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Gerald Posner's avatar

Second question needs more info about why WTC 7 came down. The WTC7 was unusual before anything happened to it. It was built over a Con Ed substation, which forced the designers to carry the upper structure on transfer trusses and use unusually long floor spans on the east side. Less redundancy than a conventional frame.

The North Tower's collapse tore a roughly ten-story gash in its south face and started fires.

The water mains were broken. Nobody fought that fire. It burned for seven hours while the FDNY set a collapse zone and told anyone who asked that the building was coming down.

And the mechanism wasn't the steel getting weak — it was the steel getting longer. Heated beams expand. On Floor 13 an expanding beam pushed a girder off its seat at Column 79, floors cascaded down to Floor 5, and that left a critical column unsupported over nine stories. It buckled. The interior failed before anything moved on the outside, which is why the facade came down looking uniform.

NIST said plainly it was the first known case of fire alone bringing down a tall building. Unprecedented is the finding, but not a hole in it.

Gerald Posner's avatar

two emailed questions: 1st one was why no video evidence from the Pentagon of a plane flying into it?

I didn't cover the video/plane/Pentagon in this guide. But, here is the answer: The tapes exist and most have been released. The Pentagon's own gate cameras came out in 2006 through FOIA, then the Citgo and Doubletree recordings.

Here's why none of them are any good.

Those were fixed cameras in 2001, recording at roughly one frame per second. A gas station camera watches the pumps. A hotel camera watches the entrance. The Pentagon's two were parking gate barriers pointed at cars. None of them were aimed at the sky.

Flight 77 was doing about 530 mph, about 780 feet per second. It crossed any camera's field of view in a fraction of a second. At one frame per second, the shutter almost certainly lands on empty sky.

We all carry 4K cameras now and assume 2001 worked the same way. It didn't.

And run the theory backwards. If someone could stage an airliner strike on the Pentagon, producing convincing video would be the easy part — and the most useful thing they could possibly have. The one thing a plot would want most is the thing that doesn't exist.

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