History: Collapse Of The World Trade Center, September 11, 2001, Lower Manhattan, New York City, United States

When completed in 1972, 1 World Trade Center (the North Tower) became the tallest building in the world for two years, surpassing the Empire State Building after a 40-year reign. The North Tower stood 1,368 feet (417 m) tall and featured a telecommunications antenna or mast that was added at the top of the roof in 1978 and stood 360 feet (110 m) tall. With the 360-foot (110 m)-tall antenna/mast, the highest point of the North Tower reached 1,728 ft (527 m). 2 World Trade Center (the South Tower) became the second tallest building in the world when completed in 1973. The South Tower's rooftop observation deck was 1,362 ft (415 m) high and its indoor observation deck was 1,310 ft (400 m) high. The World Trade Center towers held the height record only briefly: Chicago's Sears Tower, finished in May 1973, reached 1,450 feet (440 m) at the rooftop. Throughout their existence, however, the WTC towers had more floors (at 110) than any other building. This number was not surpassed until the advent of the Burj Khalifa, which opened in 2010.
Of the 110 stories, eight were set aside for technical services in mechanical floors Level B5/B6 (floors 7/8, 41/42, 75/76, and 108/109), which are four two-floor areas that evenly spaced up the building. All the remaining floors were free for open-plan offices. Each floor of the towers had 40,000 square feet (3,700 m2) of space for occupancy. Each tower had 3,800,000 square feet (350,000 m2) of office space. Altogether the entire complex of seven buildings had 11,200,000 square feet (1,040,000 m2) of space.
Initially conceived as a complex dedicated to companies and organizations directly taking part in "world trade", they at first failed to attract the expected clientele. During the early years, various governmental organizations became key tenants of the World Trade Center including the State of New York. It was not until the 1980s that the city's perilous financial state eased, after which an increasing number of private companies—mostly financial firms tied to Wall Street—became tenants. During the 1990s, approximately 500 companies had offices in the complex including many financial companies such as Morgan Stanley, Aon Corporation, Salomon Brothers and the Port Authority itself. The basement concourse of the World Trade Center included The Mall at the World Trade Center along with a PATH station. The North Tower became the home of the corporate headquarters of Cantor Fitzgerald, and it also became the headquarters of the Port Authority of New York and New Jersey.
Electrical service to the towers was supplied by Consolidated Edison (ConEd) at 13,800 volts. This service passed through the World Trade Center Primary Distribution Center (PDC) and sent up through the core of the building to electrical substations located on the mechanical floors. The substations "stepped" the 13,800 primary voltage down to 480/277 volt secondary power and further to 120/208 volt general power and lighting service. The complex also was served by emergency generators located in the sublevels of the towers and on the roof of 5 WTC.