Digitally driven

The Water Cube's structure is the outcome of applying sophisticated analysis and optimization software that Arup's engineers created in-house specifically for this project. The program helped the designers examine the space frame under various loading scenarios to determine the size, shape, weight, and other properties for each of the 22,000 steel tubes. These characteristics were automatically recorded in a database and a 3D model, which in turn were used to produce the construction documents.

Team members say that the process of digital form finding, analysis, and documentation employed to produce the Water Cube was cutting edge for a building designed largely in late 2003 and completed earlier this year. "There is a lot of talk about autogenerated architecture, but this was one of the first projects where such a process was realized," says Chris Bosse, a former project architect at PTW and now head of the Laboratory of Visionary Architecture (LAVA), in Sydney.

Vector Foiltec assembled the pillow layers from 5-foot-wide sheets of ETFE. Because the designers specified that the resulting seams, visible under certain conditions, should align from pillow to pillow, none of the patterns for the 4,000 bubbles are exactly the same. Photo © Michael Goodman

Because of the high degree of automation that the parametric process afforded, the team could generate a complete set of new construction documents in less than a week following a major change in the Water Cube's configuration, according to Carfrae. But speed was not the only advantage. The process also ensured accuracy. Prior to construction, the team issued the 3D model, traditional 2D drawings, and the database to the contractor. They did not worry about potential conflicts between the various media (or resulting construction errors) since "it was all the same information conveyed in different ways," he says.

Fabrication of the ETFE pillows depended heavily on the digital information. The digital files controlled operation of the foil-cutting equipment, the same way CAD files run a plotter. This step in the fabrication process was not completely automated, however. The data did require some manipulation, especially to address areas of the building skin where the bubble shapes are interrupted, such as those around corners and openings. "There was still a large human factor involved," says Edward Peck, director of design and development for Vector Foiltec. The international company, headquartered in Germany, partnered with a Chinese curtain-wall manufacturer to engineer, fabricate, and install the ETFE cushions.

Most of the cladding cushions are composed of three layers of of 0.008-inch-thick ETFE foil. But those located in areas of high wind loads, particularly corners, have two or three more. These extra layers provide "load sharing"-they help the building skin withstand the additional pressure and suction exerted on those spots, since making the foils thicker was not an option, says Stefan Lehnert, Vector Foiltec managing partner. At thicknesses greater than 0.01 inches, the material becomes too brittle, he explains.

To create the rounded pillow surfaces, Vector Foiltec cut 5-foot-wide sheets of ETFE into shapes that resemble sections of a banana peel. The company then assembled the pieces into larger sheets, some as wide as 30 feet, via heat welding. The pattern for each of the 4,000 cushions is unique, even though there are only 15 pillow types in the walls and seven in the roof. Since none of the pillows have exactly the same orientation, and since the design team required that the heat-weld seams run continuously from one pillow to another, from the roofline to the ground, the result is that no two cushions are alike, explains Lehnert.

Details for attaching the ETFE pillows to the Water Cube's structural frame vary depending on their location. Drawings courtesy Vector Filtec

After assembling the foils, workers transported the cushions to the site and attached the ETFE layers into aluminum extrusions that secure the pillows to the space frame. They then inflated the pillows with 18 radial ventilators permanently installed in the building. Because the pillows will gradually lose air, the building management system constantly monitors the pillows and signals the ventilators to supply filtered and dehumidified air when the pressure falls below a desired level.

Dirt typically washes off ETFE when it rains. But because of Beijing's construction dust and other particulates, the cube requires some manual cleaning. Photo © Michael Goodman

Collecting and conserving

Appropriately, some of the building's most innovative features are its systems for handling water. Unlike most swimming pools, which send filter backwash water to the municipal wastewater systems, the Water Cube collects such gray water for treatment and returns it to the pool. The system substitutes rainwater collected from the roof for the small amount of gray water lost in the treatment process. The strategy lessens the burden of the building on Beijing's wastewater infrastructure and makes it less dependent on the city's already constrained fresh-water supply. "The idea was to make it as self-sufficient as possible," says Carfrae.

To visitors, the gray-water-recycling and rainwater-harvesting systems will be invisible. For them, the bubbles (and the Olympic competitors) will steal the show, especially at night, when the Water Cube becomes a glowing blue box with the help of LEDs integrated into the pillow frames. During the day, it is sometimes soft and playful, or cold and hard-edged, depending on the weather and the angle of the sun. This ability of the building to transform is perfectly in keeping with the design team's goals: "Water has no fixed image," says Wang. "It can be still, it can reflect the sky, or it can have big waves."