Part of Willis Tower's "Ledge" cracks under visitors' feet

CHICAGO - Cracks formed in the surface of the floor of one of the four glass enclosures that form the Ledge on the 103rd floor of the Willis Tower on Wednesday night, CBS Chicago reported.

The cracks formed after a group of visitors to the Skydeck at the Willis Tower stepped out on the Ledge, but 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. The three undamaged Ledge enclosures were open while crews examined and repaired the fourth.

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.