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RE: length to be used for tension bracing kl/r

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If you were to use rods then a note in Section B7 under the L/r <= 300
states "The above limitation does not apply to rods in tension."  For this
case it is clearly stated that tension rods do not need to meet the
slenderness recommendation.  

I don't believe that this same exemption applies to angles.  If your angles
connect with a gusset plate in the middle then I would definitely consider L
as the length of each segment and not the total length.  For the case with
alternating orientation of single angle brace members that may connect in
the middle with a bolt or weld then that is more of a question.  If you
assume the only intent is as per the commentary and avoid slapping or
vibration then I would consider each segment separately again.  Again the
code commentary states that the limitations are not mandatory.  

Code wise I don't think you would even be in trouble if you didn't check the
slenderness.  However, I think there is another side to the slenderness that
should be considered.  This is the erection of the members.  Erecting an
excessively slender member will be like putting up a "wet noodle." 

Angle bracing by itself can be more difficult to erect than rods due to its
lack of adjustment.  Bolted connections can be difficult to align while a
member is sagging under its own weight and field welding requires them to
remove as much slack as possible before welding.  I would recommend, at
least on one end, the use of an erection bolt and then field weld the angles
to the connections.  I guess that way you get the best or worst of both
ways, depending upon how you look at it.

Slapping can be prevented by use of a mid span connection to the other brace
member.  This can be a bolt, weld, or as simple as a wire tie connecting
them together.

The short of it is that I think you are fine using 1/2 of the member length
(assuming the braces cross each other at midspan).  But don't overlook
erection considerations also.  I recommend watching an erection crew put up
a long slender angle if you get a chance.  It can be very enlightening.
Just watch out for thrown objects if you were the designer on that project.
:0)

HTH,
Greg Effland, P.E.
KC MO

-----Original Message-----
From: Ken Peoples [mailto:kspeoples(--nospam--at)yahoo.com]
Sent: Thursday, June 28, 2001 9:14 AM
To: seaint(--nospam--at)seaint.org
Subject: length to be used for tension bracing kl/r 


I seem to be constantly coming up with bracing member sizes that are
controlled by kl/r instead of the force in the member.  I have been
using the full length of the member when evaluating kl/r.  My question
is this: for X-bracing angles that are designed for tension only with
the kl/r<300 recommendation, is it reasonable to use a length of 1/2
the actual length when the angles are bolted together at the center.  I
know that the other angle wouldn't give it much lateral support to
prevent buckling in one of the directions if I were designing for
compression, but to satisfy the l/r<300 recommendation for tension
members, I would think that it might be OK.  I understand from the AISC
9th ed. commentary on B.7 that the recommendation is made to "afford a
degree of stiffness such that lateral movement ("slapping" or
vibration) will be avoided. What approach do most engineers use?
Thanks for your thoughts.
Ken

=====
Kenneth S. Peoples, P. E.
Lehigh Valley Technical Associates
1584 Weaversville Road
Northampton, PA 18067-9039
ph: 610-262-6345
fax: 610-262-8188
e-mail: kpeoples(--nospam--at)lvta.net


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