Sky Ledges Open Atop Sears Tower
Thursday, July 2, 2009
CHICAGO, July 1 -- Don't look down. Or do, since that's the idea. But brace for vertigo. In the city of big shoulders, this is like standing on an eyelash.
It's a glass ledge, 1 1/2 inches thick and poking out about four feet from the 103rd floor of the Sears Tower. There is no frame under the floor, only air -- 1,353 feet of it, straight down to the miniature taxis on Wacker Drive.
Picture Wile E. Coyote racing off the cliff. Think of the moment when he suddenly looks down. Only you don't actually fall.
The reason is an intriguing feat of engineering, a team of designers and builders said Wednesday, swearing on a stack of liability policies as they unveiled the project. The ledge -- actually four identical glass boxes suspended near the top of the nation's tallest building -- opens to the intrepid Thursday.
The natural instinct is to inch out onto the glass very, very slowly, said sheet metal worker Leo Thier, who took a break from another job to venture into the box. Still in his hard hat and construction boots, he delivered his verdict: "It's fantastic. It's insane."
"I've never seen a helicopter from that view: eye level," he said as a news chopper drifted in for a close-up. "See, now all the big wheels in this whole place are going to want [a ledge] in their office."
The ledges, created off the tower's enclosed Skydeck, are hardly likely to be duplicated elsewhere in the 36-year-old building. Knocking out chunks of steel and glass to install the four structures, with the wind whipping and the clock ticking, was cost and hassle enough.
If the wind is blowing at 20 mph on the ground, it is blowing two or three times harder 1,353 feet up, said construction chief Lou Cerny of MTH Industries. The building itself moves with the wind on a normal day, creating conundrums for installers seeking precision within a 16th of an inch.
"It's probably swaying seven to eight inches while we're standing here," Cerny said. "So the opening changes. You measure and you get a different dimension and you scratch your head."
The glass boxes look like square portholes on the west side of the tower -- or, from a great distance, dimpled chads on a ballot. From the Skydeck, which draws 1.3 million visitors a year, one steps onto the glass through openings 10 feet wide and 10 feet high.
Thick panels of glass are bolted to a steel frame at the top of each box. The 1,500-pound floor, which connects to the vertical pieces on three sides, is made of three sheets of glass layered with a special invisible resin called polyvinyl butyral, or PVB, the same stuff used in car windshields.
"Think of it like bulletproof glass. Even if the glass were to break, which it's unlikely to do, it stays inside the frame. It would never fall out," said Ross Wimer, the project's lead architect.