ABSTRACT
Nogami & Konagai (1986, 1988) analyzed the dynamic response of pile foundations in the time domain using a Winkler approach. Fan et al. (1991) performed an extensive parametric study using an equivalent linear approach to develop dimensional graphs for pile head deflections versus the free-field response for various soil profiles subjected to vertically propagating harmonic waves. Makris & Gazetas (1992) applied free-field accelerations to a one dimensional beam on dynamic Winkler foundation model with frequency dependent springs and dashpots to analyze the response of single piles and pile groups. The
results showed that interaction effects on kinematic loading are not significant at low frequencies but are significant for pile head loading which can be interpreted as inertial interaction.Their study was limited to equivalent linear elastic approach and one dimensional harmonic loading. Nogami et al. (1992) introduced material and geometrical nonlinearity in their analyses using discrete systems of mass, springs and dashpots. Bentley & El Naggar (2000) mentioned that elastic kinematic interaction for a single pile slightly amplifies the free-field transfer function. Cai et al. (2000) attempted to include plasticity and work-hardening of soil using a finite element technique in the time domain; however, they used fixed boundary conditions and neglected damping in the sub-system. Kim & Stewart (2003) performed an extensive empirical study of SSI effects using several strong motion data sets recorded at different building sites. They concluded that decrease in the natural frequency and increase in the damping associated with SSI might indeed affect the structural response. The response was influenced most by a parameter describing the relative stiffness between the foundation soil and the structure which is known as impedance factor. This parameter reflects dynamic characteristics of the soil, as well as the dynamic characteristics of the structure. Maheshwari et al. (2003) examined the effects of plasticity and work hardening of soil on the free field response of single piles and pile groups using the hierarchical single surface (HiSS) soil model. Maheshwari et al. (2004) extended their work to include the superstructure in order to evaluate the effects of SPSI for a fully coupled system. They conducted the analyses for both harmonic and transient excitations and compared both linear and nonlinear responses. They mentioned that the effects of nonlinearity on the responses are dependent on the frequency of excitation with nonlinearity causing an increase in response at low frequencies of excitation. The work presented in this paper is an attempt to evaluate the effects of change in piles length on the kinematic and inertial interaction of soil-pile systems which are assessed by transfer and impedance functions, respectively. The effect of foundation on the earthquake ground motion is termed kinematic interaction and the effect of foundation compliance on structural response plus the effects of inertial loads on the foundation is referred to as inertial interaction. The frequency dependent transfer functions are defined by the ratio of the foundation motion to the free field motion in the absence of structure. The flexibility of the foundation and the damping associated with foundation-soil interaction is described by a frequency dependent foundation impedance function which is defined by means of the stiffness and damping of the system and as research develops, more data are available that consider different aspects of the issue.
