The evaluation of kinematic bending moments in a pile foundation using complete 3D finite element modelling

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. The paper deals with 3D soil-pile FEM modelling. First of all the FEM modelling is compared with simplified approaches; then a parametric analysis is performed. In this last case, the pile, supposed with different lengths and diameters, is embedded in two soil layers, characterised by different stiffness ratios Vs2/Vs1. Moreover, five different seismic input motions recorded in Europe in the last 30 years are considered.