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RE: Interpreting Analysis Results

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David, Acie, Michael, et. al.;

If I translated the Mxy into a direct moment and adjust the reinforcing,
that would be O.K. My concern is that it may be more appropriate to add it
to the direct shear stress like a column supporting a flat plate with an
unbalanced load. After my post, I started doing some research on my own.

I haven't done this type of analysis in a long time. It was shocking to find
out exactly how long it's been since I no longer see the Summation(x^2*y/3)
terms in the design equations. Yikes!

It looks like, according to ACI-318, you can't rely on concrete to carry
torsion anymore, once you're over the threshold.

I went back to my "Old Reliable", Park and Paulay, where there is a method
to do a circular interaction between shear and torsion. In a sample (see:
http://www.allendesigns.com/images/ShearTorsionSlab.jpg pardon the lousy
handwriting) calculation, although Mxy is less than the threshold torsion,
it definitely is critical using the Park and Paulay analysis. Of course, the
problem with shear stresses in a slab is that you just can't fix the problem
with reinforcing; you've got to increase the thickness or increase the
strength of the concrete (f'c). Of course, my edition of Park and Paulay
references the 1971 version of ACI 318!

Comments are welcome.
	
T. William (Bill) Allen, S.E. (CA #2607)	
ALLEN DESIGNS	
Consulting Structural Engineers	
http://www.AllenDesigns.com	
V (949) 248-8588	 .	 F (949) 209-2509	
-----Original Message-----
From: Michael L. Hemstad [mailto:hemstad.ml(--nospam--at)tkda.com] 
Sent: Monday, December 27, 2004 10:38 AM
To: seaint(--nospam--at)seaint.org
Subject: Interpreting Analysis Results


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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