Dear Satish,
“Fixed bottom” does not mean a rigid foundation; you can have soil stiffness matrix enabled. But I took a brief look at your input files and I see that you’ve specified the SSIfile correctly, but I also see that you haven’t enabled any of the degrees of freedom at the reaction joint. You should enable the DOFs at the reaction joint (by setting RctTDXss = 0, etc.) for the stiffness matrix to take effect.
Best regards,