Structural Model Seismic Response Comparison for a Complex Nuclear Structure

Abstract Transactions, SMiRT-22 San Francisco, California, August 18-23, 2013

Division 3

Structural Model Seismic Response Comparison for a Complex Nuclear

Structure

Greg S. Hardy 1, Ruben Soto2, Shi Lu3, and Robert Kassawara4

1_{Senior Principal, Simpson Gumpertz & Heger Inc., Newport Beach, CA ([email protected])}

2_{Staff 1 Engineer, Structural Mechanics Group, Simpson Gumpertz & Heger Inc., Newport Beach, CA} 3_{Staff Consultant, Structural Mechanics Group, Simpson Gumpertz & Heger Inc., Newport Beach, CA} 4_{Senior Project Manager, Structural Reliability and Integrity, EPRI, Palo Alto, CA}

Introduction

The current fleet of Nuclear Power Plants (NPPs) in the United States typically have stick models (also referred to as lumped mass models) to represent the safety-related structures, both for the original design basis and for subsequent beyond design basis efforts, such as for the Individual Plant Evaluation for External Events (IPEEE) program. New plant applications are typically requiring that more modern and sophisticated finite element models be used for structural and seismic design analyses of their safety-related structures. These different structure modeling approaches used for existing/new NPPs have resulted in the question as to whether the existing stick models are adequate for use in Seismic Probabilistic Risk Analysis (SPRA) and Seismic Margin Assessments (SMA). New SPRA and SMA studies are being conducted for use in risk-informed applications as well as to respond to the new seismic hazard characterizations having much more energy in the high frequency range of the spectrum than was previously predicted. As a result, a key industry need exists to understand whether there is a technical basis for requiring the generation of new finite element models for the existing fleet of plants in support of seismic margin studies and SPRAs, and potentially for any future design basis actions that could result from the post-Fukushima 50.54f generic letters from the Nuclear Regulatory Commission (NRC).

EPRI has planned a set of research tasks to address this question related to the structural model fidelity as it pertains to seismic response requirements. To address this topic in a generic manner, four separate seismic response characterization studies have been considered:

1. Complex structure on soil site 2. Complex structure on rock site

3. Compact axisymmetric structure on soil site 4. Compact axisymmetric structure on rock site

The first of these tasks (complex structure on soil site) was undertaken as part of an EPRI research project [1] and is the subject of this paper. The EPRI project consisted of the generation of both a Finite Element Model (FEM) and stick model fixed-base responses for a complex nuclear structure. It also includes the study of soil-structure interaction (SSI) effects associated with the complex structure being founded on a soil site. If the stick models can be shown to closely represent the seismic response from a complex finite element model as a result of this first study, then it may not be necessary to complete the additional three studies.

To support this EPRI research project, a Stick Model (SM) and a Detailed Finite Element Model (DFEM) were developed for the Control Building at the Vogtle Electric Generation Plant, Units 1 and 2. The control building represents a highly complex structure due to its non-symmetric nature, its large size, and the variety of load paths inherent in its design and configuration (Figure 1). A comparison study of the Vogtle control building SM and DFEM is performed to evaluate the effects of the inherent SM modeling simplifications based on the seismic responses to uniform hazard spectra (UHS) for the Vogtle site. The purpose of this EPRI study is to determine whether a SM is adequate to develop sufficiently accurate structural response of a complex asymmetric building on a soil site compared to a DFEM. Static 1g analysis results, modal properties, and in-structure response spectra (ISRS) are used to compare the similarities and differences between the two structural models. The fixed-base model comparisons are conducted to evaluate the differences between the two different structural models. The SSI analysis includes the dynamic effects of the site’s soil profile used to identify the contribution that soil has on the overall seismic response of the structure.

Results and Conclusions

Key results and conclusions from this research project revolved around the following areas:

 Reasons for the natural frequency differences and significance assessments

 Significance of stick model simplifications

 Significance of input ground motion and corresponding spectral response shape

 Differences in the seismic in-structure floor response

 Recommendations for modeling considerations to ensure adequate response is generated from both DFEM and SM

Figure 1 – Control Building Finite Element Model

References

Structural Model Seismic Response

Comparison for Complex Nuclear

Structure

Simpson Gumpertz & Heger Inc.

Greg Hardy

Ruben Soto

Tim Lu

Electric Power Research Institute

Robert Kassawara

EPRI Research Project

Objective

-

Assess whether a LMSM provides adequate

seismic response characterization of a complex

asymmetric building compared to a DFEM



Electric Power Research Institute

(EPRI)



Dr. Robert Kassawara (EPRI Project Manager)



Principal Investigators:



Simpson Gumpertz & Heger Inc.

(SGH)

Greg Hardy, Shi Lu, Steve Short, David Nakaki, Ruben Soto, Kevin

Moore



Southern Nuclear Operating Company, Inc.



Bob Torkian, Robert Miller III, Don Moore

*EPRI Report to be published in Fall 2013

Driver for Study



Seismic Risk / Margin Assessments

Require “Adequate” Structure Models to

Develop Seismic Response



Post-Fukushima 50.54f seismic

recommendations



Risk-Informed Applications



Existing Nuclear Power Plants (NPPs)

typically only have lumped mass stick

models (LMSM)



Technical Adequacy of LMSM has been

debated in the industry for years



EPRI SPID developed some guidance for

EPRI Research Program on

Structure Models



EPRI has planned a set of research tasks to address

structural model fidelity as it pertains to seismic

response requirements



Four potential seismic response characterization

studies:

1.

Complex structure on soil site

2.

Complex structure on rock site

3.

Compact and/or axisymmetric structure on soil site

4.

Compact and/or axisymmetric structure on rock site



The first of these tasks is the subject of this paper and

consists of the generation of both the Finite Element

Model (FEM) and stick model responses for a complex

nuclear structure

Elements of the Study



Stick Model (LMSM) and a Detailed Finite

Element Model (DFEM) both developed

for Vogtle Control Building



Fixed-base and SSI responses studied



Comparisons made:



Static 1g analysis results



modal properties

Vogtle Control Building (CB)



Control Building supports both Units 1 and 2



6-story R/C shear wall structure w/ R/C slabs on steel

beams, and girders



Floor-to-Floor Height ≈ 20 ft



Height = 121 ft (bottom of basemat to roof)



Embedded (47 ft) on 3 sides



Basemat dimensions 525 ft x 169 ft

Complexities



CB has non-compact, irregular geometry



Irregularity in stiffness (tower to basement floors)



Torsional effects – 3 sides of shear walls and open

south side

22nd _{International Conference on Structural Mechanics in Reactor Technology}

Plan View

Section

Lumped Mass Stick Model (LMSM)



Southern Nuclear Company regenerated

CB LMSM from 1983 design basis model

using SAP2000



Detailed LMSM with multiple sticks



Basemat treated as a rigid foundation



Uncracked sections

Lumped Mass Stick Model



Vertical elements represents a R/C wall

(red)



Horizontal elements are high stiffness frames

linking mass to R/C wall elements

(black)

Detailed Finite Element Model

(DFEM)



SGH developed DFEM from construction drawings

supplemented by 1984 Bechtel Model



R/C shear walls and slabs modeled w/ SAP2000

thick shell elements



Basemat treated as a rigid foundation



Uncracked sections



Finite Element Model

(structure only, w/o foundation)



~18,000 nodes

Detailed Finite Element Model

(DFEM)

Vogtle Control Building Model

Static Response of Structure

1g Comparison



Fixed-base DFEM and LMSM to 1g (X,Y,Z)



Satisfactory Mass comparison (<5%)



Mass comparison per Elevation below

Elevation

(ft)

Mass-DFEM

(Kip-sec2/ft

Mass-LMSM

(Kip-sec2/ft)

% Delta

Static 1g Comparison

0 0.05 0.1 0.15 0.2 0.25 0.3

Elev

atio

n

(f

t)

1gX - Displacement (in)

DFEM - Tower NorthEast Corner

DFEM - Tower SouthEast Corner

DFEM - Center of Tower (C11/CD)

LMSM-at Mass

Y (North)

X (East) Z (Up)

Static 1g Comparison

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Elev

atio

n

(f

t)

DFEM - Tower SouthWest Corner

DFEM - Tower SouthEast Corner

DFEM - Center of Tower (C11/CD)

LMSM-at Mass

Y (North)

Modal Property Comparison

Frequency

(Hz)

X (%)

Y (%)

Z (%)

_Dominant

Modes

LMSM DFEM LMSM DFEM LMSM DFEM LMSM DFEM

7.24

6.18

72

54

1

_Y

7.66

6.73

64

45

1

_X

17.58

10.62

41

12

Vertical

Modal Property Comparison

0 10 20 30 40 50 60 70 80 90 100

Cu m u la ti ve M od al M ass P ar ti ci p ati on (%) Frequency (Hz) 0% 25% 50% 75% 100%

0 10 20 30 40 50 60 70 80 90 100

Investigation for Stiffness

Differences



Sensitivity Study on Diaphragm Flexibility

Assumptions



In-Plane Rigidity



Out-of-Plane Rigidity



Changed DFEM to Approximate the Diaphragm

Assumptions in the Stick Model



Assess the impact of the changes in the DFEM



Results show much closer correlation with the Stick

model



Thus, indication of some key differences in modeling

Modal Property Sensitivity

Plan View at Elevation 240 ft

• Unconstrained DFEM

Deformed Shape for the

First Y-mode

Modal Property Sensitivity

Plan View at Elevation 240ft

Elevation at Gridline 14 (east tower wall)

In-plane constrained DFEM

Deformed Shape for the

First Y-mode

7.5 Hz, 63% Y-MPR

Compared to LMSM, 7.2 Hz,

72% Y-MPR

Input Ground Motion



Vogtle Uniform Hazard

Spectra used for both

structural models



Time history and

strain-compatible soil properties

from seismic hazard

assessment



Site-specific in-column

response spectra at the

foundation for best

estimate soil stiffness

case



Results in peak / valley

Fixed-Base Analysis Comparison

Elevation 240 ft

DFEM

LMSM

Fixed-Base Comparison

Observations



Close comparison between the two fixed-base models was achieved

at some locations but not all locations



The LMSM’s use of rigid slabs resulted in an increased stiffness

in all three directions



The LMSM is unable to capture local modes at some locations

due to the lumping of masses at each floor level, which includes

out-of-plane slab modes



The discrepancies in the spectral accelerations directly related to

the differences in fundamental frequencies (sharp peak / valleys

augment differences)



In general, the fixed-base ISRS in the LMSM envelop the

SSI Comparison



5% damped

Foundation Response

Spectra,

DFEM

LMSM



Input Response

Spectra Purple

SSI Comparison – Examples of

DFEM >

LMSM

SSI Comparison – Examples of

DFEM >

LMSM

Y

(

N

or

th

)

X

(

Ea

st

)

Z

(

Up

)

SSI Analysis X, Y, & Z ISRS (left) at El. 220 ft, and Corresponding Plan View of El. 220 ft w/ location of ISRS

(right) 3-236E-i

SSI Response Comparison

Observations



SSI causes the natural frequencies of the system to shift to lower

frequencies compared to the fixed-base natural frequencies.



The inclusion of SSI in the seismic response analysis generally

causes the response of both models to converge as the overall

dynamic response become more similar in the presence of soft soil.



In those cases where large conservative margins exist between

responses in the LMSM and the DFEM for the fixed-base analysis,

these conservative margins are significantly decreased by the SSI

effects.



Horizontal LMSM SSI ISRS at locations out-of-plane to the CB tower

outer walls or in-plane on the tower diaphragm generally

underestimates the DFEM SSI ISRS.

(The maximum LMSM

SSI Response Comparison

Observations



Horizontal LMSM SSI ISRS at locations in-plane to the CB

tower outer walls are generally in close agreement or slightly

underestimate the DFEM response with only a few

exceptions.



Horizontal LMSM SSI ISRS at locations on the east and west

ends of the CB up to El. 220 ft, generally match or slightly

envelope the DFEM response below 10 Hz. Above 10 Hz the

LMSM response underestimates the DFEM response by a

maximum of 30% and 275%, at El. 200 ft and El. 220 ft,

respectively.



The rigid slab assumption causes the LMSM to overestimate

the stiffness & thus typically underestimate the vertical slab

response (similar to the fixed-base analysis results) because

slab out-of-plane behavior is not represented.

Conclusions



Close comparison of dynamic characteristics

and fixed-base ISRS was not achieved for this

complex structure.



Fixed-base LMSM ISRS were generally higher

than DFEM.



Closer comparison of SSI ISRS w/ fewer LMSM

exceedances due to the presence of soft soil.



However, DFEM exhibited significantly higher

Conclusions



The LMSM SSI ISRS underestimation generally

increased in directions that were sensitive to slab

flexibility (both in-plane and out-of-plane).



For large and complex structures on soil-sites, the use of

the existing LMSM is not recommended for SMA or

SPRA (unless justification is provided or modifications

are made to account for the aforementioned items).



Two primary factors for structure model inconsistencies:



rigid diaphragm modeling simplifications associated with stick

models



shape of the input response spectra

Potential Future Research



Complex Structure with Soil Site Input



SSI ISRS with multiple soil cases



Response Comparison with Stick Model

Enhancements



Broad Band Input Spectra



Other Research Studies



Compact and/or axisymmetric structure on soil site



Compact and/or axisymmetric structure on rock site



Complex Structure with Rock Site Input (higher