The tower's sway has been known to induce motion sickness in people working at the top.
Sears executives chose to build a single tall headquarters building over an alternative proposal for twin towers at half the existing tower's height.
The building's facade has been climbed twice: by Dan Goodwin in 1981 and by Alain Robert in 1999.
This is not the original Sears Tower; the previous headquarters of Sears Roebuck was a vast building on Chicago's West Side that included the Sears Merchandise Building Tower.
The structure is formed from 9 bundled square tubes, each 75 feet wide with no columns between the core and perimeter. Two of the tubes are 50 floors high, two are 66 floors, three are 90, and two are 108.
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