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Re: Effective Width for Steel Plates

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> From: "Chance, Acie" <AChance(--nospam--at)>

>      I am sure I am misunderstanding something here when you say to take =
> the actual stress and use it to determine the effective width.  Note =
> that if there is 10.o KSI "actual" stress in the member the effective =
> width is 24 inches and if I use the yield stress of 50 KSI the effective =
> width is 10.75 inches.  For the case where I have 24 inches of plate on =
> each side and 10 KSI I get an actual moment of 25.18 K-FT.  Now assuming =
> I cut off 13.5 inches of plate on each side and use the AISC with .6 FY =
> I get 64.5 K-FT.  I lose capacity with more plate.  What am I missing?

My suggestion was for the purpose of determining if the b/t ratio was
too large to properly use an effective flange width approach at the
Noncompact limit. Your flange is extraordinarily thin by any stretch of
the imagination when applying the AISC standard.

I suggest, as a measure of determining applicability:
1) b(eff_f) = t*95/sqrt(f) where f = M/S + P/A based on analysis forces
on gross properties. This is NOT intended for use in the AISC analysis.
It is just a range check. 
2) b(eff_Fy) = t*95/sqrt(Fy) Noncompact limit per ASD 9th Table B5.1

If b(eff_f) < b(gross) you may have a problem that is not adequately
considered by using b(eff_Fy) for the purposes of applying AISC

by your example:
b(gross) = 10.5"
b(eff_Fy) = (3/16)*95/sqrt(50) = 2.52"
b(eff_f) = (3/16)*95/sqrt(10) = 5.6" < 10.5"
I would probably not use an effective flange width to achieve the
Noncompact limit for AISC ASD 9th analytical purposes.

Canada CSA S16 recognizes 4 classes and will accept the effective width
approach for elements up to b/t = 60. However, the effective width and
the allowable stress would be less than AISC at Noncompact limit.

As I previously suggested, try the AISI analysis for this section.

R. Paul Ransom, P. Eng.
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)> <>

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