Hierarchical Structure Introduces Logic to Randomness

Legs, trusses and layers of secondary members shape Beijing stadium's bird's nest

10/23/2006 By Janice L. Tuchman and Andrea Ding-Kemp

	The client wanted a dramatic building that would make a statement and seat 91,000 for the opening and closing ceremonies of the 2008 Olympic Games in Beijing. Photo: MichaelGoodman/ENR

When the Olympic torch arrives in Beijing for the opening ceremonies of the 2008 Summer Olympics, the setting for the thousands of athletes, dignitaries and spectators will be a stadium that is structurally innovative, architecturally international and culturally Chinese. Swiss architect Herzog & de Meuron, collaborating with avant-garde Chinese artist Ai Weiwei, took inspiration from the auspicious natural form of a bird's nest. Façade and structure are identical, and structural elements "mutually support each other and converge into a grid-like formation—almost like a bird's nest with its interwoven twigs,the firm says.

The functional and technical requirements of an Olympic stadium are strenuous. The Beijing venue, which will be used for opening and closing ceremonies and track and field events, will have 80,000 fixed seats and 11,000 temporary seats. Elliptical in plan, the seven-level stadium will have 258,000-sq-meters of gross floor area. The 330 x 220-m roof will be saddle-shaped, and the surface of the 69-m-tall facade inclines out by about 13°.

Image: Herzog & de Meuron

The architect teamed with ArupSport for multidisciplinary engineering services and sports architecture consulting. "Clearly, the client wanted an iconic statement, a dramatic and exciting building,says J Parrish, architectural director for ArupSport. Arup had worked with the Basel, Switzerland-based architect on Munich's Allianz Arena, scene of the 2006 World Cup. Herzog & de Meuron, ArupSport and China Architectural Design & Research Group won the international design competition for the job held by the Beijing Municipal Planning Commission in 2002. "We designed from the inside out, using a combination of CATIA and purpose-designed software,Parrish adds. The team also gave a lot of consideration to "legacy uses"—how the stadium would function after the Olympics are over. A hotel is being built into the complex, for example, along with two levels of commercial development that will open after the games.

	Fabricating and erecting the frames was a challenge for contractors. Photo: MichaelGoodman/ENR

Martin Simpson, the lead structural engineer on the initial phases who is based in Arup's Manchester, England, office, says the structure is less random than it looks. The Beijing stadium has a "hierarchical structure of primary, secondary and tertiary members. The primary structure has a series of legs and trusses plus layers of secondary members that create the random appearance. The eye is drawn to the massing, not to the layers. It's a way of introducing logic to randomness,Simpson says.

The primary system has 24 column points. The roof, which springs from the columns points, is a free span of the bowl. The original design had a roof that opened and closed, but as costs came under scrutiny, the design team came up with a fixed-roof alternative, which increased the size of the opening but kept the roof frame as a single plate. "It lowered steel weight and produced a savings of about 10% on costs,Simpson estimates. No official cost figures on the stadium have been released, but published estimates put the cost at about $423 million after the redesign.

Beijing's pollution is prompting the team to install roof membrane closer to the Olympics. Photo: AP

Plates 1.2 meters wide are used to build up the box sections. The plates are 10 to 100 mm thick, depending on the forces at a particular location. "The plate thicknesses were optimized to cope with the structural loads, including earthquakes,said Michael Kwok, director of Arup Hong Kong and National Stadium project director. The seismic design includes three different load cases—for 2000-year, 475-year and 50-year earthquakes. There are different levels of performance criteria so the structure will remain elastic and withstand shaking.

Building the Nest

National Stadium's main contractor is Beijing Urban Construction Co. Ltd., and one of the firm's senior engineers reviewed progress with ENR. Work started at the end of 2003, driving piles up to 37 m deep and up to 1.2 m in diameter. There was a pause for several months in 2004 while the modifications were made to eliminate the retractable roof and enlarge the roof opening.

Steel plates for the box sections vary in thickness according to the forces at the location. Photo:Michael Goodman

By November 2005, cast-in-place concrete work for the seating bowl structure was completed. Precast concrete seating tiers will be installed. The precast work is being done at Beijing Urban Construction's own precast yard.

Erection of the structural steel columns began at the end of 2005 by Shanghai Baoye Construction Corp. and Beijing Urban Construction Jinggang Steel Structure. "Fabrication of the welded steel frames was not easy, and connecting a structure with 180 openings was a tough job," according to the contractor. The 180 sections were welded together to connect the main structure, which was finished by the end of August. A total of about 40,000 tonnes of steel was erected, which required 20,000 drawings to detail the twisting, turning frames. On Sept. 17, workers unloaded the structure from its temporary supports, and the main structure was complete.

	Main structure was unloaded from its temporary supports in September. Photo: AP

Finishing Touches

Mechanical systems are now being installed, and the entire stadium is scheduled for completion at the end of 2007. Also ahead is the installation of roof cladding—about 40,000 sq m of ETFE (ethylene tetra fluoroethylene) panels on the upper surface. An inner acoustic membrane made of 50,000 sq m of Tef-lon fabric will be installed to absorb sound.

A joint venture of Covertex and Beijing N&L Fabric Technology Co. Ltd. is fabricating and erecting the membranes. The outer membrane will be attached to a secondary structure along the roof opening using L-shaped angles welded to the steel beams to form a channel to carry water runoff. With Beijing's well-known pollution problems, contractors are holding off until the middle of 2007 to install the membrane so that it will still be clean and pretty for the Olympics.