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Ballistic glass isn’t just for stopping bullets. It crops up in occasional architectural flourishes (such as the thick glass stairs in an Apple Store) or artful interior design. But, by far, the most audacious use of ballistic glass is in the Skydeck Ledge, a set of four boxes hanging off the 103rd floor of the one of the tallest buildings in the Western Hemisphere: Willis Tower (formerly Sears Tower) in Chicago.
Mounted just seven floors below the peak of the tower, visitors to the Skydeck Ledge stand on crystal-clear glass with an unobstructed view down to the street 1300 feet below.
(Here’s a whole mess of snapshots of folks messing around on the Ledge, or check out the video embedded below: A large percentage of visitors to the Ledge jump on it!)
GOING BALLISTIC ON THE SIDE OF A SKYSCRAPER
Each “ledge” is an enclosed box made of ballistic security glass, and can support 10,000 pounds. Those walls are single sheets of triple-layer laminated tempered glass, totaling about 4.5 inches thick (in other words, somewhat thicker than the width of your palm). Each pane weighs around 2000 pounds, and is laminated both to give it more strength (the glass sheets cannot shift against each other, and are thus much more rigid) and to prevent them from flying apart if they crack. The designers even tested the glass in place, smashing it to demonstrate that even when broken the Ledges can maintain their structural integrity.
Most such “glass floor” skyscraper views rely on a single thick sheet of thermoplastic, such as acrylic or polycarbonate. These are tough, but not great. First off, thermoplastic lacks the optical clarity of true glass. Worse, these plastic windows scuff and cloud over time, substantially affecting the impact of skywalking high above the bustling city.
Sharp eyes will note that the Skydeck Ledge actually has a double floor. This doesn’t just add security–in the highly unlikely event that more than 10,000 pounds of pressure are brought to bear on the floor and the double layers of laminate is sufficiently flawed to permit it to tear. The double-floor also gives the Ledge an aesthetic edge over similar attractions: The glass you stand on is considered a “sacrificial” layer that will periodically be replaced as it becomes scuffed or worn.
Total Security Solutions vice president Jim Richards points out that, in essence, the Skydeck Ledge is an unrated Level 8 ballistic security glass system hanging off a Chicago skyscraper. “The material–laminated ballistic glass–is great, and this is well within its capabilities. It’s tested and perfectly safe.”
That said, would he step out?
“My kids would go out there in a heartbeat. Would I go out there?” Jim hesitates. “No, but I don’t like being up that high to begin with, or being able to see down like that. No thanks. But, for that matter, look at a high-rise; that amazes me as well. When you really look at that, you see that there isn’t a lot of structural material in a high-rise, compared to the amount of glass and open space you have.”
And Jim’s right; if you want to be truly blown away, check out this “making of” video for the Skydeck Ledge: