Why the World Trade Center Towers Fell on September 11

Engineers Explain the Twin Tower Collapse

Top of the New York World Trade Center, smoke, gaping hole where a plan crashed into the skyscraper
A passenger plane hijacked by terrorists struck the North Tower of the New York Trade Center. Photo © Peter Cunningham/Mission Pictures/Getty Images (cropped)

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 Twin Towers. By examining the collapse step-by-step, experts are learning how buildings fail, and discovering ways we can build stronger structures and answering the question—What Caused the Twin Towers to 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, which 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.

The impact of the aircraft and other flying objects (1) scraped off insulation that protected the steel from high temperatures; (2) damaged the sprinkler system of the building; (3) sliced and cut many of the interior columns and damaged others; and (4) shifted and redistributed the building load among columns that were not immediately damaged.

2. Heat from the Fires

Even if the sprinklers had been working, they could not have maintained enough pressure to stop the fire.

Fed by the a spray of 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° F. The steel will also become distorted when heat is not a uniform temperature. Videos of both buildings showed inward bowing of perimeter columns resulting from a sagging of heated trusses on many floors.

3. Collapsing Floors

Most fires start in one area and then spread. The fire from the aircrafts' impact covered the area of an entire floor almost instantly. As the weakened floors began to bow and then collapse, they pancaked. This means that upper floors crashed down on lower floors with increasing weight and momentum, crushing each successive floor below. With the weight of the plunging floors building force, the exterior walls buckled. Researchers estimate that the "air ejected from the building by gravitational collapse must have attained, near the ground, the speed of almost 500 mph." Loud booms were heard during the collapse, which were caused by air speed fluctuations reaching the speed of sound.

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.

Could the towers have been made stronger?

The Twin Towers were built in the 1970s. No building constructed at that time would have been able to withstand the impact of the terrorist attacks in 2001. We can, however, learn from the collapse of the towers and take steps to construct safer buildings and minimize the number of casualties in the event of a disaster.

When the Twin Towers were constructed, the builders were granted some exemptions from New York's building codes. The exemptions allowed the builders to use lightweight materials so the skyscrapers could achieve greater heights. But, the consequences were devastating. According to Charles Harris, author of Engineering Ethics: Concepts and Cases, fewer people would have died on September 11, 2001 if the Twin Towers had used the type of fireproofing required by older building codes.

A tragic legacy of September 11 is that new construction in the USA must now adhere to more demanding building codes. Tall office buildings are required to have more durable fireproofing, an extra emergency exit, and many other safety features. To learn about the new building codes and their impact on architectural design, see: Did 9/11 Change the Way We Build?


Some people believe that the Twin Towers were brought down by explosives. Researchers have shown "the allegations of controlled demolition to be absurd" and that the Towers "failed due to gravity-driven progressive collapse triggered by the effects of fire." Nevertheless, conspiracy theories continue in spite of the evidence to the contrary.

Source of Quoted Material: "What Did and Did not Cause Collapse of WTC Twin Towers in New York" by Zdenek P. Bazant, Jia-Liang Le, Frank R. Greening, David B. Benson, Department of Civil and Environmental Engineering, Northwestern University, Evanston, Illinois, Journal of Engineering Mechanics ASCE, Vol. 134 (2008), PDF.

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