What structural system is used in the Sears Tower?

The Sears Tower, now called Willis Tower, uses a bundled-tube structural system.

Structural engineer Fazlur Rahman Khan and architect Bruce Graham of Skidmore, Owings & Merrill developed the system for the Chicago skyscraper. Instead of treating the building as one enormous hollow tube, the design ties together nine smaller square tubes, allowing the structure to resist wind efficiently while supporting large office floors.

The tubes terminate at different heights, creating the tower’s famous setbacks and stepped profile. Two tubes continue to the full height, while others stop at lower levels. This arrangement reduced the amount of steel needed compared with more conventional tall-building systems and made the unprecedented scale economically practical.

A tube-in-tube system is a different arrangement, pairing an exterior structural tube with a strong interior core. A diagrid relies on diagonal members, while a space frame is a three-dimensional framework; neither is the canonical description of the Sears Tower’s primary system. The tower was completed in 1973 and renamed Willis Tower in 2009.