Influence of Soil Secondary Nonlinearity Effect on Seismic Response of Nuclear Island Structure

The effect of soil-pile-structure interaction (SPSI) is becoming one of the key issues in the seismic design of the nuclear island structures built on non-bedrock sites. The 3D lumped-mass stick (LMS) model of the AP1000 nuclear island structure considering the SPSI effect is developed to represent the steel containment vessel, the containment internal structures, and the coupled shield and auxiliary buildings supported by the pile-raft foundation. To improve the calculation efficiency, the 3D model is simplified into the equivalent 2D models in the X-Z and the Y-Z planes through vector decomposition of the 3D components and their interactions. The soil nonlinear behavior is termed the soil primary nonlinearity effect and the soil secondary nonlinearity effect (SSNE). The influence of SSNE caused by the effect of SPSI on the seismic responses of the AP1000 nuclear island structure is studied using various bedrock motions. The results show that: (1) Compared with the vertical seismic responses of the nuclear island structure, the SSNE of the horizontal responses is more obvious, and is stronger for bedrock motions with intense high-frequency components; (2) The influence of SSNE on the responses of the nuclear island structure has a positive correlation with its heights and bedrock motion levels; (3) Because of the SSNE, the peak accelerations and the peak relative displacements of the nuclear island structure decreases and increases respectively; while the 5% damped spectral accelerations of the nuclear island structure decrease in the shorter periods and increase in the mid-long periods. (4) For the peak relative displacements, the shorter the predominant periods of bedrock motions are, the higher the proportion of SSNE in the nonlinear effect of SPSI is. In general, the existence of SSNE will increase the flexibility and the energy consumption of the SPSI system of the nuclear island structure, and thus it is inappropriate to overlook SSNE arbitrarily.