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RE: FORENSICS: World Trade Center Bombing

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I was one of the 'structural' engineers involved in the reconstruction of
the WTC after the February 1993 explosion.  My time on site was
approximately 11 months.  I seem to recall an extensive article in ENR on
this topic also.  

The following is my opinion based on what I observed:

Two of the subgrade bracing diagonals were damaged.  One was next to the
bomb and completely blown off and through several walls (the steel diagonal
is now a lawn sculpture at the structural engineering company owner's house
in CT) and the other was bent.  Several of the floor beams adjacent to the
bomb suffered local web and flange crippling but did not collapse.
Approximately two to four of the main box columns had cracked welds.
Several CMU walls and other architectural elements were damaged due to the
explosion and resulting fire.  At no point do I believe that the tower was
in danger of collapsing; yes, it would not perform as designed but with
redundancy of the structural members, all loads would have found another
safe path.  The shock wave appears to have "reflected" off the more highly
loaded tower structure into the adjacent six-level below grade mechanical
and parking structure.

The mechanical and parking structure did not fare as well.  The concrete
flat plate floors collapsed progressively to the chiller plant on level B5.
The plaza level had a hole blown through it (about where the van was parked)
and several steel connections had cracked welds, but did not collapse.  The
steel columns with practically intact shearheads were still standing
unbraced about forty feet instead of the design interval of ten feet.  The
emergency effort was to brace these columns to prevent the collapse of the
plaza level above. 

Reconstruction efforts took about two years to finish.  The towers were
opened in about a month after the event after being extensively cleaned and
updating the security of the complex. 

In summary, the WTC towers were designed to withstand the impact of a large
passenger plane common in the late 1960s traveling at landing speed due to
the relative proximity to three airports (JFK, LaGuardia, and Newark).
However, the wind loads determined by wind tunnel testing were the governing
load case.  After reviewing all the information, a bomb placed adjacent to
one of the tower columns is not sufficient to endanger the tower.  Several
steps similar to imploding operations would have to be done to take down the
WTC towers which I am thankful that the "terrorists" will never have an
opportunity to do that.

William J. Keil, P.E.

-----Original Message-----
Subject: FORENSICS: World Trade Center Bombing
[QUOTE MODE ON]

In the last twenty years, the media have entertained far too many
"Chicken Little experts" proclaiming that terrorism is at our
doorstep. For that reason, we have become somewhat desensitized to the
issue and view it as a distant problem (unless, of course, you are
from New York, where Islamic terrorists came very close to collapsing
one of the World Trade Center towers with a bomb).

[QUOTE MODE OFF]

This leads me to ask: "How close" DID the bombing of the WTC Tower come to
collapsing the building? It is so long ago--and I was much less informed of
things structural then--that I can't recall the assessment made by
structural engineers.