The core of the towers housed the elevator and utility shafts, restrooms, three stairwells, and other support spaces. The core of each tower was a rectangular area 87 by 135 feet (27 by 41 m) and contained 47 steel columns running from the bedrock to the top of the tower. The large, column-free space between the perimeter and core was bridged by prefabricated floor trusses. The floors supported their own weight as well as live loads , providing lateral stability to the exterior walls and distributing wind loads among the exterior walls.  The floors consisted of 4 inches (10 cm) thick lightweight concrete slabs laid on a fluted steel deck. A grid of lightweight bridging trusses and main trusses supported the floors.  The trusses connected to the perimeter at alternate columns and were on 6 foot 8 inch ( m) centers. The top chords of the trusses were bolted to seats welded to the spandrels on the exterior side and a channel welded to the core columns on the interior side. The floors were connected to the perimeter spandrel plates with viscoelastic dampers that helped reduce the amount of sway felt by building occupants.
On September 11, 2001, 19 militants associated with the Islamic extremist group al-Qaeda hijacked four airplanes and carried out suicide attacks against targets in the United States. Two of the planes were flown into the twin towers of the World Trade Center in New York City, a third plane hit the Pentagon just outside Washington, ., and the fourth plane crashed in a field in Pennsylvania. Almost 3,000 people were killed during the 9/11 terrorist attacks, which triggered major . initiatives to combat terrorism and defined the presidency of George W. Bush.