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Bird's nest superstructure

Beijing Olympic stadium - As it nears completion China's bird's nest Olympic stadium is shouting for attention, discovers Andrew Mylius.

Beijing's architecturally flamboyant Olympic stadium is not universally admired.

'The stadium's bird's nest motif is an insult to birds, ' spluttered German structural engineering luminary Jorg Schlaich at a symposium in Beijing last month, organised by the International Association for Shell & Spatial Structures.

'It uses huge quantities of steel, ' Schlaich complained, 'far more than necessary to enclose 100,000 seats. It is a grossly inefficient structure.' Warming to his theme, Schlaich advised: 'Good engineering should be about solving a problem as economically as possible, using the least possible materials. It should never be subjugated to art. . . whatever art is.' That is a debate, of course, that Schlaich should have had with client for the stadium, the Beijing Organising Committee for the 2008 Olympic Games (BOCOG), before it decided to build the structure now nearing completion. The stadium has consumed a jaw dropping 45,000t of steel but to stunning visual effect.

Welding of the final structural members is taking place, the precast concrete supports for the seating bowl have been installed, with seating and mechanical/electrical fit out following close behind. Beijing's Olympic stadium is due to open for business this time next year.

Questions of 'good engineering' aside, design and construction of the stadium have been remarkable challenges.

Consultant Arup had the task of turning an architectural concept by Swiss architect Herzog & De Meuron into something buildable.

'Herzog & De Meuron put a lot of effort into making the structure unconventional, ' said head of Arup Sport Jay Parrish in 2004. 'Columns are skewed off the vertical. There's not a concourse with a straight edge. The whole effort has concentrated on creating an exciting space.' The stadium measures 230m wide by 330m long and 55m high. Structurally it is composed of two independent parts ? the bowl, which is a fairly conventional precast reinforced concrete structure, and the steel exterior faþade and roof.

The roof has a 'Pringle-like' geometry, derived by taking a small patch from the inside face of a vast toroid. At its edges, the roof flows into smooth corners, creating a seamless transition into the faþade, which slopes inwards towards its base at 14¦ from the vertical.

Herzog & De Meuron sketched the faþade and roof as a random scribble, from which Arup had to establish a clear structural order. This has been achieved by disguising primary structural members amid a web of secondary steelwork.

All members share the same section of 1.5m 2. This has been so effective that is difficult to see that there is a structural system that has been repeated around the stadium's perimeter.

Roof and faþade are interconnected. The stadium performs like a collection of giant portal frames. Twentyfour perimeter columns consist of two outer chords and a single inner chord ? all primary members. These rise from a single point, diverging as they go and wrap around the corner between faþade and roof to continue across the roof. At the centre of the roof there is an elliptical opening ringed by a 10m deep truss.

Each primary member strikes a tangent off the ellipse and, on reaching the far side of the roof, wraps around the corner, returning to the ground via another column.

Secondary steelwork provides triangulation between the main structural elements. Stairs threaded between the inner and outer chords of the columns provide further bracing up to sixth-floor level.

During design, 10,000t was spaired from the structure's dead weight, but Arup director Michael Kwok says that in primary members subjected to the highest loads steel thickness is up to 100mm.

Chinese main contractors Urban Construction and Citic subcontracted fabrication of the complex curved box sections making up the stadium's multiple trusses to ship builders in Shanghai and Zheianjang.

'China has some of the biggest shipyards in the world.

The hulls of ships are curved in every dimension, so this project was not especially difficult for them, ' says Kwok.

Sections were transported to Beijing by road in 12m to 15m lengths and assembled into larger units weighing up to 60t before being lifted into position.

All connections are welded.

Kwok says the pace of construction has been blistering. Ground breaking was in December 2003.

Bored pile foundations were installed through 2004 ? each column is founded on 30 to 40 piles of 800mm to 1,000mm diameter and 20m to 40m long, depending on ground conditions.

Columns are implanted in the high strength concrete pile caps and the box sections have been concrete lled to several metres above ground level, creating monolithic connections with the ground. This is necessary to help cope with horizontal forces from wind, crowd dynamic and seismic loading, says Kwok.

Erection of the first column took about a month, he recalls.

'Once the contractors had been through the learning process on the first column, though, things progressed rapidly.' Massive temporary props were required to support each column and the central ring truss. Only when all of the columns had been pieced together was the interlinking grid of primary members installed to support the ring truss.

'During design we assumed that de-propping would take place once the primary members were complete, but before the secondary bracing had been installed. This was to allow for deflection of the roof, and ensure stresses weren't locked into the structure, ' Kwok says.

The structure would sag under its own weight once temporary support was removed.

'But the contractors wanted to weld in the secondary members as they went, before de-propping.

'We did some pretty urgent re-analysis and reached a compromise. Some of the secondary members went in before de-propping, which happened in September, and they're putting in the last of the secondary members now.' Removal of the temporary props was televised ? it was a three day operation ? capturing on camera Kwok's relief that the roof behaved as expected.

'Deformation was 270mm at the centre of the roof, more or less as we'd predicted.' To cope with the stresses resulting from the deection, secondary members' plate thickness was increased locally and diaphragms were added to brace them internally.

Kwok admits that China's construction boom meant that steel and skilled fabricators were in equally short supply at times.

'Things were very tight in terms of delivery against schedule at times. That's had some impact on cost but the project's still within its Yuan3.2bn ($4084M) budget.'

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