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>I have no quarrel with Mr Wright but, to set the record straight, I seem to
>recall that William LeMessurier picked up his own error while checking into
>a contractor requested change in bracing connection details.
If the following account 
<> is correct, we're 
all about 1/3 correct. But I think the point is made--a review of the 
design caught the error. Truth to tell the biggest thing about the whole 
business is not the error but Messurier's reaction to it. Everyone make 
mistakes, but not everyone owns up in such an exemplary fashion.

-------Exerpt begins---------
 In June 1978, a month after LeMessurier was told of the switch from 
welds to bolts in the Citicorp building, he received a telephone call 
from a student. This student's professor had been studying LeMessurier's 
Citicorp design and had concluded that LeMessurier had put the building's 
nine-story supports in the wrong place. The supports belonged on the 
tower's corners, according to this professor, not at the tower's 

The professor had not understood the design problem that had been faced, 
so LeMessurier explained his entire line of reasoning for putting the 
tower's supports at the building's midpoints. He added that his unique 
design, including the supports and the diagonal-brace system, made the 
building particularly resistant to quartering, or diagonal, winds -- that 
is, winds coming on the diagonal and so hitting two sides of the building 
simultaneously. Pictured is a diagram of why perpendicular winds cause 
sway in a building.

Shortly thereafter, LeMessurier decided that the subject of the Citicorp 
tower and quartering winds would make an interesting topic for the 
structural engineering class he taught at Harvard. Since at the time the 
requirements of the New York building code, like all other building 
codes, had covered only perpendicular winds, LeMessurier did not know how 
his design would fare in quartering winds.

Interested to see if the building's diagonal braces would be as strong in 
quartering winds as they had been calculated to be in perpendicular 
winds, LeMessurier did some computations. He found that for a given 
quartering wind, stresses in half of a certain number of structural 
members increased by 40 percent.

Then he became concerned about the substitution of bolts for welds. Had 
the New York contractors taken quartering winds into account when they 
replaced the welds with bolts? Had they used the right number of bolts? 
The second question was particularly important -- a 40 percent increase 
in stress on certain structural members resulted in a 160 percent 
increase of stress on the building's joints, so it was vital that the 
correct number of bolts be used to ensure that each joint was the proper 

 What he found out was disturbing. The New York firm had disregarded 
quartering winds when they substituted bolted joints for welded ones. 
Furthermore, the contractors had interpreted the New York building code 
in such a way as to exempt many of the tower's diagonal braces from 
loadbearing calculations, so they had used far too few bolts.

Shaken, LeMessurier reviewed old wind-tunnel tests of the building's 
design against his new quartering-wind calculations (these tests had 
modeled a large part of midtown Manhattan), and found that under adverse 
weather conditions, the tower's bracing system would be put under even 
further stress. The innovative tuned-mass damper, designed to reduce the 
building's normal slight swaying, was not designed to keep the building 
from being blown down in a major storm; this further worried LeMessurier.

Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)

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