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RE: Interpreting Analysis Results

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Bill: I have attached a chunk of the help notes from my Robot software in the ways it deals with the Mxy value in getting estimates of Mx and My:
Robot  allows one also to use the Wood&Armer method (the European code supplement [ENV 1992-1-1 EC2 Design of Concrete Structures – Appendix 2, point A.2.8 Reinforcement in Slabs]).  The conception of determining equivalent moment is authored by Wood and Armer. Details concerning the method can be found, for instance, in R.H.Wood – "The reinforcement of slabs in accordance with a pre-determined field of moments", Concrete, February 1968, August 1968 (correspondence)].
Calculation procedure
For a selected direction "x" (and the corresponding perpendicular direction "y") one calculates two types of design moments M*: the "lower" ones (positive, causing mainly tension in the bottom parts) and the "upper" ones (negative, causing tension in the upper parts). The general procedure takes the following form:
Determination of the "lower" moments Mxd*, Myd*:
Mxd* = Mx + |Mxy|
Myd* = My + |Mxy|
However, if Mx < -|Mxy| (i.e. the calculated Mxd* < 0)
Mxd* = 0
Myd* = My + |Mxy*Mxy/Mx|.
Similarly, when My < -|Mxy| (i.e. the calculated Myd* < 0) (*)
Mxd* = Mx + |Mxy*Mxy/My| (*)
Myd* = 0 (*)
If any of thus obtained moments Mxd*, Myd* is smaller than zero, one should assume the zero value (the design moments for tension in the upper layers are determined further on in the text).
Determination of the "upper" moments Mxg*, Myg*:
Mxg* = Mx - |Mxy|
Myg* = My - |Mxy|
If Mx > |Mxy| (i.e. the calculated Mxg* > 0) (*)
Mxg* = 0 (*)
Myg* = My - |Mxy*Mxy/Mx| (*)
Similarly, when My > |Mxy| (i.e. the calculated Myg* > 0)
Mxg* = Mx - |Mxy*Mxy/My|
Myg* = 0.
If any of thus obtained moments Mxg*, Myg* is bigger than zero, one should assume the zero value (such moments would design the lower reinforcements, which is already guaranteed by the formerly calculated "lower" moments Mxd*, Myd*)
I would not fully rely on this but you can use it as a starting point anyway. Robot does all this for me.
Good Luck
David Handy, P.Eng.
-----Original Message-----
From: Bill Allen, S.E. [mailto:T.W.Allen(--nospam--at)]
Sent: Friday, December 24, 2004 2:12 PM
To: seaint(--nospam--at)
Subject: Interpreting Analysis Results

I’ve just modeled my first mat foundation using STAAD Pro and the foundation support option. I’m not using their foundation module, just the feature within the plain vanilla version which allows me to specify a modulus of subgrade reaction and the program applies the appropriate spring support at each node. Nice feature.

Anyway, the results I get are forces at the center of each plate. I get two shear components, Qx and Qy in psi and three bending components, Mx, My and Mxy in in-lbs/in. I know what to do with the Qx and Qy terms; I would compare these values with phi*2*sqrt(f’c). I know what to do with Mx and My. These are pretty straightforward.

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)


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