Piles-induced filtering effect on the Foundation Input Motion

The inertial interaction analysis of a structure founded on piles is conventionally performed by imposing that the Foundation Input Motion is merely that of the free field, thus neglecting the kinematic interaction between piles and soil generated by the passage of seismic waves. This would lead to unnecessary overconservatism in the design, as there is evidence that the free-field motion may be thoroughly filtered out by piles (generally reduced), especially in the case of soft soils, where piles are recurrently required to carry out the total load transmitted by the superstructure and/or to reduce foundation settlements. Results provided from analytical and numerical tools elucidate the crucial aspects controlling the mechanism of filtering effect. Reduced design spectra are also suggested to account for the beneficial effect coming from the piles when the inertial interaction analysis of the superstructure is being performed.

► Filtering effect depends on geometrical and mechanical characteristics of pile and soil as well as excitation frequency.
► Pile filtering is more pronounced in two-layer than homogeneous soils.
► A unique dimensionless parameter controls pile-to-soil acceleration ratio.
► Filtering effect is negligible for long-period structures.
► A reduction factor for design spectra is proposed to be adopted in the presence of pile foundations.