Hope this would help you, http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CCgQFjAB&url=http%3A%2F%2Fweb2py.iiit.ac.in%2Fresearch_centres%2Fpublications%2Fdownload%2Finproceedings.pdf.b14aca0b5a5a41d2.506f756e64696e672e706466.pdf&ei=UsPoU-K1HoSS7AaF-YHQDQ&usg=AFQjCNGAuWPcBrr5-3JPJFHD-gk8jAM6QQ&sig2=Cv0NNbfVUVbZzxCrPpRg9A.

Thanks

Soil may be modeled by introducing six spring elements at the base in all six degrees of freedom. Stiffness of such springs may be estimated following closed-form expressions of Gazetas or FEMA formulation. For nonlinear analysis strength assessment of such springs is necessary considering such springs as bilinear and the relevant guideline is available in FEMA. See the following paper and the references therein.

Roy, R. and Dutta, S.C. (2010). “Inelastic Seismic Demand of Low-rise Buildings with Soil-flexibility”, Int. Jr. of Nonlinear Mechanics, Elsevier, Vol. 45, No. 4, pp. 419-432.

You can use push over analysis in SAP. What's is exactly your application. is it dynamic or static?

Hi,

These papers might help:

http://thescipub.com/abstract/10.3844/ajeassp.2008.121.125

http://thescipub.com/abstract/10.3844/ajassp.2008.1793.1803

ftp://128.46.154.21/spujol/Andres/files/14-0094.PDF

http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=1632741

Thanks