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Re: UBC 1630.8.2.2.1.7 Bracing Load

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Reza,
 
What you say is essentially what I am doing now but in your example if I use a brace at, say the 10 ft. points, then I can calculate a force for the brace.  If I want to brace at 5 ft. instead then I get the exact same force.  If I want to brace at 2 ft -- again it's the same force.  Regardless of the lc or lu of the member the force is always the same -- if based on the stress in the flange.  But I think what you say is that for the force calculated it will only have to occur at either the Lc or Lu distance depending on what stress I want to use.  If I use an Lb > Lc then the brace distance, Lb, is only where I choose to place it using the allowable respective stress at that length.  Any lesser length between braces is redundant but the force would be the same.
 
Jim Persing
 
On 10/10/07, Paul Ransom <ad026(--nospam--at)hwcn.org> wrote:
> From: "Kevin Below" <kbofoz(--nospam--at)gmail.com>

> Jim, it seems to me too that if the load is applied on the bottom flange
> then the beam is stable and cannot rotate.  I had the same scenario some
> months ago with a small foot-bridge (30 ft span) using through-trusses.  i.e.,
> the supporting trusses also act as the guard-rails, and the traffic surface
> is supported directly on the bottom chords of the trusses.  The top chords
> have no lateral support at all, but the trusses cannot rotate.

Kevin,
Check out the general beam moment capacity development as described in the
SSRC Guide to Stability of Steel Structures (I'm going from memory as I
don't have it at my fingertips). Loading below the N.A. improves stability
(e.g. longer unbraced length for same capacity) but may not remove the need
to stabilize the compression flange.

In your example, there may be other contributing factors such as moment
restraint at the deck level that provides a torsional brace to restrain the
compression chord.

Regards
Paul
--
Paul Ransom, P.Eng.
ad026(--nospam--at)hwcn.org

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