Dynamic soil–structure interaction of polypod foundations

The paper concerns the importance of dynamic structure–soil–structure interaction (SSSI) for structures with two or more foundations. Firstly, analysis of such polypod foundations is performed in frequency domain, considering the range 0–50 Hz and employing Green’s function for wave propagation in layered soil. Normalised dynamic stiffnesses related to individual foundations and cross-coupling between two foundations are presented. To allow time-domain analysis, frequency-independent consistent lumped-parameter models (LPMs) are developed, proposing a criterion for determining which components must be included within an LPM to account adequately for the SSSI. While the methodology can be applied to polypods with any number of footings, the examples concern bipods with two footings or piles. However, the examples illustrate the methodology and prove its accuracy and efficiency. Firstly, the response is evaluated for monolithic masses placed on each foundation such that interaction can only occur via the soil. Secondly, a structure is introduced to examine and compare the coupling between the two foundations via the soil and via the structure. Different modes of dynamic SSSI are studied, finding that LPMs can provide nearly exact results for the considered problems, but with extremely few degrees of freedom compared to a finite-element model.