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Interpreting Analysis Results

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Bill Allen wrote:

"I've just modeled my first mat foundation using STAAD Pro and the 
foundation support option.

....

The value that has me a little baffled is the torsion term, Mxy. It =
appears
to me that I need to do some sort of Tc/J calculation and add this value
=
to
the Qx and Qy term, but I'm not certain.

Thanks in advance,


T. William (Bill) Allen, S.E. (CA #2607)"


Bill,
I took a Plates class at the end of my graduate school (it nearly was
the end of me), and asked that same question.  The professor said not to
worry about it.  I asked what it actually was that I was ignoring, so
that I could still sleep nights.  It apparently is, as you have guessed,
some kind of torsion within the element, which will thus result in shear
in some direction.  But, if I remember correctly (it's been 5 or 6
years), she said the calculated magnitude kind of depends on the size of
the element chosen and really doesn't mean much.  She said that in the
steel or aluminum plates that whe was used to dealing with, it never
amounted to enough stress to worry about.  I asked, what about in
concrete?  She said it was still usually just ignored.

I'd love to hear the real answer, but that's the one I was given.  In
fairness, I didn't entirely trust it then and I still don't now.  Later
on that same day, I asked what side of the slab the reinforcing should
go on.  We never did figure out the sign convention, and she advised me
to put the same rebar on both sides.

FWIW,

Mike Hemstad, P.E.
TKDA
St. Paul, Minnesota

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