The Willis Tower, formerly known as Sears Tower, is a 110 storey skyscraper in Chicago, United States of America, was once the tallest building in the world, occupying the top spot for a period of around 25 years, from 1973 to 1998, until the Petronas Twin Towers in Kuala Lumpur, Malaysia, broke the record. While Burj Khalifa, Dubai, holds the current world record, Willis Tower is now the eighth tallest building in the world, and the second tallest building in USA, surpassed only by the recently reconstructed One World Trade Centre in Manhattan. Willis Tower stands tall at an impressive height of four hundred and forty three metres, which is over sixty metres taller than the Empire State of Building in New York City. The Tower was originally built as the national headquarters of the Sears, Roebuck Company.

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Most people generally agree that the skyscraper was invented in Chicago, which is the third largest city in USA.  In 1871, a fire destroyed a third of the city, and architects set about rebuilding it. By this time, steel was being mass produced and, in 1852, Elisha Otis had invented the elevator, also known as the lift. New buildings could safely and practically be made much taller. The first high-rise building appeared in the city in 1882. This was the Home Insurance Company Building. It was the first building to have a skeleton structure made entirely of steel and iron. It had ten floors – a hundred fewer than Sears Tower which would be built ninety years later. As the demand for land in city centres grew, people needed to build fast and economically. Skyscrapers were the perfect solution to this challenge. With new materials and developing technology, the design of skyscrapers was gradually improved and perfected. Steel girders were used in different ways to build a strong framework. Some buildings were made of tall steel tubes, designed to withstand the winds that blow around the top levels. Inside the tubes, individual floors and rooms are rigid so that people can live and work safely and comfortably.

It took around two thousand and four hundred workers around three years to build Willis Tower, and eventually finish it in 1973. The unique framework of the tower is made up of nine steel square tubes, which form a large square at the base. This rests on concrete, rock-filled shafts that are securely fitted into the solid rock beneath. The nine welded steel tubes rise fifty floors up. Then the building starts to narrow as you go upwards. Seven tubes continue on up to the sixty-sixth floor. Then five rise on to the ninetieth floor, leaving just two tubes to form the top twenty floors. Two television antennae take the building’s total height to a whopping five hundred and twenty metres – over half a kilometre. The amount of steel used to build the framework alone was enough to make more than fifty two thousand cars. Each of the steel tubes that form the top stage of the tower is 23 metres square in size. Viewed from above, the third strafe makes up a cross shape of five tubes. The second stage of the “step back” design has seven tubes that rise for 65 metres. Each side of the tower’s base, made up of nine tubes, is 69 metres long, with the total structure weighing over a 200,500 tonnes. During construction, up to eight floors were added to the building every month. The building’s steel frame was put together in sections, and were then welded and bolted tightly into place. In 1985, a four storey hall was built a base of Willis Tower, to give extra space. There are also shops and restaurants in the buildings.

This huge building has over four hundred and eighteen thousands square metres of space, which amounts to around sixty football pitches. The Tower has a hundred and four high-speed lifts, which divide the building into three separate zones and make it easier for people to find their way around. The fireproofed frame is covered in a metallic skin made of black aluminium, and more than sixteen thousand bronze-tinted glass windows. Six automatic window-washing machines clean the whole of the building eight times a year. Willis Tower makes the high-rise buildings around it look small. It is designed to sway up to ninety centimetres at the top in the occurrence of strong winds. About 1.5 million tourists visit Willis Tower each year, making it the most sought after tourist attraction in the city. Two express lifts take them up to the 103rd floor in just over a minute. From the Tower’s Skydeck, visitors have a spectacular view of the Chicago panorama. In the building’s lobby there is an enormous “wallmobile” by the famous sculptor Alexander Calder. The Skydeck is situated a four hundred and twelve metres up in Willis Tower. On a clear day, visitors can see over eighty kilometres, with a breath-taking view of four American states – Illinois, Indiana, Michigan, and Wisconsin.

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The official address for the building is 233 South Wacker Drive, Chicago, Illinois 60606, and is the “most impressive office space in Chicago” as it rightfully claims on its official website. Its pinnacle stands tall at a height of 1,729 feet. While the Sears sold the building way back in 1994, and completely vacated the building by the following year, it still held the naming copyrights to the building till 2003. That year, the London based stockholdings company Willis Group Holdings Limited bought around three floors’ worth of area, and thus won the naming rights without any conflict. Currently, United Airlines is the largest occupier of the Willis Tower, and has about twenty floors under its ownership which are home to its headquarters and operations centre.