Photo by Alejandro Faribay

Protective glass pane shattered, frightening sightseers, buy underlying structure remained intact.

A group of tourists won't soon forget it, but a glass pane that shattered beneath them on a ledge atop Chicago's Willis Tower, the nation's second-tallest structure, posed no structural damage to the all-glass attraction, according to its fabricator and installer.

Designed "to prevent structural glass beneath it from being scratched, [the pane] allows visitors to walk out on the ledge in shoes rather than booties," says Mike Swanberg, president of MTH Industries, which also performs maintenance on the Skydeck on Willis' 103rd floor. The bay with the cracks is one of four that project 4.3 ft from the deck.

The unit, made of ¼-in.-thick low-iron tempered glass, shattered in late May, causing people on the ledge to believe it was collapsing. The cause is unclear, but Swanberg speculates a tourist dropped a camera on the unit. "When you fracture its surface it shatters," he says. "An underlying clear vinyl film prevents shards from scratching the structural glass. It's not unlike safety glass found on windshields." A less brittle assembly such as polycarbonate would "scratch up very quickly and obscure views," Swanberg adds.

MTH replaces units every six to nine months as a result of scratches, a task that requires an hour per unit. Units have shattered "two or three times" since ledges were built in 2009, says Swanberg.

The ledges, situated 1,353 ft above grade, are constructed of three layers of ½-in. tempered glass and designed to withstand loads of five tons. About 1.5 million visitors per year visit the Skydeck.

All four ledges were closed for a day while inspectors evaluated them and workers replaced the shattered unit.