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Heritage site

Cover Story

Beijing's £300M Olympic stadium was inspired by ancient Chinese 'scholar stones' - heavily veined pebbles mounted on small plinths - and by traditional crackle glazed ceramics, claims head of Arup Sport Jay Parrish. Dubbed 'the bird's nest' by less culturally literate commentators, its form is described by, and its structure composed of, an apparently random scribble of steel lines.

Arup, with Swiss architect Herzog & De Meuron and the Chinese Architecture & Design Research Group, won an international competition for the 100,000 capacity venue in 2003.

'Herzog & De Meuron has put a lot of effort into making the interior structure unconventional, ' Parrish says. 'Columns are skewed off the vertical. There's not a concourse with a straight edge. Columns are flying at strange angles across them.

The whole effort has concentrated on creating an exciting space.'

Sited in the middle of a rapidly developing area of central Beijing, the stadium sets out to 'blur internal and external space' with a perforated exterior facade.

'The whole idea was that people would be able to move freely in and out of the stadium - make it a part of the city.'

Structurally the stadium is composed of two independent parts - the bowl, which will be constructed in the main from precast reinforced concrete elements, and the steel exterior facade and roof. While the bowl has involved intensive iterative design to optimise sight lines and bring spectators as close as possible to the field, it is pretty conventional structurally, notes Arup Sport lead design engineer Martin Simpson.

The exhibitionist steelwork, on the other hand, has been a structural challenge. 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 facade, which slopes inwards at 14infinity from the vertical.

Arup had to establish a clear structural order while maintaining the appearance of chaotic randomness. This has been achieved by carefully disguising primary structural members amid a web of secondary steelwork. It comes as some surprise that the stadium is symmetrical and that there is a high degree of repetition.

'We've established a pattern for one section and repeated it over the whole structure, ' says Simpson.

The facade and roof break down into interconnected elements. The central roof opening is described by an ellipse, composed of a 10m deep truss. Twenty four columns around the stadium's perimeter consist of two outer chords and a single inner chord. These rise from a single point, diverging as they go. At the corner between facade and roof, the outer chords wrap around and continue across the roof, striking a tangent off the central ellipse. On reaching the far side of the roof, each line wraps around into and returns to the ground via another column.

'Every primary member emerges from and returns to the ground, ' summarises Parrish. In the roof these form the upper chords of massive trusses, turning the structure into a collective of vast portal frames capable of withstanding the earthquakes that periodically rock the seismically active region. 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.

The structure was designed to accommodate a moving roof. On such a large stadium - it measures 230m wide by 330m long and 55m high - Parrish and Simpson opted for a simple and potentially reliable mechanism, accepting the heavier weight in exchange for better performance than earlier retractable stadia roofs, which have a long history of failure. At the bidding of BOCOG, the moving roof has been dropped to reduce cost.

This has enabled Arup to enlarge the opening above the field, allowing in more light and air, and has helped meet BOCOG's demand that total steel tonnage be reduced from 55,000t to 45,000t, a figure apparently selected arbitrarily.

Despite chopping 10,000t of deadload from the structure, it is still enormously heavy. 'There are lighter roof structures, ' Parrish admits. 'But we were insistent that it had to be practical.' This was achieved by painstaking computer analysis using CATIA software.

'Design is to Chinese building codes but was carried out using our IT. We had to write a special software loop to translate back and forth between codes and the CAD model, ' Simpson notes.

Main structural members are made up of 1.2m by 1.2m steel boxes with plate thicknesses varying from 15mm to 60mm.

Depth of the bottom chord box sections reduces towards the centre of the stadium from 1.2m to 800mm.

Steel fabrication will be no mean challenge, says Simpson.

'Box sections have to be parallel to the surface - we wanted a perfect geometry of flush surfaces. That means that as you run over the corners you get these box sections curving and twisting in three dimensions.

'We've spoken to a lot of Chinese fabricators but the job will probably go to shipbuilders who are used to manipulating large pieces of steel and getting them into pretty much any 3D shape you want. Shanghai yards are producing a massive proportion of the world's ships at the moment.'

As yet it is undecided whether the highly stressed corner elements, where columns and roof trusses meet, will be fabricated or cast.

Steelwork erection will involve setting out the central ellipse on temporary platforms and holding columns in place with guy cables.

The rest of the interconnecting primary structure will be fully welded insitu. Secondary elements are likely to have bolted connections with cover plating, Simpson says.

Construction is being let as a build-operate-transfer package with French firms Bouygues and Vincy advising on procurement.

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