This paper studies resonance-sensitive seismic demand in a base-isolated nuclear reactor building. The baseline target spectrum is the design response spectrum (DRS) defined in ASCE/SEI 43-19 for seismic design category 5 (SDC 5), which corresponds to a target performance goal of 1×10−5 mean annual frequency of exceedance. Beyond-design-basis earthquake (BDBE) levels are represented by controlled multipliers of the same DRS shape (150%, 167%, and 200%). In parallel, a critical excitation (CE) procedure is used to derive a CE-based response spectrum (CE-RS) by maximizing a selected response measure within an admissible input set constrained by physical intensity measures, including peak ground acceleration (PGA), peak ground velocity (PGV), and Arias intensity. The CE constraints are anchored using a recorded event. Response-history analyses are performed using 11 spectrum-compatible horizontal record sets. The reported outputs are peak isolation displacement, isolation-plane base shear, and peak absolute acceleration at the lumped superstructure mass as a global indicator. Results show that BDBE scaling increases demands relative to the DRS baseline, as expected. However, CE-RS produces the largest demands across all metrics, indicating that standard DRS/BDBE scaling may not fully capture resonance-driven isolation demands for long-period isolated systems. A validation check shows that CE results agree closely with CE-RS-based results (differences below about 1%). The proposed workflow provides a fast, traceable supplement for early-stage design screening and for evaluating strengthening or rehabilitation options in existing isolated systems.
