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RE: LG Truss bracing steel beams

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Bruce,
 
My steel support beams are supporting opposite ends of a mono-type truss where the truss slopes from 0 at end to whatever the high elevation is at the other end.  Bracing through the roof diaphragm would therefore only work at the low end.
 
With trusses spanning 30 ft and spaced at 2ft or 4ft on center (yet to be determined), the lower chord light gage member would have nil strength without itself being braced.  I know something would need to be added to the truss lower chords to develop this stiffness, I just don't know if there is an economical way to do it.
 
The only other option I have is to provide structural steel bracing.  But there isn't any infill framing on the 30ft grid, so it would be a lot of extra steel for the sole purpose of flange bracing to support light gage metal.
 
 
Regarding the 2% v. tributary concept, and at the risk of bastardizing my notes from Yura and Helwig's Bracing for Stability seminar, I am reading that each brace is designed for the full flange force, or a value near there.  LRFD 3rd edition commentary for C3.4a sheds a little bit of light on this with a Ct value = 1+(1.2/n) factored into the required brace strength where n=number of braces.
 
Regards,
Jim Wilson, PE
Wilsonengineers(--nospam--at)yahoo.com
Stroudsburg, PA
 

Harold Sprague <spraguehope(--nospam--at)hotmail.com> wrote:
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" 
>Reply-To: 
>To: 
>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
>
> _____
>
>Do you Yahoo!?
>New and Improved Yahoo! Mail
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