Willis Tower (former Sears Tower)
This 110-storey building, a real feat of engineering, held the record for the tallest building in the world for 20 years. Called the Sears Tower until 2009, the Willis Tower is nowadays the second tallest skyscraper in the US and fifth in the world. The tower is composed of nine rectangular tubes resting on more than 100 steel and concrete caissons that are anchored into the bedrock dozens of metres below ground, providing excellent resistance to high winds.
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