As the 2012 Olympic Games’ gateway venue, the $405-million Aquatic Centre is the 250-acre park's most flamboyant building, adorned by an aluminum-clad roof with a sweeping profile. But many agree the center will not look its best until workers clip its temporary wings, after the July 27 to Aug. 12 games. During the events, nearly two thirds of the 17,500 Olympic spectators will sit in two temporary wings, rising steeply from either side of the center.
The curvaceous building is running 20% over its 2007 budget. Officials at the U.K. Dept. of Culture Media and Sport attribute the rise to the center's “complicated nature� and constrained site.
The center's design competition in late 2004 aimed to show the International Olympic Organizing Committee that London was "serious� about the games, says Mike King, project director with the building’s designer Arup Group, London.
London-based architect Zaha Hadid won the competition. Her original concept called for placing all seating, temporary or permanent, under the permanent roof. After the games, crews would have removed seats and "pulled in" the long sides of the facades to create a venue more suitable for local use.
But plans changed. “We reduced the area of the roof in the legacy mode so...there are not massive overhangs for the rest of the existence of the building," says King.
The design change, emerging from a review ordered after London’s 2005 designation as the games' host, resulted in a 160-meter-long curved roof, with a width varying up to 80 m. In plan, it resembles a contoured seashell.
In addition to "shrinking" the permanent roof, engineers also abandoned early thoughts of framing the roof with two huge arches because of the large resulting outwards thrusts, says King.
In parallel, designers consulted contractors and then simplified the original, curved-member structural steel roof system. One goal was to reduce the risk for the bidders by making the roof more constructible.
“One of the things we got very strongly from [talking to contractors] was a perception that this was going to be quite a complicated structure," says King.
In the end, "we totally reconfigured the way the roof works," he says.
The engineer designed a structure composed of only 2D trusses, not more complex 3D trusses, laid out on an simpler orthogonal, not curved, structural grid. The resulting straight structural members—the straight lines that generate the roofs curves—are simple I-section plate fabrications 4.5 to 9 m long.
The design team also made a key early decision to separate all of the purlins from the primary structure. This avoided "fiddly bits" impeding construction, says King. Repetition of structural elements also increased.
The simplified roof spans 120 m between a 28-m-long concrete wall at the south end and two tower cores at the north. A combination of sliding and fixed spherical bridge-bearings eliminates horizontal thrust.
The Olympic Development Authority used a project delivery model similar to what is known as bridging design-build in the U.S. The system, popular among public owners, employs a design architect, typically through design development. The owner then bids the project based on the design. When Balfour Beatty Construction Ltd., London, won the center's design-build contract in early 2008, it took on the original designers as subcontractors to continue with detailed design.
During steel erection, to reduce overhead work, the contractor had the fabricator-erector Rowecord Engineering Ltd., Newport, site-preassemble the trusses in sections, typically 30 m long and 70 tonnes, on the ground. In the spring of 2009, crews started steel erection, setting trusses onto rows of temporary trestles using a 230-tonne-capacity crane. Worker moved from the south support wall toward the two core towers to the north.
Last October, they transferred the roof loads from temporary to final supports by lifting the roof at the south support by over 1 m, with the other end turning on rotating bearings. Crews the removed the upper portion of the intermediate trestles and the roof settled into its final position. As the roof took its final shape, the southern wall moved out some 20 cm, as predicted on sliding bearings.
With the roof now done, the contractor aims to have the swimming pools filled with water next spring and the project complete later in the year.
Construction complications extend beyond the roof, says Fraser. There is a high water table and two large tunnels immediately below the building. And workers had to replace 140,000 tonnes of soil contaminated with gasoline, oil, tar, solvents and heavy metals.
The Aquatic Centre is the second major U.K. building for Hadid. The first, the Evelyn Grace Academy in Brixton, London, opened last month.