SOM / Fazlur Rahman Khan · 1973 · Chicago
Nine bundled tubes that touched the sky
Completed in 1973 as the Sears Tower, Chicago's Willis Tower was the tallest building in the world for nearly twenty-five years. Its breakthrough was structural: engineer Fazlur Rahman Khan and architect Bruce Graham of Skidmore, Owings & Merrill conceived it as a "bundled tube" — nine square steel tubes bound together at the base, dropping away at different heights as the tower rises, so that only two continue to the 108th floor. The system braced the 442-metre building against wind with far less steel than a conventional frame, and quietly rewrote how tall buildings are built.
That stepped silhouette became one of the most recognisable in the world, and Khan's bundled-tube idea underlies supertall towers to this day. Renamed Willis Tower in 2009, it still draws crowds to the glass Skydeck ledges cantilevered over the city, 412 metres above the streets of Chicago.