Seismic soil-structure interaction analysis of tall slender structures

The seismic response of soil-structure system depends on the characteristics of the input motion, site conditions and structure itself. In a seismic soil-structure interaction analysis, it is necessary to consider the infinite extent and layered nature of the soil, and the nonlinear behaviour of soils. This paper presents the seismic soil-structure interaction (SSI) analysis of a ventilation stack located in a nuclear power plant site. The SSI analysis was carried out by the flexible volume substructure method using computer program SASSI 2000. The influence of various parameters such as relative stiffness of the layers of the site, thickness of soil layer and foundation embedment on the interaction behaviour of the stack is investigated. The nonlinear soil behaviour is modelled by using site-specific modulus reduction and damping ratio curves developed from the laboratory experiments. It is found that the seismic response at the various levels of the stack shows a strong dependence on the relative stiffness of site and the depth of the soil layer to bedrock. The spectral accelerations and top displacements decrease with increase in embedment ratio. This aspect has to be taken into account while proportioning the stack like structures.