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RE: Question about brace design forces for columns

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The rule of thumb is most widely attributed to Joseph Yura from the
University of Texas at Austin.  He has lectured extensively on bracing, and
has published in the AISC Journal as well as the Structural Stability
Research Council Conference.  The proofs always go back to Timoshenko's
Theory of Elastic Stability.

Braces must have adequate strength and stiffness.  Most all braces are
governed by strength, but occasionally they are driven by stiffness as was
the case in the Hartford Civic Center.  So just checking the 2%
approximation will generally keep you out of trouble, once in a while
stiffness will control.

There are bracing requirements, but they are hard to find.  They are a
little more apparent in the AISC LRFD.  I guess the code developers feel
that it is a first principle.   

Regards,
Harold O. Sprague

-----Original Message-----
From: Clifford Schwinger [mailto:clifford234(--nospam--at)yahoo.com] 
Sent: Wednesday, September 24, 2003 9:51 AM
To: seaint(--nospam--at)seaint.org
Subject: Question about brace design forces for columns

Could someone please explain to me where the
rule-of-thumb that column braces be designed for 2% of
the column vertical load comes from? (I think in Great
Britain there is a code requirement to use a value of
2.5%)

If 1:500 out-of-plumbness is allowed, then a brace
that's restraining the top of one column and the
bottom of another column (which are each kinked 1:500
out of plumb in a "V" shape) will have a theoretical
brace force of 0.4%.  Assuming that you impose a brace
stiffness requirement that restricts brace deformation
(and additional associated horizontal column movement
at the brace connection) to another 1:1000, you would
still only have a theoretical brace force of 0.6%. Is
the 2% rule of thumb used to provide a safey factor
(of about 3) on the very critical brace member, or to
account for other unanticipated lateral deformations
at or near the brace connection?  Maybe I just
answered my own question, but I'd like to get some
opinions on this issue.

Also, does anyone know why there are no codified brace
force design requirements in U.S. codes and standards?
The European codes all seem to provide specific design
criteria for braces.

Thanks!


Cliff Schiwnger 
 

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