Give Now

A Moment of Science

9/11 Twin Towers

In this Moment of Science, we explain how and why the collapse of the Twin Towers occurred.

The Twin Towers memorial at night.

Photo: dliberty3 (Flickr)

Built in the 1960s, each Twin Tower had a tube structure, meaning that the concrete-encased steel columns that supported the building and bore its weight were on the building's outside.

Amid the shocking events that occurred on September 11th, one of the most startling was the sight of the collapse of the Twin Towers soon after hijacked planes hit them.

Structure And Design

Designed and built in the 1960s, each tower had a tube structure, meaning that the concrete-encased steel columns that held the building up and bore its weight were on the building’s exterior. Inside, each Tower had a vertical steel core at its center that housed stairwells and elevators.

Steel beams radiated outward from the core, supporting the floors and attaching to the exterior columns. Part of the reason that the buildings collapsed as they did is because aside from the floor reinforcements, there were no columns holding the building up from the inside.

Cause Of Collapse

When the planes hit they severely damaged the exterior columns. But the impact alone was not what caused the collapse; it was the intense heat generated by exploding airplane fuel. Each plane carried 24,000 gallons of fuel, enough to create fires hotter than 1472 degrees Fahrenheit.

Exposed to such intense heat, both the exterior columns and floor beams began to weaken. Consequently, the upper floors caved in, collapsing the floors below. The accumulated weight was enough to destroy the entire building.

What Could’ve Been

Even though they eventually gave way, the Towers’ structure saved thousands of lives by withstanding the heat for as long as they did, allowing many of the buildings’ occupants to reach the ground.

Had they been designed differently, the Towers might have fallen over like giant trees, creating a disaster of even larger proportions.

  • Anonymous

    Sorry! I don’t speak english.  It’s wonderful… United States, our big brother is stand up again. Before, the Twin Towers were material US biulding icon … today its bigger and greater than before because the 3D Twin celestial lights Towers it’s a heavenly US icon, non destructible never.  Thanks for the American Great Spirit and Hearts.  Love,  isolda

  • Anonymous

    Sept. 12, 2011Sorry! I don’t speak english.  It’s wonderful… United States, our big brother is stand up again. Before, the Twin Towers were material US biulding icon … today its bigger and greater than before because the 3D Twin celestial lights Towers it’s a heavenly US icon, non destructible never.  Thanks for the American Great Spirit and Hearts.  Love,  isolda

Stay Connected

What is RSS? RSS makes it possible to subscribe to a website's updates instead of visiting it by delivering new posts to your RSS reader automatically. Choose to receive some or all of the updates from A Moment of Science:

Support for Indiana Public Media Comes From

About A Moment of Science

Search A Moment of Science