Willis Tower, formerly the Sears Tower, Chicago, the second tallest building in the United States. Until the completion of the 1,483-ft (452-m) Petronas Towers (1998) in Kuala Lumpur, Malaysia, it was the world's tallest building. Constructed from 1970 to 1974 for Sears, Roebuck & Co., it rises to a height of 1,451 ft (442 m) and has 108 floors as well as a 278-ft (85-m) television antenna. 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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