黑洞社区

It is said that one in three of the world鈥檚 population tunes in for the opening ceremony to an Olympic Games. With 4 billion eyes watching, the pressure is on to make sure things run to plan. And if you鈥檙e the man responsible for the 80,000-seat stadium, you probably feel more pressure than most to get it right - particularly as the previous opening ceremony took place in an iconic structure widely hailed as a modern engineering miracle, while this project has been variously dismissed as both unambitious and unachievable.

The wind was predominantly coming in from the south-west and just dumping onto the field of play and causing a lot of problems

Tony Aikenhead, team stadium

And that鈥檚 not even to mention the time factor, which the man in question - Ian Crockford, the Olympic Delivery Authority鈥檚 project sponsor - describes as the biggest challenge in building the stadium. While 27 July 2012 is the date etched on most people鈥檚 minds, Crockford鈥檚 target is just 12 months from now. By July next year, the stadium should be completed and commissioned and ready for the first test events.

The clock started ticking four years ago when bids were invited for design and construction teams. By the end of 2006, negotiations were under way with Team Stadium, a consortium including architect Populous - formerly HOK Sport - contractor Sir Robert McAlpine and structural and building services engineer Buro Happold. They went on to win the scheme and according to Tony Aikenhead, Team Stadium鈥檚 project director, they were able to hit the ground running. 鈥淭he team was already used to working with each other, having done the Emirates stadium and just coming off the O2 arena redevelopment project [when it changed from being the Millennium Dome].

We had that relationship and all knew each other and knew how to work together, which meant the design could be fast-tracked and the packages procured much earlier.鈥�

But while the time pressures have had a big influence on how the stadium has been designed and constructed, Team Stadium also had to contend with a brief that bucks the conventions of stadium building. First, it鈥檚 an incredibly compact site, and second, a large chunk of the stadium had to be demountable. As Crockford recalls, nothing like this had been tried before and when the brief was developed, it wasn鈥檛 known if it was achievable.

The stadium is the antithesis of Beijing鈥檚 Bird鈥檚 Nest. Instead of a dramatic architectural statement, the focus is much more on functionality. To get an idea of its scale, Sydney鈥檚 Olympic stadium or Wembley, placed on the same piece of ground, would overhang the rivers hemming the island site 30-40m away. The height of the Bird鈥檚 Nest, meanwhile, matches the lighting towers that project 30m above the roof.

The stadium鈥檚 position in the park was questioned at the concept stage, says Crockford, but it was part of the masterplan and 鈥渂asically we were stuck with it鈥�. As a result, the team was on a mission to make it as compact as possible. To achieve this, all the public concourses, toilets, food outlets and concession stands that you would normally find inside a stadium have been stripped out and placed in standalone pods on the concourse around the perimeter. This frees up vital space to wrap the seating tightly around the field of play, ultimately giving a more intimate atmosphere. But there is a further benefit as it also removes risk, as these are fiddly facilities to build.

To address the eventual scaling back of the stadium after the Games, a hybrid of permanent and temporary structures was developed. There鈥檚 nothing novel about Olympic stadiums having their capacity boosted by temporary seating, but it鈥檚 the scale that鈥檚 making the difference in London. It challenges the idea of what makes a temporary facility.

What鈥檚 been constructed is a sunken, 25,000-seat bowl of precast concrete. This is the permanent element. Above this sits a single upper tier of bolted lightweight steel sections supporting precast concrete terraces, which is the demountable element.

When I鈥檓 racing I do block things out, but it鈥檚 that initial walking onto the track when you feel it. When I walked out onto Beijing I had to pinch myself to believe it

Lisa Dobriskey, athelete

At the early stages, the design team hit a snag. The intention, like the stadium built for the Barcelona Olympics, was to do without a roof. This would be simpler and save money. But the design team was forced to change its plans when it started analysing the wind conditions on the park.

For a wind speed above 2m/s, any world record set on track isn鈥檛 eligible. 鈥淲e were finding that the wind was predominantly coming in from the south-west and just dumping onto the field of play and causing a lot of problems,鈥� says Aikenhead. 鈥淎dding a roof would keep this at bay and also double up as protection against weather.鈥�

The initial idea was to cantilever the roof from the perimeter, but concerns emerged over the length of the cantilevers, plus it wasn鈥檛 the most aesthetically pleasing solution. So the designer switched to a cable net structure.

This had the potential to slow progress, so to enable Team Stadium to press on, it split the stadium in two - the substructure and superstructure - and, after the client signed off the concept design in late 2007 and the project went to scheme design, it applied for planning permission for the two separately.

This meant it could get on with building the permanent bowl much earlier. By May 2008 permanent piles were being installed on site, just six-and-a-half months after the completion of the design, three months earlier than planned. It was very fast track, says Aikenhead. 鈥淲hen we were out tendering this project, Wembley was still very much in the press, so we were conscious of the timetable and getting things started on site.鈥� It was also essential to get the stadium ahead of the other projects on the park to help spread out the procurement and logistics on what is Europe鈥檚 biggest construction site.

So how is it being constructed? To streamline the build process, Team Stadium has tried to maximise prefabrication and use as many plug-and-play building elements as it can. To avoid disturbing the contaminated ground beneath the site, about 5,000 driven piles are being used to support the seating tiers, while around the perimeter 150 concrete columns were installed to support the podium slab. This will act as the stadium鈥檚 concourse and allow spectators to circulate.

A precasting concrete plant was set up on the park to churn out the standardised raking beams used in the lower bowl. At the same time, the precast concrete terrace units, which they will support, were being trucked in from batching plants in Somerset and Lincolnshire. The seating will be fixed onto these in the coming months.

Once the construction team reached the podium level, they could begin to install the 40m-long steel terracing supports for the demountable upper tier. There are 112 of these in total, which took nine months to install but, says Aikenhead, as soon as they had enough of the concrete terracing units installed to tie the temporary structure together, they were able to begin the roof structure. Although the roof is a completely independent structure, the upper tier needed to be in place to tie the roof supports to it temporarily.

The roof design is similar to a bicycle wheel. A 15m-high compression truss sits around the outer edge supported on steel columns from the ground - this is the wheel rim. From here, steel cables - the spokes - connect to an inner tension ring - the wheel spindle - which sits 30m above the field of play. PVC panels are then fixed between the 鈥渟pokes鈥� to finish it off.

The compression truss was match cast (meaning one section is cast and then the next one is cast against it so the team know it fits) by Watson Steel in Bolton in 30m-long sections to get the close tolerances required. Once on site, a superlift crane positioned on the stadium floor raised the 28 sections into place.

The tricky bit was getting the inner tension ring in place. A temporary scaffold was first constructed around the bottom of the upper tier on which the 450-tonne elliptical tension ring was built. Strand cables were then attached from the compression truss down to the ring and, using 56 jacks in unison, it was gradually pulled into position in a six-week operation. This was a delicate process, recalls Aikenhead. 鈥淚t had to be jacked simultaneously to keep the loads in the cables consistent; otherwise there was the danger that one might become overloaded and break.鈥�

Only with the cable net roof in place could work begin to raise into position the 14 lighting towers, which will flood the field of play with the 1MW of lighting needed for high-definition television broadcasting. These are located on the inner tension ring of the roof, which was necessary to beam light at the right angle down onto events.

Each tower weighs 34 tonnes and includes integrated walkways, power supplies and cabling, together with 44 floodlights. They were first constructed and tested at ground level before being hoisted to their position 60m above the field of play.

This was the trickiest operation of the entire stadium construction, says Aikenhead. A 41m long, 600mm diameter temporary stay was first fixed to the back of each unit, which acted as a temporary support when the tower was initially lowered into position. This allowed a pair of cables to be connected to the compression ring to prevent it falling forward; another pair was attached to the inner ring to stop it falling backwards. With these in place, the centre of the stadium could be cleared of cranes and equipment so work could begin on preparing the ground where the track and field will be.

The home stretch is certainly in sight. The fit-out and commissioning of the 700 internal rooms, including changing rooms, training areas and media facilities, is under way and designs are being worked up for the wrap that will cloak the upper tier of temporary seating.

Contractually, the stadium has to be completed by June next year but, according to a confident Aikenhead, the bulk of it will be done by February. The only exception is the final surface of the running track, which will have to wait until April or May when the temperature and humidity conditions are right for laying. Once in place, the countdown can begin in earnest.

Photographer Tim Crocker has filmed the stadium and other venues exclusively for 黑洞社区. Watch the videos at www.building.co.uk