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RE: Modeling the subgrade modulus in Staad

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Gail,
 
Speaking specifically about Staad, I believe "k" is treated as a simple spring constant (Winkler foundation type).....it will automatically calculate the tributary area of the node to arrive at a total spring stiffness at a single node ..... (kips/in^3) * (in^2)  = kips / in......it does no "coupling" of the springs to the best of my knowledge, however, bear in mind the deflection is "coupled" ....I have used STAAD frequently and have found it to produce results fine for use in the design office (now they have the option of specifying "compression" only springs...in the past I had to use the multilinear spring option).....I think you can get more complicated in your modeling if desired (maybe using Algor or Nastran ?).....if you want, I have attached a paper I wrote on the subject a couple of years ago where I used STAAD.......I can e-mail that to you if you would like....
 
Also, check out Section 9-8 of the Bowles "Foundation Analysis and Design" book....5th Edition......it speaks directly to FEM modeling of soil-structure interaction.....
 
Robert Rogers, PE
 
 
-----Original Message-----
From: GSKWY(--nospam--at)aol.com [mailto:GSKWY(--nospam--at)aol.com]
Sent: Tuesday, May 04, 2004 8:46 AM
To: seaint(--nospam--at)seaint.org
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