Approximate 3-D structure-soil-structure interaction parametric analysis for seismic design of nuclear power plants

The need of truly 3-D numerical models in structure-soil-structure interaction analyses has been already pointed out by several researchers in the case of both aligned and misaligned buildings. The subject is dealt with, for instance, in a paper presented by some of the Authors at the 5th SMirt Conference in Berlin. It was recognized, however, that 3D models often can reach prohibitive computational efforts, compared with simpler 2-D models. This consideration is also to be related to the accuracy, reacheable with presently available computer codes, in the evaluation of the coupling terms of the foundation stiffness matrices. In the present paper an approximate procedure, which was developed on the basis of the previous research work, is presented to account for the 3-D structure-soil-structure interaction phenomenon. This procedure allows for a fairly accurate representation of the mutual interaction effect on the dynamic response of a system composed by a heavy building, usch as the Reactor Building of a Nuclear Power Plant (NPP) and a lighter building nearby (as a Turbine or Auxiliary Building). The method is classifiable among the so-called indirect or three-step approaches and consists in the approximate evaluation of the dynamic stiffness matrix of the foundation system. The approximation technique requires the calculation of the static stiffness matrix of the coupled foundation system and of the dynamic stiffness parameters of the buildings considered isolated. The proposed procedure, which is suitable for the case of aligned buildings when the seismic motion is coincident with the direction of the alignement, is inserted in a computer code for the complete analysis of the phenomenon. The program includes the modeling of the buildings and the treatment of seismic input motions. (orig./RW)