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RE: Questions About the Events of September 11th (off topic)

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I would ask those to consider charlie's comments as more correct than
mine. Mine are based on a documentary I saw about 6 months ago, I have
not read any of the papers which may now contradict the documentary or
come to more definitive conclusions.

In the documentary, they had two scenarios as to what went first, and
the there was some comments about the smoke and some slight twist prior
to collapse that supports the columns went first theory. Each building
may have failed differently depending on the fireproofing and location
of the impact. The documentary never really said definitively which
theory was more correct.

The other theory was that either the truss connections failed or the
columns deflected/distorted to the point that the trusses fell off their
supports one after another and the diaphragm restraint was lost as each
truss fell. As the domino effect of each truss falling (one end hitting
the floor below) compounded, the diaphragm could no longer support the
columns laterally and thus the extremely long buckling length of the
column. There was an animation in the program showing each truss
"slipping" off the bracket and the columns being unbraced for many
stories at a time. Then column buckled and all the floor above come
crashing down to start the pancaking of the floor below. It looked like
the columns came down after the floors on some of the video, not sure
which tower, which would make me think the floors went then the columns.

It seems to me that some of both had to happen with each one
contributing to the other's loss of integrity. Charles Thornton's belief
seemed to be that the floor truss connections were the culprit (followed
by his criticism of the design (I thought that was unfair)).

But as others have suggested, the FEMA document is likely the best
source and is likely the most technical in content.

I won't say anymore on this, I feel a little strange commenting on the
technical aspects of this since so many lost their lives. However, I
think there is much to learn structurally from this and I am grateful
for those who have educated the rest of us on the behavior of the
building in this terrible attack on innocent people. I hope we can
prevent these attacks rather than trying to make bunkers for every tall
building that becomes a target for those who hate our country.
Unfortunately, I think this is only the beginning.

-gerard
Santa Clara, CA

-----Original Message-----
From: Carter, Charlie [mailto:carter(--nospam--at)aisc.org] 
Sent: Monday, November 11, 2002 11:42 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Questions About the Events of September 11th (off topic)

A few important clarifications based upon what I know....

> ... the blast from the explosion blew off the
>fireproofing on the floor trusses.

This assumption is being questioned extensively right now. Some experts
have
advocated that the reason for differences in the collapse times for the
towers is that the North tower had twice as much spray-on as the South
tower. The only way that could make any difference is if the
fireproofing
were still in place.

> The heat from the fire caused weakening of the
>interior columns that had re-distributed loads
>and the lack of fireproofing & caused the
>trusses and their shelf angle supports to
>become over loaded. The shelf
>angle brackets then failed.

We will never know for sure, but I don't think those truss-to-exterior
wall
connections failed. I saw them still attached to the structural elements
(perimeter columns and truss chords) in the piles of debris. And from
what I
saw, it appeared to me that they would have had to have been torn apart
in
the collapse, not at the initiation point, though. Many times, it was
the
truss chords that had fractured from the welded gussets (not the
two-bolt
erection connections that were widely reported as being the "cause" of
the
collapse).

>If the fireproofing could have stayed on the
>trusses, I think this only would have made
>the building stand up maybe an hour longer
>(saving more lives)

As mentioned above, it may be that the fireproofing did remain adhered
much
more extensively than at first thought. Where it did come off, I do not
think anybody has anything that would have remained intact. Some
advocated
that we should return to wrapping the steel in concrete, but the
concrete
cover was obliterated in the Pentagon attack. The 14-in.-square RC
columns
became round with exposed rebar in the path of the plane there.

Unfortunately, the popular press is either lost on these and similar
aspects
or does not care.

Charlie






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