Document: NUREG-0800
Document ID: 671effeb-08bf-46ab-b6de-dd19a026ebb6
Document Type: srp
Title: -
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1235/ML12353A354.pdf
Revision Date: 2023-06
Chapter: 3
Section ID: 3.7.2
CFR Part: 
CFR Title: 

Content:
Draft Revision 4 - December 2012 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. (2) The subtraction method (SM), in which a simplification is made such that only the nodes on the outer boundary 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 connected to the free-field system, the excavated soil volume may not have compatible motions with the part of the free-field being replaced, especially at frequencies higher than the fundamental frequency of the excavated soil volume. This may lead to limitations in the application of the SM and potential errors if the method is not implemented appropriately. 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, technical justifications should be provided to demonstrate the adequacy of the SSI analysis based on the SM. These technical justifications should include the following elements: (1) An assessment of the excavated soil volume should be performed to identify its vibratory frequencies and mode shapes. These frequencies and mode shapes may be spurious in the SM solution, which can lead to unconservative or erroneous results. They can be identified as spikes in the transfer functions computed using the SM, which do not