WORLD TRADE CENTER

WORLD TRADE CENTER, a seven-building complex that was located on a sixteen-acre site in lower Manhattan in New York City. The Port Authority of New York and New Jersey financed the $958 million cost of construction. The architect Minoru Yamasaki designed the two 110-story towers (numbers 1 and 2 World Trade) in the International Style; the Twin Towers, as they were called, were to be the tallest buildings in the world, a record they held in 1973. To achieve that height, the engineering firm of Worthington-Skilling recommended a tube structure in which columns on the exterior walls, and the inner core of the skyscrapers, bore the gravity load. A grill of lightweight steel trusses connecting the perimeter and core supported the floors. Given the proximity of the Twin Towers to two major airports, each tower was built to withstand the impact of a Boeing 707 aircraft.

Relying on the perimeter and core columns to provide vertical support created 10 million square feet of open commercial space, which was leased to import and export businesses, government agencies, financial firms, and restaurants. Groundbreaking occurred in 1966; tenants moved into the World Trade Center in December 1970. The last building in the complex, 7 World Trade, a forty-seven story building, was completed in 1985.

On 23 February 1993, a truck bomb tore through an underground parking garage beneath the Vista Hotel (3 World Trade), killing six people. The explosion produced a crater six stories deep and destroyed lateral supports throughout the damaged area. As a result of the bombing, building modifications were introduced to improve evacuation, with tenants receiving evacuation training, and additional

fire command centers were established in the lobbies of the Twin Towers. On 11 September 2001, two hijacked Boeing 767 commercial airliners were flown into the Twin Towers, causing the collapse of both skyscrapers; 2,830 people, including 403 emergency personnel, died.

The Federal Emergency Management Agency (FEMA) led the effort in trying to determine the progression of the collapse, a task complicated by the removal of the beams to recycling centers and scrapyards during the recovery effort. The prevailing hypothesis is that the impact of the airliners sheared off the fireproofing on the trusses, which softened in the subsequent blaze; jet fuel pouring into the elevator shafts spread the fires to lower decks. With the integrity of the sagging floors compromised, it is believed that an unsupportable gravity load was redistributed to the core columns, leading to total structural failure. The damage of the initial impact was also being assessed.

Eight surrounding buildings either partially or totally collapsed that day, crushed by falling debris (3 World Trade) or gutted by fire (7 World Trade). Discussions about the future use of the site, referred to as "Ground Zero"—whether it should be dedicated solely as a memorial or reopened for mixed-use purposes—were ongoing at the end of 2002.

WORLD TRADE CENTER

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