Sears Building Glass Walkway

http://www.dailymail.co.uk/news/worldnews/article-1196967/Dont-look-Terrifying-view-glass-box-balcony-jutting-skyscrapers-103rd-floor.html

The glass is crystal clear and even though it's tempered, it lacks most of the greenish tint you're used to seeing in tables and other furniture made with similar material. Unlike pretty much every other "glass" floor in skyscrapers around the world, this one is actually glass, not plastic that gets scratched and scuffed so much that it's hard to tell what you're looking at. The floor of each of the four pods has an additional layer of "sacrificial glass" on it to absorb the scuffs, dings, chips, and dirt that come from millions of yearly tourists. When this layer gets sufficiently scratched, it can be removed and replaced, thus preserving the view. It's a far superior solution than is used on the glass walkway over the Grand Canyon. There, people are required to don little fabric booties to keep from scratching the bridge. The designers of The Ledge decided doing the same thing in Chicago would detract from the magic of the experience.

The way the pods seem to hover in mid-air is certainly magical, but "hover" is probably not the right word. "Hang" is better, because that's what they do. Each of the pods is a five-sided box hanging from a steel truss projecting from the ceiling out into free air. There is no reassuringly supportive steel frame to look at while taking in the views of the city below. That's all done above you, and in a way that you hardly notice it, until you're safely back inside.
Once safely back in Skydeck Chicago (formerly known as the Sears Skydeck), you can see the mechanical system in the ceiling. It allows the pods to be smoothly retracted back into the building for cleaning and maintenance. The mechanism is similar to ones used to move sets around at plays, so it's able to move very heavy objects with precision.
And the pods are heavy. Each of the sides is formed from three layers of glass bonded together. They are designed to hold at least 10,000 pounds. But even though they're mostly glass, the ledges are incredibly strong. In early testing done on the 103rd floor, the glass was deliberately broken. But the bonding material was enough to maintain the structural integrity of the pod and support its load.

A few notes:

• The ledges are on the west side of the building because that's the only side that goes all the way down to ground level uninterrupted by setbacks.
• The ledges extend four feet, four inches from the building's facade.
• The ledges were designed by Skidmore, Owings & Merrill; the same architecture firm that designed the building.
• The glass and metal engineer was MTH Industries, the same company that brought Cloud Gate to life. Halcrow Yolles was the structural engineer, and Berglund Construction was the general contractor.