The World Trade Center towers used neither a steel skeleton nor reinforced
concrete. They were designed as square tubes made of heavy, hollow welded
sections, braced against buckling by the building floors. Massive Foundations
descended to bedrock, since the towers had to be safe against winds
and other lateral forces tending to overturn them. All this was taken into
consideration in the design and construction, which seems to have been
first-rate. It
was ensured that its foundation are strong enough to with stand any
attempt to damage the buildings by a bomb at the base had
negligible effect. The strong base and foundation would repel any such assault
with ease, as it indeed did. The impact of aircraft on the upper stories had
only a local effect, and did not impair the integrity of the buildings, which
remained solid. The fires caused weakening of the steel,
and some of the floors suddenly received a load for
which they were not designed.
What happened next was unexpected and catastrophic. The slumped floors
pushed the steel modules outwards, separating them from the floor beams. The
next floor then collapsed on the one below, pushing out the steel walls, and
this continued, in the same way that a house of cards collapses. The debris of
concrete facing and steel modules fell in shower while the main structure
collapsed at almost the same rate. In 15 seconds or so, 110 stories were reduced
to pile 9 stories high, mainly of steel wall modules and whatever was around
them. The south tower collapsed 47 minutes after impact, the north tower 1 hour
44 minutes after impact. The elapsed times show that the impacts were not the
proximate cause of collapse; the strong building easily withstood them. When
even one corner of a floor was weakened and fell, the collapse would soon
propagate around the circumference, and the building would be lost.
It is clear that buildings built in this manner have a catastrophic mode of
failure ("house of cards") that should rule out their future
construction. It is triggered when there is a partial collapse at any level that
breaks the continuity of the tube, which then rolls up quickly, from top to
bottom. The collapse has a means of propagation that soon involves the whole
structure, bypassing its major strengths and impossible to interrupt. There is
no need for an airliner; a simple explosion would do the job. There were central
tubes in the towers, for elevators and services, but they appeared to play no
substantial role in the collapse, and were not evident in the pictures or
wreckage.
Detailed explanation of the Collapse from videos
released
It was very interesting, but
seemed to concentrate on the effects of fire and on building safety, rather than
on the collapse mechanism. When the collision of an airplane with a tower was
shown, the bang was simultaneous with the flash, showing that we were viewing an
edited version. The connections of the floor trusses with the external wall were
indeed not very substantial--two 5/8" bolts each, so the floors and the
wall could easily be separated.
The role of the fire in weakening the steel where the collisions occurred is
undoubted, as is the ineffectiveness of the fire protection foam, which seems to
have been blown off. However, this can only cause a local collapse, depositing
an unusual load on the floors below. It is their response, of a structure
unweakened by fire or impact, that is significant, and this topic was brushed
aside in the program. In fact, an erroneous graphic of floors collapsing on one
another successively, "pancaking," was shown, while the collapse of
the towers was quite different, the upper floors ending up on the bottom of the
pile and the lower floors on the top. One commentator actually mentioned the
buckling of the wall (without mentioning buckling), but did not follow up.
More detail was presented on the core, which contained the stairways and
elevators, plus building services such as firefighting water (which was only
interrupted in the North Tower). This core never appears in the videos as an
element of strength, though the floor trusses were supported on it. One might
suspect that when the outer walls failed, the core was
simply pulled apart and collapsed. The collapse of the North Tower
shows the TV antenna initially falling, though the walls were already clearly in
collapse. The conclusion that the central core failed in this case pulling down
the outside seems very ill founded. In the views of the South Tower, there is no
evidence at the entire core. In both cases, the collapse was simultaneous around
the building, not asymmetrical.
It should be recognized that the damage to the towers was different, as the
program made clear, so we have two examples of this kind of failure, not just
one. The program stays away from the embarrassing conclusion that this kind of
structure has an inherent failure mode, as it has been suggested. Perhaps more
examples can be given to make this clear. The Empire
State Building was also struck by an airplane (a bomber) that did
considerable damage, but there was no hint whatever that the building was in
danger. One suspects that if an airplane struck a building with a volume
skeleton, there would be no total collapse, only local damage. Rubbish from the
collapsed part would fall outward to the sides, not pry the building apart from
the inside.