I believe * the planes and incompetent illegal construction did it.
I have to disagree about the incompetence and/or illegality of the
WTC construction. The buildings were built by a new (at the time)
method--that of using the central tower as the main support for the
floors and not using the outer walls as structural elements. This
design was neither incompetent nor illegal, and is now a fairly
common practice. By using it, buildings can be made stronger, taller
and of lighter weight (stronger and lighter, I'm happy with; taller
may be a questionable goal). The insulation applied to the structural
members of the tower was clearly not sufficient to resist the heat
from the massive fire that the spilled fuel created, and that was
what ultimately caused the buildings to collapse. But the possibility
of a fully loaded aircraft hitting the buildings was not contemplated
at the time, since that was not on the route out of either LaGuardia
or JFK. Inbound aircraft did come in that way, but they would not
have nearly the fuel loads of an outbound plane and so would not give
rise to the level of fire that was seen on 9/11. The design took that
possibility into account, but not the one that actually happened.
It was, in fact, the central tower supporting structure that allowed
the buildings to fail in the way they did, that is, to not topple to
the side, but to pancake on themselves. Had they toppled, the damage
and loss of life would have been much greater than it was. One design
flaw was the concentration of elevators and stairs around the central
tower, which meant that once the fire started, there was no escape
from above the level where the impact occurred. Had they put the
stairs on the corners of the building, or somewhere on the outer
walls, at least two of them would have remained open from above,
allowing many more people to escape. However, I'm not sure that such
an eventuality would have occurred to the designers then (although I
cannot imagine it not occurring today). It does seem reasonable,
however, that any number of possible problems could have made it
useful to have a distributed exit system from the buildings, so that
something happening near the central tower would not block exit from
the floors above, as happened on 9/11. It is likely that the corners
of the building were thought to be of more value as occupied space
rather than as escape routes, so that decision might have been made
in spite of the designers' wishes rather than because of them.
I think one can say that, given the construction standards at the
time, the WTC buildings were well and competently built, and even
failed within the parameters envisioned by the designers.
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