From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
Date: Wed, 7 Feb 2001 13:12:16 -0500 (EST)
Keep in mind that the shape factor is indepentent of the relationship
between Fy and Fu. That is to say that the shape factor DOES not depend
on what type of steel is being used.
The difference between Mp and My is that My is the moment at which the
extreme fibers of the section begin to yield while Mp is the moment at
which the entire cross section has yielded (i.e. a plastic hinge has
formed). This is all a function of the members cross section.
Therefore, the type of steel should not change the "allowable stress" for
a WF bent about the weak axis. Think about it. When you bend a WF about
its strong axis, there is more "stuff" (ie the entire flange) closer to
the extreme fiber of the section, which means that more of the section
will yield faster. Thus, the difference between My and Mp is small, which
leads to the smaller shape factor. Now bend the same member about the
weak axis. There is less "stuff" (ie the tip of the flanges) closer to
the extreme fiber of the section, which means that it takes longer for a
significant part of the section to yield. Thus, the difference between My
and Mp is greater, which leads to the larger shape factor.
Hope that helps,
On Wed, 7 Feb 2001 Stanley_P_Johnson(--nospam--at)dot.ca.gov wrote:
> 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)mplusl.com>
> > To: <seaint(--nospam--at)seaint.org>
> > 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