For years the tallest building in the world, the 110-story Willis Tower (formerly Sears Tower) marked a major step toward exemplifying and defining SOM's belief that a building’s structure should naturally inform its exterior profile. The tower's structure comprises nine squared tubes, each rigid within itself without internal supports. The tubes are bundled together as a closed square below the first 50 stories. Above the first 50 stories, the tubes terminate at varying heights and create a 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.

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 enabled it to become a premier address for a host of Chicago companies and firms. Today, the tower is well-maintained and remains a center of activity in Chicago’s Loop.