Architects: SOM Architects
Role: Structural glass design
Halcrow Yolles has been entrusted with renovating Chicago’s emblematic Willis Tower, formerly known as the Sears Tower; still the tallest skyscraper in the western hemisphere, 35 years after its completion.
The new Skydeck observation area – nicknamed The Ledge – features a series of all-glass observation boxes that extend 4.3 feet out from the existing tower wall.
The new glass observation boxes are comprised of 3 x 0.5 inch thick glass panes, and have no structural steel framing elements used along the face, sides or floor of the box. This innovative structure provides unobstructed views of up to 50 miles and four states, including Chicago’s cityscape and thrilling views straight down the face of the tower, which stretches 1,353ft below.
The nearly invisible support system that is responsible for these spectacular views were accomplished by hanging the boxes from cantilevered steel frames and strategically hiding structural fames behind ceilings and drywall. The visible hint of support appears as small clips on the sides or floor of the structures, which measure 12 feet long by 10 wide and have floor space of approximately 4.5 feet by 10.5 feet. The boxes are also retractable for servicing and cleaning.
Each box can withstand wind pressures up to 125 pounds per square foot and loads of up to 5 tons. The low-iron, clear glass walls and floors are fully tempered and heat-soaked for durability. In addition, they include redundant laminate for additional safety and have heated tracing elements installed along the edges to prevent ice formation.
The firm’s sound design philosophy and extensive and relevant experience working with speciality glass were important aspects in the process of successfully obtaining the essential building permit for this cutting edge project from the City of Chicago’s Materials Testing Committee.