in front of their eyes. they feared the ledge was about to fall to the ground. because you can see through it obviously, you have the psychological effect that, you know, oh, my gosh, something broke. nobody was ever in danger. reporter: the engineer, michael swanberg, says there s a thick panel of glass that didn t break. designed to withstand 10,000 pounds, or 5 tons of weight. we watched as crews replaced the thin layer above it that caused such a scare. so this is the new sheet of glass that will go on the sky deck. if you look here you can see exactly how thin that is. crewing tell me this is meant to be scratched and scraped up. in fact, it s replaced every six to nine months, depending wear and tear. from outside, at a helicopter s vantage point, you can see how the observation ledge extends just beyond the willis tower. with the new glass panel finally in place. you put it to the test. so we are stepping on to that thin sheet of glass now. that s correct.
that didn t break. designed to withstand 10,000 pounds or 5 tons of weight. we watched as crews replaced the thin layer above it that caused such a scare. so this is the new sheet of glass that will go on the sky deck. if you look here you can see exactly how thin that is. crews tell me that this is meant to be scratched and scraped up. in fact, it s replaced every six to nine months depending on wear and tear. from outside, at a helicopter s vantage point, you can see how the observation ledge extends just beyond the willis tower with the new glass panel finally in place, we put it to the test. so we are stepping on to that thin sheet of glass now. that s correct. the quarter inch top piece of glass is designed to protect the 1 1/2-inch thick structural glass. oh, my god, take the picture. reporter: the ledge on the sky deck now back in business. and even these guys admit after the scare of their lives for the record, it s an