Kinematic pile bending during earthquakes: analysis and ®eld measurements S. NIKOLAOU Ã , G. MYLONAKIS { , G. GAZETAS{ a nd T. TAZOH} The passage of seismic waves through the soil surrounding a pile imposes lateral displacements and curvatures on the pile, thereby generating`kinematic' bending moments even in the absence of a superstructure. These moments are concentrated in the vicinity of interfaces of alternating soft and stiff soil layers and, in the case of restrained-head piles, at the pile head. The scope of this paper is threefold: (a) to critically review some existing design methods for kinematic pile loading; (b) to develop new analytical results for piles in homogeneous and layered soils; (c) to present a case study in which theoretical predictions are tested against ®eld measurements. To this end, an approximate beam-on-dynamic-Winkler-foundation (BDWF) model is implemented, speci®cally developed for the seismic response of piles in layered soil. Both ®xed-and free-head piles, and different boundary conditions at the pile toe, are considered. It is shown that the magnitude of kinematic moments depends mainly on the stiffness contrast between the soil layers, the pile±soil stiffness contrast, the excitation frequency, and the number of excitation cycles. A unique case history involving the instrumented pile foundation of a multistorey building in Japan is presented. Time histories of bending and axial strains recorded at six locations along two piles are successfully compared with results computed from simple formulae and methods presented in the paper.

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Several aspects of the seismic response of groups containing nonvertical piles are studied, including the lateral pile-head stiffnesses, the "kinematic" pile deformation, and the "inertial" soil-pile-structure response. A key goal is to explore the conditions under which the presence of batter piles is beneficial, indifferent, or detrimental. Parametric analyses are carried out using three-dimensional finite-element modeling, assuming elastic behavior of soil, piles, and superstructure. The model is first used to obtain the lateral stiffnesses of single batter piles and to show that its results converge to the available solutions from the literature. Then, real accelerograms covering a broad range of frequency characteristics are employed as base excitation of simple fixed-head two-pile group configurations, embedded in homogeneous, inhomogeneous, and layered soil profiles, while supporting very tall or very short structures. Five pile inclinations are considered while the corresponding vertical-pile group results serve as reference. It is found that in purely kinematic seismic loading, batter piles tend to confirm their negative reputation, as had also been found recently for a group subjected to static horizontal ground deformation. However, the total ͑kinematic plus inertial͒ response of structural systems founded on groups of batter piles offers many reasons for optimism. Batter piles may indeed be beneficial ͑or detrimental͒ depending on, among other parameters, the relative size of the overturning moment versus the shear force transmitted onto them from the superstructure.