9/11 conspiracy theories misconstrue how World Trade Center buildings collapsed

Twenty-one years after the 9/11 terrorist attacks, conspiracy theories about how the World Trade Center buildings in New York City collapsed still persist.

On Sept. 11, 2001, two hijacked airplanes hit the twin towers of the World Trade Center, causing their destruction. A third building, World Trade Center 7, also collapsed that day due to uncontrolled fires.

But conspiracy theorists have been claiming for more than two decades that the collapse of WTC 7, a 47-story building that housed offices for the CIA and Secret Service, was nefarious. Some claim planted explosives caused it to fall; others question how the building collapsed when no plane struck it.

This is one in a long line of conspiracy theories that have proliferated since the attacks that killed nearly 3,000 people. This year, on the attacks’ anniversary, the claim resurfaced. In one Facebook post, an edited image of Tom Hanks in the movie "Forrest Gump" read, "I may not be a smart man, but I know two planes can’t cause three buildings to collapse in free-fall time."

A second post on Instagram said, "We are supposed to believe two planes destroyed three buildings that were specifically built to withstand a plane being flown into them."

These posts were flagged as part of Facebook’s efforts to combat false news and misinformation on its News Feed. (Read more about our partnership with Facebook.)

Debris from the nearby destruction of the twin towers started fires on at least 10 of the floors of WTC 7, according to the National Institute of Standards and Technology.

These fires became uncontrollable because the sprinkler system failed, the institute’s report said. The building’s primary and backup water supplies came from New York City’s water lines, which were damaged by the collapse of the main WTC buildings.

After burning for almost seven hours, WTC 7 collapsed at 5:20 p.m. on Sept. 11, 2001. An airplane never struck the building.

The heat from the blazes caused the building’s steel floor beams and girders to thermally expand, causing a key structural column to fail. Without this column’s support, Floor 13 collapsed, triggering a series of structural failures until the whole building fell.

This was the first time a building taller than 15 stories was brought down primarily by uncontrollable fires.

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In videos taken outside the building, WTC 7’s collapse appears to happen suddenly and uniformly. This is because the building’s interior failings did not cause the exterior framing — which was stiffer and stronger than the former — to crumble until the collapse’s final stages.

The recent Facebook post claimed WTC 7 collapsed in "free-fall time," or at the rate of gravitational acceleration, but this is inaccurate. The National Institute of Standards and Technology’s final 9/11 report, released in September 2008, analyzed the building’s collapse and found it fell at a rate 40% slower than free-fall time.

The analysis described three distinct stages of the building’s 5.4-second collapse. In the first stage, until 1.75 seconds in, the acceleration was less than that of gravity; in other words, slower than free fall. In the second stage, between 1.75 and 4 seconds, the building disintegrated at free fall. Then, in the third stage, after 4 seconds, the building’s collapse decreased in acceleration to again be slower than free fall.

If the entire collapse had occurred at free-fall time, the report said it would have taken 3.9 seconds, a second and a half shorter than the actual time.

Yes, but that plan did not account for crashes that caused fires.

Structural engineer Leslie Robertson designed the World Trade Center complex, which was completed in 1973, to withstand the impact of an airplane. In his calculations, he used a Boeing 707, the largest aircraft in service at the time, but smaller and lighter than the Boeing 767 models that struck the towers in 2001.

When the planes hit the twin towers, the buildings remained standing for some time, allowing thousands of people to escape. However, Robertson’s calculations did not account for the possibility of collisions leading to fires, which are what led to the buildings collapsing.

Social media posts called into question the destruction of three World Trade Center buildings on Sept. 11, 2001.

A Facebook post shared on the terrorist attacks’ 21st anniversary claimed "two planes can’t cause three buildings to collapse in free-fall time." And an Instagram post said, "Two planes destroyed three buildings that were specifically built to withstand a plane being flown into them."

World Trade Center Building 7 was not struck by an airplane on 9/11; it collapsed because of uncontrollable fires ignited by debris from the twin towers. WTC 7 collapsed at a rate 40% slower than free-fall time.

The main World Trade Center complex was designed to withstand the impact of a Boeing 707, a smaller plane than the ones that struck the towers. The engineer also did not account for fires, which ultimately caused the buildings’ downfall.

We rate this claim False.