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RE: UBC 1630.8.2.2.1.7 Bracing Load

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Jim,
 
If you are choosing to brace the beam at 10' intervals then your Lb is 10' and you choose a beam that can resist the moment with 10' un-braced length. Now if you choose to brace it @ 5' O/C you may be able to use a lighter beam which will result in a smaller force for bracing. But if you are not changing the beam, as you mentioned, these new braces will be redundant. If this is a special case, you may choose to design your main braces (@10' O/C) for the brace force and the redundant ones for whatever you want. But let's look at another example:
 
If you need a 30' beam with phi_bMn>=200 K-ft and you have typical braced conditions at 5' O/C you may choose a W14x34 and design the braces for F1. Now if you have to choose a W14x43 for deflection reasons you only need one braced point at 15' with a design force of F2>F1(for un-braced length of 15' you get 208 k-ft for W14x43)but if we still have to have similar braced conditions at 5' (like a connection from joists or...) and we want to verify or design the braces I would probably design all of them for F2 or simply switch to a W14x61 with no braced points! (for un-braced length of 30' and 215 k-ft) It may actually end up to be a cheaper option.
 
You may argue that for braces at 5' O/C and your max moment you did not need the W14x43 and F2 but only W14x34 and F1 and as such design the braces only for F1. You may have a point here since for a beam supporting only gravity load, a capacity design concept for brace connections may not be required. But things can change and some decades later the next engineer, not requiring the same exact deflection criteria in a renovation, may assume that you had a W14x43 braced at 5' O/c and add up to 30% load to your beam! I know that he will be required to check everything but...
 
Reza Dashti P.Eng
Vancouver, BC





Date: Thu, 11 Oct 2007 15:21:18 -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

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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