and the Washington, DC area (Bruneau et al. 2002).
On that day, 19 terrorists from the Islamist militant
group al-Qaeda hijacked four passenger jets. The hijackers intentionally crashed two planes into the Twin
Towers of the World Trade Center in New York City;
both towers collapsed within two hours. Hijackers
crashed a third jet into the Pentagon in Arlington,
Virginia, and ensuing fires caused one section of
the building to collapse. The fourth jet crashed into
a field near Shanksville, Pennsylvania, after passengers
attempted to take control before it could reach the hijackers’ intended target. Nearly 3,000 people died in
the attacks.
The Twin Towers were built to maximize useable
office space; many support columns were placed toward the exterior of the building. The airplanes’ impact blew spray-on fireproofing off the trusses, leaving
them vulnerable to the heat. Had the retardant remained, the steel trusses may have held firm until
the fire burned itself out. Sheetrock was used to wall
off the central core area, which included the emergency stairwells. Drywall is effective in resisting heat
and flames but is weak and did not withstand the impact of the planes (Kennedy and Klein 2011).
Following the collapse of the Twin Towers, building laws and practices for high-rise buildings in
New York City were revised. Tall buildings must
now incorporate improved sprinkler systems, better
exit signs, an additional stairway, and other features
to help people exit quickly during emergencies. The
International Code Council approved new safety regulations in the International Building Code and the
International Fire Code that include additional stairways, more space between stairways, stronger walls in
stairwells and elevator shafts, reinforced elevators for
emergency use, stricter standards for construction materials, better fireproofing, backup water sources for
the sprinkler system, glow-in-the-dark exit signs,
and radio amplifiers for emergency communications.
These safety changes were incorporated into the design
and construction of the Burj Khalifa in the United
Arab Emirates, which has a building height of
828 m and was completed in 2009. The skyscraper
construction incorporated multiple evacuation lifts,
super-high-speed elevators, thick concrete reinforcement in the stairways, and many other safety features.