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Re: Shape Factors and AISC ASD

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The LRFD Specification handles the elastic plastic behavior of ductile
steel beams flexure a little differently.  The plastic moment strength,
which is a function of Z-sub-y, is limited by some factor times the elastic
moment strength, a function of S-sub-y.  In other words, the flexural limit
state DIRECTLY considers both the elastic and plastic section properties.

As far as the ASD Specification is concerned, the shape factor concept
(0.66 times F-sub-y for compact strong-axis and 0.75 times F-sub-y for
compact weak) INDIRECTLY considers both the elastic and plastic section

In your example your steel has a special requirement for 70 ksi yield.
What is the specified ultimate strength?  If your F-sub-u is not at least
90 ksi, I would be concerned that the ASD approach adequately addresses
your situation.

Also, make sure that you have a compact section at 70 ksi.  If you have a
noncompact section bent about the weak-axis, ASD Section F2.2 will limit
your allowable bending stress to 0.60 times F-sub-y.

Rick Drake, SE
Fluor Daniel, Aliso Viejo, CA


Stanley_P_Johnson(--nospam--at) on 02/07/2001 09:30:25 AM

Please respond to seaint(--nospam--at)

To:   seaint(--nospam--at)

Subject:  Shape Factors and AISC ASD

If I am working with a steel where the ratio of min Fy to min Fu is closer
to unity than with the usual structural steel, is this likely to preclude
the section from realizing the advantages of a high shape factor because
the steel ruptures prior to developing ZFy?  In my particular case I am
dealing with a steel that is specified as ASTM A572 grade 60, but a special
requirement has been placed on the steel that the min Fy is 70 ksi.  I
question whether 0.75 * 70ksi is an appropriate allowable stress.

> From: "Gerard Madden" <GMadden(--nospam--at)>
> To: <seaint(--nospam--at)>
> Subject: Re: Shape Factors and AISC ASD
> Stan,
> The shape factor is the ratio of the plastic section modulus Z to the =
> section modulus S ( Zx / Sx ) or (Zy / Sy). It is basically a measure of
> how much inelastic stress you can withstand above and beyond yield.
> Hope that helps.
> -Gerard

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