Willis Tower, formerly the Sears Tower, Chicago, the tallest building in the United States. Until the opening of the 1,483-ft (452-m) Petronas Towers (1997) in Kuala Lumpur, Malaysia, it was the world's tallest building. Constructed from 1970 to 1974 for Sears, Roebuck & Co., it rises 110 stories to a height of 1,450 ft (442 m); the 253 ft (77 m) television antenna topping it makes it the world's third largest freestanding structure at 1,703 ft (519 m). Designed by the firm of Skidmore, Owings and Merrill, the building is supported structurally by square tubes of welded steel with floors suspended within the tubes, a technological innovation that was developed specifically for the mammoth skyscraper by architectural engineer Fazlur Kahn. The tower has an exterior of black aluminum and bronze-toned glass cut by black bands. It was renamed in 2009.
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