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Re: Interpreting Analysis Results

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Dear Bill,
 
Yes its really fun to work on a mat foundation modeled with finite elements, but a note of caution on the following:
 
1) The value that you use for the spring constant (i.e. the modulus of subgrade reaction), as that defines the stiffness of the soil, & has quite an impact on the depth & reinforcement of the mat.
 
2) Be sure to check the joint displacement & support reactions of all the supports, as that tells you whether the mat is stiff enough to transfer forces effectively to the soil. When we design manually we assume the structure to be rigid (which is not exactly true). Opposed to that when we use FEM, the interaction between the stiffness of soil (its subgrade reaction) & the stiffness of the mat comes into play, and gives us a pretty good picture of what should be the depth of the mat foundation for proper transfer of loads to the soil (weaker the soil, the stiffer the mat has to be).
 
3) Be sure that you don't have sharp re-entrant corners in your mat, or you will have a lot to be baffled about.
 
As far as Mxy is concerned, I think in a normal situation it can be safely ignored, as that is the in-plane torsion, which for the size of the mat becomes inconsequential, unless you have a reason to believe otherwise.
 
Regards
 
Pankaj Gupta
Structures Online
India.
 
 
----- Original Message -----
Sent: Saturday, December 25, 2004 12:41 AM
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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