Thursday, July 12, 2007

China's Bubbly Aquatics Center

China's Bubbly Aquatics Center Nears Completion

A solid block of water appears to have rained down on Beijing's Olympic Green. While most architecture buffs have been focused on Herzog & de Meuron's National Stadium -- dubbed the Bird's Nest for its curved shape and overlapping structural supports -- its neighbor, the National Aquatics Center, just might steal the show come opening day. Designed by Australia's PTW Architects, engineering firm Arup, and China State Construction Design International, the so-called Water Cube has a structural system unlike any other building. No wonder -- it's based on an age-old physics problem related to bubbles.

Left: Bird's Nest or bubbles? The refreshing Water Cube rises in front of the main Olympic stadium in Beijing. PTW, Arup and CCDI won an international competition to build the National Aquatics Center with a design that resembles a heavy-duty block of flavor-free Jell-O.


"Our engineers became obsessed with the concept of bubbles," says PTW project director John Bilmon. Combing through existing literature on the structure of bubbles and foam, PTW and Arup discovered an old physics problem originally elaborated by Lord Kelvin in the 19th century that concerns the most efficient method of subdividing space in equal-volume cells. Kelvin proposed that the answer involved identical geometric bubbles. But in 1994, Denis Weaire and Robert Phelan, physics professors at Trinity College, did one better by finding a more efficient way of subdividing space with a foam that used two bubbles of equal volume but different shape. That mathematical foam became the basis for the Water Cube.


Arup translated Weaire and Phelan's discovery into architecture by shifting emphasis from individual planes to a series of lines and nodes -- elements that could later be represented by steel beams and connectors on a construction site. "Arup was able to take Weaire and Phelan's models and convert them into equivalent formulas using structural language and algorithms," says Bilmon. "They were then able to produce a geometrical model that could be used for an architectural application."


Arup's digital model offered an infinite web of bubbles. PTW lopped off a perfect square, and then carved out the spaces needed for pools and other facilities. But there was a problem -- the façade and roof showed repeating patterns, which didn't look like the random soapy mounds found in your bathtub.

To create seemingly random patterns for the façade and roof, Arup and PTW rotated the digital foam along different axes and took snapshots as it turned. "The computer power to do that was immense. It took the combined computers of PTW and Arup together, working over weekends, just chugging out these calculations," says Bilmon. "Ultimately, we found a pattern that provided a variety of bubble sizes on the façade while also retaining a number of repeating patterns that allowed the building to be built more easily." In fact, the connectors used to build the structure share common geometries and were prefabricated off-site.

The foam structure is about more than good looks. "It's a very appropriate structure to use in Beijing because of the earthquake conditions that exist there," says Bilmon. "It's a highly flexible structure and it responds extremely well to the seismic loadings."


Two layers of a high-performance plastic film called ethylene tetrafluoroethylene, or ETFE, fill in the spaces between structural members and are inflated from within to create pillows with a rounded surface. Both the exterior and interior of the 12-foot-thick walls and 22-foot-thick roof have ETFE skins -- the hollow space in between is used for ventilation. This design allows the structure to act like an enormous greenhouse. When the building needs heat, air is circulated from the warm walls into the interior spaces; when the building needs to be cooled, air is pulled up from ground level and expelled through the roof.


Construction of the Water Cube is scheduled to be finished this October, in time for a few trial events. The full-scale splashdown happens August 2008 with the start of the Olympic Games. Considering Beijing's summer heat, don't be surprised if the National Aquatics Center gets mistaken for a mirage.


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