Evaluation of kinematic bending moments in a pile foundation using the finite element approach

During earthquakes seismic wave crossing through soft soil can lead to significant curvatures on pile foundations, which in turn lead to significant bending moments. These bending moments are commonly named “kinematic bending moments” to be distinguished from the “inertial bending moments” due to horizontal forces transferred from superstructures to pile heads. Approaches to carefully evaluate inertial bending moments have been developed world-wide; on the contrary, up to now different simplified approaches have only led to different evaluations of the kinematic bending moments on pile foundations. Thus, the evaluation of the kinematic bending moments is still questionable. Nevertheless, the European technical code EC8 [1] and the new Italian technical code, D.M. 14/01/2008 [2], underline the importance, in the geotechnical and structural design of pile foundations, to take into account not only inertial bending moments but also kinematic bending moments, in order to avoid significant structural damage. In this paper a 3D soil-pile FEM system is analysed. The system is subjected to a seismic input motion, applied at the base of the system, which represents the conventional bedrock. The FEM analyses lead to the evaluation of the kinematic bending moment distribution along the pile. Finally, the numerical results are compared with those coming from some simplified approaches available in Geotechnical literature.