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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.

Greg Effland, P.E.

-----Original Message-----
From: Ken Peoples [mailto:kspeoples(--nospam--at)]
Sent: Thursday, June 28, 2001 9:14 AM
To: seaint(--nospam--at)
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.

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)

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