Willis Tower in Chicago was designed by architect Bruce Graham and structural engineer Fazlur Rahman Khan.
The tower, originally called the Sears Tower, was completed in 1973 for Sears, Roebuck and Company. Its bundled-tube structural system groups nine square tubes into a stable megastructure, allowing the building to rise efficiently while reducing the amount of exterior material required.
The tubes step back at different heights, creating the tower’s familiar tiered silhouette. This structural approach became one of the most influential developments in modern skyscraper engineering.
The building was officially renamed Willis Tower in 2009 after the Willis Group became a major tenant. Many people still call it the Sears Tower, but both names refer to the same Chicago skyscraper.