COMPARATIVE STUDY AND APPLICABILITY ASSESSMENT ON SSI SIMULATION METHODS FOR DYNAMIC ANALYSIS OF COOLING TOWERS

Natural draft cooling towers are critical structures in nuclear power plants, characterized by large height, slender configuration, and thin-walled shell. Their dynamic characteristics are significantly influenced by soil-structure interaction (SSI). To investigate the effects of SSI under different modeling approaches and soil conditions, a typical cooling tower was modeled using four SSI finite elementapproaches: a fixed-base model, an equivalent spring model at the ring foundation, a pile-soil spring model, and a solid soil model with piles. Both the equivalent spring at the ring foundation and the pile-soil spring models are simplified spring methods. Four soil types—soft soil, medium soft soil, stiff soil, and rock—were considered, and the dynamic characteristics were compared via modal analysis. Results showed that the relationship between the natural period of the cooling tower and the predominant period of the site soil governs the SSI effect. Under otherwise identical conditions, the lower the shear wave velocity of the soil, the longer the predominant period of the site soil, the longer the natural period of the cooling tower, and the earlier the global translational modes appear, indicating SSI influence on the dynamic characteristics. When the natural period of the cooling tower was shorter than the predominant period of the site soil, the cooling tower, the pile foundation, and the surrounding soil exhibited collaborative swaying or rockingThe soil participated in the soil-pile-cooling tower mode shapes, indicating that SSI altered the dynamic characteristics of the cooling tower. Accordingly, when the natural period of the cooling tower obtained from simplified spring methods is shorter than the predominant period of the site soil, the solid soil model with piles is recommended to better capture the soil-pile-cooling tower mode shapes and account for SSI effects.