Relevance of the Foundation Input Motion in the assessment of the seismic response of pile-supported bridge piers

The seismic response of pile-supported bridge piers is commonly studied avoiding the assessment of the Foundation Input Motion (FIM) (i.e., the motion at the foundation level, which is different compared to that in the free-field, due to the filtering effect exerted by the piles) and assuming fixed-base conditions. A better approach is represented by the substructure method which consists of 3 steps: 1) the assessment of the FIM; 2) the computation of the impedance functions of the soil-foundation system, 3) the analysis of the superstructure response resting on a compliant-base at which is applied the FIM. In many cases this method is not used in a rigorous way (as the assessment of the FIM is not commonly considered an easy task by the structural engineers) and the seismic motion at the base of the model is assumed to be the same as in the free-field. Thus, the effect of Soil-Structure Interaction (SSI) is neglected. Nevertheless, this effect might be beneficial, as piles can filter out some high-frequency components of the free-field motion, especially in the case of large-diameter piles and soft soil conditions (i.e., in those cases in which piles are generally necessary). On the other hand, special attention should be given to those foundations composed by very small pile groups, since the FIM will consist of both a horizontal and a rotational component (which would play a detrimental role). Moreover, it should be remembered that considering SSI would lengthening the fundamental period of the system compared to that under fixed-base hypothesis. This aspect would lead to both positive and negative effects based on the seismic motion properties. This contribution will attempt to show the relevance of a proper assessment of the FIM while studying the seismic response of pile-supported bridge piers. To this end a specific software has been developed to properly compute the FIM considering the filtering effect exerted by the piles.