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RE: LG Truss bracing steel beams[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: RE: LG Truss bracing steel beams
- From: "Harold Sprague" <spraguehope(--nospam--at)hotmail.com>
- Date: Thu, 08 Jul 2004 23:15:07 +0000
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 _____ Do you Yahoo!? New and Improved Yahoo! Mail <http://us.rd.yahoo.com/mail_us/taglines/100/*http:/promotions.yahoo.com /new_mail/static/efficiency.html> - 100MB free storage!
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- RE: LG Truss bracing steel beams
- From: Jim Wilson
- RE: LG Truss bracing steel beams
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