Cloud Gate - Construction and Maintenance

Construction and Maintenance

The British engineering firm Atelier One and freelance engineer Chris Hornzee-Jones (who later went on to form the engineering firm Aerotrope in 2005) provided the sculpture's structural design, and Performance Structures, Inc. (PSI) was chosen to fabricate it because of their ability to produce nearly invisible welds. The project began with PSI attempting to recreate the design in miniature. A high-density polyurethane foam model was selected by Kapoor, which was then used to design the final structure, including the interior structural components. Initially, PSI planned to build and assemble the sculpture in Oakland, California, and ship it to Chicago through the Panama Canal and St. Lawrence Seaway. However, this plan was discarded after park officials deemed it too risky, so the decision was made to transport the individual panels by truck and to assemble the structure on-site, a task undertaken by MTH Industries.

The sculpture's weight raised concerns. Estimating the thickness of the steel needed to create the sculpture's desired aesthetics before fabrication was difficult. Cloud Gate was originally estimated to weigh 60 short tons (54 t; 54 long tons) when completed. However, the final figure was almost twice as heavy at 110 short tons (100 t; 98 long tons). This extra weight required engineers to reconsider the sculpture's supporting structures. The roof of the Park Grill, upon which Cloud Gate sits, had to be built strong enough to bear the weight. The large retaining wall separating Chicago's Metra train tracks from the North Grant Park garage supports much of the weight of the sculpture and forms the back side of the restaurant. This wall, along with the rest of the garage's foundation, required additional bracing before the piece was erected. Cloud Gate is further buttressed by lateral members underneath the plaza that are anchored to the sculpture's interior structure by tie rods.

Inside Cloud Gate's polished exterior shell are several steel structures that keep the sculpture standing. The first structural pieces, two type 304 stainless steel rings, were put into place in February 2004. As construction continued, crisscrossing pipe trusses were assembled between the two rings. The trusses and supporting structures were only present for the construction phases. The finished sculpture has no inner bracing. The supporting structural components were designed and constructed to ensure that no specific point was overloaded, and to avoid producing unwanted indentations on the exterior shell. The frame was also designed to expand and contract with the sculpture as temperatures fluctuate. As a result, the two large rings supporting the sculpture move independently of each other, allowing the shell to move independently of the rings.

When Cloud Gate's interior components were completed, construction crews prepared to work on the outer shell; this comprises 168 stainless steel panels, each 3⁄₈ inches (10 mm) thick and weighing 1,000 to 2,000 pounds (450 to 910 kg). They were fabricated using three-dimensional modeling software. Computers and robots were essential in the bending and shaping of the plates, which was performed by English wheel and a robotic scanning device. Metal stiffeners were welded to each panel's interior face to provide a small degree of rigidity. About a third of the plates, along with the entire interior structure, were fabricated in Oakland. The plates were polished to 98 percent of their final state and covered with protective white film before being sent to Chicago via trucks. Once in Chicago, the plates were welded together on-site, creating 2,442 linear feet (744 m) of welded seams. Welders used keyhole welding machines rather than traditional welding guns. The plates were fabricated so precisely that no on-site cutting or filing was necessary when lifting and fitting them into position.

When construction of the shell began in June 2004, a large tent was erected around the piece to shield it from public view. Construction began with the omphalos, where plates were attached to the supporting internal steel structure, from the inside (underside) of the sculpture downward to the outermost surfaces. This sequence caused the structure to resemble a large sombrero when the bottom was complete.

The shell of Cloud Gate was fully erected for the grand opening of Millennium Park on July 15, 2004, although it was unpolished and thus unfinished, because its assembly had fallen behind schedule. The piece was temporarily uncovered on July 8 for the opening, although Kapoor was unhappy with this as it allowed the public to see the sculpture in an unfinished state. The original plan was to re-erect the tent around the sculpture for polishing on July 24, but public appreciation for the piece convinced park officials to leave it uncovered for several months. The tent was again erected in January 2005 as a 24-person crew from Ironworkers Local 63 polished the seams between each plate. In order to grind, sand and polish the seams, six levels of scaffolding were erected around the sides of the sculpture, while climbing ropes and harnesses were used to polish harder-to-reach areas. When the upper and side portions of the shell were completed, the tent was once again removed in August 2005. On October 3, the omphalos was closed off as workers polished the final section. Every weld on the Cloud Gate underwent a five-stage process, required to produce the sculpture's mirror-like finish.

The sculpture was finally completed on August 28, 2005, and officially unveiled on May 15, 2006. The cost for the piece was first estimated at $6 million; this had escalated to $11.5 million by the time the park opened in 2004, with the final figure standing at $23 million in 2006. No public funds were involved; all funding came from donations from individuals and corporations.

Kapoor's contract states that the constructed piece should be expected to survive for 1,000 years. The lower 6 feet (1.8 m) of Cloud Gate is wiped down twice a day by hand, while the entire sculpture is cleaned twice a year with 40 U.S. gallons (33 imp gal; 150 L) of liquid detergent. The daily cleanings use a Windex-like solution, while the semi-annual cleanings use Tide. In addition to the scheduled cleanings, the work endures periodic vandalism, including a notable February 2009 incident during which two names were etched in letters about 1 inch (25 mm) tall on the northeast side of the curved sculpture. The graffiti was removed by the same firm that did the original polishing.