Tourists taking photos in the glass ledges

of Chicago's Willis Tower, which hang 103 stories above the street below, got a hell of scare this week when it looked like the transparent floor beneath them was about to collapse. A protective coating covering the 1.5-inch-thick, 1500-pound glass panel cracked, leading those standing on the The Ledge at the time to believe the entire glass structure was breaking.

The visitors to the observation deck were in no danger. Fortunately for them, the web of cracking extended only through the 1/8-inch-thick protective coating, not the glass itself. But the playful fear of heights people experience on the 103rd floor momentarily turned to real terror for a few people, and made us wonder: Just how much stress can glass support when it's hovering 1353 feet above the city? Turns out, unless a pair of white rhinos decide to take a Windy City vacation, The Ledge is going to be fine—it's even back open for business today.

Skidmore, Owings, and Merrill, the architecture firm behind the building formerly known as the Sears Tower, designed The Ledge with four fully enclosed clear boxes that can retract into the building, allowing easy access for cleaning and maintenance—something now needed. Halcrow Yolles designed and detailed all the glass, which is comprised of three half-inch layers laminated into one seamless piece of low-iron glass that's tempered for durability. It's held in place with steel to easily support five tons, but the engineers want to preserve the freakiness of standing inside a clear box. So they tweaked the original design and eliminated steel that would be visible at the sides and along the floor, creating a near-invisible support system that's safe but also allows for the acrophobic illusion of standing above the city.

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Ledge of Willis Tower's Skydeck cracks during tourists' visithttp://t.co/hxMOHoOwkq pic.twitter.com/tj2fZGSPQG — BJ (@bjlutz) May 29, 2014

To protect the clear visuals of Wacker Drive and the Chicago River below, crews installed the protective coating to guard against scratches. If punctured, which can happen occasionally, the coating is designed to crack but stay in place, like windshield glass does, and still protect the glass. The cracking might have been scary, but it works, Willis Tower spokesman Bill Utter says.

"The protective coating on the glass cracked in one section, but the glass itself did not," reads a Willis Tower statement. "This coating does not affect the structural integrity in any way."

Still, we'd understand if the people who were on The Ledge when it cracked want to stay on ground level for a while.

Tim Newcomb covers infrastructure and sports for Popular Mechanics. Follow him on Twitter at @tdnewcomb.

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