The Willis Tower (formerly and informally: Sears Tower, its title until 2009) is a 110-story, 1,450-foot (442.1 m) skyscraper in Chicago. At completion in 1973, it surpassed the World Trade Center in New York City to become the tallest building in the world, a title that it held for nearly 25 years; it was also the tallest building in the Western Hemisphere for 41 years, before the new One World Trade Center surpassed it in 2013. While it held the title of'Tallest Office Building' before 2013, it dropped the title of'Tallest Man-Made Construction' after only 3 decades. The CN Tower in Toronto, which functions as a communications tower, took over the title in 1976.
The Willis Tower is considered a seminal accomplishment for engineer Fazlur Rahman Khan. It is currently the third-tallest construction in the USA and the Western hemisphere -- and the 23rd-tallest in the world.
Radio masts and towers are, typically structures designed to support antennas for telecommunications and broadcasting. There are two kinds: structures that are guyed and self-supporting. They are among the tallest human-made structures. Masts are often named after the broadcasting organizations that currently use them or initially built them. In the event of a mast radiator or radiating tower, tower or the whole mast is the transmitting antenna.Source: Wikipedia