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Re: Strength of Non-Compact HSS

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The "Specification for the Design of Hollow Structural Sections (1997)
should be followed for HSS members acting as beams. This spec has
slightly different requirements than the LRFD spec. You will find that
the limiting b/t ratios will control strength long before the unbraced
lengths.

For example, an HSS 8x8x3/8, Fy=46ksi has a b/t ratio of 19.9 which is
less than the limiting value of 28.12.  ØMn is 101 ft-k, Lp=29.38 ft and
Lr=533.61 ft.

"Jones, Mark A (Battle Creek)" wrote:

>
>
> What is the formula for reducing the strength of non-compact HSS?
>
> LRFD, pg4-8 gives the formula
>  phi.b Mn = phi.b Mp - phi.b(Mp-Mr)[(Lb-Lp)/(Lr-Lp)].
> But the section says that this is for I or C rolled shapes only.
>
> LRFD, App. F, pg 6-11 gives (substituting l for lambda)
> Mn = Cb[Mp - (Mp-Mr)[(l-lp)/(lr-lp)]
>
> Can I use the second formula for any shape?
>
> Mark Jones
> Jacobs Engineering