The Willis Tower, still the Sears Tower to Chicago
It was the tallest building in the world for about 25 years, and the stepped shape that makes it recognizable from the lake is a direct readout of how it holds itself up.
The numbers
The Willis Tower is 442 m (1450 ft) tall with 108 floors, at 233 South Wacker Drive in Chicago. It was completed in 1973. Bruce Graham designed it and Fazlur Rahman Khan engineered it, both at Skidmore, Owings & Merrill, the firm that later designed the Burj Khalifa. Two antennas rise above the roof; they are taller than the building's official height but do not count toward it, which turns out to matter a great deal later.
Nine tubes in a square
Khan's structure is a bundled tube. Instead of one hollow tube resisting the wind, the tower is nine square tubes, each 75 ft on a side, set in a three-by-three grid and tied together so they act as one stiff unit. Each tube carries its own share of the wind load, and because they brace each other, none has to be as heavy as a single tube of the same height would be.
The useful part is that the tubes do not all have to go the same distance. Two of the nine stop at the 50th floor, two more at the 66th, three at the 90th, and two continue to the top. That is why the profile steps back twice on the way up: every setback is a pair of tubes ending. The shape is the structure, visible from outside.
Khan's system is also what made the height affordable. A tall steel frame spends most of its steel fighting wind rather than carrying floors, and the bill grows faster than the building. Bundling the tubes cut the steel per square foot enough that Sears could justify the top of the tower at all. The same reasoning had already produced the braced tube of the John Hancock Center a few blocks north, and thequestion of how much a tower movesis the one these systems exist to answer.
Sears built it, Willis renamed it
Sears, Roebuck and Company commissioned the tower as a headquarters for a retail business that was then the largest in the world, with the idea that its buyers and merchandisers would fill the wide lower floors. They never filled it. Sears began leasing space out almost immediately, moved its merchandise group to suburban Hoffman Estates, and left the building entirely in 1992.
The name lasted longer than the tenant. In 2009 the London insurance broker Willis Group Holdings took about 140,000 square feet and negotiated the naming rights as part of the lease, so the Sears Tower became the Willis Tower. Chicago mostly declined to go along. Twenty years on, cab drivers, radio traffic reports and locals giving directions still say Sears Tower, and people searching for the building still type it that way.
Twenty-five years as the tallest
On completion in 1973 the tower passed the World Trade Center in New York and became the tallest building in the world. It held the title until 1998, a run beaten only by the Empire State Building, and it was the tallest in the United States for even longer.
How it lost the record still annoys people in Chicago. The Petronas Towers in Kuala Lumpur took the title in 1998 on architectural height, which includes spires but excludes antennas. Petronas has a lower roof, fewer floors and a lower highest occupied floor than the Willis Tower. Its spires are structural and counted; the Willis Tower's antennas are equipment and did not. Change the rule and the record would have stayed in Chicago, which is why the measuring convention gets argued about as hard as the buildings do. The full chain of record holdersshows how often that definition has decided the answer.
The Skydeck and The Ledge
The Skydeck is on floor 103, at 412 m (1,353 ft), and it is the reason most visitors come. On a clear day the view reaches into Indiana, Michigan and Wisconsin, though the honest version is that genuinely clear days over Lake Michigan are not common.
The Ledge opened in July 2009: four glass boxes that cantilever about 1.3 m (4.3 ft) out from the facade, with glass floors, so you stand outside the building line with 412 m of air below your shoes. Each box is laminated glass in three layers and engineered for loads far beyond anything a crowd of tourists applies. The glass has spooked visitors once or twice when a protective outer coating cracked underfoot, which is a surface layer rather than the structural pane, and the boxes reopened after inspection.
Where it ranks now
The Willis Tower is still the tallest building in Chicago and the third tallest in the United States, behind One World Trade Center and Central Park Tower in New York. It has not been the tallest in the world since 1998, and the gap has grown; the current record holder is nearly twice its height. Chicago's own skyline has filled in around it without passing it, and the city's tall buildings are mostly the reason the Willis Tower has company rather than competition.