A kinematic interaction model for a large-diameter shaft foundation. An Application to Seismic Demand Assessment of a Bridge Subject to Coupled Swaying-Rocking Excitation

The aim of this paper is to illustrate an analytical model for the assessment of kinematic interaction of large-diameter shaft foundations. The model is derived using recently obtained solutions of soil structure interaction problems of rigid walls and fixed base cylinders subjected to a dynamic excitation. The proposed model constitutes an extension to a deformable base of the elastodynamic solution of a rigid, fixed-base cylinder imbedded in a homogeneous or inhomogeneous soil stratum with different lateral boundary conditions. The analytical model has been validated by means of a finite elements code and it has been implemented in a consistent seismic soil-structure-interaction analysis procedure. An application of the model to a long, multi-span continuous prestressed concrete viaduct with tall piers has been carried out focusing on the importance of kinematic interaction. The main finding of the study is that the foundation input motion is characterised not only by a translational horizontal component which is usually of a reduced amplitude if compared with the free-field ground motion, but also by a rotational component that is responsible for a large seismic demand in the superstructure. The proposed model represents an effective tool to be used in the engineering practice to assess both the seismic actions induced by the ground shaking on the foundation system and the effective input motion of a superstructure founded on massive and large diameter shafts.