Olympics showcase: translating the architects' vision for the Aquatics Centre
Talent pool: the striking roof of the 2012 aquatics centre will be a showpiece for British architecture.
Compared with some recent Olympics, London’s games will be a stripped-down affair. The organisers decided to avoid the architectural fantasias of Beijing’s improbable Bird’s Nest Stadium and shimmering Water Cube in place of more utilitarian buildings, exemplified by the no-frills bowl of the athletics stadium now looming above Stratford Olympic Park.
However, top-notch architecture is on display, most notably in the Aquatics centre. Perched to the west of the stadium across the Plaza where 250,000 visitors will enter the park every day, the building’s swooping roof spreads its winged form over three pools (one for the main swimming events, one for diving and one for practice purposes in the basement) and 18,000 spectators. The design, by Anglo-Iraqi architect Zaha Hadid, was chosen in a competition. It will be the first of her projects to be built in her home country.
But the striking roof, with its three-dimensional curves, presented a real challenge to the engineers who had to realise Hadid’s design. Measuring 160m in length and 90m in width, it appears to float unsupported above the pools and seating. How do you build a structure like that? And worse, how do you build it in a recession, under the scrutiny of an organising body determined to pare costs down as much as possible, and pressure from the media determined to seize upon any evidence of extravagance?
The striking roof was actually designed before the Olympic Games were assigned to London
Addressing a recent meeting organised by the Royal Academy of Engineering, project director Stuart Frazer of Balfour Beatty said that there were many challenges on the site. ’First, there was the remediation from the site’s use pre-war and into the 1950s and 1960s - Stratford used to be a big railway town and there were the remains of facilities for manufacturing steam locomotives and carriages,’ he added. ’We then inherited the remediated site in June 2008 to start work, much of which had to be carried out below the water table. That presented another problem: there are two tunnels that run immediately below our site, 20m underground.’ Known as the PLUG (power line underground) tunnels, these carry electricity to the north end of the site.
However, the roof presented the biggest challenge: a highly complex structure, it also serves as a showpiece for British architecture. Part of the problem was that the competition was actually part of the London 2012 bidding process and was therefore designed before the games were assigned to London, according to Mike King, an associate director of engineering consultancy Arup, who led the project to build the aquatics centre. ’This was to show the International Olympics Committee [IOC] that London was serious about the games and was prepared to invest in cutting-edge architecture,’ he said. However, once the games were won, it became obvious that certain parts of the design would need to be rethought.
The design presented to the IOC showed the venue in games and legacy modes, but there wasn’t much difference between the two. ’For the games mode, it was a case of tucking in the seating under the roof, then, after, the Olympics installing a new facade some distance back from where it had been during the event,’ added King. ’And that was how the design looked in 2005. After that, we went back and looked at where the focus should be. The advice was that we should focus on the legacy and think about how we could adapt that for the Olympics.’
This meant that the size of the roof was reduced, cutting back the overhanging wings to a scale more in proportion with the building’s post-games capacity. The seating was also redesigned, from sweeping curves to a right-angled grid that was easier and cheaper to model and build.
The focus then came back to the roof. ’One of the key things in the project was the reduction in risk, or the perception of risk, so the ODA created a dialogue process where the design team would sit down with potential contractors to instil confidence that the scheme was buildable,’ explained King. The result of this was that architects and engineers spent approximately four months revisiting the design to make the structure less complex, while retaining its distinctive shape.
In the previous design, the two arches that form the winged side sections of the roof transferred forces into the ground. ’There was a huge thrust into the ground and we had a tie buried in the substructure beneath the ground to cope with that,’ he added. The roof itself was to be built from support beams that followed its double-curved shape.
“It’s an elegant, closed form in that all the forces are contained within the roof”
This part of the design did not survive the rethink. Even with its reduced scale, the roof was still huge and had to sit on main supports that were only 22m wide, and the wings overhang these supports on both sides. ’If you draw a line down from the tip of the roof, you have a 27m overhang from your last line of support, bang in the middle of this 120m span,’ said King.
The result was a structure formed from two-dimensional roof trusses that travel from the front to the back of the roof’s long axis, perpendicular to the wings. These are made from sections of fabricated steel, all of the same dimensions, to ensure that all the nodes - where the trusses are fixed to other sections - are the same. The wings are still arches but, rather than transferring thrust into the ground, they are connected to in-plane trusses and transfer their weight via tie into the central trusses. ’It’s an elegant, closed form in that all the forces are contained within the roof,’ added King.
The complex three-dimensional curve of the roof is created by the purlins, which run perpendicular to the trusses, but even these only have to be curved in one dimension. ’If you look at the top surface, which is clad in aluminium, and the bottom, which is clad in timber, we get that great seductive curve from simple 2D elements that are universal beam sections - cheap and cheerful, if you like, because it’s economical and easily fabricated.’
The roof is supported on spherical bearings, fixed at one end and sliding at the other. One allows movement longitudinally but not laterally, which helps the structure to withstand the wind; the other two can slide in both directions.
Balfour Beatty found the new design relatively simple to work with, said Frazer. ’We had visions of erecting the roof on the floor and lifting it, but it’s 11m high at its highest point, so that wasn’t practical,’ he said. ’We made the steel pieces flat, lying on their sides, then towed them down to the site and lifted them onto three trestles that provided temporary propping.’
Two years ahead of the games and the roof is complete, with pods containing security cameras, lights and speakers ready to be installed in the space between the outer and inner cladding. Inside, the warm timber panels of the roof will form a smooth, uninterrupted curve.
’Come 2012, we’ll have a series of venues that are a testament to UK construction and engineering,’ said Frazer.
The engineers were in charge of creating the centre’s main structure
While the sweeping roof dominates the aquatics centre, it is not the whole story. The main structure of the building consists of 50,000 tonnes of concrete, using 25 per cent recycled aggregate and 50 per cent cement alternative to reduce the environmental impact. This rests on 1,800 25m-long piles, placed to avoid the two PLUG tunnels.
’We also did the services and lighting design, which required a lot of CFD in terms of the environment inside,’ said King. ’We had to understand the temperature distribution in the roof space because it gets quite hot and that influences how it expands and contracts. We also had to create two distinct environments, one at pool level for the athletes and officials and one for the spectators, to ensure the comfort of both groups. We’ve also done a lot of work to reduce the glare from the Sun coming in through the facades in legacy mode.’