Willis Tower (formerly Sears Tower) – Structural Engineering

Willis Tower (formerly Sears Tower) consists of a structural steel frame that was pre-assembled in sections and then bolted in place on the site. The lightweight building skin — a black aluminum and bronze-tinted glare-reducing glass curtain wall — serves as an insulator between the interior and exterior structure to maintain a relatively constant temperature, in turn minimizing the expansion and contraction of the frame.

Structurally, the building pioneered the use of bundled tube construction. The tower is composed of nine bundled structural tubes resting on reinforced concrete caissons that go down to bedrock. The caissons are tied together by a reinforced concrete mat.

The iconic setback design of the structure was conceived as a direct result of the client's space requirements. The designers were required to develop a building that incorporated not only very large office floors, which were necessary for the company's operations, but also a variety of smaller floors for tenants requiring less floor area. The basic structure developed for this program of the tower consists of nine 75-foot-by-75-foot column-free square tubes at the base, forming a cellular-tube frame. Floor sizes were reduced by eliminating 75-foot-by-75-foot increments at varying levels.