News: cracked protective coating on the Willis Tower's glass viewing deck, 410 metres above ground level, leads to temporary closure.

According to local reports, all four of the glass box viewing areas of The Ledge attraction on the 103rd floor of the 440 metre tall Willis Tower skyscraper were temporarily closed yesterday morning for a "routine inspection", following reports of cracking glass.

Tourist Alejandro Garibay told NBC Chicago that he had been in one of the boxes on the west side of the building with his brother and cousins when they heard cracking. Images showed a floor panel that appeared to have shattered.

A spokesperson for the building said that there was no danger, and that the cracked surface was a layer of protective coating over the structural glass designed to prevent the glass from damage and scratching.

"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," said Brian Rehme, a spokesman from public relations firm FleishmanHillard.

By 4pm Chicago time, the attraction's operators had announced that the protective coating on box four of the ledge had been replaced. It is due to re-open later today.

The Ledge was designed by Skidmore, Owings & Merrill with structural glass designers Halcrow Yolles and opened in 2009 as an addition to the tower's existing 1974 sky deck.

Made from three layers of 12mm thick tempered glass laminated together, each panel used in the boxes weighs over 680 kilos. The boxes can retract into the  building for cleaning and maintenance.

Glass viewing platforms have become increasingly popular as tourist attractions on tall building projects.

Earlier this month, the owners of the observation deck at Chicago's Hancock Building opened a new addition to rival Willis Tower's Ledge – a glass box that tilts visitors forwards at a 30 degree angle to experience a bird's eye view of the city more than 300 metres below.

Called The Tilt, the box can accommodate up to eight people and is located on the building's 94th floor. It is operated using three hydraulic lifts.

Paris-based Montparnasse Group 56 bought the John Hancock Observatory in 2012 for $44.2 million. It operates as a separate business from the rest of the spaces in the 100-storey tower, which was also designed by Skidmore, Owings & Merrill and completed in 1969.

"It's no secret that observation decks as a business are very profitable, as opposed to leasing square footage in the building," Daniel Thomas, executive director of the World Federation of Great Towers and a former general manager of the Hancock Observatory told Crain's when plans for The Tilt were revealed.

Thomas estimated that the Observatory was making over $10 million a year from ticket sales, while the Willis Tower might be making as much as $25 million.

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