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Last edited 23 May 2017
The Pentagon is the headquarters of the United States Department of Defence and one of the largest office buildings in the world. Located in Arlington County, Virginia, it has become a symbol of military power.
The Pentagon was designed by architect George Bergstrom and built by contractor John McShain. It provides around 600,000 sq.m (6.5 million sq.ft) of accommodation, of which 340,000 sq.m (3.7m sq.ft) serve as offices for approximately 23,000 military and civilian employees.
A large defence hub was urgently needed during the Second World War. Initially, President Roosevelt was said to favour a building without windows to protect against possible air raids, although he was persuaded otherwise by engineers, in favour of the five-sided design by Bergstrom.
The building has five sides, five storeys above ground, two levels below ground, and five ring corridors per floor, creating a total of 17.5 miles (28.2 km) of corridors. The building has a height of 23 m (77 ft), and each of the five sides is 281 m (921 ft) long.
 Design and construction
Bergstrom was forced to use an asymmetrical five-sided shape for his design because of the position of existing roads on the site. The site itself previously contained housing occupied by the descendants of former slaves. However, it was not large enough for the massive complex being planned, and so a further 150 families were evicted from their homes and their land appropriated.
Because the site is located on the floodplain of the Potomac River, the ground conditions presented a number of engineering challenges. So too did the varying elevations across the site, which ranged from 3-12 m (10-40 ft) above sea level.
One of the first design stipulations was that the building be no taller than five storeys due, in part, to concerns over disrupting the views to and from the Virginia site and nearby Washington D.C. Another reason was because of the steel shortage as a result of the ongoing war effort.
Ground was broken and construction began on September 11 1941. It was built as a reinforced concrete structure, using 680,000 tons of sand dredged from the nearby Potomac River. The elevation variations were compensated for by two retaining walls, while cast-in-place piles were used to deal with the ground conditions.
As a result of the urgent need for the building, the architectural and structural design work proceeded simultaneously with construction, with construction overtaking design at certain points, with the result being that different materials were used from those specified in the plans. A workforce of more than 15,000 worked on the project around the clock, and the first employees moved in to the complex before it was completed.
The Pentagon became synonymous with protests during the Cold War, and particularly during the Vietnam War.
The Pentagon underwent a major renovation, beginning in 1998. This involved bringing the building up to modern standards, removing asbestos, improving security, sealing all office windows, and so on. This replaced the original system of cross-ventilation from openable windows with air conditioning for cooling.
In the aftermath of the attack, the Phoenix Project was launched. This was a repair and renovation initiative, completed in February 2003 at a cost of some $5bn. In 2008, a memorial garden was opened to the public.
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