Partitioned analysis of soil-structure interaction for Nuclear Island Buildings

In the nuclear industry, soil-structure interaction (SSI) analysis of safety-related structures has been typically conducted using the three-step solution proposed by Kausel (usually in the frequency domain), and the equivalent shear moduli and damping ratios are used to resolve the nonlinear behavior of the soil-structure system.

With the advances in computing technologies, a significant improvement in the representative computer program-based analysis approaches has been reported. For example, the frequency-domain analysis method is computationally efficient and requires only a few parameters to model the dynamic soil properties, and it generally leads to appropriate designs of nuclear power plants that can withstand low-to-moderate amplitude earthquakes. However, for design basis and beyond-design basis shaking, this method cannot address the gapping, sliding, and other nonlinear effects caused by intense earthquakes, which are expected at the sites of many nuclear facilities.

Although the dynamic SSI analysis in the time domain has been successfully performed by solving the motion equations via a direct numerical integration scheme. The main shortcoming of this standard approach is the use of the same time integrator (homogeneity) and timescale (synchronous) for all finite elements of the grid, which causes computational inefficiencies.

In order to address these challenges, Dr. Shaolin Chen, Dr. Hao Lv and Dr. Guoliang Zhou from Nanjing University of Aeronautics and Astronautics proposed a partitioned analysis of SSI (PASSI) to improve the computing efficiency. Their approach 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.  Their research work is currently published in the journal, Earthquake Engineering and Structural Dynamics.

The authors showed that the proposed method was highly efficient for SSI analysis of nuclear structures. 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. The seismic response analysis of a nuclear power plant is presented to validate the feasibility and efficiency of the intrafield and interfield parallel procedures.

In summary, the study presented a time-domain PASSI for nuclear buildings, and two parallel procedures are proposed. The feasibility and efficiency of the proposed method were tested using two numerical examples. In a statement to Advances in Engineering, first author Dr. Shaolin Chen said their findings would contribute to efficient SSI analysis and the safe design of major projects such as nuclear plant.