Document: NUREG-0800
Document ID: 78905d69-1945-4638-99b9-2db68eb3da77
Document Type: srp
Title: SEISMIC SYSTEM ANALYSIS
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0520/ML052070318.pdf
Revision Date: 2023-06
Chapter: 3
Section ID: 3.7.2
CFR Part: 
CFR Title: 

Content:
tic stress distribution under the foundation and transmitting 3.7.2-13 DRAFT Rev. 3 - April 1996 the frequency content of interest. The two-dimensional approximation of three- dimensional problems may have to be justified in some special situations. Two mathematical representations of the model side boundaries are available for use in the direct solution approach-simple or viscous boundaries and transmitting boundaries. The location of the simple or viscous boundaries is dependent on strain and damping in the soil and is typically thrice the base dimension from the structure. The side boundary nodes can be either "constrained," in which case free-field displacements are specified, or "free," in which case forces are specified. When using the transmitting boundaries, it is possible to place the boundaries immediately adjacent to the structure if secondary nonlinearities in the soil are ignored. The following limitations should be observed for deep soil sites: - The model depth, generally, should be at least twice the base dimension below the foundation level, which should be verified by parametric studies. - The fundamental frequency of the soil (or backfill) stratum should be well below the structural frequencies of interest. - All structural modes of significance should be included. Half Space or Substructure Solution Technique The substructure (3-step) approach comprises the following steps: (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 soil-structure interaction analysis. Step (1) requires that assumptions be made about the mechanism of wave motion at the site. The foundation motion may be determined by a number of techniques, including: - Analytic functions - Boundary integral equations - Finite element and difference methods. DRAFT Rev. 3 - April 1996 3.7.2-14 In calculating the foundation motion