Inferno Heat, Not Impact, Brought Down Towers, Experts Say

Bijal P. Trivedi
for National Geographic Today

September 17, 2001

Had it not been for the raging inferno that consumed the core of each of the World Trade Center towers in New York City last week, the buildings might still be standing, although severely damaged.

The structures withstood the initial impact of the two jetliners that crashed into them after being hijacked. But the steel columns supporting the damaged floors could not withstand the intense temperatures produced by the fireball of burning jet fuel, experts say.

Jet fuel fires burn unusually hot, and engineers believe the fire may have led to temperatures as high as 1,600 degrees Celsius (2,900 degrees Fahrenheit).

At temperatures above 500 degrees Celsius, steel loses its strength and "turns to Play-doh," said engineer Ted Krauthammer, of Penn State University in University Park, Pennsylvania.

As the steel columns at the core of the Twin Towers collapsed, the floors they supported fell on each other like two stacks of pancakes. "I was surprised and horrified to see them collapse," said Brooklyn-born physicist Frank Moscatelli of Swarthmore College in Pennsylvania.

In an effort to understand why the upper 20 stories that were instantly damaged didn't just "peel away," Moscatelli did some calculations. He estimated that the combined amount of energy released from the impact of the planes hitting the towers, the jet fuel explosions, and the collapse of both buildings was roughly equivalent to exploding 200 tons of TNT—about 2 percent to 4 percent of the explosive energy of the bomb detonated at Hiroshima.

Together, the energy from the force of the planes hitting the towers and the subsequent fuel explosion wasn't great enough alone to immediately knock down the towers, Moscatelli believes. What caused the buildings to collapse entirely, he said, was the combined weight and momentum of the falling floors.

"Basically, the aircraft and the fire destroyed the top 20 stories and gravity destroyed the rest," he said.

Once the top stories had begun to collapse, the undamaged steel supports of lower levels did not have enough strength to prevent the domino collapse of the upper stories.

Ultimately, the buildings' majesty was their own undoing, said Moscatelli.

Skyscrapers such as the Twin Towers are storehouses of gravitational energy, he explained. Once the structural support of the upper floors is removed, a few falling floors can bring down an entire building.

The rectangular design of the buildings probably contributed to their destruction, he added.

The Empire State Building, for example, has a more pyramidal structure, which makes it more resistant to the kind of demolition-style vertical collapse that brought down the Twin Towers. The architectural design of the older building also has greater "redundancy" in column supports, said engineer Marc Hoit of the University of Florida in Gainesville.

On July 28, 1945, a B-25 military bomber, disoriented in a heavy fog, crashed into the 79th floor of the 102-story Empire State Building. Eleven occupants and three flyers were killed, but the landmark—the world's tallest building at that time—was not permanently damaged.

Hoit believes that while the destruction of the World Trade Center may lead architects to consider some subtle changes in the design of major buildings, the disaster is not likely to have a large impact on global architecture. The result may be greater attention to ease of evacuation or more structural redundancy, for example, but not a diminished desire to build big.

"People want tall monuments, tall buildings," said Hoit. "People want architecture that symbolizes strength and power."