A procedure for the computation of seismic fragility of NPP buildings with base isolation

The research work here described is devoted to the development and testing of a numerical procedure for the computation of seismic fragilities for equipment and structural components in Nuclear Power Plants (NPP). Given the very low damage probabilities which are required in modern nuclear industry, attention is focused on the comparison between the performance of traditional and seismically isolated buildings. The procedure is based on the hypothesis, typical of nuclear structures, of linear behaviour of the building in the traditional case; the behaviour of isolation devices, on the other hand, is modelled taking mechanical non-linearities into account. The proposed procedure for fragility computation makes use of the Response Surface (RS) Methodology to model the influence of the random variables on the dynamic response. To account for stochastic loading the latter is computed by means of a simulation procedure. Given the RS, the Monte Carlo method is used to compute the failure probability; a risk-based procedure for refining the RS is also proposed and tested in an illustrative example. For the isolated case, an overall experimental/numerical methodology for fragility assessment is summarized and an example of fragility estimation is finally shown.