9/11 Controlled Demolition or Progressive Collapse?
I share this article not only because of my natural interest as a Structural Engineer
dedicated to failure analysis, but because more than 20 years after one of the greatest
tragedies in recent history, a certain veil of uncertainty still persists in public opinion
regarding the causes of the collapse of the Twin Towers in New York in 2001.
Controlled Demolition
The theory of a controlled demolition has not only been supported by conspiracy
theorists, but even by professionals in Architecture and Structural Engineering, who
even produced a documentary called “9/11 Explosive Evidence ” ( 9/11: Explosive
Evidence - Experts) . Speak Out (Free 1-hour version ) AE911Truth.org ), a hypothesis
that is mainly supported by the detection of “ nanotermite ” at the collapse site, a
material commonly used in the controlled demolition of buildings and in the
acceleration of the fall of the towers, very close to that of free fall.
Other evidence, such as a video of Larry Silverstein (Master Concessionaire of the WTC
in 2001) saying that in conversation with the Fire Department, the decision was made
to " pull it " the WTC-7 building, coupled with the purchase of a terrorist insurance policy
just two months before the attack, only increase doubts about a possible involvement
in the collapse of the towers. InsuranceCoverageNewsletterSummer2004.qxd
“I remember getting a call from the fire chief telling me they weren't sure they
could contain the fire. And I said, 'We've already lost too many lives... maybe the
sensible thing to do is pull it down. '” “ It ) '. They made that decision, and we
saw how the building collapsed.” Larry Silverstein admits WTC7 was pulled
down on 9/11
For this reason, I feel almost obliged to share (in a very condensed manner) with the
professional community the official version, as well as the results of subsequent
research, since before taking a position on the causes, it is best to be informed of the
available evidence.
Structural System
The structural system of the towers is called "Tube within a Tube." It consists of a central
core of 47 steel columns, providing high rigidity to the building, surrounded by a
perimeter system of columns, allowing for ample open spaces for offices.
Each floor consisted of a concrete slab on a steel truss , which extended from the core
to the perimeter. These trusses were connected by brackets with viscoelastic dampers,
which allowed them to dissipate deformations caused by earthquakes and wind.
The Fire
The planes that hit both towers became embedded in the steel cores, causing a fire
fueled by the aircraft. Recovered evidence indicates that the steel reached
temperatures exceeding 250°C. Steel begins to lose significant strength at 500°C;
however, thermal degradation in steel also depends on expansion, the slenderness of
these elements, and the existing protection. The following image shows how the lattice
system supporting the building's slab presented deficiencies in its protection scheme,
which could easily lead to heating of the section due to its high thermal conductivity.
The deformation of the lattice due to the expansion, the weight of the slab, and the
resulting failure of the floor-level support connections could have induced
deformations in the perimeter columns of the building, triggering the buckling
phenomenon, caused by the weight of the upper area subjected to the impact of the
aircraft.
The mechanism described above would also explain why the second tower impacted
collapsed first, which would be because the impact it received was at a lower level than
the impact of the first, so the weakened area of the structure had to support, in this
case, a greater weight of the upper structure.
Once the top block began to fall, the rest of the structure could not withstand the
impact of the descending mass. The lower floors offered minimal resistance to the
collapse, resulting in a rapid, vertical fall.
Unfortunately, the available evidence does not confirm the level of damage suffered by
the central core columns as a result of the impact, which could undoubtedly have been
another determining factor in the observed mode of collapse.
Conclusions
The official version offers significant evidence that would explain the observed failure
mode; however, as with any investigation of this type, the lack of sensors and
monitoring prevents access to precise data on the stress distribution in the structure
prior to collapse, which represents a limitation for its analysis.
Structural maintenance is a key factor, especially when a structure is subjected to
extreme events. In this case, an adequate fire protection system could have delayed
the collapse, saving a greater number of lives and allowing time for evacuation.
Finally, believing one hypothesis or another could also reflect the trust people have in
government institutions, which can be a bias that can distort our engineering judgment,
so it's worth asking the question:
I don't believe the collapse mode...or I don't believe whoever is saying it?
Note: This article is for informational and reflective purposes only; it does not attempt
to address the full range of aspects involved in this type of event.
Gabriel Canales H.
Civil engineer
https://www.instagram.com/structural_learning_/
BONUS TRACK:
• The expression “ pull-it ” in the context of Larry Silverstein ’s statement could
also have meant “pull them away,” referring to the fire crews on the scene, to
prevent further loss of life.
• According to NIST ( National Institute of Standards and Technology ), regarding
the “nanotermite ” found at the site, indicates that there is no chain of custody
supported by such evidence, which would allow us to conclude that it really
belongs to the collapse site.
