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The Ledge extends out of Willis Tower more than four feet and 103 stories above Chicago. (photo credit: Skydeck Chicago)
Imagine you’re standing on a glass ledge that’s situated 1,353 feet in the air. The views are amazing and then you hear it. Crack! Having a piece of glass break below your feet so high up would make anyone’s heart skip a beat. Thank goodness for a sacrificial lite. That was the news yesterday when the sacrificial lite at Chicago’s Willis Towers’ Ledge swept the internet. This isn’t the first time it’s happened at The Ledge. A sacrificial lite in the same glass box broke in May 2014, and both times it did what it was supposed to do. Lyle Hill, who ran the glazing contracting company that handled the original glazing installation, tells USGNN™ that the glass breaking isn’t a big deal. That’s because the sacrificial lite is installed on top of the structural glazing to protect it from damage.
“You don’t want to damage the expensive, hard-to-replace piece of glass that serves as the floor of the boxes. Replacing it would be costly and a major undertaking because it’s made of a custom piece of laminated glass with strengthened interlayers. It would cost a few thousand dollars minimum to replace. So it’s designed with sacrificial glass on top,” he says.
Historically, Hill says these types of projects include film in between the sacrificial lite and the expensive structural glass underneath to keep any broken pieces of the sacrificial lite from scratching the glass underneath.
“The whole thing cleans up and is replaced very quickly. Historically, we had extra pieces of sacrificial glass onsite because these do break or get scratched up from people walking on them, and so they would be replaced every so often whether they broke or not,” says Hill. “We used to pre-film them and had four to five of these things in the building. It’s not a big deal and the lites would be replaced in a matter of minutes.”
Hill explains that the typical cause of a sacrificial lite breaking is someone dropping something on it or stepping on the glass with a sharp stone or something similar stuck on the bottom of a shoe.
Background
The Ledge’s four glass boxes extend 4.3 feet out from the Willis Tower on the 103rd floor, providing unparalleled views of Chicago. The attraction opened in 2009.
Hill says that Chicago officials had engineers create a test to ensure that the boxes would be safe for visitors.
“We loaded 3,000 pounds of steel into the box and then broke one of the real glazed lites, not the sacrificial layer, to see what would happen. It deflected about an 1/8 of an inch but held together,” he says.
The project team then had to remove the steel, break another layer and then put all of the steel back into the box.
“It did deflect more but it didn’t fall through,” he continues. “I think at that point they thought that it’s holding up 3,000 pounds of steel with two broken lites and not going anywhere, so they decided it was good.”
Hill emphasizes that when projects such as the Ledge are designed, the engineering is thorough and based on statistical probability and anticipated performance with safety in mind.
“You’re confident it’s going to work,” he says.
The Associated Press has reported that the sacrificial lite has already been replaced and the Ledge has reopened.
