The Willis Tower is the second tallest building in the United States, and its Skydeck observatory, on the 103rd floor, lets visitors step out over the city while supported only by a few layers of glass. It’s designed to be a safe, if terrifying, tourist experience—but after cracking once in 2014, the protective glass flooring on the observatory deck cracked again this week.
#skydeckchicago just broke!https://youtu.be/n-W8LY2wHBU
Posted by Jesús Pc on Monday, June 10, 2019
As CNN reports, the company in charge of the popular city attraction says that the glass “did what it was supposed to do.” What does that mean, exactly? The Skydeck floor is made of two types of glass. The main layer is the one that actually supports the weight of people. The secondary top layer—which is much thinner—is meant to shatter if something, from a set of keys or or perhaps a shoe sole with a protruding nail or a little rock stuck on it, hits it with enough force. As a Willis Tower spokesperson put it in 2014, “Occasionally this happens, but that’s because we designed it this way. Whatever happened […] is a result of the protective coating doing what’s designed to do.” Back then, the attraction’s principal engineer, Terry McDonnell, spoke with Fast Company, saying, “you could park a [10,000-pound] car on the thing.” But since they don’t make people wear disposable, scuff-proof slippers like other glass tourist attractions (like the Grand Canyon Skywalk), the engineers “added a sacrificial piece of quarter inch glass.”
In other words, while the shattering effect is scary, it’s a design feature meant to protect the structural glass below it. At the same time, according to researcher Andrew Dent of the consulting firm Material ConneXion, glass is “unpredictable” and susceptible to “catastrophic failure.” Michael Ra, a partner at glass engineering expert Front, seemed to agree: “Because glass can be damaged and failure is instantaneous.” We have reached out to Willis Tower for comment and will update this post if they respond.
But just to be safe, the next time you go to Chicago, consider asking someone to hold your hand as you step onto the Skydeck.