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Re: Structural failure of NY's WTC



At 10:34 -0700 9/16/01, Tucker Hiatt wrote:

Moral astonishment aside, I was completely taken by surprise upon
observing the physics of collapse of the World Trade Center
buildings. Reading reports such as
<http://abcnews.go.com/sections/scitech/DailyNews/WTC_towers_structure010911.html>
has helped very little.

Would any PHYS-Ler care to speculate regarding why a structural
failure (due to intense heat) at or above mid-section would lead to a
domino-like structural failure all the way down? Clearly, many
experts in structural integrity were surprised when the collapses
occurred -- some of those experts were rescue workers toiling
underneath the doomed buildings.

I think Ludwik's response (not copied here) is pretty much it. I
heard a discussion from a structural engineer on the radio Tuesday.
He said basically that the structure gave way because the intense
heat substantially reduced the tensile strength of the steel used in
the frame, which then failed under the weight of the structure above.
When the stucture above, weighing upwards of 30,000 tons, hit the
undamaged floors below, now with some momentum, the impulse thus
imparted to the lower structure was sufficient to cause it to
progressively fail as the increasing mass from above hit each
successive floor with increasing impulse--just like an avalanche.

I don't know if the planners of this event anticipated the result, or
not. The fact that they chose to hijack aircraft that had just
departed and thus were still heavily loaded with fuel, which would
certainly start large fires, indicates that they may well have done
so.

Shortly after WWII, when a B-26 bomber on final approach to LaGuardia
airport in bad weather, hit the Empire State Building, there was no
fire since the plane was low on fuel, and the resulting damage was
significantly less. Of course, there were other factors in play here
as well. The B-26 was considerably lighter and slower than the
airliners used on Tuesday, so the energy of the impact was a lot
less. But if the plane had been loaded with fuel (in those days, the
much more volatile aviation gasoline), the resulting fire could have
had the same catastrophic results with the ESB that the airliners had
with the WTC. In other words, it was the fire and not the collision
itself which led to the collapse of the buildings.

It also seems to me that the escape routes were partly to blame for
the loss of life in the buildings. Several of those who did make it
out said that the stair wells quickly filled with smoke and dust, and
that they also were jammed with the people trying to escape which was
further complicated by the fire and rescue people trying to get up
the stairs. The smoke problem in the stairwells has been reported in
other building fires as well. It would seem to me that some means of
keeping the smoke out of the stairwells would be vitally important in
these emergencies, and either making the stair wells wider as one
descends or increasing the number of stair wells from the lower
floors would be prudent design features in future high-rise
buildings. I doubt that we will ever again fail to evacuate an entire
building when a fire starts on an upper floor, anticipating a
possible repeat of Tuesday, so without some increase in stair
capacity in the lower floors, traffic jams such as occured on Tuesday
will be inevitable.

Hugh
--

Hugh Haskell
<mailto://haskell@ncssm.edu>
<mailto://hhaskell@mindspring.com>

(919) 467-7610

Let's face it. People use a Mac because they want to, Windows because they
have to..
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