The two pieces of art on Millennium Park's western edge are monumental, and so is the work that it took to get them there.

The two works are "Cloud Gate," a sculpture by British artist Anish Kapoor, and the Crown Fountain, a creation of Spanish artist Jaume Plensa. As impressive as these works are in completed form, the story of their design and construction is equally compelling.

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A Surpassing Cloud

"Cloud Gate," a 66-ft.-long, 33-ft.-high bean-shaped sculpture, began with the effort to produce a composite model of the work, said Ethan Silva, president of Oakland, Calif.-based Performance Structures, which produced the finished piece. The company toiled through several attempts to reproduce in miniature a design acceptable to the England-based Kapoor.

The models were molded of high-density polyurethane foam weighing 16 to 20 lbs. per cu. ft.

Once a model received Kapoor's blessing, it was used to design all parts.

Those included the outside shell, as well as the stainless steel "stiffeners" and internal structural components.

"We needed to find a way to support the shell without overloading any given point," because to do otherwise would have risked the eventual formation of indentations, Silva said. "We also had to allow for expansion and contraction. You have over 120 degrees of temperature variation, and different parts of the sculpture at any time could be different temperatures."

The secret to dealing with temperature changes was to allow each part to move relative to other parts without excessive constraint on any one component. Two large rings inside the sculpture move somewhat independently of one another, in turn supporting the shell. That means the shell had to be permitted some movement independent of the rings, Silva said.
Three-dimensional modeling software was used to create the 168 stainless steel exterior plates, each with stiffeners welded on, that make up the shell.
"We used a computer-controlled cutting method to cut all the parts, so they would fit in the shape we needed," Silva added.

To bring those exterior plates together requires full-penetration welds at the seams. After that welding, seams are to be ground perfectly flush and polished to a mirror finish.

Some of that work was completed before Millennium Park's opening, but the bulk of the sculpture has yet to be welded and polished, which is why visitors can still see seams in the shell.

Keeping Sculpture Up

The shell of the sculpture alone weighs 80 tons, the entire structure 115 tons. That heft has spurred the question of what keeps such a heavy object from crashing through the roof of the Park Grill, the restaurant below.

The answer is in the retaining wall separating the Metra train tracks from the North Grant Park garage. The wall travels along the back side of the restaurant and supports the sculpture. The Kapoor is further buttressed by lateral members underneath the plaza anchored to the sculpture's interior structure by tie rods.

All the interior structure and about one-third of the shell components were assembled in Oakland to test their fit. Then these parts were disassembled and shipped to Millennium Park in containers aboard trucks.

Workers began assembling the interior structure at the park in February. Assembly work began on the shell in April and was complete by the end of June.

"The sculpture is completely unique in terms of what it is, what it looks like, its setting and what it means to the people of Chicago," Silva said.

"But all that being said, we'd like to do another one."

Fountain Overlooks Pool

The Crown Fountain features two glass towers rising from a reflecting pool.
Each tower is 50 ft. tall, with footprints measuring 16 by 23 ft. They soar out of a 0.25-in.-deep pool that is 42 ft. wide and 222 ft. long and are centered on a plaza created from the same granite as the pool and measuring 84 ft. wide by 276 ft. long.

Upon the towers' facades are moving images of faces of typical Chicagoans. The images are made possible through the same light-emitting-diode technology that lights scoreboards in outdoor stadiums.

Water rises through the middle of each tower, spills over all four sides and cascades down the glass bricks sheathing the LED screen.

Piping also carries water up the tower and spouts it from a gargoyle-like nozzle in the middle of each tower.

"When we filmed all the subjects' faces, we converted the video to an electronic file, and electronically manipulated the image so the mouth would come at or near this nozzle," said Roark Frankel, vice president and project manager of Chicago's U.S. Equities, a real estate company that also handles project management and development management for third parties. "So when they purse their lips, it appears water is coming from their mouths."

U.S. Equities custom engineered a solution that allowed the piping to pass through the LED screen without interfering with the display. "It was eight months worth of trial and error, testing and going through wrong turns," Frankel said.

The towers were constructed of a thin stainless steel, created in the form of a grid. The glass bricks were inserted into each opening in the grid. The front joints of the glass were then sealed with silicone to waterproof the structure. Silicone was chosen for its clarity.

"You don't really see the stainless steel grid, and you don't see the mortar," Frankel said.

Barcelona-based artist Plensa insisted that the fountain represent a cross section of the Chicago population. So U.S. Equities studied city demographics and assembled a cast that would closely reflect the city's gender, ethnic and age makeup.

All the filming was handled by School of the Art Institute faculty advisers and students. To obtain the required 1,000 images, many more than 1,000 faces were filmed, Frankel said.

The fountain's design goal was to capture and reuse as much as 97 percent of the water. At the base of the towers and in the reflecting pool are large reservoirs that collect and recirculate water.

Approximately 11,000 gallons of water are recirculated per minute, and the pool is self-leveling, with automatic adjustments taking place about every five minutes.