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Re: Queries on ASD

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Check with Abe Rokach at AISC (rokach(--nospam--at)aiscmail.com). I think this limit
was changed to 970/sqrt(Fy) to eliminate the confusion you are
experiencing.

Charlie

Allen Adams wrote:
> 
> Regarding the use of Fb in the Noncompactness equation for Webs:
> It is common to use Fb = 0.60Fy in that equation (760/sqrt(Fb)). It is
> conservative to do so, although not unreasonably so. However, it is simple
> enough to be more precise if you deem it necessary: If the section does not
> qualify as compact, assume the web qualifies as noncompact. Then calculate
> the allowable Fb from the appropriate equations from Section F1. Then use
> this value in the equation 760/sqrt(Fb) and determine if your assumption of
> noncompactness was correct. If the section is now determined to be
> noncompact, you can use the value of Fb that you calculated for the design
> of the member. If the section is slender, you should either select a larger
> size (which is normally what is done) or you must follow the requirements
> for slender members found in Appendix B. (Note that if the section's h/tw
> is less than 760/sqrt(0.60Fy), then the web is definitely not slender, and
> it is not necessary to re-check it against 760/sqrt(Fb) after the actual Fb
> is calculated.)
> 
> Allen Adams
> RAM International
> 
> >From: John Smith <smith_john_1(--nospam--at)yahoo.com>
> >Subject: Queries on ASD
> >To: seaint(--nospam--at)seaint.org
> >
> >
> >I am an Australian Engineer and have a project requiring a design
> >conforming to ASD.
> >
> >I have a query regarding the interpretation of the use of Fb in Table
> >B5.1 - Limiting Width-Thickness Ratios for Compression elements page
> >5-36.
> >
> >In rows 10 & 11 of the table, regarding the use of webs in flexural
> >and axial compression, the table requires the use of an Fb in the
> >NonCompact limit of 760/Sqrt(Fb).
> >
> >My inquiry is:
> >Is it common practice to use an Fb = 0.60Fy ??
> >(from equation F1-5)
> >
> >Note:
> >I notice that in the LRFD that this limit is now 970/Sqrt(Fy) which
> >corresponds to an equivalent Fb = 0.614Fy
> >
> >JS
>