Volume 44, Issue 10 - October 2009


A View Unlike Any Other

The Willis Tower’s Skydeck Opens Its All-Glass Ledge to Rave Reviews
by Megan Headley


Maybe the building owners were tired of the nose prints on the glass.

“You only need to see the forehead prints on the windows to know that visitors are constantly trying to catch a glimpse below,” says Randy Stancik, general manager of Skydeck Chicago, on the 103rd floor of the Willis (formerly Sears) Tower.

Whatever the motivation for the owners, The Ledge, a series of glass observation boxes, or bays, that extend out 4.3 feet from the west side of the building’s 103rd floor, is a new one-of-a-kind enticement for tourists.

Standing 1,450 feet and 110 stories tall, Willis Tower is the tallest building in the Western Hemisphere. Originally opened in September 1973, the landmark on Wacker Drive boasts spectacular vistas. More than 1.3 million visitors come each year to check out those views from the Skydeck. And, as Stancik notes, “Now, they have a fifth view right at their feet.”


Wide Open Views
The Ledge opened on July 2 to a flurry of coverage in the consumer press. The Chicago office of Skidmore, Owings and Merrill (SOM), the building’s original designers, provided the design plan for this engineering achievement.

“The Sears Tower set architectural and engineering standards when it was first built and now we are able to carefully craft new elements that expand the capabilities of the original design while retaining its integrity,” says Ross Wimer, design partner with SOM.

“The Sears Tower has always been about innovation,” adds Bill Baker, structural engineer partner at SOM. He explains, “Cantilevering out from the side of America’s tallest building, the viewing platform allows visitors to see the incredible city of Chicago literally beneath their feet.”

Halcrow Yolles, the engineers of record for the observation boxes, fully designed and detailed all the glass and steel components. According to John Kooymans, senior principal and structural glass engineer, “People like the thrill of being able to look straight down from so high up—the clue lay in the hundreds of forehead prints left by visitors each week on the windows.”

Upon first hearing about this project, Ludek Cerny, vice president of glazing contractor MTH Industries in Hillside, Ill., “thought it was pretty unusual.”

Once MTH signed onto this project, its glaziers began to learn just how extraordinary this project would be.

“Everything was out of the ordinary,” Cerny says. Because of that, MTH wound up taking on a design-assist role.

How They Say
“The Ledge” Down Under

As John Kooymans, Halcrow Yolles’ senior principal and structural glass engineer, notes, “The concept [for The Ledge] was partially inspired by the glass floors at the Grand Canyon and Toronto’s CN Tower.” But the Willis Tower’s Ledge isn’t the first such glass box to be installed.

Melbourne, Australia’s Eureka Tower opened in 2006 as the world’s tallest residential tower. Its façade consists of approximately 430,560 square feet of glass and aluminum, and the top 11 floors feature 24-carat-gold infused glass. The Skydeck is situated on the 88th floor and is the highest public vantage point, at 935 feet, in the Southern Hemisphere.

On the Eureka Skydeck, visitors will find the Edge, a glass cube that moves with passengers onboard. The Edge was designed by Dick Baird, consulting engineer and chairperson on the Australian Standards Committee for amusement devices, and built by G&G Engineering of Melton, Victoria. The Edge features a switchable glass substrate provided by iGlass of Ballarat, Victoria, with glass manufactured and laminated by Pilkington. It features 2.2 tons of glass, 45 millimeters (approximately 1.7 inches) thick, reinforced by steel framework. The cube is designed to hold at least 11 tons.

In addition to the differences in aesthetics, the Edge functions a bit differently than Chicago’s Ledge. Once visitors snap onto their shoes protective booties to prevent scratching the glass, up to twelve people can step into the cube—while its walls are fully opaque. The cube moves outward at a pace of one meter every 10 seconds. Once fully extended, the cube’s glass floor changes from opaque to clear, followed by the other surfaces.

Fabrication Facts
Each of the Ledge’s glass boxes are comprised of three tempered ½-inch-thick lites of Pittsburgh-based PPG’s Starphire low-iron glass. The walls and roof are laminated with clear PVB supplied by Solutia in St. Louis, while the floors were laminated with Wilmington, Del.-based DuPont’s™ SentryGlas® Plus interlayer. Representatives of the glass fabricator, Prelco of Rivière-du-Loup, Québec, note that the wall lites are approximately 60 by 126 inches, while the floor and roof panels measure approximately 48 by 120 inches; each laminated unit is 15/8 inches thick.

Prelco began its involvement on the project in November 2008. “After cutting the glass, every lite went to one of our CNC edging machine for polishing the edges, making the notches and drilling the holes,” explains Christian Fournier, project manager for Prelco. “Then the glass was tempered and heat-soak tested. After a successful heat-soak test, the glass was laminated. Every lite went through a more-than-usual inspection before being sent to be crated for shipment.”

Heat-soak tests to detect nickel sulfide inclusions were performed to avoid thermal breakage.

Yet, for such an unusual project, Prelco director of marketing Bill Marchitello says that the fabrication was business as usual. “We already had the equipment to handle such thick and heavy glass that is laminated with the specific detailing of holes and edgework,” Marchitello says.
“We have been doing structure walls or spider walls that must be produced with tolerances that are much less than the norms allowed for holes, sizes, etc.”


Strength in Standards
According to information from Halcrow Yolles, the thick lites were specifically designed, fabricated and tested to meet multiple international code standards. Each box can withstand design wind pressures of 125 pounds per square foot and loads of up to 5 tons—3 tons more than Chicago’s human occupancy load requirements. Or as Cerny puts it, it can essentially hold more people than it can fit.

Load tests done in-house by MTH involved loading a glass lite that was half the size of the actual floor of the bays to 2½ times the required code load for a 24-hour duration.

“The test was later repeated with fracturing one of the lites with the actual design load,” Cerny says. Evidently that wasn’t enough for this team. “Out of curiosity,” Cerny says, “we actually went as far as breaking more lites and realized that you could still stand on this with the all of the lites broken.”

Whether or not that’s a comforting thought for visitors, the tests show that the bays are, in fact, more than secure. In addition to damage from breakage, the team installed some ways of protecting the bays from daily wear as well.

“Most people don’t realize there is anti-graffiti film on the inside of these boxes,” Cerny says.

In addition, he adds, “The floor itself has a ¼-inch sacrificial layer of fully tempered, heat-soaked glass on it that can be removed or replaced if it gets scratched, cracked or damaged.”

Visitors to the Grand Canyon’s Skywalk will recall sitting to pull booties over their shoes before proceeding onto the glass; the booties keep the glass floor free of scratches. The owners of the Skydeck didn’t want the same hassle for its crowds of visitors. “They want to make sure people can walk in there with their shoes on, but obviously that means you’re going to damage the floor glass,” Cerny says.

The sacrificial layer is one more bit of evidence that the view needed to be completely unencumbered—by scuffs, scratches and certainly by frames and support. According to Kooymans, “Halcrow Yolles engineers took [the SOM design] one step further by eliminating all perimeter structural steel at the sides and along the floor of the glass enclosures and created a near-invisible support system. This was accomplished by hanging the boxes from cantilevered steel frames and hiding structural frames behind ceilings and drywall. In fact, the only visible hint of support appears as small clips on the sides or floor.”

Cerny says that those small, stainless steel fasteners were all custom-machined by MTH. “We did that, which is kind of unusual,” he says. “Usually when you do a job like this you try to use what’s available. In this particular case, everything was very custom.”

Perhaps more unusual yet is the fact that these seemingly unsupported all-glass boxes are retractable.


On the Move
Cerny explains that the observation bays move into three different positions. “[There’s] the viewing position, which people typically see … There’s a position that’s referred to as a window washing position, where the face of the glass is approximately flush with the face of the building; the building has an automatic window washing machine that goes up and down and has to clear it. And there’s a third position, called a maintenance and inspection position, which brings the face of the glass about six feet inward of the face of the building.”

The motorized system that projects and retracts the boxes from the building utilizes steel LinearBeam mechanical linear actuator systems. The systems operate with the company’s patented rigid chain technology. The rigid chain is a mechanical actuator that is flexible in one direction and forms a steel beam in the other direction. MTH worked with the supplier to design the locking pins and the control systems that secure the bays.

“We ended up designing … roller bearings, wheels and assemblies—all manufactured custom,” Cerny says.

Because of the movement, the perimeters of the bays are lined with inflatable seals supplied by Weiler Rubber Technologies LLC in Chicago. When the bay is in the viewing or maintenance position, the seals inflate to create a secure air and water lock for the building.

Movement proved to be one of the bigger installation challenges for MTH—although not the movement of the bays, per se.

Cerny explains, “One of the biggest challenges you have is you’re installing these boxes that are, let’s say, 10 by 10 by 4 feet, the frame itself is twice that size and you’re trying to get this material up 103 floors—and there are no elevators made big enough for this.”

As a result, the installers moved the glass and 18-foot suspension beams up more than 100 floors on the tops of elevator cars.

“The material moving was definitely a major challenge. We’re talking about a 1,200- to 1,500-pound piece of glass. Just rotating it in the shop is easy, but when you get onto a jobsite it’s a little bit more difficult,” Cerny says.

To ease the material handling, MTH wound up creating custom tools to help hoist and carry. “We pretty much built everything custom,” Cerny says. “Everything” involved a range from tip cars for carrying materials to jacking and hoisting frames inside of elevators—everything except for the necessary vacuum cups. “It was all a conglomeration of things that already existed modified to work under these conditions in the space allowed,” Cerny adds.


Walking on the Ledge
The Skydeck is open daily September-March from 9 a.m.-10 p.m.
and April-October from 10 a.m.-8 p.m. If you make your way to The Ledge, share your trip photos with USGlass by e-mailing mheadley@glass.com. For more information visit www.theskydeck.com.

Looking Back
Despite the challenges, it seemed that the team had completed its project before they could say “Hold that door.”

“This was a very fast track job,” Cerny says. Prelco delivered its last panel in April 2009, six months after the company began fabrication.

While this highly visible portion of the landmark tower may have moved quickly, the Tower’s owners are still looking at further renovations; in June it was announced that the Tower would undergo a $350 million green renovation. Among the planned changes are efficiency improvements to the building’s 16,000 single-pane windows. Strategies to achieve a thermal break of the curtainwall also are being investigated.

“We’re working with the ownership on details and hope to have some, maybe a lot, of involvement, but this is a very large project,” Cerny says.

While work on The Ledge may have been over quickly, the attention seems certain to last for some time. In fact, Cerny says, “The thing that surprised me most is the amount of publicity this is getting and the success in attendance.”

Shortly after completing the project, Cerny returned to see the finished product as it was meant to be enjoyed. “It’s amazing when you see the place literally jam-packed with lines standing the whole width of the building,” Cerny says. “The most fun part is watching the kids’ reactions. Kids will jump on and put their faces on the glass and everything else and look—and half the parents stand about two feet away.”

Megan Headley is the editor of USGlass

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