Volume 21, Issue 2 - April/May/June 2007

Making Metal Gleam
The sculpture in Chicago’s Millennium Park does just that

Cloud Gate in Chicago’s Millennium Park is a 110-ton elliptical sculpture composed of highly polished stainless steel plates welded together, giving the appearance of being seamless. It is British artist Anish Kapoor’s first public outdoor work installed in the United States. 

Inspired by liquid mercury, the sculpture is among the largest of its kind in the world, measuring 66-feet long by 33-feet high. A 12-foot-high arch provides a “gate” to the concave chamber beneath the sculpture, allowing visitors to touch its mirror-like surface or see their reflected image. 

“What I wanted to do in Millennium Park is make something that would engage the Chicago skyline…so that one will see the clouds kind of floating in, with those very tall buildings reflected in the work,” said Kapoor.

For MTH Industries, the Chicago-based 120-year-old glass and architectural metal contractor that installed the sculpture, the project was a totally unique, two-year-long experience.

Lou Cerny, vice president of engineering and the project manager for MTH, explained the process. “We started installing the truss system with two large fabricated 304 stainless steel O-shaped rings—one at the north end of the structure, and one at the south end. The rings are held together with criss-crossing pipe trusses. The ring-core subframe is built in sections and field-bolted with welded reinforcements. So there’s a big superstructure that is strictly for structural framing.” 

The 168 metal plates were fabricated by Performance Structures Inc. (PSI) in Oakland, Calif., and then shipped to Chicago. However, because of the uniqueness of the project and the distance between the two sites, “probably about 75 percent of the plate-hanging assemblies were fabricated or modified in the field,” according to Cerny. 

Installation work started with the dome on the underside of the sculpture. Cerny explained that the dome was suspended from the truss using a temporary four-point hanging spring support system composed of hangers, cables, and springs. The springs provided give-and-take as more plates were added to the structure. To help balance the structure, after a new plate was added, the springs would be readjusted based on the new weight. 

Plate Hanging
Each plate had its own four-point hanging spring support system for individual support as it was put in place. “The idea was not to overstress any joint, because these were put together to a 0/0 clearance,” Cerny explained. “If a plate hit the one below it, it could cause problems. This had never been done before—in fact, we were told it couldn’t be done. But we did it.”

When all the plates were fully installed and the exterior plate welding completed, permanent weld-ups were done on the inside.

The final phase of the project was the grinding and polishing. “The goal was to eliminate any visible trace of the seam,” Cerny explained, to maintain the reflective quality of the sculpture. “People can walk up to it from every possible angle. It’s fully exposed under the brightest light available—the full sun at high noon. Our work had to remain invisible for the piece to work effectively,” he stated. 

Architects' Guide to Glass & Metal
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