###### Abstract  
Soil-structure interaction (SSI) leads to a modification in the dynamic properties of structure, but due to the complexity of analysis, it is traditionally assumed in seismic designs that the structure is fixed-supported on the ground, which brings about potential risks to the seismic performances of structure. The study works on the possibility of SSI having detrimental effects by comparing the dynamic responses of the SSI system to a fixed-base structure, and presents charts for an evaluation of the system frequency of SSI for the purpose of engineering practice. In order to reveal the physical nature, the SSI model is reduced to its simplest form, consisting of a SDOF oscillator, a three-dimensional rectangular foundation, and a multi-layered half-space. The energy dissipation in the soil is achieved by foundation impedances and the substructure method. Previously, the foundation impedances are usually acquired by two-dimensional or axisymmetric three-dimensional models in uniform half-space to avoid the high cost of the more realistic, fully 3D models, while a high-precision indirect boundary element method is employed, combined with the non-singular Green's functions of distributed loads to calculate the foundation impedances. Although SSI dampens the peak amplitude of structure response in the frequency domain, case studies on four buildings' responses to 42 earthquakes in the time history show a possibility of 15-20% that SSI amplifies the dynamic responses of structures, such as the maximum and the mean values in the time history, depending on the properties of the structures and the site, as well as the frequency component of incident waves. soil-structure interaction; indirect boundary element method; foundation impedances; system frequency; system responses; detrimental effects; beneficial effects; seismic design +
Footnote †: journal: Applied Sciences  
on SSI reveals that SSI increases the damage index before a threshold period, and the conventional fixed-base model underestimates the damages of buildings with periods less than the threshold [4]. A study using fragility curves shows that neglecting SSI results in nonconservative predicted damage levels of structural and non-structural elements [5]. Although the analysis of SSI is necessary for structural seismic designs, due to its complexity and cost, it currently only applies to a few very important infrastructures, such as nuclear power plants [6], huge dams [7] or long bridges [8; 9]. Actually, it was the development of the nuclear power plants in the 1970s that promoted the extensive research on SSI. The analysis of SSI applies to a very small number of civil structures used only for some special purposes; for example, the influence of adjacent topography on buildings [10], the influence of a poroelastic site on deep piled foundations [11; 12], and so on. For most civil structures, however, little attention is paid to seismic designs involving SSI. The conventional fixed-base assumption is usually adopted for convenience, and it is only possible to very roughly determine the potentially detrimental effects of SSI by the shear wave velocity, or the period, of the site. Th method, although widely used among civil engineers, is not accurate and cannot be parametrically quantified. For example, many studies using numerical simulations [13; 14] and earthquake damage investigations [1; 15] have revealed that structures with some of the same features in a certain local area exhibit much more serious damage than other structures without those features. Nevertheless, there is an ongoing effort to integrate SSI analysis into the framework of coded seismic designs in engineering practice. In ASCE/SCI-7 [16] and NEHRP (FEMA P-1050-1/2015) [17], there are coded provisions in recommended chapters in all versions after the 1990s (v1997, v2000, v2003, v2009, v2015) to involve SSI analysis, which is based on the substructure method and foundation impedance functions.