In this study, an improved method for calculating the effective seismic loads used in substructure Soil-Structure Interaction (SSI) analysis under earthquake loading is proposed. This method, called the Multiple Boundary Reaction Method (MBRM), is an extension of the existing Boundary Reaction Method (BRM). By applying effective seismic loads on two different boundary interfaces, the proposed method effectively cancels out the amplitude of low-frequency wave components transmitted to the far-field soil. This mechanism further enhances the wave-absorbing performance of the Perfectly Matched Layer (PML), which serves as the Absorbing Boundary Condition (ABC) in this study. To verify the effectiveness of the proposed method, seismic response analyses were carried out on a Korean standard nuclear power plant structure. The MBRM-PML method demonstrated clear improvements compared to the conventional BRM-PML method. Specifically, the average error in the low-frequency range decreased from 10.17% to 3.47%, while the error over the full frequency range decreased from 4.27% to 3.12%, and the maximum error decreased from 5.28% to 3.80%. The accuracy improvement is most evident at lower frequencies compared to previous methods.
Lee et al. (Thu,) studied this question.