Ground motion considerations for nuclear power plant design with emphasis on soil-structure interaction aspects

Nuclear power plant structures tend to be massive, stiff structures with fundamental frequencies in the 2 to 10 Hz range depending on oil conditions. Whereas damage to conventional structures probably correlates best with velocity content of the ground motion, the seismic vulnerability of nuclear power plants is probably more related to acceleration content. For this reason, the ground motion input for seismic design of nuclear power plants has been primarily expressed in terms of acceleration. It has often been noted, particularly in connection with near-source motions due to low-to-moderate magnitude earthquakes, that structures have performed much better than would be predicted considering the free-field instrumental peak acceleration to which the structures were subjected. In such cases, the differences in measured ground motion, design levels, and observed behavior are so great that they cannot be reconciled with typical safety factors associated with elastic seismic analyses for design even when typical ductility considerations are included. The problems with free-field instrumental peak acceleration and free-field instrumental-based elastic response spectra as measures of potential seismic damage are discussed. Because of these considerations, the US Nuclear Regulatory Commission (USNRC) has sponsored research work on the subject of engineering characterization of ground motion. This paper presents a summary of this work with emphasis on SSI and spatial variation of ground motion effects