Partitioned Nonlinearity Soil–Structure Interaction Analysis for Nuclear Power Plant Structures with Pile Foundations

Accurate seismic safety assessment of nuclear power plant (NPP) structures with pile foundations on soft soil sites requires consideration of soil nonlinearity and pile–soil–structure interaction (PSSI). This study develops an efficient partitioned SSI framework, where the nonlinear soil response is simulated using the Davidenkov skeleton curve combined with a modified Masing rule and solved by an explicit time integration scheme, while the structural dynamics are evaluated using the modal superposition method. The framework is applied to a pile-supported CAP1400 NPP model on deep soft soil, with both piles and the superstructure modeled as elastic. Two computational schemes are examined: (a) explicit integration of the soil while treating the piles and structure as an integrated system analyzed via modal superposition; and (b) explicit integration of both soil and piles, with the structure analyzed using modal superposition. Under pulse excitation, both schemes yield comparable dynamic responses, whereas scheme (b) improves computational efficiency by over threefold (88 h vs. 293 h). Results using scheme (b) under RG1.60 excitation show that soil nonlinearity reduces and delays structural responses but increases pile bending moments and stress concentration, demonstrating the framework’s effectiveness and practicality for nonlinear SSI analysis of NPP structures.