Analysis of Characteristic Frequencies of Coupled Soil-Pile-Structure Systems

A combination of the multiple-shear-spring soil model and a rigorous soil-pile interaction spring was implemented through a finite-element computer code to ascertain characteristic frequencies dominating the nonlinear seismic response of soil-pile-structure (SPS) systems. Different piles, represented by their rigidities, are assumed to be embedded in homogeneous or inhomogeneous soil profiles and to support single degree-of-freedom (SDOF) structures, which are each represented by the natural frequency of the structure on a rigid base. The results indicate that the pile-head motion was dominated by two sequential frequencies: the fundamental frequency of the coupled SPS system, f(SSI), at which the pile motion was noticeably magnified, and the pseudonatural frequency, f(pSSI), at which the response was significantly reduced in reference to that of the free field. These results confirmed those observed in a geotechnical centrifuge using actual and simulated ground motion. In addition, strong soil-structure interaction (SSI), manifested by a considerable reduction in f(SSI) of the coupled SPS system, had been noticed for systems consisting of rigid structures supported by flexible piles. The nonlinearity and inhomogeneity of the ground induced further reduction in f(SSI), indicating the strong need to revisit the current interaction methodologies that had been based on linear elastic SSI. However, the f(pSSI) did not show considerable variation with excitation amplitude.