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Actually i think explained fairly well why the building did not collapse because of gravity acting alone
if it collapsed from just the gravity alone there is actual slowling process and the main structure below would be absorbing the kinetic energy
when the top is cruching down there is what is called a Crush up effect.
I have a little a home experiment for you....take some paint cans....
the towers were not symetrical in thier construction...the base of the towers used more robust materials in the first twenty floors of the towers...so it had even more structural integrity than the upper...
Mechanics of Progressive Collapse:
Learning from World Trade Center and Building Demolitions
Zdenˇek P. Baˇzant 1 , F. ASCE, and Mathieu Verdure 2
Abstract: Progressive collapse is a failure mode of great concern for tall buildings, and is also
typical of building demolitions. The most infamous paradigm is the collapse of World Trade Center
towers. After reviewing the mechanics of their collapse, the motion during the crushing of one
ﬂoor (or group of ﬂoors) and its energetics are analyzed, and a one-dimensional continuum model
of progressive collapse is developed. Rather than using classical homogenization, it is found more
eﬀective to characterize the continuum by en energetically equivalent snap-through. The collapse,
in which two phases—crush-down followed by crush-up—must be distinguished, is described in each
phase by a nonlinear second-order diﬀerential equation for the propagation of the crushing front of
a compacted block of accreting mass. Expressions for consistent energy potentials are formulated
and an exact analytical solution of a special case is given. It is shown that progressive collapse will
be triggered if the total (internal) energy loss during the crushing of one story (equal to the energy
dissipated by the complete crushing and compaction of one story, minus the loss of gravity potential
during the crushing of that story) exceeds the kinetic energy impacted to that story. Regardless of
the load capacity of the columns, there is no way to deny the inevitability of progressive collapse
driven by gravity alone if this criterion is satisﬁed (for the World Trade Center it is, with an order-
of-magnitude margin). The parameters are the compaction ratio of a crushed story, the fracture of
mass ejected outside the tower perimeter, and the energy dissipation per unit height. The last is the
most important, yet the hardest to predict theoretically. Using inverse analysis, one could identify
these parameters from a precise record of the motion of ﬂoors of a collapsing building. Due to a
shroud of dust and smoke, the videos of WTC are useless here. It is proposed to obtain such records
by monitoring the precise time history of displacements in diﬀerent modes of building demolitions.
The monitoring could be accomplished by real-time telemetry from sacriﬁcial accelerometers, or
by high-speed optical camera. The resulting information on energy absorption capability would be
valuable for the rating of various structural systems and for inferring their collapse mode under
extreme ﬁre, internal explosion, external blast, impact or other kinds of terrorist attack, as well as
earthquake and foundation movements.
Dr. Bazant attempts to explain the balance of energies at a point in time immediately after
collapse initiation. He states that,
" To arrest the fall, the kinetic energy of the upper part, which is equal to the potential
energy release for a fall through the height of at least two floors, would have to be
absorbed by the plastic hinge rotations of one buckle, i.e., Wg/Wp would have to be less
than 1. Rather, Wg /Wp ?= 8.4 (3) if the energy dissipated by the columns of the critical
heated floor is neglected."
3. Pancake Theory Not Supported
NIST: “NIST’s findings do not support the “pancake
theory” of collapse, which is premised on a progressive fail-
ure of the floor systems in the WTC towers… Thus, the
floors did not fail progressively to cause a pancaking phe-
Agreed: the “pancake theory of collapse” is incorrect and
should be rejected. This theory of collapse was proposed by
the earlier FEMA report and promoted in the documentary
“Why the Towers Fell” produced by NOVA . The “pan-
cake theory of collapse” is strongly promoted in a Popular
Mechanics article along with a number of other discredited
ideas [8, 9]. We, on the other hand, agree with NIST that the
“pancake theory” is not scientifically tenable and ought to be
set aside in serious discussions regarding the destruction of
the WTC Towers and WTC 7.
Originally posted by -PLB-
I can not really believe you are a structural engineer, or if so you are terrible at it. Statements like "the structure below will..Resist the falling mass....SIMPLE" are simplistic to such a degree, I would expect it to come from a total layman, but not somebody with an education in structural analysis....
This isn't simple at all, and you can't just intuitively say what will happen.
Originally posted by Drunkenparrot
The WTC towers both fell because of gravity acting alone. There were a series of events preceding the collapse that allowed gravity to do its work but the collapse was soley due to gravity.
Originally posted by pteridine
I didn't make any such claim. You misinterpreted what I wrote and didn't bother to read any responses.