Willis Tower, formerly (1973–2009) Sears Tower, skyscraper office
building in Chicago, Illinois, that is one of the
world’s tallest buildings. The Sears Tower opened to tenants in 1973, though
construction was not actually completed until 1974. Built for Sears,
Roebuck and Company, the structure reaches 110 floors and a height of 1,450
feet (442 meters), excluding broadcast antennas and their supports. (See Researcher’s
Note: Height of the Willis [Sears] Tower) The architectural firm of Skidmore, Owings & Merrill was responsible for the
design and construction of the tower; Bruce Graham served
as architect and Fazlur Khanas structural engineer.
Welded steel frames form vertical tubes
that provide the rigidity needed to limit the lateral sway from wind forces.
This system minimizes the amount of structural steel required for a building of
its great height. The steel was prefabricated, with nearly all welding done off
the erection site and bolt connections made at the site.
The building is modular in plan, with nine 75-foot-square, column-free
units. These nine square units compose a 225-foot-square base. At the 50th
floor two diagonally opposite units stop, forming the first step back. The
second step back is at the 66th floor with the other two diagonal units stopping
and the last at the 90th floor with three units stopping, leaving an upper
tower of 20 stories.
The exterior is sheathed in black aluminum and bronze-tinted glass.
Black bands appear around the building at the 30th–31st, 48th–49th, 64th–65th,
and 106th–108th floors, at which point's louvers clad the areas
devoted to mechanical operations of the building. In the lobby is a major work
by the American sculptor Alexander Calder, an enormous motorized mural named Universe, which he called a “wall mobile.” The tower’s
observation deck, the Sky deck, is located on the 103rd floor. In the early
21st century the Sky deck underwent a major renovation that included addition
of The Ledge, four glass boxes that extend 4.3 feet (1.3 meters) from the
building; The Ledge opened in 2009, offering unobstructed views of Chicago and
the outlying area.
The Sears Tower was the world’s tallest building until 1996, when
it was surpassed by the PETRONAS Twin Towers (1,483
feet [451.9 meters) in Kuala Lumpur, Malaysia. (See Researcher’s Note:
Heights of Buildings.) In 2009 incoming tenant Willis Group Holdings, an
insurance brokerage firm based in London, changed the building’s name to Willis
Tower.
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