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RE: Modeling the subgrade modulus in Staad

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In addition to the programs Craig notes below..STAAD also has most if not all of the functionality you describe.
Utilizing the mesh generation and plate mat/elastic mat commands you can refine the mesh to find the appropriate influence area.  You can also account for uplift/loss of contact or "tension" by the use of the spring compression command. 
If you supply the subgrade modulus, the program will generate the soil springs at the nodes of the elements based upon influence area.  It will also generate the base pressures/base pressure plots.
I've used all of these features with good results.
Hope this helps.
John T. Whitty, P.E.

-----Original Message-----
From: Craig Leech [mailto:cleech(--nospam--at)]
Sent: Tuesday, May 04, 2004 9:16 AM
To: seaint(--nospam--at)
Subject: RE: Modeling the subgrade modulus in Staad

Some of the programs I have used allow definition of the spring constant per unit area (modulus of subgrade reaction, I think it is called).  Thus the spring is applied to an element area.  The program then models this as a spring at each node, based on the tributary area.

The springs are typically linear elastic, sometimes with no tension.  To use a non-linear spring (I think this would cover the "coupling") typically requires a non-linear program.
MIDAS, SAFE and ETABS all have some of these options.

From: GSKWY(--nospam--at) [mailto:GSKWY(--nospam--at)]
Sent: Tuesday, May 04, 2004 8:46 AM
To: seaint(--nospam--at)
Subject: Modeling the subgrade modulus in Staad

When you model the subgrade modulus (k)  in Staad or any other finite element programs, say to design a raft foundation,  does it treat "k" as a simple spring constant?

I.e.,  does it consider anything like coupling between adjacent springs due to lateral pressures,  does it adjust "k"  for its location with respect to the edge of the foundation, etc.

Or do you just put in a mesh size and a "k" value and have the program model in as an independent, identical spring at each node?

Is there any program that is particularly good for this type of modeling?

Gail Kelley