In the years since September 11, 2001 terrorist attack in New York City, engineers and other experts have been studying the collapse of the World Trade Center towers. By examining the collapse step-by-step, experts are learning how buildings fail, and discovering ways we can build stronger structures. When Boeing jets piloted by terrorists struck the Twin Towers, some 10,000 gallons (38 kiloliters) of jet fuel fed an enormous fireball. But, the impact of the planes and the burst of flames did not make the Towers collapse right away. Like most buildings, the Twin Towers had redundant design. The term redundant design means that when one system fails, another carries the load. Each of the Twin Towers had 244 columns around a central core that housed the elevators, stairwells, mechanical systems, and utilities. When some columns were damaged, others could still support the building.Heat from the FiresThe sprinkler system was damaged by the impact of the planes. But even if the sprinklers had been working, they could not have maintained enough pressure to stop the fire. Fed by the remaining jet fuel, the heat became intense. Jet fuel burns at 800° to 1500°F. This is not hot enough to melt structural steel. However, engineers say that for the World Trade Center towers to collapse, their steel frames didn't need to melt, they just had to lose some of their structural strength. Steel will lose about half its strength at 1,200 degrees F. The steel will also become distorted when heat is not a uniform temperature. Most fires start in one area and then spread. The fire from the terrorist planes covered the area of an entire floor almost instantly. As the weakened floors began to collapse, they pancaked. This means that floors crashed down on floors with increasing weight and momentum, crushing each successive floor below. With the weight of the plunging floors building force, the exterior walls buckled.
Why did the collapsed towers look so flat?Before the terrorist attack, the twin towers were 110 stories tall. Constructed of lightweight steel around a central core, the World Trade Center towers were about 95% air. After they collapsed, the hollow core was gone. The remaining rubble was only a few stories high.
Why did the collapsed towers look so flat?Before the terrorist attack, the twin towers were 110 stories tall. Constructed of lightweight steel around a central core, the World Trade Center towers were about 95% air. After they collapsed, the hollow core was gone. The remaining rubble was only a few stories high.
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