A lumped parameter model for the time domain inertial soil-structure interaction analysis is proposed with reference to square pile group foundations. Simplified formulas are presented for estimating its parameters. The model is able to reproduce the coupled rotational-translational behaviour of the soil-foundation system. Formulas are calibrated from results of an extensive non-dimensional parametric analysis considering head-bearing pile groups. The closed-form expressions may be readily adopted to define the compliant base restraints of a generic structure for the non linear dynamic analysis carried out with commercial software
