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RE: UBC 1630.8.2.2.1.7 Bracing Load

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Jim,
 
I was not following this topic so you may already have your answers, but here is my 2(Canadian)Cents:
 
Not mentioning the stiffness requirement for the brace, that force is what we need to create a "braced point". The force is different for different sections as you mentioned. As for the distance between the "braced points" in mid-span and where we would have them; let's assume that we have a 30' span. If the beam we choose (for force or deflection reasons) can resist the factored moment with the 30' un-braced length (see Beam Selection Tables, page 5-71, LRFD 3rd Ed. or similar)then we don't need any mid-span "braced points". Now if we choose another beam that can resist the moment with 15' (or 10' or L_sub_b < L_sub_P) un-braced length then we need one (or two or more) mid-span braced points. As these beams are different, the bracing force will be different. That is how you get to pounds of force from feet of distance between braced points. 
 
 
HTH,
Reza Dashti P.Eng
Vancouver, BC





Date: Mon, 8 Oct 2007 08:07:51 -0700
From: omega.two.0(--nospam--at)gmail.com
To: seaint(--nospam--at)seaint.org
Subject: Re: UBC 1630.8.2.2.1.7 Bracing Load

Wesley and Jim,
 
Thanks for the AISC link but I have always been curious about this force.  The AISC design force (or the 2%, etc force) is in pounds but say, for a simple beam with only the bottom flange continuously braced, how often should the top flange be braced?  At the Lu points? Lc distance? An arbitrary distance?  If say I picked 2 ft. increments then is that same force required at each one?  Or if I doubled that length then is it still the same force but only every 4ft.?  This has always puzzled me since the force is in pounds but the application distance is in feet.
 
I also noticed in Appendix 6 that the force is based on Mr, not the capacity of the beam.  Most of my beam selections are based on deflection not stress so it stands to reason that the brace force should be based on the required stress in the beam not the capacity of the beam.
 
One more comment.  Appendix 6 requires a brace on the tension flange at the end of a cantilever beam.  This seems reasonable for a beam loaded on the top flange but what if the load to the end of the cantilever is applied at the bottom flange?  It seems to me that this is a condition where it would be difficult or impossible for the end of the beam to rotate.  Any comments?
 
Jim Persing

 
On 10/3/07, Bill Allen <T.W.Allen(--nospam--at)cox.net> wrote:
In bracing the top and bottom flange of a steel beam, what force do I use to design the brace connection?
 
TIA,
 
T. William (Bill) Allen, S.E.
ALLEN DESIGNS
Consulting Structural Engineers
 
V (949) 248-8588 F(949) 209-2509



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