eye-popping physical presence in Beijing but to the prospect of its image being broadcast to billions of television viewers across the globe. "So we didn't want to do a stadium like anywhere else." More particularly, he wanted to break away from "the insistent sameness of technocratic architecture dominated by large spans and digital screens".

"We wanted it to be in the tradition of great Chinese architecture," he continues. "And we also wanted something that would be structure, space and ornament at the same time. We were inspired by antique Chinese ceramic vases with cracks. And that's why it has a bird's nest structure, too." To Herzog, Olympic stadiums in general are not only ugly but usually left half-abandoned after the games. "Nobody looks at them, and they're closed off from the city. Ours will be the venue for the Olympic Games, but with a permanence, too." To prevent it from shutting up shop after the 2008, Herzog & de Meuron has conceived the whole building as a civic space freely accessible to the public. "Unlike the Japanese, the Chinese are extrovert and like using public spaces," Herzog says. "We wanted the stadium to be a place where people would go just to hang out. Something like the very large but totally unexpected public space created below the Eiffel Tower." So rather than a hard shell encasing the building and fencing it off from the surrounding city, the perimeter envelope will be made as permeable as possible. Two layers of translucent membrane such as ETFE will form external and internal linings to the roof and a wide perimeter zone, and will fill the spaces between the structural members, or "twigs", that dominate the stadium's appearance.

At entrance level, even this membrane will be largely removed for most of the time, creating in effect an open arcade that will allow visitors and fresh air to pass straight through. Nor will these visitors be confronted by the undersides of seating tiers or arrays of shops and restaurants, as the entrance concourse will be level with the top of the lowest tier of seating, whereas the shops and restaurants will mostly be self-contained pods relegated to upper levels.

We always felt the Chinese would take pride that it was made in China, and the Chinese people we work with have taken the design to their hearts

Stephen Burrows, director, Arup Sports

This arrangement has been achieved by setting the stadium on E E a wide plinth and partly sinking the seating bowl into it.

Six extensive storeys of ancillary accommodation, including changing rooms, broadcast equipment, shops, storage and car parking, have been packed into the plinth, yet all of this has been skilfully concealed by forming the plinth as a wide, gently rising mound rather than an abrupt megastructure. And into its top surface, several small, intimate gardens have been sunk, and stone squares have been created, to provide relief from the overwhelming scale of stadium and plinth.

As for Arup's role, this has been to make Herzog & de Meuron's vision stand up and perform properly. More crucially, Arup's sports consultancy has played the lead role in configuring the seating bowl for the benefit of spectators. As Arup Sports director J Parrish puts it: "An architect designs a stadium from the outside; an engineer designs it from the inside." "The seating bowl at Beijing is driven by athletics, not football," says Parrish. "Since it is designed around an athletics pitch, it is closer to an elliptical bowl." Even so, football was the critical factor when configuring the upper seating tiers. "Viewing distances relate more to football than to athletics, as a ball is smaller and moves faster than the human body," says Parrish. "No seat should be further than 190 m from the action, and that means that the top tier is near circular in plan and 280 m in diameter." The seating bowl is configured by a computer modelling program that optimises sight lines for every seat. Parrish says: "The key is not to make the rows too tight, because when tiering is steep you find it precarious to move past other people to get to your seat." Access to and from the upper rows is by the straightforward means of staircases housed in the perimeter zone, with exits clearly visible to spectators in case of an emergency. Once the games are over, the capacity of the stadium will be reduced to 80,000.

We wanted the stadium to be a place where people would go just to hang out. Something like the very large but totally unexpected public space created below the Eiffel Tower

Jacques Herzog, architect

High-tech solutions for reducing capacity were discarded in favour of simply removing the upper tiers and increasing spacing between rows of seats to give more leg-room.

The translucent roof overhead will include two movable sections not dissimilar to those at Cardiff Millennium Stadium. The movable sections will be housed at either end, and when it rains or during cold winter spells, they can be rolled across on tracks like concertinas to cover the entire central open area.

As for making the crazy bird's nest roof structure work, this was largely up to Arup Sports' other director, Stephen Burrows. Surprisingly, Burrows regards this unprecedented structural solution not as revolutionary but as evolutionary. In fact Arup has reduced the bafflingly complex-looking structure to a few commonplace elements. "It simply consists of a series of trussed steel portal frames, all of 12 m depth, the longest with a clear span of 330 m," Burrows says. "There are basically three types of stadium roof design – the cantilever structure, the roof supported by masts and cables, and the 3D space frame, and this one fits into the third category. The unusual thing here is the random pattern in which the portal frames are arranged and cross over each other." No less surprisingly, Burrows claims the random bird's nest arrangement of interwoven twigs, which seems so much like an architectural whim, makes sense structurally too. "The biggest problem lies in resisting an earthquake. This could make a large mass some distance above ground, like this, accelerate sideways. In such conditions, this roof will behave structurally just like a giant bird's nest if you turned it upside down and shook it. It distributes stresses in a similar way, and is inherently stable." A seemingly more straightforward aspect of the structural design are the steel portal frames. Each member of these portal frames E E is a box frame of 1200 mm square section. This makes the structure appear regular and benign to the eye, and on the face of it, looks relatively simple to fabricate by folding and welding single pieces of steel plate.

The portals, however, are deceptive. For a start, the thickness of the steel plate varies according to the stresses borne by the member. It is further complicated by the fact that the bird's nest has been jazzed up, or as Herzog puts it, "radicalised", since the competition design. All the vertical columns of the portal frames in the original design have transformed into columns that rake giddily in various directions, in the spirit of the random bird's nest concept, before curving round at the eaves to form the horizontal roof trusses. As Burrows explains, this causes the columns' steel box frames to twist and bend while retaining their constant 1200 mm profile, and the irregular bird's nest arrangement means that hardly any columns are alike. This in turn creates headaches in plotting the pattern of steel plates to form the box section for each member. As in the fabrication of the irregular 3D geometry of steel car bodies, these difficulties that can only be resolved by using a sophisticated CAD modelling system Last year, the entire construction project was awarded to a Chinese design-and-build contractor, CITIC, to which the design team, including the local Chinese design institute, has been novated. Completion is due for December 2006, a full 20 months before the start of the Olympic Games. Detailed design, including full independent verification, was completed last month so that trade packages are now ready to go out to tender. "We're already further ahead than Athens," says Parrish.