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RE: Lateral Brace Deflection

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Ed

Salmon & Johnson has a good treatment of point bracing for beams.  You want
to make the brace stiff enough to inhibit undesirable buckling shapes.  The
strength of the brace is then calcualted as a function of the crookedness
of the brace.


Mark Gilligan



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Message text written by INTERNET:seaint(--nospam--at)seaint.org
> 
I know that many use a factor of 0.02 of the design load on a compression
member to brace it, but what kind of deflection criteria should be used?

I'm bracing the top chord of a bowstring truss for a bridge (discussed
earlier on this list) with a diagonal fastened to an outrigger.  The
outrigger extends 6' out, so it will deflect close to an inch under the 3
kip vertical reaction from the brace (using a TS6x6x9/16).  With a 10' high
truss, that's  about 1.5" lateral movement of the top chord (2L6x4x5/8
LLH).
That's the center brace.  Each following brace would move successively
less.
So, I guess you could look at it as 100'*12/1.5 = 800; L/800 deflection.  I
don't want to over-simplify it here, but I'm guessing that most people
aren't doing P-delta analysis in each case, and there is some conventional
approach.  I haven't been able to find a discussion of it in any of my
books...unless I suppose a unbraced frame analogy could be made.

Advice from any of the members of this list is much appreciated.

Regards,

Ed Fasula
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