ML24215A042

LLC, Response to Sdaa Audit Question Number A-3.8.5-10

ML24215A042
Person / Time
Site:	05200050
Issue date:	08/02/2024
From:	NuScale
To:	Office of Nuclear Reactor Regulation
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Response to SDAA Audit Question Question Number: A-3.8.5-10 Receipt Date: 08/07/2023 Question:

It is noted that 5.0 thick foundation mat is modeled for the CRB with single layer shell elements (SHELL181, a 4 nodes element with 6 DOF at each node). The staff is seeking to understand the following modeling topics related to the CRB foundation in order to complete its safety review:

1.

Given the variation of material (concrete and steel) and material properties (concrete compression and tension) through mat thickness and SHELL181 stress and strain profile through element thickness is not available (only average stress is available), explain in detail why using a single layer of shell elements is adequate to account for the behavior of steel reinforced concrete mat. Discuss whether the simplification of using a single-layer SHELL181 elements for a 5-foot-thick reinforced concrete member might affect the CRB structural analysis.

2.

It is noted that there are many SC walls on the CRB foundation mat (Figure 3B-47), which divide the foundation mats into several smaller areas, resulting in lower span-to-thickness ratios. Discuss how a 5 thick RC concrete mat, with its bottom surface supported by soil and top surface separated by a wall, can react like a shell and is therefore modeled as a shell, and explain why the assumption of a thin structure using shell elements is reasonable.

3.

Provide information on how to model the contact between SHELL181 elements and soil elements, or how the nodes of SHELL181 elements are connected to the nodes of soil elements.

i.

Please explain in detail how to analyze uplift and slip by modeling the interface between SHELL181 elements and soil elements or by other methods.

ii.

If nodes of SHELL181 element at midplane are directly connected to the corresponding soil nodes, justify how the model can still reasonably represent a 5' thick CRB mat foundation.

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Response

Item 1:

The foundation basemat is modeled as a shell structure with gross section properties corresponding to the concrete only, which is in agreement with reinforced concrete design practice and the corresponding design codes (i.e. ACI 349-13). ANSYS shell elements (SHELL181) are used to simulate shell behavior.

To support this audit response, a mesh-density and element technology evaluation was performed to test the adequacy of using single-layer SHELL181 elements in the control building (CRB) basemat deformation. The CRB model is placed on solid elements to represent the underneath soil similar to the CRB differential settlement analysis. ((2(a),(c) Item 2: In comparison to legacy shell elements that used Kirchhoff classical plate theory, modern shell finite element technologies in ANSYS (e.g, SHELL181 and SOLSH190) follow the higher order Reissner-Mindlin plate theory which takes into account transverse shear strains. Thus, modern shell elements can be used in either thin or moderately thick-plate applications. There are no steel-composite (SC) walls on the CRB foundation basemat. The SC labels on the US460 Standard Design Approval Application (SDAA) Figure 3B-47 identify section cuts used for the design. The SDAA Figure 3.7.2-11 shows the RC walls that are on the CRB basemat. These are also represented with SHELL181 elements (with the ANSYS option set to visualize shell thickness in Figure 3.7.2-11). There are only two interior walls. Thus, division of the foundation into smaller areas for compatible meshing with the walls is not an issue for the CRB. Further, ACI Committee 336 (Footings, Mats and Drilled Piers) developed a guide for the analysis and design of mat foundations, ACI 336.2R-19. Section 6.11 and 6.12 of the ACI guide discuss mat thickness and two versus three-dimensional analysis and concludes that thin plate NuScale Nonproprietary NuScale Nonproprietary

analysis is generally used and is adequate. Thus, the assumption of a thin plate structure using shell elements is reasonable. Item 3: i) Uplift and slip modeling for the foundation-soil interface is considered for the CRB Stability Analysis only. The performed transient analysis have a nonlinear nature that stems from the frictional boundary condition between the CRB basemat and the soil layer under it. (( }}2(a),(c) ii) For the settlement and harmonic soil structure analysis of CRB, there is no non-linearity stemming from the boundary conditions and the foundation-soil interface behavior is fully bonded. ((

}}2(a),(c)

No changes to the SDAA are necessary. NuScale Nonproprietary NuScale Nonproprietary}}