“It’s a glass ledge, 1 1/2 inches thick and poking out about four feet from the 103rd floor of the Sears Tower. There is no frame under the floor, only air — 1,353 feet of it, straight down to the miniature taxis on Wacker Drive.” The trick to it “is an intriguing feat of engineering, a team of designers and builders said Wednesday, swearing on a stack of liability policies as they unveiled the project. “