New York's World Trade Center Twin Towers were designed to withstand fire and hurricane-force winds. Some engineers believed the Twin Towers could even survive impact from a Boeing 707. But no engineer or architect could have anticipated the terrorist attack that turned the Twin Towers to rubble, and experts often don't agree what steps might have been taken to make the buildings stronger. For the most recent findings and detailed technical information, be sure to visit the World Trade Center Coverage page at icivilengineer.

How did the Twin Towers fall?

1. Impact from the Terrorist Planes
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.

2. Heat from the Fires
The 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. Most fires don't get hotter than 900 to 1,100 degrees F. The World Trade Center fire may have reached 1,300 or 1,400 degrees F. Structural steel does not easily melt, but it will lose about half its strength at 1,200 degrees F. The steel structure of the Twin Towers was weakened by the extreme heat. The steel also became distorted because the heat was not a uniform temperature.

3. Collapsing Floors
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 crashed into the floors below. With the weight of the plunging floors accelerating, 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, they were about 95% air. After they collapsed, the hollow core was gone. The remaining rubble was only a few stories high.