An enduring Chicago landmark

For decades the tallest building in the world, the 110-story Willis Tower (formerly Sears Tower) marked a major step in exemplifying and defining SOM’s belief that a building’s structure should inform its exterior profile. The 4.5-million-square-foot tower was the headquarters for Sears, Roebuck & Co. from the time of its completion in 1973 until 1995, when the merchandiser relocated to suburban Chicago. The building’s design, flexible floor plan, and iconic stature have made it a premier address for a host of companies. Today, the tower is well-maintained and remains a center of activity in Chicago’s Loop.

Pragmatic engineering

The iconic setback design of the structure was conceived in response to the original client’s space requirements: Sears wanted a building that incorporated not only large office floors, necessary for the company’s operations, but also a variety of smaller floors for tenants requiring less floor area.

Pioneering the use of bundled tube construction, the basic structure consists of nine 75-foot-by-75-foot column-free square tubes at the base, forming a cellular-tube frame. The nine bundled structural tubes, each rigid within itself without internal supports, rest on reinforced concrete caissons that go down to bedrock. The tubes are bundled together as a closed square up to the first 50 stories, and then terminate at varying heights to create the tower’s distinctive, multi-tiered form.

The structural steel frame was pre-assembled in sections and then bolted into place on site. The lightweight building skin, a black aluminum and bronze-tinted glare-reducing glass, serves as an insulator between the interior and exterior structure in order to maintain a relatively constant temperature, which minimizes the expansion and contraction of the frame.

Above the clouds

In 2009, the Ledge at Willis Tower was created to enhance the building with an innovative new observation deck on the 103rd floor. In 2021, SOM continued its 50-year stewardship of Willis Tower with a transformative renovation of the Ledge. Interior enhancements, improved lighting, building technology upgrades and a new exhibition space further immerse visitors in Chicago’s historic skyline.