COMPARATIVE STUDIES ON SEISMIC INCOHERENT SSI ANALYSIS METHODOLOGIES
Dan M. Ghiocel
The paper discusses the effects of the 3D seismic random wave propagation (incoherent motion) vs. the 1D seismic deterministic wave propagation (coherent motion) on SSI response. Both theoretical and practical aspects are presented. The presented incoherent SSI analyses were performed using deterministic and stochastic incoherent SSI approaches with and without complex response phase adjustment as implemented in the ACS SASSI code (Ghiocel, 2013). The incoherent SSI approaches with phase adjustment were validated by EPRI (Short, Hardy, Merz and Johnson, 2007) and accepted by US NRC (2008) for their application to the seismic SSI analysis of the new nuclear power plant structures in United States. The paper investigates the effects of complex response phasing on the incoherent in-structure response spectra (ISRS) for different nuclear island SSI models. It is shown that EPRI validated incoherent SSI approaches are always conservative, but sometimes they could be overly conservative for complex FEA SSI models. Finally, the effect of the foundation basemat flexibility that increases the incoherent ISRS and the bending moments in the baseslab is discussed.