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Re: UBC 1630. Bracing Load

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I had never heard of the SSRC Paul.  Thanks for the reference. 

In my case, there certainly is moment restraint at the base of the bottom chord.


On 10/11/07, Paul Ransom <ad026(--nospam--at)> wrote:
> From: "Kevin Below" < kbofoz(--nospam--at)>

> 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.

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.

Paul Ransom, P.Eng.

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