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The complete collapse of the Twin Towers has more to do with
the construction than anything else. Yes, it was the loss of
fireproofing and subsequent fires that collapsed the
building but if it were not for the construction, a total
collapse may not have occurred. Ironically, the construction
also saved many lives by letting the towers stand as long as
they did. These pages explain the construction and provide
evidence for collapse by fire.
"Tube in a
Tube"
The
designers coined the term "Tube in a Tube" to describe the
buildings’ construction. The design was an innovation of its
day. Thick steel columns ran up the middle of the building.
This housed the elevators, stairwell, electrical conduits,
water, sewer and other services which ran up and down the
length of the buildings.
For
you to understand the collapse, you will have to remember
four main elements.
Core
columns
Perimeter columns
Floor Trusses
Fire
proofing
The
columns of the building normally found evenly spaced out on
a given floor became the outer wall of the building. This
left large open areas for renting. A good explanation of
this can be found on
PBS.
Instead of encasing each column in heavy concrete, (normal
fire proofing) the designers relied on 'sprayed on' fire
proofing with a 2 hour rating to protect the load bearing
columns and trusses from fire.
Each
WTC tower was essentially bolted together like an erector
set. Each average floor had 376 5/8" bolts, 188 Truss seats,
120 ViscoElastic pieces and 120 gusset plates.
Below is a diagram of the truss system with connections.
Trusses connected
Perimeter columns to the core. Without this connection
neither one could stand alone. Note how the columns are
constructed in threes known as column trees. Each column is
bolted onto the column beneath and to the tree beside it.
There was a 3 to 4 inch layer of concrete on the average
floor. There were reinforced concrete floors in the core as
well. Contrary to what some conspiracy theorists say, the
core walls were NOT concrete reinforced. The columns in the
core were also not incased in concrete. This was an error
made by the BBC which grew a life of its own. Here is the
article. Note the date it was created, Sept. 13, 2001, just
2 das after the collapse.
Here is a photo
of the core sub levels during construction. You can see just how deep
below ground level the buildings went. You can also see no
concrete around the core or columns anywhere.
This type of
construction makes a very open floor plan.
The fire
proofing was sprayed on after the steel went up. This is
called "Passive" fire proofing.
There is a big difference between this type of construction
and others like the Landmark Towers in Fort
Worth,
Texas,
which is often used as a comparison by conspiracy theorists.
There are
large I-Beam columns encased in concrete. Large I-Beams also
hold up the floors. The photo below was taken during
preparations to demolish the building.
Note the
vertical column above still has some concrete around it.
If the towers had concrete around the columns, you would
have seen columns with damaged concrete around them similar
to this.
Now you have
the general idea of the differences in construction between
the towers and other high rise buildings. So why did it
collapse the way it did?
A major
factor was the fire proofing on the impact floors. The
impact blew off the fire proofing as in the above PBS
special.
