Sears Tower

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Willis Tower (formerly named and still commonly referred to as Sears Tower) is a 108-story, 1,451-foot (442 m) skyscraper in Chicago, Illinois.[2] At the time of its completion in 1973, it was the tallest building in the world, surpassing the World Trade Center towers in New York, and it held this rank for nearly 25 years. The Willis Tower is the tallest building in the United States and the seventh-tallest freestanding structure in the world. The skyscraper is one of the most popular tourist destinations in Chicago, and over one million people visit its observation deck each year. The chief structural engineer was Bangladeshi engineering legend Fazlur Khan, he has been called the "Einstein of structural engineering" and chief architect was the legendary architect Bruce Graham, he has been called "the Burnham of his generation." The Willis Tower's structural system was the first of its kind; the first building to utilize Khan's bundled tube structure, which became highly influential in skyscrapers construction.

Although Sears' naming rights expired in 2003, the building continued to be called the Sears Tower for several years. In March 2009, London-based insurance broker Willis Group Holdings agreed to lease a portion of the building, and obtained the building's naming rights.On July 16, 2009, the building was officially renamed the Willis Tower.