London 2012: The aquatics centre
E&T describes the engineering behind 'the jewel of the Olympic Park' - the Aquatics Centre.
The Aquatics Centre is going, organisers claim, to be the jewel of the Olympic Park, 'The Gateway to the Games'.
Internationally acclaimed architect Zaha Hadid has created a sinuous, wave-shaped building, which will hit spectators as they enter the Park across the Plaza and the large bridge - the main pedestrian entrance. She said that the 'S' design is meant to be a reflection of the flowing movement of water, while Mike King, the project engineer for Arup, sees it as an organic free-form structure inspired by marine animals and the ray family.
The original concept was more free-flowing than the current one where the roof has had to be altered for legacy reasons. The main difference is in the two wings, or temporary stands, attached to the east and west sides. John Nicholson, the project sponsor for the Olympic Delivery Authority, stressed that the Centre had been designed for the legacy and adapted for the duration of the Olympic and Paralympic Games. It will host swimming, diving, synchronised swimming, the water polo finals and the swimming section of the modern pentathlon. There will be two 50m pools, a 25m diving pool, permanent seating for 2,500 spectators and temporary stands for an additional 15,000. Warm-up pools will be incorporated and nearby will be temporary ones for the water polo events.
John Nicholson says the Aquatics Centre has been the most challenging of all the Olympic venues to be constructed. From the moment the site was chosen, difficulties were encountered, for it is bordered by a railway line to the east, a canal to the west and the water polo site just to the north.
About 160,000t of soil had to be excavated - much of it contaminated with petrol, tar, lead and arsenic - from the 55,000sq m site; four skeletons were recovered from the remains of a prehistoric settlement there, and 140,000t of clean soil had to be imported from other parts of the site before construction could begin.
Late in 2008, electricity cables were relocated underground into tunnels, so that the overhead power cables and 52 pylons crossing the site could be removed. These 'plug' (power lines underground) tunnels run right through the aquatics site. Substantial foundations work - a 25m-wide, 3m-deep transfer structure - was built in the part where one of the roof supports was positioned on top of a plug tunnel.
Rising water was also a problem. The water table is high and the area has been subject to flooding. In the early stages, pumps had to be used before work could begin. King says that the risk of flooding in the area has been calculated to be once in 100 years. There is also the probability of a 'buoyancy effect,' that is, if the pools are left empty for a considerable time for cleaning and re-tiling, there would be a tendency for them to be lifted up.
To safeguard against these concerns, 1,200 piles - mostly tension rather than compression piles - of reinforced concrete were driven in to give support to the pools and the building. Tension piles were used because they transmit the load by means of skin friction between the soil and the surface of the pile.
The wave-shaped roof
The outstanding feature of Zaha Hadid's plan for the Centre is the wave shape of the roof. Its area, 11,200sq m, and weight, 2,800t, involved complex design and construction challenges. It is shaped roughly like a diamond, stretching 120m north to south and 90m east to west. Such a long span is usually supported by columns, but to give spectators as unimpeded a view as possible, it was thought that they could not be employed. Instead, a bold plan to use trusses resting on only three points was designed to bear the load of the roof.
At the southern end, a 10m-high, 30m-long and 5m-wide concrete wall has been built. Fifty-four metres away are the two northern core supports, each approximately 10m by 4m.
Two primary trusses rest on these supports. Between them, another series of trusses fan out to span the 120m. They cantilever out to 32m past the base cores, north and south, and overhang the east and west sides by 27m, giving the unique undulating shape. The temporary stands, on the two east and west overhangs, are constructed with 11.5m-deep fan trusses that also contain inspection and maintenance galleries and house television lights.
At first, constructing the roof on the ground and 'stand jacking' it up into place was considered, but this was rejected because the shape of the roof would need too many temporary supports. Instead, they built the roof on site. The trusses were assembled from fabricated elements brought in, and a 1,350t super crane was constructed on the spot to lift each section into place.
The shape of the building, glorious as it may be, can bring a problem. Winds can swirl and create turbulence underneath the overhangs. This could lead to a build-up of pressure, if the resonant frequency of the building is reached. All buildings have a resonant frequency and care must be taken to ensure that the natural frequency of the roof structure does not correspond to that of the turbulence, generated by the wind swirling under the overhangs. If the resonant frequencies do coincide, there is the possibility of damage, caused by increased roof vibration.
Structural engineers quote 'Galloping Gertie', Tacoma Narrows Bridge in Washington State, US, which collapsed in November 1940. One of the reasons given for its collapse was that it was "an example of elementary forced resonance with the wind providing an external periodic frequency that matched the natural structure frequency". Arup say there is little probability of this happening to a building with a roof of this size, but there is a possible "amplification effect on the static wind loads applied to the roof design".
The shining exterior of the Centre is made of Kalzip aluminium standing seam, which is fixed to an aluminium structural decking. The underneath of the ceiling is made of red lauro wood from Brazil. It is said to combine the sturdiness and weathering characteristics of teak with the colour of redwood. However, some critics have reservations because it has not been tested in the humid conditions of a swimming pool.
The temporary stands are for the Games period only and will be dismantled after the Olympics. The additional 15,000 seats will bring the capacity to 17,500.
The seats in incline terracing will be mounted on a ply and rubber finishing over a decking with a steel profile. The roofs of the stands are supported by 85m-deep primary trusses, spanning north-south between the perimeter walls. To facilitate dismantling and for possible use after the Games, the whole structure will be bolted steelwork and designed in sections.
Again, for ease of disassembling, the exterior of the stands is made of a fabric façade wrap rather than steel sheeting. After the Games, when the temporary stands have been dismantled and the seats removed, a glazed façade will be erected for the legacy mode.
The Olympic Delivery Authority have taken the wishes of the swimmers into consideration when designing the pool. They want a 'fast pool' - by that they mean 10 lanes, each 2.5m wide. Following the example of the pool in Beijing, they want an optimum depth of 3m, deeper than in the past, but not too deep. If it is deeper, they can lose their sense of vision and location in the pool. When it is just right, turbulence and waves are dissipated down to the bottom, making progress through the water much easier.
They do not want the outside lanes to be used in competitions. The reason is that waves, churned up during races, can bounce off the sides of the pool and impede swimmers in the outside lanes, giving those in the middle lanes an unfair advantage. Any such waves are now siphoned off into perforated gutters along the pool sides.
Another device to dissipate turbulent water is the use of 'wave eaters' in the lane lines that separate the swimmers. The aim is to make the water as flat and clear an environment as possible. This is done by introducing independently rotating discs in the lines that allow waves to be dispersed along the length of the lanes.
The water in the pools, King says, is "fresh potable". It is circulated continuously and treated with "UV-based chlorine". The 'backwash greywater' is filtered and used again for toilet flushing.
Pool environments are notorious for their corrosive atmosphere. This is kept under control, says King, by the ventilation systems operating 24-hours a day, and all materials within the pool halls have been selected to be suitable "for a corrosive pool environment to a level appropriate for the element lifespan".
There is still much to do: the roof is to be completed and the pools excavated and finished off by the scheduled completion date in 2011. Initially, the budget was £75m, but the latest estimate has climbed to £250m. However, when the Games are over, the image of the Aquatic Centre will probably be the one that lingers.