"Hard Science and the Collapse of the World Trade Center"

From "Garlic and Grass":

The odd, swift collapse of WTC7 made me reconsider the Twin Towers and how they fell. As I had with WTC7, I first studied video footage available on the web. Then I acquired and watched a DVD of the collapses, frame by frame.

What struck me first was the way the second plane hit WTC2, the South Tower. I noticed that this plane, United Airlines Flight 175, which weighed over 160,000 pounds and was traveling at 350 mph, did not even visibly move the building when it slammed into it. How, I wondered, could a building that did not visibly move from a heavy high speed projectile collapse at near freefall speed less than an hour later?

Next, I turned my attention to steel beams that fell in freefall next to the building as it collapsed. The beams were falling at the same rate that the towers themselves were descending. Familiar with elementary physics, including principles of conservation of energy and momentum, this seemed quite impossible if the towers were indeed “pancaking,” which is the official theory.

The height of the South Tower is 1362 feet. I calculated that from that height, freefall in a vacuum (read, absolutely no resistance on earth) is 9.2 seconds. According to testimony provided to the 9-11 Commission, the tower fell in 10 seconds. Other data shows it took closer to 14 seconds. So the towers fell within 0.8-4.8 seconds of freefall in a vacuum. Just like WTC7, this speed seemed impossible if each of the 110 floors had to fail individually.

As I was considering this, another problem arose. There is a principle in physics called the Law of Conservation of Energy. There is also the Law of Conservation of Momentum. I'll briefly explain how these principles work. Let’s assume there are two identical Honda Civics on the freeway. One is sitting in neutral at a standstill (0 mph). The other is coasting at 60 mph. The second Honda slams into the back of the first one. The first Honda will then instantaneously be going much faster than it was, and the second will instantaneously be going much slower than it was.

This is how the principle works in the horizontal direction, and it works the same in the vertical direction — with the added constant force of gravity added to it. Jim Hoffman, a professional scientist published in several peer-reviewed scientific journals, took a long look at all of this. He calculated that even if the structure itself offered no resistance — that is to say, even if the 110 floors of each tower were hovering in mid-air — the "pancake" theory would still have taken a minimum of 15.5 seconds to reach the ground. So, even if the building essentially didn't exist — if it provided no resistance at all to the collapse — just the floors hitting each other and causing each other to decelerate would've taken 15.5 seconds to reach the ground.

But of course the buildings did exist. They had stood for over 30 years. The floors weren't hovering in mid-air. So how did the building provide no resistance?

Yet another observation one makes in watching the collapsing towers is the huge dust clouds and debris, including steel beams, that were thrown hundreds of feet out horizontally from the towers as they fell. If we are to believe the pancake theory, this amount of scattering debris, fine pulverized concrete dust, and sheetrock powder would clearly indicate massive resistance to the vertical collapse. So there is an impossible conflict. You either have a miraculous, historical, instantaneous, catastrophic failure that occurs within a fraction of a second of freefall and that kicks out little dust, or you have a solid, hefty building that remains virtually unaffected after a massive, speeding projectile hits it. You either have a house of cards or a house of bricks. The building either resists its collapse or it doesn't.

And we know the WTC Towers were made of reinforced steel and concrete that would act much more like bricks than cards.

Thus, put simply, the floors could not have been pancaking. The buildings fell too quickly. The floors must all have been falling simultaneously to reach the ground in such a short amount of time. But how?

The author is Dave Heller "who has degrees in physics and architecture, is a builder and engaged citizen in Berkeley, California."

I think this argument is the key to why the collapses of WTC 1, 2 and 7 can't have been progressive "pancake" collapses: the extreme speed at which they happened.