Updated 05/29/14 - 10:33 p.m.
CHICAGO (CBS) -- Three of the four glass enclosures that extend out the side of the 103rd floor of the Willis Tower reopened Thursday afternoon, a day after cracks formed in the floor of the fourth box. Officials expected to reopen the fourth enclosure on Friday.
The cracks formed around 10 p.m. Wednesday, after a group of visitors to the Skydeck stepped out on one of the Ledge boxes.
Tony Saldana, of Crest Hill, was visiting with his cousins from California when he and three other men decided to pose for a photo on the Ledge shortly before closing time at the Skydeck. They sat, and someone took a photo.
"All of us collectively start getting up, and it's at that point that we hear and feel the glass start shattering. Mind you, this is happening in an instant. Incredibly fast," he said. "We see it completely just shatter all the way through, and at that point, all four of us just completely rushed right out of it."
Willis Tower officials said the glass itself did not crack, only the protective coating. A spokesperson said visitors never were in danger. The protective coating is designed to crack when damaged to protect the glass underneath, and has done so before after visitors went out on one of the enclosures.
"The Ledge was designed with a protective coating that completely covers all glass surfaces to protect against scratches. This coating does not affect the structural integrity of The Ledge in any way. Occasionally, the coating will crack, as it is designed to in order to protect the surface of the glass," Willis Tower spokesman Brian Rehme said in an email.
The Skydeck remained open Thursday, while crews worked to replace the protective coating that cracked. Initially, the three undamaged Ledge enclosures were open while crews examined and repaired the fourth, but late Thursday morning, Rehme said all four boxes were being closed temporarily "for routine inspection."
The other three were reopened by Thursday afternoon, and tourists continued to visit the attraction. The fourth enclosure was expected to reopen on Friday.
Ledge Surface Cracks Under Visitors' Feet
It was unclear what caused the coating to crack, but officials said it's not the first time this has happened when someone stepped out on the Ledge.
The glass boxes that form the Ledge are made from three half-inch thick layers of glass laminated together.
They can withstand up to four tons of pressure, and hold up to 10,000 pounds, according to a fact sheet from the Willis Tower.
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