Design of piles under seismic loading

Seismic forces on piles may be distinguished from a phenomenological viewpoint in two modular contributions stemming from different yet simultaneous interaction phenomena. Along with bending moments arising from the oscillation of the superstructure (the so-called inertial interaction), piles experience additional bending moments which are induced by the deformation of the surrounding soil during the passage of seismic waves (the so-called kinematic interaction). However, piles are traditionally designed to withstand only inertial actions, thus neglecting kinematic moments which may be equally or even more important. To shed light on these aspects, kinematic interaction is first rationalized as the combination of two simpler physical mechanisms. Then, ready-to-use formulae for estimating kinematic- and inertial-induced pile bending moments are reported, and key parameters governing the relative importance of the two sources of loading are highlighted. The role of pile diameter in controlling the relationship between bending demand and capacity is emphasized toward pile design under combined inertial and kinematic loading. In addition, the kinematically-induced potential of piles to filter out high-frequency components of free-field signals is physically explained, and simple expressions are provided for routine engineering to assess the seismic demand transmitted to the superstructure. The importance of pile group effects on these topics is discussed. Finally, a step-by-step procedure which provides guidance on the design of piles under seismic loading is presented as a flowchart encompassing all the issues covered in this chapter, followed by a numerical example.