News
06.16.2009
Get Vertigo!
SOM-designed observation platforms at the Sears Tower's skydeck allow visitors to walk on air
Not for the faint of heart: The Ledge at Sears Tower.
Courtesy SOM

On the occasion of the release of AN's first ever Midwest Edition, we will be posting stories from the issue all week, beginning with yesterday's Renzo Piano interview.

As a Chicago tourist attraction, the Skydeck at the 103rd floor of the Sears Tower stands second to Shedd Aquarium. “We haven’t been able to keep up,” explained Randy Stansik, the observation platform’s general manager. “Every time they add a new critter, they get people coming back, but what can we add?”


As if the glass boxes were not impressive enough, they will retract into the building to allow for window washing.
COURTESY SOM
 
The Sears Tower.
Soakologist/Wikimedia Commons
 
 

Indeed, visitors come from all over the world to survey the tower’s unimpeded, panoramic views, but once these are taken in, there’s really nothing else to see. This summer, however, in a daring game of brinksmanship, the Skydeck is upping the ante on its rival by opening The Ledge, an addition that will add yet another vantage to its mix: straight down.

Following such glass-bottomed North American predecessors as Toronto’s CN Tower and The Skywalk at the Grand Canyon, The Ledge is made up of four glass boxes that cantilever out from the west face (the only elevation that features a sheer drop from parapet to sidewalk).

Once installed, the stout-of-heart will be able to walk more than four feet “outside” of the building’s curtain wall and experience the sensation of floating 1,353 feet above Wacker Drive. “This is one view that hits you right between the eyes,” boasted Stansik.

Choosing an architect was a no-brainer for the Skydeck management. They went straight to the tower’s original design firm and structural engineer, Skidmore, Owings & Merrill (SOM). “We’ve done this on a number of our classic projects,” said SOM design partner Ross Wimer. “We’ve had a chance to revisit and try to stay consistent with the aesthetic, but also do these updates.”

In designing The Ledge, however, SOM placed a greater value on creating 21st-century transparency rather than mimicking the building’s 1970s vocabulary. The team did not even consider replicating the existing bronzed windows, and instead selected triple laminated water-white glass, which transmits 98 percent of light.

There is no visible frame, either. The roughly one-inch-thick structural glass panels, each made up of tempered lites with plastic interlayers, are joined by small, unobtrusive pinned connections. These pinned connections also join the ledges to steel frames concealed within the ceiling, which cantilever from beams at the 104th floor.

The choice of material and minimal structure do not equate to frailty, though. Tested for wind and snow loading, the ten-foot-high by ten-foot-wide by four-foot-deep glass boxes are built to code for areas of public assembly, meaning they are capable of supporting live loads of 100 pounds per square foot.

In addition to delivering a gut-wrenching experience while ensuring no one actually plummets to the ground, the designers had to contend with one other factor: the window washing system. The glass boxes promised to be a real impediment to the facade-access baskets, which drop straight down the building’s face.

In answer, SOM designed The Ledge’s modules to retract into the building. Linear chain drives in the ceiling can pull each box inside or push them out along tracks that function much like a sliding drawer’s mechanism. Pneumatic gasket systems around the boxes’ perimeters deflate when the assembly is in motion and inflate when in place to create a seal.

Aaron Seward