TOP 10 US MEGASTRUCTURES

DEVELOPED BY ARCHITECTS George E . Bergstrom and David J . Witmer , the Pentagon was constructed to accommodate the growing number of military and civilian employees , who were spread across 17 different office buildings . Contractor John McShaine was also commissioned to undertake the construction . Built at a cost of over $ 35 billion , the concrete build was swiftly finalised in two years with the use of over 4,000 workers , becoming the largest office building in America , spanning 29 acres and over 17 miles of corridor over four storeys .

The five-sided concrete structure , constructed through over 680,000 tonnes of sand and gravel from the Potomac River has been reconstructed as a result of the September 11 2001 attacks , with the first floor undergoing extensive renovation .

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