CHICAGO (CBS) — Visitors at the Willis Tower were again standing in the Skydeck’s glass enclosures on Friday, perched 1,300 feet over the sidewalk below, after crews repaired one of the boxes that was damaged late Wednesday.

Cracks formed in the protective surface of one of the enclosures around 10 p.m. Wednesday, after Tony Saldana and his cousins sat and posed for a photo on one of the four glass boxes shortly before closing time at the Skydeck. As they stood up, they noticed the surface of the floor start to crack under their feet — more than 1,300 feet above the sidewalk below.

“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.

All four glass enclosures were closed for a few hours on Thursday while crews performed an inspection. The three undamaged boxes reopened by Thursday afternoon, but the damaged one remained closed while the protective coating was replaced.

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.

Watch & Listen LIVE