The present methods for three-dimensional time-domain soil-structure interaction (SSI) analysis are often uneconomical because they are performed with a single time integration scheme and a single time step, which prohibits their application to large-scale SSI problems. In this study, a partitioned analysis of SSI (PASSI) is proposed for enhancing SSI computational efficiency. This is accomplished by partitioning the soil-foundation-structure system into the soil (foundation) and structure subsystems and implementing the continuity conditions of the displacements and reaction forces at the soil (foundation)-structure interface in a primal way. A lumped-mass explicit finite element method and a transmitting artificial boundary are used to model the unbounded soil, the structure is analyzed via the implicit finite element method, and the response of the rigid foundation is calculated through an explicit time integration scheme. The solution is separately advanced over time for each subsystem. Different time steps can be chosen for the explicit and implicit integration schemes, which can greatly improve efficiency. Interaction effects are accounted for by the transmission and synchronization of the coupled state variables. In addition, intrafield and interfield parallel procedures for PASSI are developed, and their theoretical efficiencies are analyzed. A simple example is provided to verify the performance of the partitioned approach with an explicit-implicit co-computation and to compare it to the fully explicit approach. Finally, the seismic response analysis of a nuclear power plant is presented to validate the feasibility and efficiency of the intrafield and interfield parallel procedures.

Soil-structure interaction (SSI) is often neglected in seismic design because of its inherent complexity. Therefore, SSI-related research is crucial for ensuring that more significant issues reach engineering practice. This study focused on torsional motion to investigate the accidental eccentricity caused by SSI effect by comparing the proportional relationship between the torsional motion of an SSI system and the translational motion of a fixed-base oscillator. The SSI model was created using a three-dimensional rectangular foundation embedded in a layered half-space with an oscillator mounted on the top. We employed an indirect boundary element method (IBEM) combined with non-singular Green's functions of distributed loads to calculate the system responses of the SSI. Parametric analysis-based results in the frequency domain revealed that the accidental eccentricity factor owing to SSI was typically in the range of 0.03-0.15. Additionally, a case study of four buildings subjected to 42 earthquakes revealed that the accidental eccentricity factor exceeded 0.05 in several cases, and 0.2 in some cases. Furthermore, it depended on the excitation spectral shape and dynamic characteristics of the SSI system. Therefore, the coded accidental eccentricity factor value of 0.05 is insufficient for ensuring the safety of buildings from the torsional effect of SSI, and a value of 0.1 is more reasonable.