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RE: 9-11 free-fall

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David, 

I think FEMA 403 and NIST's follow-up report both stated that the time
was slightly longer than freefall and consistent with the collapse
observed.

The damaged core framing had started descending as the hat truss and
floor framing acting in catenary action could no longer support it. The
downward progression and inward pull on the perimeter walls created a
global instability in the perimeter wall, which initiated the fall of
the mass of the entire segment of the tower above the damage point onto
the tower structure below. That is, essentially, the demolishing
mechanism was a ten-some and twenty-some story building falling down on
the structure underneath it, picking up speed with every successive
floor.

Much has been made by conspiracy theorists about the comparison to a
freefall, mostly to support the idea that explosive charges had to have
been placed in the building. I think there is little question that the
collapse was by disintegration from top down due to debris inertia. One
conspiracy theorist argued that the fall was faster than a freefall. I
wonder if he ever heard of Isaac Newton.

A fall time noticeably longer than freefall is not likely in this case
because how can you stop the inertia of 10 and 20 stories once they are
in motion? Comparisons to story-shear failures in earthquakes were
initially made and some tried to find why the same thing didn't happen
here (that is, why the towers didn't simply lose one story and have the
top sit down on the structure below). But a seismic story shear is less
immediate as the columns disintegrate and usually limited to one story.
The tower collapse mechanism occurred over many stories -- the global
buckle in the perimeter wall appears from the videos I've seen to be at
least 10 stories tall. The first couple floors below had the best chance
to stop the progression, but we know from the result that there was just
too much inertial force to resist.

If you look closely at the collapse video, you can also see the outside
walls of each tower tended to funnel the debris in, at least initially.
As the unsupported vertical protrusion of the remaining tower walls
approached about 30 stories, the sides peeled. The peeling wall system
followed after the core and floor debris. Thus, the horizontal spread of
debris was limited to slightly more than the arc dimension of the peel.
I think the containment provided by the strong interconnection of
elements in the failing structure was what caused the collapse to be
mostly vertical.

Charlie



-----Original Message-----
From: David Merrick [mailto:MRKGP(--nospam--at)winfirst.com] 
Sent: Monday, April 24, 2006 12:16 PM
To: SEAINT
Subject: 9-11 free-fall

Has anyone predicted and measured the fall time of the 9-11 twin towers?

I have listened to a summary of an analysis that concluded that the 
world trade towers, fell (9/11/01) at the speed of a free falling 
object. They state that at free-fall speed, the complete decent should 
conclude in less than 10 seconds. Varying reports timed the fall, 
ranging from 8.2 to 9.4 seconds.

The lower floors would have had to crush under the falling floors above.

This resistance should have slowed the fall. The claim is that the 
energy absorption would have increased the fall time 3 to 4 times. It 
was concluded that the simplest solution is that the columns were 
destroyed, manually as the building fell, like in a controlled
demolition.

An asymmetrical airplane crash did result in a perfect vertical fall.

-- 
David B. Merrick, Structural Engineer


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