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Why the Towers Fell
Image of the World Trade Center Falling
Photograph courtesy AP/Wide World Photos—Gulnara Samoilova

The towers of the World Trade Center were designed and built in the late 1960s to survive hurricanes, earth tremors, and even the impact of a Boeing 707. The original design did not anticipate the impact of a much larger Boeing 767 filled with fuel.

On September 11, 2001, Flight 11 hit between the 94th and 98th floors of the North Tower on its north face and Flight 175 sliced through the 78th to 84th floors of the South Tower on its south side. The impact initially damaged the exterior load bearing columns and severed steel interior columns of the central core. Concrete floors were pulverized and steel support trusses that held up the floors were obliterated. The remaining structural elements were able to carry the load for a short time before instability began. Due to the destruction of exterior columns, the floors above the impact level sagged downward and the undamaged core columns were forced to carry an increased compression load.

graphic of Flight 11 crashing into the North Tower
This visualization illustrates the devastating impact of Flight 11 into the North Tower. The animation, courtesy of Purdue University, depicts over one second of real time at 1/27 speed using structural data from Boeing, the Port Authority and FEMA among others.
Hi-Res Video 3.2MB
Under extreme heat, steel melts. The fireballs of exploding jet fuel burned off within minutes, but the fires the fuel ignited fed upon combustible office contents. Sprinklers and standpipes were disabled and a coating of fire-resistant vermiculite plaster, which covered the steel components to keep heat from weakening the metal, was dislodged with the impact. Temperatures rose to over 1,300°F and caused the steel structure to weaken and the trusses to sag. If the fire had been more contained, the beams could have cooled down and thus regained their strength as the fire moved and consumed fuel.

As the structural steel frame began to melt, it collapsed downward onto the already weakened floors below. Each falling floor added to the downward forces. The accumulated weight of collapsing floors bent steel not subjected to the fires. Full collapse was inevitable.

Despite the degree of structural damage to the World Trade Center and the scorching temperature of the fires, the two towers stood for over an hour, allowing thousands of people to escape.