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RE: Increaing double-angle brace compressive strength

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How do you bolt in case B?

Are the long legs back to back?

Can you weld instead of bolt?

You are trying to reduce kl/r right? r=sqrt(I/A) Need to carefully
choose the orientation of the new steel to maximize the I/A ratio.

Can you decrease K by doing something with the end connections to create
a fixity?

Are you treating these as individual angles? Or are they acting together
Via a stitch plate? Might be easier/cheaper to simply replace the braces
or install them as X-braces so they only need to act as tension only
braces.

Is it out of plane buckling or in plane buckling? (this gets back to
whether the long legs are back to back.)

Just some thoughts,
-gerard
Santa Clara, CA

-----Original Message-----
From: Dave Evans [mailto:DEvans(--nospam--at)tnh-inc.com] 
Sent: Thursday, August 01, 2002 7:57 PM
To: seaint(--nospam--at)seaint.org
Subject: Increaing double-angle brace compressive strength

We need to increase the compressive strength of some double 
angle braces. Y-Y axis buckling is the problem. Our first thought 
was to bolt a channel to each one so that the channel web is in 
contact with the angle outstanding legs and the angle outstanding 
legs nestle between the channel flanges thus (hope this art comes 
thru):
------------------
|  ----   ----   |
       | |
       | |

Call this Case A.  It is easy to analyze using the "lean-on" bracing 
concept.

Where this arrangement is physically impossible because of 
attachments to the brace, it would be possible to invert the channel 
and bolt it to the back-to-back legs thus:

   ----   ----   
       | |
|      | |        |
------------------

Call this Case B.

I like Case A much better than B, mostly because in Case A, the 
shear centers of the channel and double angles are relatively close 
to each other.   In Case B, they are quite distant.  The problem is 
figuring out whether or not there is a significant difference in the 
effect of the reinforcement.  

I've considered the column-constrained-to-buckle-about-a-fixed-axis 
concept for Case B, but that's not completely accurate.  Might be 
good enough, though.  Any comments or other ideas on how to 
approach Case B would be appreciated.

Dave Evans, P.E.
TNH, Inc.

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