UNRESTRICTED / ILLIMITÉ Analysis of ACR ® Nuclear Island Seismic SSI: Challenges & Experiences Analysis of ACR ® Nuclear Island Seismic SSI: Challenges & Experiences N. Allotey, R. Gonzalez, A. Saudy & M. Elgohary OECD SSI Workshop Ottawa, Canada, October 6-8, 2010
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UNRESTRICTED / ILLIMITÉ
Analysis of ACR® Nuclear
Island Seismic SSI:
Challenges & Experiences
Analysis of ACR® Nuclear
Island Seismic SSI:
Challenges & Experiences
N. Allotey, R. Gonzalez, A. Saudy & M. Elgohary
OECD SSI Workshop
Ottawa, Canada, October 6-8, 2010
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Presentation Outline
• Introduction
• Design Basis Parameters
• Analysis Overview
• Issues & Challenges
• Summary
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Introduction
• ACR® & EC6TM standard designs are based on existing
CANDU design
• Light water cooled, heavy water moderated pressure-
– Primary nonlinearity due to wave propagation is captured by with strain-compatible modulus and damping ratio curves
–However, secondary nonlinearity effects including wave-trapping in weak backfills
– Additional structural elements to model near-field soil
– Very challenging & its effect has been proven harmfully unpredictable
– Issue: Would not time-domain approaches be a better alternative?
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Summary
• Seismic SSI analyses of ACR nuclear island have
recently been completed
• Standard design basis parameters
• Overview of SSI analysis model development, analysis
procedures and sample results
• Discussion of learnt lessons, encountered challenges
& gained experiences
• Identified few issues that, authors believe, remain
open-ended & require clearer industry guidance
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Issues & Challenges
• Interaction Nodes
–Located on soil-structure interface; i.e. shared between
structure & foundation medium
–Need for guidance on uniform distribution of interaction nodes
over interaction surface
– A highly non-uniform distribution of interaction nodes could
introduce unrealistic torsional effects
– Lead to inconsistent SSI results
– After a peer-review, a more uniform distribution of interaction
nodes was adopted instead
– Inconsistencies were rectified
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Issues & Challenges
• Transfer Function Acceptability –SSI analysis evaluates transfer function of input motion to
desired location at discrete frequency points
– Interpolation algorithm to develop continuous transfer function
– Accurate interpolated transfer function, over the desired frequency range, leads to accurate SSI results
– Act of balancing: very few (poor TF) vs. too many (expensive) frequency points
–Logically, refined models requires far more frequency points then lumped mass stick models
– 200 frequency pts. Refined model/coherent motion
– 300 frequency pts. Refined model/incoherent motion
– 50 frequency pts. Lumped mass model/coherent motion
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Issues & Challenges
• Transfer Function Acceptability (cont.) –Assessing adequacy of interpolated transfer functions is
important and labour-intensive
– Useful tools are available, such as viewing TF & ITF, and frequency search
– Still, a developed in-house macro worksheet was used to reduce considerably time spent on checking and cross-checking
– Particularly, in SSI analyses with incoherent motion, with many interpolation and smoothing parameters
– A Restart option that uses stored impedances from initial run, as input for subsequent runs
– Without this feature, SSI analyses with incoherent motion would not have been possible
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Analysis Overview
• Sample Results (cont.)
* Letter code relates to locations in the analysis model # Combinations of design soil profile and design ground response spectra;
e.g., “B3-S” refers to B3 profile & CSA-based Soil motion
Floor acceleration (g) Location Symbol
*
H1 H2 V
Basemat: square part a 0.64 B3-S# 0.52 B3-S 0.58 B3-S
Basemat: rectangular part b 0.50 B3-S 0.47 B3-S 0.40 B3-S CS top c 2.53 B2-R 2.36 B2-R 0.98 HR-E IS top d 1.6 HR-R 1.6 B2-R 0.84 B2-R RAB square part top e 1.55 HR-R 0.84 B2-R 0.57 HR-E RAB rectangular part top f 0.98 HR-R 1.15 HR-R 0.76 HR-R
Maximum peak of 5% damped ISRS (g) Location Symbol
*
H1 H2 V
Basemat: square part a 2.53 B3-S 2.45 B3-S 2.36 B3-S Basemat rectangular part b 1.74 B3-S 1.74 B3-S 1.44 B2-R CS top c 14.85 B2-R 12.45 B2-R 5.86 HR-R IS top d 8.71 B2-R 8.76 B2-R 3.01 B2-R RAB square part top e 8.0 HR-R 3.78 B2-R 2.17 B2-R RAB rectangular part top f 4.54 HR-R 5.72 HR-R 3.12 HR-R