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Re: ASD Torsion

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Jeff,

As Charlie pointed out, there is no "special" specification for torsion.
Torsion will produced shear stresses and possibly axial (normal) stresses
depending on the shape being considered.  Thus, you would just make use of
the "normal" ASD or LRFD specs.

The "trick" in dealing with torsion is to understand what stresses you get
on the member.  It is becomes a function of what your cross-section is.
Generally speaking, for "closed" sections (i.e. tubes, pipes, etc) the
torsional effects due to warping torsion with be able to be neglected and
you will mainly be dealing with "true" (St. Venant's) torsion.  In this
case, then you will really only have shear stresses to deal with.  If you
are dealing with an "open" shape (i.e. wide flanges, channels, angles,
etc), then the warping torsion portion can be quite significant (and
generally is), which means that you will not only get shear stresses due
to the torsion, but also significant axial (normal) stresses.

In either case, the shear stresses from torsion get added to the shear
stresses from actual vertical (and horizontal) shear and checked against
the "permitted" (i.e. allowable in ASD) values.  In a similar fashion, the
axial (normal) stresses due to torsion get added to the axial (normal)
stresses from flexural and any axial loadings and checked against the
permitted values in the spec.

As Charlie pointed out, the AISC Design Guide on torsion is a good
publication to use to deal with torsion.  While I have never seen it, I
have seen and used the publication from Bethlehem Steel (I think it was
them) on torsion that I believe was the starting point for the AISC design
guide.  Plus, after I had tons of fun "playing" with torsion at a job in
the real world, I was back in school and took an advanced steel course
that covered torsion.  Let's just say that after taking the class, I
understand torsion in steel a WHOLE LOT BETTER now.  As I understand it,
the AISC Design Guide is much more thorough than the original Bethlehem
publication, so it should be much more helpful.

Another reasonable source for some basics on torsion in steel is
Blodgett's "Design of Welded Structures".

After you delve a little more into torsion, what you will find is that it
is generally a good idea to stick with closed sections (tubes and pipes)
when you are dealing with torsion.  Makes life a whole lot easier.

HTH,

Scott
Ypsilanti, MI


On Mon, 24 Feb 2003, J+R wrote:

> Hello list:
>
> Is there an AISC spec for torsion analysis per ASD?
> I am aware of the LRFD spec for steel hollow structural sections, but not one dealing with ASD.  Does AISC design series 9 cover this?
>
> Thanks for any help.
>
> Jeff


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