Willis Tower

Completed in 1973, the Willis Tower was commissioned by Sears, Roebuck & Co., then the world's largest retailer, to consolidate its thousands of employees into a single headquarters. Designed by the architectural firm Skidmore, Owings & Merrill, the project was led by architect Bruce Graham and structural engineer Fazlur Rahman Khan. At the time of its completion, it surpassed the World Trade Center in New York City to become the tallest building in the world, a title it held for 25 years until 1998. The tower is famous for its revolutionary 'bundled tube' structural system, an innovation by Fazlur Rahman Khan that allowed for greater heights with less steel. The design consists of nine square tubes of varying heights, bundled together to provide immense rigidity against Chicago's powerful winds. This architectural breakthrough not only defined the building's distinct stepped silhouette but also paved the way for the supertall skyscrapers of the modern era, including the Burj Khalifa. In 2009, the London-based insurance broker Willis Group Holdings leased a significant portion of the building and obtained the naming rights, leading to its official name change from the Sears Tower to the Willis Tower. Despite the name change, it remains a beloved symbol of Chicago's industrial might and architectural heritage. Today, it attracts over 1.7 million visitors annually to its Skydeck, which features 'The Ledge'—glass-bottomed balconies that extend out from the 103rd floor, offering a dizzying view of the city below.