Try the calculation assuming:
1) initial imperfection (sweep) at the limit of fabrication tolerance
2) considering second order effects of eccentric compression
Web members in compression may also influence compression chord stability.
If the bottom chords are bridged then there is probable redundancy due to
counteracting directions and magnitude of the initial imperfections.
OWSJ will typically require minimum brdging/bracing for stability reasons.
Regards
Paul
--
Paul Ransom, P.Eng.
ph 905 639-9628
cell 905 802-3707
fax 905 639-3866
ad026@hwcn.org
> From: "Wesley Werner" <wwerner@conewago.com>
> 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?
> =20
> Wesley C. Werner, P.E.=20
> wwerner@conewago.com=20
> Phone (717) 632-7722=20
> Fax (717) 632-8665=20
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