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Compressor Foundation

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Dear all,
          I am designing a compressor foundation as table top foundatin with low tuned system my problem is that 
the soil is very stiff and value of dynamic shear modulus is 580 MPa. i have modeled the structure with soil structure 
interaction be I have used the equations of Barkan for modelling the soil springs i am getting 90 % mass participation 
only at a frequency of 22 cycles per second i have listed the machine details below Where as the fundamental frequency is 7 cycles 
per sec. I have modelled the structure on Staad Pro with soil springs in all degrees o freedom. When i reduce the 
spring values of spring constants i am getting better results. Can any one explain does a stiff soil profile in crease the 
fundamental frequency of the foundation . I cannot go for a high tuned system because the frequency of the compressor is 8260rpm.
The machine is a compressor with gear an electric motor the operating speed is 1800 rpm.
The equipment details 
are Motor Gear and Compresser on a skid the total weight of equip ment is 1120KN
The bottm of skid elevation is 4 m from FGL
Motor Speed 1800 rpm
Compressor Speed 8260
I any one can explain the details to be taken care of while modelling in staad to get more accurate 
results.Is it possible to design the system as a high tuned system where as per
design citeria a mixed tuned system is not allowed.

Best regards

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