## 5 Conclusions  
The problem of SSSI with two structures was investigated by a series of geotechnical centrifuge experiments. Dense sand ground was prepared with several instruments. Three structures were made of aluminum with different mass, height, and fixed-base frequency. Different configurations of two structures located on the ground surface were tested under two earthquake input motions at various input intensities. Major observations are as follows:  
1. Roof motion amplified at a lower frequency compared to fixed-base natural frequency indicating a period lengthening of the soil-structure system. FIM was de-amplified at high frequency due to base-slab averaging of kinematic interaction effects. Flexible-base frequencies of all structures decreased at larger input intensity.
2. SSSI affected structure response in both time and frequency domains indicated by reduction in both \(\Sigma\)PSD and I\({}_{\alpha}\) of S1 structure when it was located adjacent to S2 or S3 structures representing both lighter and lower structures. However, SSSI phenomena almost had no influence on the resonant frequency of a more massive structure during an earthquake. More reduction in response of S1 structure was observed at L_Input excitation compared to that at higher input intensities.
3. Severe SSSI effects were observed in distance smaller than the width of the foundation. Center-to-center unfavorable distance could be \(\lambda/4\) (\(\lambda\) is the wavelength emitted from an adjacent structure) where the most significant increment in structure response of smaller structure was observed.
4. Fixed-base frequency ratio between structures had an insignificant influence on SSSI phenomena. Height ratio (\(\chi\)) and mass ratio (\(\psi\)) reflecting relative inertial interaction behavior between structures controlled SSSI effects between two structures in this study. Depending on whether \(\chi\) and \(\psi\) values were smaller or larger than 1, SSSI effects could reduce or increase structure motions, respectively. SSSI effects were found to be more severe at a low-intensity earthquake because of significant inertial interaction mechanism of the structure. With increasing input intensity, less significant SSSI effects were observed.  
Conceptualization, C.L.; Data Curation, V.-L.N.; Formal Analysis, C.L. and V.-L.N.; Funding Acquisition, J.-M.K.; Methodology, C.L.; Project Administration, J.-M.K.; Supervision, C.L. and J.-M.K.; Validation, S.-H.C.; Experimental Program, C.L. and V.-L.N.; Resources, C.L. and J.-M.K.; Writing--Original Draft Preparation, V.-L.N.; Writing--Review & Editing, C.L. and S.-H.C.  
This research was funded by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20161520101130).  
This work was a part of project titled \"Earthquake Response Analysis of Nuclear Structure Considering Structure-Soil-Structure Interaction\", supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20161520101130).  
The authors declare no conflict of interest.