Willis Tower

The Willis Tower (formerly named the Sears Tower) is a 108-story, 1,451-foot (442 m) skyscraper in Chicago, Illinois. 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.

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.

Planning and construction
Breakdown of the bundled tube structure of the Willis Tower with simplified floor plans.

In 1969, Sears, Roebuck & Co. was the largest retailer in the world, with approximately 350,000 employees. Sears executives decided to consolidate the thousands of employees in offices distributed
throughout the Chicago area into one building on the western edge of Chicago's Loop. With immediate space demands of 3 million square feet (279,000 m²), and predictions for future growth necessitating more space, Sears commissioned architects Skidmore, Owings and Merrill (SOM) to produce a structure to be one of the largest office buildings in the world. Their team of architect Bruce Graham and structural engineer Fazlur Khan designed the building as nine square "tubes", each essentially a separate building, clustered in a 3×3 matrix forming a square base with 225-foot (75 m) sides. All nine tubes would rise up to the 50th floor of the building. At the 50th floor, the northwest and southeast tubes end, and the remaining seven continue up. At the 66th floor, the northeast and the southwest tubes end. At the 90th floor, the north, east, and south tubes end. The remaining west and center tubes continue up to the 108th floor.

Sears executives decided that the space they would immediately occupy should be efficiently designed to house their Merchandise Group. But floor space for future growth would be rented out to smaller firms and businesses until Sears could retake it. Therefore, those floor areas had to be designed to a smaller plate, with a high window-space to floor-space ratio, to be attractive and marketable to prospective lessees. Smaller floorplates required a taller structure to yield sufficient square footage. Skidmore architects proposed a tower with large 55,000-square-foot (5,000 m²) floors in the lower part of the building, and gradually tapered areas of floorplates in a series of setbacks, which would give the Sears Tower its distinctive look.

As Sears continued to offer optimistic projections for growth, the tower's proposed height soared into the low hundreds of floors and surpassed the height of New York's unfinished World Trade Center to become the world's tallest building. Restricted in height by a limit imposed by the Federal Aviation Administration (FAA) to protect air traffic, the Sears Tower was financed by the company. It was topped with two antennas to permit local television and radio broadcasts. Sears and the City of Chicago approved the design, and the first steel was put in place in April 1971. The structure was completed in May 1973. Construction costs totaled approximately US$150 million at the time, which would be equivalent to roughly US$950 million in 2005. By comparison, Taipei 101, built in 2004 in Taiwan, cost around the equivalent of US$1.76 billion in 2005 dollars.

Even though regulations didn't require a fire sprinkler system, the building was equipped with one from the beginning. There are about 40,000 sprinkler heads in the building. The sprinkler system cost 4 million dollars.
Black bands appear on the tower around the 29th–32nd, 64th–65th, 88th–89th, and 104th–108th floors. These are louvres which allow ventilation for service equipment and obscure the structure's belt trusses.

In February 1982, two television antennas were added to the structure, increasing its total height to 1,707 feet (520 m). The western antenna was later extended, bringing the overall height to 1,730 feet (527 m) on June 5, 2000 to improve reception of local NBC station WMAQ-TV.

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