Soil-Structure-Interaction Study and Safety Assessment of Ventilation Stack for Extreme Wind Events

Ventilation Stack structure of a typical Nuclear Power Plant (NPP) is analysed and designed for a design basis wind velocity corresponding to 1000 years return period. Subsequent to the Fukushima Event, as a part of safety assessment of NPP structures, assessment of structural integrity of stack structures has been taken up against extreme natural events i.e., beyond design basis wind loading in both along and across wind direction using non-linear. In the present work, detailed Soil-Structure-Interaction (SSI) analysis of typical stack structure of NPP modeled as 3D finite element along with soil has been carried out in Abaqus software under sustained plus along and across wind load corresponding to extreme wind condition. Mohr–coulomb plasticity model is used for continuum soil medium. Elasto-plastic behaviour of the raft-soil interface is simulated using Coulomb friction model available in Abaqus. Steel reinforcements have been introduced at relevant layers/sections of the stack. Transverse shear deformation in element formulation and geometric non-linearity due to large deformation has been considered during the analysis. Non-linear analysis with detailed material characterization and failure models for both concrete and reinforcement has been carried out for a realistic estimation of post-cracking margins and to understand the non-linear behaviour of the stack at various load steps. This paper presents the analysis methodology for safety assessment of wind sensitive stack structures against beyond design basis wind with due consideration to SSI effects. The analysis procedures, material models adopted for the analysis, etc. and the analysis results under extreme wind have been discussed in this paper. Based on the results of the nonlinear analysis safety margin with respect to beyond design basis wind speed has been established.