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9/11 Commission

The Pile Driver

In the wake of the attack experts, including structural engineers, asserted that the Twin Towers were destroyed by a "pile driver" effect, in which the top of the building crushed the rest of the building, from top to bottom.

e x c e r p t
title: How the World Trade Center fell
authors: Sheila Barter
Science of collapse

From then on, the collapse became inevitable, as each new falling floor added to the downward forces.

Further down the building, even steel at normal temperatures gave way under the enormous weight - an estimated 100,000 tonnes from the upper floors alone.

"It was as if the top of the building was acting like a huge pile driver, crashing down on to the floors underneath," said Chris Wise.

e x c e r p t
title: Structural engineer describes collapse of the World Trade Center towers
authors: Mark Shwarz

Hamburger and his colleagues have not yet determined which of these scenarios occurred on Sept. 11, but there is little doubt that the collapse of the upper floors of the WTC towers brought down both structures.

"Think of the impact of dropping a 25-story building straight down," Hamburger told the audience. "It was like a pile driver, which is why it collapsed as it did."

e x c e r p t
title: WTC collapse forces skyscraper rethink
authors: Dr. David Whitehouse
The towers' central steel spines were weakened by the intense heat from the burning aviation fuel, and eventually gave way when they could no longer support the weight of the floors above the crash zones - that is the accepted analysis. When those upper floors began to fall, they forced everything below them to collapse in a "piledriver" effect.

Exercise: Find the Pile Driver

Can you find the pile driver in this image or any of the other images of the collapses of the Twin Towers?

Even if the pile driver is hidden within the dust cloud, it would only have a fraction of the mass of the former top of the building, since most of it was clearly falling outside of the building's profile.

Moreover it is noteworthy that the rubble falling outside of the tower's profile is falling at about the same rate at which the Tower is disappearing. Even the largest and heaviest pieces of rubble at the bottom edges of the rubble cloud are falling less than twice as fast as rubble falling inside the Tower's profile. But the rubble falling inside the Tower's profile is, according to the official explanation, crushing the building.

page last modified: 2005-11-28
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Ronald Hamburger, proponent of pile driver theory