Photo courtesty of NBC Chicago’s Twitter, @nbcchicago
My Twitter and Facebook feeds exploded last week when the news broke (pun intended) that the glass floor cracked on one of Willis Tower’s 1,353-foot-high ledges. Heart-stopping: yes. Dangerous: no.
The headlines were sensational: “The Ledge Cracks and Shatters During Family Visit;” “Tourist tells of ‘this-cannot-be-it type of moment’ during Willis Tower incident.” And the comments were hilarious: “Chicago, Chicago! New attraction: Fall from the 103rd floor or 1,353 ft!” However, anyone familiar with glass floors knows that they are made to withstand breakage and remain structurally safe.
I called Ludek Cerny, who was the glazing project manager for The Ledge glass boxes when they were installed in 2009. Cerny explained that The Ledge’s glass floors are made up of three ½-inch lites of low-iron glass, laminated with DuPont’s Sentry Glass Plus. The laminated floor is topped with a ¼-inch sacrificial layer of fully tempered heat-soaked glass. It was the sacrificial glass lite that broke underfoot of the tourists last week.
“The sacrificial layer is there to protect the main structural glass floor against minor impacts and scratches. It did exactly what it was supposed to do,” Cerny said. “It can be replaced relatively quickly. If the glass is stocked in the building, it should only take 4-6 hours.”
A statement last week from the Skydeck Chicago echoed Cerny’s explanation. “The Ledge was designed with a protective coating that covers the glass surfaces to protect against scratches. This coating does not affect the structural integrity of The Ledge in any way. Occasionally, the protective coating will crack, as it is designed to in order to protect the surface of the glass. We are replacing the protective coating on the one affected Ledge.”
Photo courtesy of The AP
Cerny went on to explain the incredible testing that was performed on The Ledge system prior to installation. For the structural load test, “we created a Ledge box with all the fittings used in the actual construction and loaded it with steel weights, 500 pounds at a time, measuring the deflection throughout the process,” Cerny said. “Several thousand pounds of steel weights were loaded. And, when the entire load was on, we broke the top [laminated] lite. The deflection increased, but not tremendously. We then went ahead and broke the second lite. There was a little more deflection, but it still held several thousand pounds.”
At that point, the system went beyond the requirements of the structural tests, as it supported several thousand pounds of weight with two of the three laminated lites broken. However, out of curiosity, the team decided to see what would happen if the final third laminated lite was broken. “After the third lite was broken, we walked across it. We were easily supported with just the structural interlayers,” Cerny said.
After having walked on The Ledge just a few months ago, I can certainly understand why it would be an alarming experience to see and hear the glass beneath you break, while looking down more than 100 stories. However, it’s nice to be reminded of how safe and strong those systems really are.
Devlin is editor of Glass Magazine. Write her at email@example.com.