SEISMIC SSI ANALYSIS COMPARISON BETWEEN DETAILED ... - …
Transcript of SEISMIC SSI ANALYSIS COMPARISON BETWEEN DETAILED ... - …
Transactions, SMiRT-24
BEXCO, Busan, Korea - August 20-25, 2017
Division X
SEISMIC SSI ANALYSIS COMPARISON BETWEEN DETAILED
AND DISCRETIZED MODELING OF AN AUXILIARY CONTROL
BUILDING
Samer El-Bahey1, PhD., P.E., Stephane Damolini2, Todd Radford1, Ph.D., Cory Figliolini2
1 Senior Engineer, JENSEN HUGHES, USA 2 Principal Engineer, JENSEN HUGHES, USA
ABSTRACT
In recent years, the nuclear industry and the Nuclear Regulatory Commission (NRC) have made a
tremendous effort to assess the safety of nuclear power plants (NPPs) as advances in seismology have led
to the perception that the potential earthquake hazard in the United States may be higher than originally
assumed. The Seismic Probabilistic Risk Assessment (S-PRA) is a systematic approach used in NPPs in
the U.S. to realistically quantify the seismic risk by identifying the dominant contributors to seismic risk
and core damage. One major aspect of realistically quantifying the seismic demands is by performing
Seismic soil-structure interaction (SSI) analysis.
SSI analysis of NPPs has been historically performed in the frequency domain using a lumped-
mass stick model of the structure to capture the global dynamic response of the system required to
calculate realistic in-structure response spectra (ISRS). With the recent advances in computer software
and hardware technology, it is now possible to perform SSI analysis of detailed finite element structural
models in the frequency domain.
This paper presents the results of a seismic SSI analysis of an Auxiliary/Control Building (ACB)
using both lumped mass stick and detailed 3-dimension Finite Element (FE) representations of the
structure. Because of the complexity of the structure, the stick model consists of multiple interconnected
sticks developed using GT-STRUDL and calibrated against a detailed FE model of the structure with a
fixed base developed using ANSYS before introducing the SSI effects. SSI effects were analyzed using
EKSSI for the stick model and ACS/SASSI for the detailed FE model.
The results of the detailed FE model in terms of response spectra, are calculated and compared
against those of the lumped-mass stick model.
24th Conference on Structural Mechanics in Reactor Technology
BEXCO, Busan, Korea - August 20-25, 2017
Division IX (include assigned division number from I to X)
INTRODUCTION
Seismic Probabilistic Risk Assessment (S-PRA) studies have been performed in many of the US
Nuclear Power Plants over the last two decades. The S-PRAs were initially performed to answer safety
concerns in heavily populated areas, then evolved to satisfy the NRC’s request for information regarding
severe accident vulnerabilities in Generic Letter 88-20, Supplement 4 USNRC., (1991). The NRC
encourages the use of PRA for making risk informed decisions and has developed a Risk-Informed
Regulation Implementation Plan USNRC., (2000) and associated regulatory guides. Most of the initial S-
PRAs performed in the US in the 1980s, contained a level of uncertainty arising from the seismic hazard
and uncertainty in the fragilities of structure, systems and components (SSCs) which resulted in the
spread of the level of uncertainty in the calculated Core Damage Frequency (CDF).
Following the March 2011 Great Tahoku Earthquake and its catastrophic consequences on the
Fukushima Daiichi NPP, it was clear that relying on uncertainties in the design could lead to catastrophic
consequences. From which, the Nuclear Regulatory Commission (NRC) established a Near Term Task
Force (NTTF) to conduct a systematic review of NRC processes and regulations and to determine if the
agency should make additional improvements to its regulatory system. The NTTF developed a set of
recommendations intended to clarify and strengthen the regulatory framework for protection against
natural phenomena. Subsequently, the NRC issued a 50.54(f) letter on March 12, 2012 requesting
information to assure that these recommendations are addressed by all U.S. nuclear power plants. The
50.54(f) letter requests that licensees and holders of construction permits under 10 CFR Part 50 re-
evaluate the seismic hazards at their sites against present-day NRC requirements and guidance.
Advances in characterizing earthquake source, travel path, and local site effects have led to the
perception that the potential free field earthquake hazard in the United States may be higher than
originally assumed. The effect of SSI is yet still a major uncertainty in the seismic design of nuclear
power plants.
SOIL-STRUCTURE INTERACTION OVERVIEW
The ground motion observed by any structure is different than the free field motion due to the
following interactions:
• Inertial Interaction: Inertia developed in the structure due to its own vibrations gives rise to
base shear and moment, which generates displacements and rotations of the foundation relative to the
free-field due to the flexibility of the soil-foundation system. This added flexibility affects the building
frequency by shifting it towards the flexible range. The system overall damping is also affected by the
added displacements as energy dissipation via radiation damping and hysteretic soil damping rises
affecting the overall system damping.
• Kinematic Interaction: The presence of stiff foundation elements at or below the ground surface
cause foundation motions to deviate from free-field motions as a result of ground motion incoherence,
wave inclination, or foundation embedment.
Commonly used methods for capturing the SSI effects are either:
Direct Analysis: where the soil and superstructure are included in the same finite element model
and analyzed as one system. This could be performed using multiple SSI software like FLUSH by
representing the soil as a continuum along with foundation elements. The direct analysis method is rarely
used in practice due to the computational complexity.
24th Conference on Structural Mechanics in Reactor Technology
BEXCO, Busan, Korea - August 20-25, 2017
Division IX (include assigned division number from I to X)
Substructure Approach: where the structure is initially analyzed having a fixed-base, from which
the dynamic characteristics of the structure are calculated including the modal frequencies, Eigen vectors,
and Eigen values. The kinematic effects are then addressed using frequency dependent transfer functions
relating the free-field motion to the foundation input motion (FIM) taking into account the soil column
properties. The inertial interactions are then addressed by calculating frequency dependent impedance
functions to represent the stiffness and damping of the soil-foundation interface depending on the soil
column properties. The superposition inherent in a substructure approach requires an assumption of linear
soil and structure behavior, although in practice this requirement is often followed only in an equivalent-
linear sense.
AUXILIARY AND CONTROL BUILDINGS
The chosen Auxiliary Building (AB) is a multi-story, structural steel and reinforced concrete
structure which houses the safety injection system, residual heat removal system, CVCS monitoring
system, auxiliary feedwater pumps, steam and feedwater isolation and relief valves, heat exchangers,
other pumps, tanks, filters, and demineralizers, and heating and ventilating equipment.
The chosen Control Building (CB) is a rectangular structural steel and reinforced concrete
structure which houses the access control areas, control room, upper and lower cable spreading rooms,
electrical and mechanical equipment rooms, and locker rooms. The intermediate floors and roof are
reinforced concrete slabs supported by structural steel beams and girders. The floor and roof framing are
supported by exterior reinforced concrete bearing walls and interior steel columns.
The AB shares a common base mat and wall with the CB. The building interior is enclosed on
one side by the reactor building wall.
SITE CONDITIONS AND SEISMISITY
The geologic column underlying the chosen site consists of soil deposits over a sequence of older,
competent sedimentary lithified rock formations. The soil deposits are related to glacial and postglacial
processes during the Quaternary Period and consist of loess, clay, and till. In the area of the Nuclear
Island, these deposits are removed, and as needed, replaced by engineered fill. The uppermost competent
rock layer is the Graydon Chert Conglomerate. Underlying the Graydon Chert Conglomerate are the
Burlington and Bushberg formations of Mississippian age. The Burlington Formation consists of
limestone. The Devonian-age Snyder Creek Formation underlies the Bushberg Formation throughout the
site area.
No faults have been identified within 12 miles of the site, and no historic earthquake epicenters
have been reported within approximately 30 miles. The closest earthquake of any size was a md = 2.3
event at a distance greater than 20 miles from the site.
SITE-SPECIFIC GROUND MOTION AND SOIL PROPERTIES
In accordance with the 50.54(f) letter and following the guidance in the SPID EPRI., (2013A), a
probabilistic seismic hazard analysis (PSHA) was completed in a separate effort using the recently
developed Central and Eastern United States Seismic Source Characterization (CEUS-SSC) for Nuclear
24th Conference on Structural Mechanics in Reactor Technology
BEXCO, Busan, Korea - August 20-25, 2017
Division IX (include assigned division number from I to X)
Facilities (CEUS-SSC, 2012) together with the updated EPRI Ground-Motion Model (GMM) for the
CEUS EPRI., (2013B). For the PSHA, a lower-bound moment magnitude of 5.0 was used, as specified in
the 50.54(t) letter. Information pertaining to the Hazard Consistent Strain-Compatible Properties for upper
bound, UB, best estimate, BE, and lower bound, LB, soil cases are obtained from the PSHA and used
herein.
The site-specific ground motion considered herein is based on the new Ground Motion Response
Spectrum (GMRS) developed as part of the PSHA effort along with its associated Foundation Input
Response Spectrum (FIRS) and the Soil properties for the Lower-Bound (LB), Best Estimate (BE) and
Upper-Bound (UB) cases as shown in Figure 1. The properties include hazard-consistent strain-
compatible properties (HCSCPs) for the full column profile for FIRS hazard level.
Artificial time histories corresponding to the FIRS are generated herein based on the fitting and
enveloping requirements of NUREG. (1987), Section 3.7.1 Option 1 Approach 2 at 5% damping.
A convolution analysis is then performed using the SOIL module of ACS/SASSI for each
combination of earthquake direction and soil profile. The developed artificial FIRS hazard level
foundation input time histories are input as outcrop motions in soil column convolution analyses, to
generate surface time histories. Time histories are convolved to the surface of the soil profile for the X, Y,
and Z directions using the appropriate hazard consistent LB, BE, and UB soil properties as shown in
Figure 2.
Surface time histories are then converted to 5% damped Response Spectra and compared against
the Review Level Earthquake Surface Response Spectra (RLESRS) that corresponds to the GMRS as
shown in Figure 3. It can be seen from the plots that the envelope of the RS from the 3 time histories for
each horizontal direction is a good match to the horizontal RLESRS (RLESRS_H). The envelope of the
RS from the 3 time histories for the vertical direction is a good match to the vertical RLESRS
(RLESRS_V). The conservatism in higher frequencies results from additional soil column amplification
above the outcrop and it will not significantly affect the structure response. The comparisons verify that
there is no significant deficiency in the envelope of the time histories when compared to RLESRS.
24th Conference on Structural Mechanics in Reactor Technology
BEXCO, Busan, Korea - August 20-25, 2017
Division IX (include assigned division number from I to X)
Figure 1. Full Column Profile for shear-wave velocity (Vs), compression-wave velocity (Vp), and
shear
24th Conference on Structural Mechanics in Reactor Technology
BEXCO, Busan, Korea - August 20-25, 2017
Division IX (include assigned division number from I to X)
Figure 2. Artificial Time Histories Corresponding to the FIRS (X, Y, and Z)
24th Conference on Structural Mechanics in Reactor Technology
BEXCO, Busan, Korea - August 20-25, 2017
Division IX (include assigned division number from I to X)
Figure 3. Surface Time Histories Response Spectra compared against the RLESRS at 5% Damping
24th Conference on Structural Mechanics in Reactor Technology
BEXCO, Busan, Korea - August 20-25, 2017
Division IX (include assigned division number from I to X)
STRUCTURAL MODEL DESCRITIZATION
To include eccentricities in a lumped mass stick model between centers of gravity and centers of
stiffness as well as to adequately capture local vertical modes in the Control Building where floor systems
may be relatively flexible, the response of the Auxiliary/Control building was captured through
decoupling the horizontal and the vertical responses through two structural fixed base GT-STRUDL
models.
The horizontal model developed is a three-dimensional stick model and shown in Figure 4. The
mass eccentricities were accounted for at each major floor elevation by lumping the floor masses at their
respective centers of mass and connecting them through rigid members to the vertical sticks located at the
centers of rigidity. The model is used to calculate the natural frequencies of the structure and its
eigenvectors.
No Auxiliary/Control horizontal model concrete stick elements were anticipated to be
significantly cracked at the Ground Motion Response Spectrum (GMRS) review level earthquake (RLE).
Therefore, no reduction in stiffness parameters for these elements are necessary per the guidance of
ASCE 4-13 Table 3-2 ASCE., (2013).
Figure 4. Schematic Diagram for Enhanced Horizontal Model of Auxiliary/Control Building
Although there are no major structural irregularities in the ACB, the floor system in the Control
Building is relatively flexible. The aforementioned features made it necessary to model the ACB in the
vertical direction considering two sticks connected through horizontal beams. The intent of the vertical
stick model is to adequately capture the global wall and column modes in addition to the local Control
Building floor modes. One vertical stick represents the Auxiliary Building and the outer structural walls
of the Control Building and the other stick represents the Control building supporting steel columns.
Vertical floor flexibility is represented by the horizontal members. A schematic of the vertical model is
shown in Figure 5.
24th Conference on Structural Mechanics in Reactor Technology
BEXCO, Busan, Korea - August 20-25, 2017
Division IX (include assigned division number from I to X)
The 1984’ elevation of the Control Building was found to have vertical frequencies greatly
exceeding 20 Hz and therefore modeled integrally with the ACB at that elevation. The length of the
horizontal elements representing the floor system was taken to be very small (0.1 ft) to eliminate the
unrealistic rocking effect due to the eccentricity of the model from the chosen coordinate system. Floor
inertias were adjusted accordingly. Given the framing of the slabs into perimeter walls, the composite
floor beams were considered fixed to the structural walls; whereas they were modeled pinned to the
columns since the spans on opposite sides of the columns may feasibly oscillate out of phase. The
horizontal translational degrees of freedom were restrained for all nodes. Effective inertias of the
perimeter walls were also calculated in order to account for their additional vertical floor flexibility.
Figure 5. Schematic Diagram for the Auxiliary/Control Vertical model
LUMPED MASS STICK MODEL SSI ANALYSIS
The EKSSI computer programs used herein for SSI analysis were developed by Professor
Eduardo Kausel of the Massachusetts Institute of Technology (MIT), and verified by Stevenson and
Associates (S&A). The EKSSI software package includes multiple modules. The following two modules
were used for the current analysis. The SUPELM program module computes the frequency-dependent
dynamic impedance of the foundation. The foundation is assumed to be rigid and cylindrical in shape,
which is reasonable. SUPELM can also compute transfer functions allowing for the determination of time
histories at the bottom of the foundation using the SUPELM KININT module. The EKSSI program
module provides the frequency domain solution, including SSI effects, to a dynamically-loaded structure
that is supported on compliant soil. The EKSSI program performs the SSI analysis by combining the
building model and the foundation impedance matrix, subjecting the combined model to input
acceleration time histories, and determining the response at required nodes.
Fixed-base modal properties for the ACB are calculated using GT-STRUDL software. The time
histories applicable to the free field surface are calculated using SPECTRASA software.
Impedance functions for the substrata are calculated using SUPELM. The transfer functions are
used by the KININT module to generate time histories at the foundation bottom.
24th Conference on Structural Mechanics in Reactor Technology
BEXCO, Busan, Korea - August 20-25, 2017
Division IX (include assigned division number from I to X)
The structural model and the foundation impedance functions are combined in EKSSI to form the
soil-structure interaction model. The models are then analyzed in EKSSI using the input time histories.
Resultant response time histories are calculated separately in the X, Y, and Z directions at all levels of
interest. Structural inherent damping was considered at 4% for un-cracked concrete structures.
DETAILED FINITE ELEMENT SEISMIC MODEL
A best estimate fixed base seismic model of the ACB was first created using the ANSYS finite
element analysis package as shown in Figure 6 and a modal analysis was performed. Structural walls and
floors were individually modeled with shell elements. Beam elements were used for columns and for floor
framing. Structural dead load mass and distributed equipment load were taken into account. The
maximum mesh size was set to 48 inches for all 2D elements. Composite action for floor framing was
credited where applicable. Material densities were used for dead loads and adjustments to mass were
made to account for modeling simplifications. Mass for fixed equipment and live load were also added.
Fixed boundary conditions at the base of the foundation elements (top of bedrock) were applied.
Figure 6. ACB ANSYS Model Overview
DETAILED FINITE ELEMENT MODEL SSI ANALYSIS
The ACS/SASSI program was used for the SSI analysis of the detailed FE model to account for
coupling between the structure and the soil, effects of flexibility in the underlying media, and effects of
flexibility in the structural basemat. Embedment of the ACB was considered and the flexible volume sub
structuring approach was used. A fixed-base version of the created ACS/SASSI model is compared to the
fixed-base ANSYS model for validation of the conversion. The transfer functions for a representative set
of nodes of the converted ACS/SASSI model were compared to the ones of the original ANSYS model.
The ACS-SASSI model was modified to be a fixed-based model similar to the ANSYS model. To
simulate a fixed base in the ACS/SASSI model, the volumic soil elements and the embedded soil layers
were removed and the ground elevation was set at the foundation of the building. All the remaining layers
of soil were turned into extremely stiff hard rock. The same properties were applied to the half-space. A
24th Conference on Structural Mechanics in Reactor Technology
BEXCO, Busan, Korea - August 20-25, 2017
Division IX (include assigned division number from I to X)
comparison between the transfer functions from ACS/SASSI and ANSYS, for the fixed-based model is
presented in Figure 7 at the Control Building Elevation 2090’ for the X, Y and Z transfer functions.
Figure 7. ACS-SASSI and ANSYS fixed-base models X, Y and Z transfer functions, CB El. 2090’
24th Conference on Structural Mechanics in Reactor Technology
BEXCO, Busan, Korea - August 20-25, 2017
Division IX (include assigned division number from I to X)
Preliminary runs were performed using the Fast-Flexible Volume method. For this method, the
interaction nodes were comprised of the excavated volume boundary nodes, along with user-determined
internal node layers within excavation volume. This method is a good compromise between
computational resources and modelling refinement.
For each soil case, for each direction, results were inspected for reasonableness. Out of the 9
analyses (3 soil cases, 3 directions each), the Lower Bound X-direction transfer functions and response
spectra were not satisfactory, as the transfer functions had high, unexpected spikes, at around 8.2Hz. This
is attributed to local soil modes, which can result from a Fast-Flexible Volume analysis, as not all the
excavated volume nodes were defined as interaction nodes.
A full Flexible Volume analysis was then performed instead of the Fast-Flexible Volume for the
Lower Bound, X-direction. Results were inspected and no unexpected spikes in the transfer functions
were observed.
SEISMIC RESPONSE ANALYSIS COMPARISON
Average floor In-Structure response spectra for the detailed FE Model were generated by creating
a nodal envelope of the LB, BE, and UB response and then averaging all nodal envelopes across each
floor elevation for each of the three directions. The In-Structure response spectra for the lumped mass
stick model was generated by enveloping the LB, BE, and UB responses at each node representing a floor
elevation for each of the three directions. Comparison between results for select floor elevations are
shown in Figure 8 and Figure 9. The maximum observed accelerations for select floors are presented in
Table 1. The comparisons indicate a generally close agreement between the calculated maximum
accelerations at major floor elevations specially in the horizontal direction. However, in terms of the
calculated in-structure response spectra, the results of the two models slightly differ despite an overall
similarity in spectral shapes.
Table 1. Comparison of Maximum Accelerations
Location
LMSM FEM
Maximu
m Sa (g)
Maximu
m Sa (g)
X Y Z X Y Z
C
ontrol
1
974’
0
.79
0
.83
2
.82
0
.72
0
.84
0
.82
1
984’
1
.03
1
.08
2
.56
0
.88
0
.88
1
.87
2
000’
1
.31
1
.25
2
.38
1
.1
1
.33
2
.93
A
uxiliary
2
047’
2
.47
2
.15
1
.41
2
.15
2
.21
1
.98
24th Conference on Structural Mechanics in Reactor Technology
BEXCO, Busan, Korea - August 20-25, 2017
Division IX (include assigned division number from I to X)
Figure 8. ISRS Comparison between LMSM and FEM for the Auxiliary Building
24th Conference on Structural Mechanics in Reactor Technology
BEXCO, Busan, Korea - August 20-25, 2017
Division IX (include assigned division number from I to X)
Figure 9. ISRS Comparison between LMSM and FEM for the Control Building
CONCLUSIONS
A 3-D detailed FE model of an Auxiliary Control Building was developed and analyzed to
evaluate the seismic SSI response using ACS/SASSI. The FE model consisted mainly of solid/shell
elements representing concrete floors, walls and basemat. The model considers spatial distributions of
mass and stiffness in the structure, including the flexibility of the basemat, walls and floors. In addition,
the model is made sufficiently refined to capture the out-of-plane flexural response of floors and walls.
The results of the detailed model at key locations in the structure have been computed and compared with
those of a comparable lumped-mass stick model typically used in SSI analysis. The comparisons indicate
a generally close agreement between the calculated global responses, such as maximum accelerations at
major floor elevations. However, in terms of the calculated in-structure response spectra, the results of the
two models slightly differ despite an overall similarity in spectral shapes. The differences in the spectral
responses are more pronounced in the vertical response of the structure. This may be attributed to the
flexibility of the floors, walls and basemat; the spatial distribution of mass in the structure and the limited
number of modes that can be represented by the stick model.
24th Conference on Structural Mechanics in Reactor Technology
BEXCO, Busan, Korea - August 20-25, 2017
Division IX (include assigned division number from I to X)
REFERENCES
ASCE., (2013), “Seismic Analysis of Safety-Related Nuclear Structures and Commentary, ASCE 4-13”
EPRI., (2013A), “Seismic Evaluation Guidance: Screening, Prioritization and Implementation Details
(SPID) for the Resolution of Fukushima Near-Term Task Force Recommendation 2.1: Seismic.”
EPRI 1025287, Palo Alto, CA
EPRI., (2013B), “Ground-Motion Model (GMM) Review Project.” EPRI 3002000717
NUREG. (1987), “Commission, N.R., NUREG-0800,". Standard Review Plan for the Review of Safety
Analysis Reports for Nuclear Power Plants."
USNRC., (1991), “Supplement 4, Individual Plant Examination of External Events (IPEEE) for Severe
Accident Vulnerabilities—10CFR 50.54 (f)(Generic Letter No. 88-20)”, Rockville.
USNRC., (2000), “Risk-Informed Regulation Implementation Plan”, Washington, DC: Document US
NRC SECY-00-213