# Seismic Response Effect on Base-Isolated Rigid Structures by Mass Eccentricity in Nuclear Plants
## 5 Shaking Table Test for Base-Isolated Structures with Mass Eccentricity
The comparison includes sensitivity analysis (SA), finite element model (FEM) analysis, and the seismic response in the three-axis vibration table test model. The measurement locations are the four corners (A2-A5) and the center point of the upper structure, and the results are given in terms of acceleration responses. This table reveals that, for the base-isolated structure, as the eccentricity of the upper structure's mass increases, the responses at individual corners may exhibit irregular behavior, but the response at the center tends to decrease. This behavior is likely due to increased torsional behavior with increasing eccentricity, which can offset translational motion. While the responses at each corner of the base-isolated structure show relatively minor differences between the sensitivity analysis and the finite element model analysis, the response of the test model shows differences of more than 30% in some locations compared with the analysis results. This difference is attributed to the fact that, in the sensitivity and FEM analyses, only horizontal mass eccentricity was considered, while in actual tests, both horizontal and vertical mass eccentricities were affected. Therefore, a combination of horizontal rotation and vertical rocking behavior could have lead to variations in the vertical direction response [17]. Figure 16 shows graphs illustrating the acceleration-response spectra of a beam-shaped secondary structure installed on top of the base-isolated upper structure and the shaking table. Additionally, the responses of beams installed on top of the base-isolated non-eccentric and eccentric upper structures are also compared. So to speak, the figures explain the effects of base isolation and the mass eccentricity of the upper structure on secondary structure response through acceleration-response spectra. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Non-Ecc (0\%)**} & \multicolumn{3}{c}{**Ecc1 (12.5\%)**} & \multicolumn{3}{c}{**Ecc2 (25\%)**} \\ \hline
**Loc** & **SA** & **FEM** & **Test** & **SA** & **FEM** & **Test** & **SA** & **FEM** & **Test** \\ \hline A2 & 1.89 & 1.91 & 2.15 & 2.12 & 2.03 & 1.63 & 2.14 & 1.92 & 1.73 \\ A3 & 1.89 & 1.91 & 1.53 & 2.12 & 2.03 & 1.56 & 2.14 & 1.93 & 1.72 \\ A4 & 1.89 & 1.89 & 1.81 & 1.60 & 1.52 & 1.09 & 1.41 & 1.36 & 1.01 \\ A5 & 1.89 & 1.89 & 1.69 & 1.60 & 1.52 & 2.04 & 1.41 & 1.38 & 1.12 \\ Center & 1.89 & 1.89 & 1.80 & 1.86 & 1.78 & 1.58 & 1.78 & 1.65 & 1.40 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Analysis and test results for mass-eccentric base-isolated structures. Figure 15: Acceleration responses of base-isolated structures by mass eccentricity. for the 10 Hz beam, and this is attributed to the seismic energy input being significant at around 10 Hz. Figure 16a,b demonstrates that the acceleration response of the secondary structure with natural frequencies of 5 Hz and 10 Hz, installed on top of the eccentrically base-isolated structure, shows minimal influence from eccentricity. These differences are not considered to be very significant due to mass eccentricity across various frequency ranges. However, the mass eccentricity effect of the primary structure on the secondary structure can increase by around 30-40% in the frequency range above 10 Hz, and this should be considered in the design. The base isolation effect is preserved effectively. However, as shown in Figure 16c, for the 30 Hz secondary structure, the acceleration response mainly increases above 4 Hz due to the mass eccentricity of the base-isolated upper structure. Therefore, in the case of a secondary structure with high stiffness and high natural frequencies, the base isolation effect may be reduced due to the mass eccentricity of the upper structure. It is essential to be cautious in such situations.