SEMI-ANALYTICAL SOLUTIONS FOR EVALUATING THE FIM FOR PILE FOUNDATIONS

In this contribution, the kinematic filtering effect of free-head and fixed-head single piles in layered soils is investigated to define new simplified formulas for evaluating the foundation input motion (FIM). These solutions have been derived numerically by employing a boundary element method code (KIN SP, Stacul and Squeglia [11]) and a finite element method code (VERSAT-P3D, Wu [12]). The proposed solutions allow to compute the transfer functions in translation (Iu) and rotation (I?), representing the ratio between pile-head and free-field motion as a function of the excitation frequency, for the following soil profiles: homogeneous, two-layered, parabolic and Gibson soil profiles. These solutions have been developed assuming a linear elastic behavior for the pile material and a linear viscoelastic soil model. Nevertheless, these solutions are expected to be valid also in the case of non-linear soil response and large earthquake-induced shear strains. In fact, as shown in a recent work (Stacul et al. [10]), kinematic soil-pile interaction is a stiffness-controlled mechanism while dynamic and non-linear effects simply modify soil response at free-field conditions.