## 1 Introduction
[PERSON] and [PERSON] (2005) validated [PERSON]'s method by comparison with the coupled analysis result for an NPP containment building connected to an SDOF oscillator [13]. [PERSON]'s method was derived by serially connecting the dynamic stiffnesses of the primary structure and the secondary system, but the acceleration response of the decoupled primary structure is not applied to the connection between the secondary and primary systems but to the boundary between the primary system and ground. So, the analytical model is difficult to explain physically. Recently, the interaction impact on the response of equipment in NPPs has been investigated. It was reported that the ISRS computed through decoupled analysis significantly increased in the proximity of the dominant natural frequency of the primary structure, a containment building in this case, compared to the coupled analysis [13]. [PERSON] et al. (2015) computed ISRSs at different locations of a reinforced concrete building using a finite element (FE) model and investigated the difference between the ISRSs computed by coupled and decoupled analysis, respectively [2]. [PERSON] and [PERSON] (2020) applied the  
Figure 1: Procedure of calculating ISRS. methodology of [PERSON] (1995) to the lumped mass stick (LMS) model of an NPP structure to investigate the reduction of the ISRS by coupled analysis [3; 14]. [PERSON] et al. (2022) build a coupled analysis model based on the LMS model of an NPP containment building and investigated response reduction effects obtained by coupled analysis [15]. [PERSON] compared the responses of coupled and decoupled analyses for petrochemical facilities [16]. [PERSON] et al. (2019) investigated the response reduction effect of coupled analysis at various locations and equipment masses in an NPP auxiliary building using a three-dimensional FE model [17]. In EPRI 3002009429, [PERSON]'s coupled analysis method using transfer functions was applied to an idealized structure in which only slabs have flexibility while the other components are rigid [18]. The coupled analysis effect was greater in the vertical direction than in the horizontal direction because the mass ratio of the equipment to the modal mass was large in the local vibration mode of the slabs. Model fidelity makes difference in the accuracy of ISRS [19]. Some researchers utilized shaking table test results to validate model adequacy and ISRS. [PERSON] et al. (2017) investigated the influence of inelastic deformation on the ISRS and showed that an equivalent linear model of a shaking table test specimen produced an ISRS matching test results [20]. [PERSON] et al. (2022) investigated the influence of location on the floor response using a three-dimensional FE model updated to fit the shaking table test result of a reinforced concrete building specimen [21]. Although they considered equipment-structure interaction in the analysis, their approach was based on the repeated analysis of a model with entire DOFs. In this study, a two-degree-of-freedom (two-DOF) equation of motion was derived in the frequency domain using a substructuring technique for efficiently calculating ISRS considering the equipment-structure interaction. The ISRS of the auxiliary building was efficiently calculated using only a single calculation of the impedance function and structural response conducted for the decoupled primary system. Repeated analyses of the coupled model for different equipment frequencies were conducted using the model with only two DOFs, instead of analyzing the coupled model with full DOFs. Compared to the transfer function method, the proposed two-DOF model enables clear physical interpretation and computes the response of the primary structure at once in addition to the secondary system response. Apart from the coupled analysis technique, decoupling criteria is another important issue in equipment-structure interaction. Therefore, the adequacy of the three decoupling criteria was investigated in this study.