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Queries on ASD

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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