World Trade Center.
On September 11, 2001, terrorists crashed two hijacked commercial jets into the Twin Towers of New York City's World Trade Center in a coordinated attack on this symbol of American financial power and influence. Remarkably, both towers survived the initial impact of Boeing 767 jets traveling at speeds in excess of 400 mph, and remained standing long enough for most occupants below the impact floors to escape.
The 110-story Twin Towers of the World Trade Center embodied many engineering advances. They were the first major structures to be designed using wind tunnel testing and digital computers, the first to use structural dampers, and, at the time, the tallest ever built.
In each tower, 236 perimeter columns formed a highly redundant structural tube that redistributed loads around severed columns, preventing immediate collapse. The force of the impacts atomized the jet fuel in both aircraft and also stripped fireproofing from the intact steel elements of the towers. Approximately a third of the jet fuel was consumed in the highly visible fireballs at the points of impact. The rest of the fuel was forced into the towers and saturated the combustible contents. The fires caused by the ignition of these materials directly heated the exposed and overloaded columns and trusses, rapidly causing the steel to lose strength. After 56 minutes for the south tower and 102 minutes for the north tower, the strength of critical members fell below the loads they were carrying, initiating collapse.
Thousands of tons of structural debris destroyed the surrounding and below grade portions of the World Trade Center complex, severed vital underground utility and communications links and damaged numerous surrounding buildings. The smoke and dust plumes from the collapse were visible from outer space.
The third plane of the attack damaged the Pentagon in Washington, D.C. while the fourth was brought down by its passengers in southwestern Pennsylvania, but the majority of the 9/11 fatalities occurred at the World Trade Center. Ultimately, 2,749 innocent people died in the two hijacked jets and the Twin Towers.
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2 周Semion Gengrinovich Thank you for reminding us all of the importance of strong engineering rigor to minimize the adverse impact when disaster hits.