A note on a pseudo-natural SSI frequency for coupled soil-pile-structure systems

摘要

The notion of a pseudo-natural SSI frequency was introduced in a recent publication by the authors, as the frequency where foundation motion is minimized with respect to the free field surface motion. This frequency is determined analytically in this paper, for a single-degree-of-freedom structure supported on a pile foundation. The analytical solution is compared to numerical results from rigorous finite element analyses for different pile and structural configurations. The relationship between pseudo-natural (f_(pssl)) and effective natural SSI frequency (f_(SSI)) of the coupled system is also analytically quantified. It is concluded that f_(pSSI) may deviate substantially from f_(SSI) when a stiff squatty structure is founded on a stiff and/or short end-bearing pile for which foundation translation prevails. Conversely, when a flexible tall structure is supported on a flexible pile, f_(PSSI) and f_(SSI) nearly coincide due to dominant base rocking effects. In the latter case the effective natural SSI frequency can be predicted by standard identification procedures even when free-field recordings are missing. Effective damping effects are also discussed.