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RE: LG Truss bracing steel beams

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Bruce,
What you are describing is what has been the practice for bar joists and structural steel for years. Engineers often extend a bar from the bottom chord of the joist to the bottom tension flange of the structural steel member. The bars extend from the bottom chord of joists on each side of the beam. That way the braces are tension only. Under gravity load they can not create a fixed end condition for the joists. The bar joist manufactures hate this detail because it can "potentially" create a fixed end condition for the bar joists. But they have been living with it for years.

In your related question, I would think that you could size each brace for the tributary area that it is bracing. Keep in mind that the 2% approximation is just that. You can invoke Timoshenko and go back to the theory of elastic stability and check both stress and stiffness. Often times, no bracing is required at all. The Yura 2% approximation is very conservative, but you must be careful because it does NOT account for stiffness. This can cause a problem. It is rare, but it is real.

Regards,
Harold Sprague


From: "Bruce Holcomb" <bholcomb(--nospam--at)brpae.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: RE: LG Truss bracing steel beams
Date: Thu, 8 Jul 2004 13:23:10 -0500

I have used LGS trusses to brace the beam they are bearing on.  The roof
was sheathed with plywood providing a load path for the lateral flange
brace force and the trusses were at 2'-0" o.c.  The beam flanges weren't
extremely large (I don't recall what the beam sizes were).  For small
W-shapes, the lateral flange force is very small and typical roof truss
framing and connections should have no problem resisting this lateral
load, though reviewing the truss shop drawings should help to decide if
the bottom chord has adequate axial capacity based on size, brace
locations, etc.  For larger W-shapes, I would recommend specifying the
connection from the truss to the W-beam and providing the lateral flange
bracing force to the truss designer.  I would also recommend using an
unbraced length adequate to allow 1 or 2 truss connections to "fail"...
just an additional factor of safety.



I have a related question... If I design a beam which requires bracing
only at the mid-span, but the actual detail will provide more bracing
locations, (such as at 4' or 5' o.c.), does each brace have to be
designed for 2% of the lateral flange force or can I assume that the
brace locations share the load?





Bruce D. Holcomb, P.E., S.E.

Structural Engineer

Butler, Rosenbury & Partners

300 S. Jefferson, Suite 505

Springfield, MO 65806

ph. 417-865-6100

fax 417-865-6102

www.brpae.com

Architecture, Engineering, Interior Design, Planning & Development

Your Vision.  Our Focus.



-----Original Message-----
From: Jim Wilson [mailto:wilsonengineers(--nospam--at)yahoo.com]
Sent: Thursday, July 08, 2004 9:29 AM
To: seaint(--nospam--at)seaint.org
Subject: LG Truss bracing steel beams



Seaint,



Is it practical and efficient to use the lower chord of a light gage
truss to brace the top flange of a steel supporting member?



To specify this, wouldn't it then be necessary to put the brace force
required on the structural drawings for the light gage truss
manufacturer to incorporate?  Thinking out loud, the required stiffness
of the lower chord would also need to be specified.  If this goes beyond
the realm of conventional truss design software, then it might be
necessary to provide member selection guidelines to meet these stiffness
and force requirements.



TIA for the thoughts,



Jim Wilson, PE

wilsonengineers(--nospam--at)yahoo.com

Stroudsburg, PA

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