n.
Huge five-sided building (194143) in Arlington, Va.
, that is the headquarters of the U.S. Department of Defense. Designed by George Edwin Bergstrom, it was, on its completion, the world's largest office building, covering 34 acres (14 hectares) and offering 3.7 million sq ft (about 344,000 sq m) of usable floor space for as many as 25,000 workers. Built of structural steel and reinforced concrete with some limestone facing at a cost of $83 million, the five-story structure actually consists of five concentric pentagons, with 10 spokelike corridors connecting the whole. In 2001 more than 180 people were killed and many more were injured when part of the Pentagon's southwest side was destroyed in the September 11 attacks .