Large Accelerations at base of mat foundation over soil

[jaimercado]

Hello

Quick question, im modeling a mat foundation over a PDMY02 soil. The mat is just an elasticbeamcolumn, and the soil elements are Nine_Four_Node_QuadUP. To connect them Im using equalDOF only. After running the earthquake, i found there are large accelerations values specifically at those nodes. For example, for a normal PGA of 0.2g, the value calculated is, per say, 0.6g. Do you probably know what is going on?

Some modelling facts:
constraints Penalty 1.e18 1.e18
test NormDispIncr 1.0e-5 300 1
algorithm KrylovNewton
numberer RCM
system ProfileSPD
integrator HHT 0.6
analysis Transient
This happens as well with uniformexcitation, multisupport, dashpot...

Thanks.