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Sky Ledges Open Atop Sears Tower

To make sure, workers used a center punch to shatter first the top layer of the glass, then all three layers, Cerny said. The floor held. As installed, each floor is designed to hold several times more weight than would ever be placed on it.

"There's no problem with putting 5,000 or 10,000 pounds without anything happening," Cerny said. "It's been tested."

Then there is the mechanical part. Engineers had to figure how to clean the glass for paying customers and, just as important, allow a clear path for the building's window-washing platforms, which drop from cables anchored on the roof, seven floors above.

So, using equipment designed to move theater sets, they made the ledges retractable. The boxes can be hauled into the openings in the 103rd floor to allow the window washers to do their work, then are rolled back out.

Wimer, who typically designs airports and skyscrapers, was intrigued by the project. Working with colleagues at Skidmore, Owings and Merrill, the first instinct was a mesh floor and sides so visitors could "step out there and feel the wind."

That proved impractical -- and potentially painful in a city with merciless winters. Mesh would also impede the views. Wimer was seeking the sensation of "floating in space," as well as something that was not, as Prince Charles once said of a proposed London gallery extension, "a monstrous carbuncle on the face of a much-loved and elegant friend."

"How do we do this in a way that's consistent with the logic of the building?" Wimer said designers asked. "We didn't want this to be an amusement park ride, but simple, elegant and natural, so it felt it belonged in the Sears Tower."

One reason for designing glass ledges just four feet deep was to prevent more people from crowding onto them. Not for safety reasons, Wimer said, but to preserve an open feeling easier to establish if "they're not standing two-deep."

As the designers pulled back a curtain on a cloudy Wednesday morning and reporters moved gingerly toward the ledges, a television reporter said to his cameraman: "I'm not going to get out there. I'll get close."

"Close," it turned out, was a foot away. He extended his microphone to interview 11-year-old Isaac Moldofsky, who stood without fear on the glass and pronounced the experience "pretty cool."

Later, a visitor named Viesha Arbes was having none of it. In dressy clothes and heels, she lowered herself indelicately to hands and knees and crawled toward the opening. When she caught her first glimpse of the street 103 floors below, she gasped and lunged away.

"I don't think I have that much confidence," said Arbes, visiting from Raleigh, N.C. "It feels too real."

Reality worked for her 12-year-old daughter, Sarah.

"It's scary like no other. It's like you're floating," she said, long after Wimer had departed. After 20 minutes of coaxing, Sarah led her mother backward onto the ledge to pose for a photo. Her mother never looked down.

Staff writer Kari Lydersen contributed to this report.

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