Some cornerstones of dynamic soil–structure interaction

Salient features of dynamic soil–structure interaction are discussed. A criterion for the presence of radiation damping in a site is formulated. The radiation condition at infinity of outwardly propagating energy can for certain sites correspond to incoming waves. The consequences that the dynamic behaviour of the unbounded soil depends on the dimensionless frequency, which is proportional to the product of the frequency and the radial coordinate, are discussed. The partition of the radiated energy of surface waves and of body waves for increasing frequency is addressed. In addition, procedures to analyse the dynamic soil–structure interaction are outlined, ranging from the approximate simple physical models (cones, spring–dashpot–mass representations) for the soil to the damping-solvent extraction method and to the rigorous forecasting method and the scaled boundary finite-element method. Convolution integrals can be avoided by constructing a dynamic system with a finite number of degrees of freedom for the soil. Extensions for moving concentrated loads and an increase in efficiency using a reduced set of base functions are presented. The damping ratio of an equivalent one-degree-of-freedom system representing the interaction of the structure with the soil for a horizontal earthquake reflects the effect of the cutoff frequency for a soil layer. The same simple model can be extended to consider the partial uplift of a basemat for seismic excitation (nonlinear soil–structure interaction). Two-dimensional versus three-dimensional foundation modelling is examined.