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Unbraced truss compression flange

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Title: Conewago Stationery

 

    I am being asked to design a custom truss with a completely unbraced bottom flange. The dead load on the truss is so light that there is a net uplift with wind load. This puts the bottom chord in compression. I don't like this condition, but I would like to have more than "I don't like it" for a answer, and if there is a safe way to design for this condition I would like to know how. I checked the bottom chord as a unbraced column the length of the joist and it worked fine. Also, KL/r would be less than 200 if K is 1.0. Is there some reason that K would be more than 1.0? Is there some reason other than structural stability to brace the bottom chord? The truss is basically a bar joist made out of tube steel. I have two competing ideas in my mind. The one is bar joists where any joist with uplift always has uplift bridging. The other is a crane beam where the compression flange has no bracing. Is there a way to calculate a stiffness that would make compression chord bracing unnecessary?

 

Wesley C. Werner, P.E.
wwerner(--nospam--at)conewago.com
Phone (717) 632-7722
Fax (717) 632-8665