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Re: Lateral Brace Deflection

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-----Original Message-----
From: Ed Fasula <tibbits2(--nospam--at)metro.lakes.com>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Saturday, August 14, 1999 7:26 PM
Subject: Lateral Brace Deflection


>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?

This brings to mind a comment regarding bracing from Jack Boukamp. who said
something like "A brace to an initially straight member could be a piece of
cardboard, since the buckling force is nil until the chord is allowed to
move"
>
>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

Clearly, the chord IS allowed to move in this case, but why so much?: An
inch for a 72 inch outrigger is, on the face of it, L/72, and seems quite
flexible.  Obviously, "seems flexible" doesn't establish a criteria, but
isn't a numerical evaluation of the top chord for p-delta  fairly simple?
Assume a parabolic shape and find the incremental deflection due to 3 inch
lateral movement. If  this additional deflection is "slight" it's OK if not,
stiffen the outrigger.  See the slender wall procedures in the Masonry
section of the UBC for help in determining "slight" that denotes
convergence.