As Unbreakable as ... Glass?

CHICAGO — To truly appreciate how glass can be used structurally, make your way to 233 South Wacker Drive in downtown Chicago. More precisely, make your way 1,353 feet above South Wacker, to the 103rd floor of the Sears Tower.

Once there, take a few steps over to the west wall, where the facade has been cut away. Then take one more step, over the edge.

You’ll find yourself on a floor of glass, suspended over the sidewalk a quarter-mile below. If you can’t bear looking straight down past your feet, shift your gaze out or up — the walls are glass, too, as is the ceiling. You’ve stepped into a transparent box, one of four that jut four and a half feet from the tower, hanging from cantilevered steel beams above your head. The glass walls are connected to the beams, and to the glass floor, with stainless-steel bolts. But what’s really saving you from oblivion is the glass itself.

The boxes, which opened last week as part of an extensive renovation of the tower’s observation deck, are among the most recent, and more outlandish, projects that use glass as load-bearing elements. But all glass structures have at least a bit of daring about them, as if they are giving a defiant answer to the question: You can’t do that with glass, can you?