Document: NUREG-0800
Document ID: d1a90472-98c9-447c-aeba-d2fe7d322ac5
Document Type: srp
Title: -
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1319/ML13198A223.pdf
Revision Date: 2023-06
Chapter: 3
Section ID: 3.7.2
CFR Part: 
CFR Title: 

Content:
Revision 4 – September 2013 (1) Determine the motion of the massless foundation, including both translational and rotational components. (2) Determine the foundation stiffness in terms of frequency- dependent impedance functions. (3) Perform SSI analysis. The procedures, modeling assumptions and analytical bases adopted for performing the half space or substructure analysis, including use of frequency- independent soil spring parameters, and the spring and damping coefficients, is reviewed on a case-by-case basis. In the SSI analysis of embedded structures, some computer implementations of the substructure approach use two alternative methods to model the excavated soil volume: (1) The direct method (DM), in which the foundation impedance is calculated for the free field at all nodes of the excavated soil volume that is discretized into finite elements. These nodes, termed “interaction nodes,” connect the excavated soil volume and the free field soil system to ensure compatible motions. DM is also referred to as the flexible volume method (FVM) in frequency domain solutions. (2) The Subtraction Method (SM), in which a simplification is made such that only the nodes on the outer boundaries of the excavated soil volume are treated as interaction nodes. This simplification reduces the computational effort needed for solving large problems typically encountered in NPP applications. However, because the interior nodes are not treated as interaction nodes, the compatibility of displacements is no longer imposed at every interaction nodes in the excavated volume. This may lead to limitations in the applications of the SM and potential errors induced in computed foundation compliance functions as well as transfer functions. In light of the above discussion, the DM should be used to the extent practical to perform the SSI analysis of embedded structures. In cases that require the use of the SM, due to limitations of the DM in handling very large computational models,