Artist impression of London Olympics stadium

Olympics Watch - The main stadium

In the second article of his Olympics Watch series, E&T takes readers on a tour of London's main stadium and points out the impressive engineering behind it.

The Olympic stadium is the focal point of any Games. It is the place where, arguably, all the highest profile events take place, and where the world's top performers set about smashing the iconic records set by the legendary athletes of the past. The stadium is also the place that remains etched onto the memory long after the Olympic torch has passed on. Memories of 2012 will be of a stadium in London.

The city's aspirations are summed up by Lord Coe, a legendary athlete in his own time and now the chairman of the London 2012 organising committee.

"The stadium will be inspiring, innovative and sustainable - the theatre within which the Olympics and Paralympics will be played out. It will be leaving behind top class sporting and community facilities after the Games," he says. "We genuinely believe that this creates a new blueprint for building Olympic stadia, one which integrates Games' time requirements with a long-term legacy vision." So, it is going to be different. It won't be an iconic building but a sustainable, minimalist structure, with temporary features and an emphasis on its legacy.

Some 80,000 spectators will pack the stadium in July 2012. The day after the Games, deconstruction will begin and its capacity will be reduced to 25,000 for the legacy stadium. It will be the first stadium to downsize on such a large scale, the first one to use temporary structures, yet still meeting the standards for high-level performance. It will use only 10,000t of steel, making it the lightest of any comparable structure. Previously, Olympic stadia have resounded to the cheers of spectators before becoming empty relics, a burden to the host nation's taxpayers. This time the thinking is different. The stadium that remains will be open and viable. When the Olympics are over, it will become a national athletics arena, with a sports training, science and medicine centre.

Location

The questions and considerations that spring up around a project as significant as an Olympic stadium are endless, and they begin with the choice of location. A suitable site must be surveyed, and computer models constructed.

Meteorologists must be consulted to gauge an opinion about the likely weather conditions. Will it rain? What sort of wind will prevail, and from which direction? Will there be wind turbulence? Can it be an open site or will a roof be needed? It goes without saying that the position of the Sun must be anticipated - these Games are going to be broadcast around the world, and television broadcasters will not tolerate an inconvenient shadow.

Dignitaries, VIPs and International Olympic Commission authorities will want to sit in boxes in the comfort of shade. This means they will be on the west side of the stadium; the 100m straight and the finishing line must be there too.

These are all universal considerations. But there are also the special circumstances to take into account. The London stadium stands at the south of the Olympic Park, almost surrounded by water in a location that has been prone to flooding. Ian Crockford, project sponsor of the stadium, says that this problem has been closely monitored and a flood alleviation scheme is in place.

Foundations

Building work on the London site began in May 2008 - three months ahead of schedule - after 800,000t of soil were dug out and taken away for use in other parts of the park. A sunken bowl was scooped out and 4,000 holes were drilled into the site, each around 25m deep. They were filled with reinforced concrete to create piles on which the stadium will be erected.

Later, concrete floor slabs were laid to construct the base of the bowl and the floor of the lower tier. Over 100 columns, each 5m tall, have already been erected and act as the support for the podium of the stadium's west and south stands. The podium level also forms the lower ground floor of the west stand, which houses the athletes' changing rooms, a 60m-long call track and other behind-the-scenes facilities, such as doping control and treatment rooms. Early in 2009, the steel structures that will support the stadium roof were built, giving an indication of the final height and scale of the structure.

Pods

The designers have a vision to bring the people into as close contact with the action as possible. They call it an 'inside-out stadium'. Being innovative, the designers have moved away from the traditional style of building arenas by stripping out all but the essentials from inside the stadium. They have put toilets and shops, stalls that sell merchandise outside the stadium on the concourse. These 'pods' are painted in fluorescent colours and are clustered together making them much easier to demount and de-rig when the Games are over. There are plans for these extension units to be boxed up and sold to other countries who want to expand the facilities of their stadia in the future.

Podium

It is the way in. This is where the build-up starts - a flat concourse that rides completely round the stadium. The area can be used as a carnival space, with events and acts to entertain spectators. The idea is to charge up the atmosphere and for people to throng on through the vomitories - the openings into the stadium wall - into the arena itself.

Emergence into the arena should have the strongest 'wow' factor. Spectators will emerge one-third of the way up the stadium; below will stretch the arena, the running track, jumping run-ups and the brilliant green grass. Sunlight will slant through the opening of the roof - the English July permitting.

This entire experience is created on 145 support columns, each 5m in height, which have been craned into place, and surrounded by 1,300m3 of concrete poured to make the slabs for the concourse floor.

The tiers

The tiers are where the structure becomes a stadium of two parts: the permanent and the temporary; concrete and steel.

The lower tier is permanent; it has concrete seats. Some 12,000 pre-cast concrete rakers, or terracing units, will hold the 25,000 permanent seats of the lower bowl. These units come from Tallington in Lincolnshire and Henlade in Somerset. They can weigh up to 10t and contain 20 per cent recycled concrete.

The upper tier is temporary and has steel seating rakers - 40m-long steel terracing supports, each weighing 35t. This year will see 112 rakers erected on the south stand. Manufactured in Bolton, they were delivered to the site in sections and will be bolted together and lifted into place by a 200t crawler crane. All steel girders will be bolted together, not welded, for ease of demounting. Team Stadium is also working closely with fabricator Watson Steel to ensure the steel is light and easily demountable. "We're looking at every single joint to see how it will come apart," says project sponsor Crockford.

Stadium architect Rod Sheard says there is no need for a permanent shell or cover for the upper tier, as it is to be dismantled. During the Games, the outside of the upper tier of rakers will be hidden by a 900m fabric wrap. On this canvas artists will create a large mural showing Olympic heroes and exploits as well as flags and sponsors' logos. It will also act as additional protection and shelter for spectators. There is talk that the wrap might be cut up, recycled and turned into fabric bags after the Games.

Roof

Work has already begun on the roof, with 60m cranes lifting a 30m-long, 85t, steel compression truss into place. A further 27 sections are scheduled to be in place by October. Fabric or sail, just as in the Colosseum of ancient Rome, will be clamped to the trusses. When finished, the roof will stretch 28m the whole way round the stadium covering an area of 24,500 sq metres and providing shelter for two-thirds of the spectators. After the Games, the roof will be lowered to fit over the reduced structure, but enough room will be left to refit a second tier, should an increased capacity of 45,000 be needed.

Sightlines and acoustics

Spectators' view of the action - the sightlines - are always a problem, as ideal conditions usually conflict with safety requirements. Spectators must be close to the action and their view should not be impaired by columns and other obstructions. The maximum viewing distance from seat to arena is usually calculated at 190m, with the optimum being 150m. Designers use a 'C' value for sightlines; this is the height of a spectator's eye line above the head of the spectator in front. The best solutions are a rake angle for the tier of 35-42°. Seats have a role to play in determining the angle of rake. The comfort of a seat depends on its rise and tread. The ideal dimension for the rise of a step for a seat is 475-500mm on treads, the width of a step measuring 760-800mm. So the higher these numbers, the higher the rake. In the UK, there is safety limit of 34° for the rake angle. So they will have to work out a compromise. There is also a limit of 28 seats per row between aisles.

Acoustics give the stadium atmosphere. Sound must reverberate and rebound, amplify and rise. The stadium must not have a wind-tunnel effect but must have the sensation of an enclosed space to produce a signature Olympic atmosphere. Tests have shown that the best impact will come from a roof that does not enclose the stadium fully but only in part. This is the reason for the peculiar profile of the roof - keeping rain off spectators was only a secondary consideration.

Safety

The safety of 80,000 spectators is obviously at the front of organisers' minds. In an emergency, spectators must be able to get to safety quickly. Designers work out what they call an 'evacuation time' of 5-10 minutes. This is the time spectators need to move from their seats to, first, a temporary safety zone inside the stadium and then to a permanent area outside it. This is based on an assumption of the number of people per minute who can pass safely through a particular width of passage or gate. A clear width of 600mm is regarded as the minimum sufficient for one person, 1,200mm for two to pass side by side.

Toilets

Spectators have to be comfortable. There are never enough toilets, particularly for women.

Most venues have a ratio of 80:20 male/female, because women's toilets take up significantly more space than men's. At the Sydney Olympics, the ratio was 70:30. Also, there was one male toilet per 600; one hand basin per 30 and one urinal per 70. The ladies had one toilet and one hand basin per 35. Many - at least half the population - would argue there is a good deal of scope for London to improve on these statistics.

Legacy

If the London stadium has an ethos, it is its legacy. The way it has been conceived and constructed is based on the premise that there must be a sustainable product once the Games have finished. As we have seen, the upper tier seats are founded on steel supports that can be easily dismantled; the fabric wrap can easily be stripped off. Then the permanent lower tier of concrete-based seats will be revealed. The 80,000-seater will be refined to a 25,000-seat permanent top-class athletics venue. It is hoped that other sporting organisations will take the stadium over in the winter months. The problem here is the running track. Athletics and football stadiums have different requirements. The running track makes for a distant experience. In Germany, Bayern Munich quit the Olympic Stadium because it was lacking in atmosphere. Perhaps the key to success is a mechanism that will cover the running track in winter and can be removed for the athletics season in the summer.

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