Why the World Trade Center Towers Fell on September 11

Engineers Explain the Twin Tower Collapse

Smoke pours from the twin towers of the World Trade Center after they were hit by two hijacked airliners in a terrorist attack September 11, 2001 in New York City.
September 11, 2001 in New York City. Photo by Robert Giroux/Getty Images News/Getty Images (cropped)

In the years since the September 11, 2001 terrorist attacks in New York City, individual engineers and committees of experts have studied 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—all by 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 heat; (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 temperature 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 from the intense heat. Steel will lose about half its strength at 1,200° F.

The steel will also become distorted (i.e., buckle) when heat is not a uniform temperature—the exterior temperature was much cooler than the burning jet fuel inside. 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 aircraft's 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 between 1966 and 1973. 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 skyscrapers 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. Some say that 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.

Others say that the architectural design actually saved lives. These skyscrapers were designed with redundancies—anticipating that a small plane could accidentally penetrate a Twin Tower skin and the building would not fall down.

Yet the buildings actually withstood the impact of a large aircraft. The North Tower was hit at 8:46 AM, between floors 94 through 98—it did not collapse until 10:28 AM, which gave most people over an hour-and-a-half to evacuate. Even occupants of the South Tower, which was hit later at 9:03 AM but collapsed first at 10:05 AM, had over an hour to evacuate (the South Tower was hit on lower floors, between floors 78 and 84, and became structurally compromised earlier than the North Tower).

The Towers could not have been designed any better or stronger. Nobody anticipated the deliberate actions of an aircraft filled with jet fuel. Perhaps the real question is why can't aircraft use solid fuels?


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.

The Legacy of 9/11 on Building

Architects want to design safe buildings; developers don't always want to pay for over-redundancies to accommodate events not likely to happen. The legacy of September 11 is that new construction in the United States must now adhere to more demanding building codes. Tall office buildings are required to have more durable fireproofing, extra emergency exits, and many other fire safety features. To learn about the new building codes and their impact on architectural design, see: Did 9/11 Change the Way We Build?

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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