Why the Towers collapsed

Louis Proyect lnp3 at panix.com
Tue Sep 11 13:16:24 MDT 2001


[One of the things that puzzled me was how the crashes could have 
brought the two buildings down, especially since the planes hit a 
fairly high altitude. This exchange on Henwood's list explains it, 
for what it's worth.]

>From: kelley <kwalker2 at gte.net> 
> 
><forwarded> 
> 
>A word on the structure of the WTC towers: 
> 
>The WTC towers had a distinctive structural system which utilized 
>the exterior wall framing for lateral bracing -- a so-called lattice 
>framework. This allowed minimization of internal lateral bracing 
>and opened up the floor plans. You can see the effect of that when 
>the buildings collapsed, with the lattice framework crumbling and 
>the interior imploding. The lattice works so long as it remains 
>intact as a system: if a part of it goes, then the whole system 
>goes. 


Engineering issues aren't at the top of anyone's mind, I know, but 
this structural factor offers an interesting contrast with what 
happened to the Empire State Building (relatively little) when it was 
hit by a B-25 in 1945. The following is from one web source on this: 

Mitchell Bomber vs. Empire State Building 

© Copyright 1999, Jim Loy 

On Saturday, July 28, 1945 (a few days before the Atomic bomb was 
dropped on Hiroshima), a B-25 Mitchell bomber ran into the Empire 
State Building, then the tallest structure in the world. The bomber, 
piloted by Lt. Colonel William F. Smith, was flying under clouds, 
from Massachusetts to New Jersey. At about 10 A.M., the bomber hit 
the 79th floor, killing the three men aboard instantly. One of the 
two engines went through the building and out the other side, and 
through the roof of a 20-story building on the other side of 34th 
Street, starting a fire. The other engine, and part of a landing gear 
entered an elevator shaft and fell to the basement, onto an 
unoccupied elevator. Two women in another elevator fell 75 stories, 
and survived with serious injuries. Eleven people died in the fire on 
the 79th floor. 

Why didn't the Empire State Building fall down? Well, an airplane 
(even a bomber) is fairly insignificant compared to the massive steel 
and concrete building. And, as explained by Levy & Salvadori, in Why 
Buildings Fall Down, the Empire State Building had built-in 
redundancy. No single beam held up the building. As it turned out, 
none of the vertical beams was severed, although two of them were 
struck by wings. 

On May 20, 1946, another military airplane, lost in fog, hit the 58th 
floor of a building on Wall Street, killing the five men on board, 
and injuring no one else. 

(end) 

Carl 


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