Seismic Input and Soil-Structure Interaction (Ch. 5 of TBI report, PEER 2010/05)
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Seismic Input and Soil-Structure Interaction
(Ch. 5 of TBI report, PEER 2010/05)
TBI Committee MembersY. Bozorgnia C.B. CrouseJ.P. Stewart
October 8, 2010
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Outline1. Seismic Hazard Analysis
Probabilistic Deterministic Site-Response Analysis
2. Soil-Foundation-Structure Interaction Kinematic Inertial Input Motion Specification
3. Ground Motion Selection and Scaling Identification of Controlling Seismic Sources Ground Motion Selection Accelerogram Modification
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Two SHA Approaches
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RecommendationUse General Procedure if geotechnical engineer is inexperienced or unqualified to perform site-specific probabilistic and deterministic SHA.
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Two SHA Approaches (cont.)2. Site-Specific (Preferred)
Probabilistic Deterministic
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Probabilistic Seismic Hazard Analysis (PSHA)
Source models Eqk locations M range Recurrence
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Probabilistic Seismic Hazard Analysis (PSHA)
Source models
Ground motion prediction equations (GMPEs):
mSa, sSa | (M, r, S, …)
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PSHA Output: Ground-Motion Hazard Curves
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Uniform Hazard Spectrum
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Recommendations for PSHA For experienced PSHA users only
Use QA-checked software
Account for alternate seismic source parameters and GMPEs (epistemic uncertainty)
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Logic Tree
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GMPEs Recommended for Shallow Crustal Western U.S. Earthquakes
NGA GMPEs (2008) Abrahamson & Sliva Boore & Atkinson Campbell & Bozorgnia Chiou & Youngs Idriss
See EERI Spectra Journal (Feb. 2008, v. 24, no. 1)
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Empirical GMPEs Recommended for Subduction Earthquakes Atkinson & Boore (2003) – Site Class B,
C, D Crouse (1991) – Soil Youngs et al. (1997) Soil and Rock Zhao et al. (2006) Soil Classes I – IV and
Hard Rock
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Deterministic MCE Calculation Req’d per ASCE 7 Ch 21 Provides “cap” near major faults Arbitrary decisions regarding:
Ruptured fault segment (closest) Magnitude (use average of Mmax from logic
tree) Use same GMPEs & wts from PSHA Different sources may be most critical at
short and long periods
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Site-Specific Deterministic MethodASCE 7, Sect. 21.2.2
Find Fault à largest median Sa
Compute 1.5 x median Sa (ASCE 7-05)
Compute Sa84th >1.5Sa
median (ASCE 7-10)
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Site Response AnalysisASCE 7-05; Ch.21
Site-Specific Ground Motion
`
PSHA/DSHA – Vs30
PSHA/DSHA – Ref. Vs30
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Recommendations SRA not needed in absence of pronounced
impedance contrast (often the case for stiff soil sites)
Site effect can be accounted for in such cases through GMPE site terms
SRA advisable/required for:
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Recommendations SRA produces amplification factors, AF(T)=
Sa,soil/Sa,rock Typically applied as deterministic modification
of UHS (Hybrid proc.): Sa,soil=AF(Sa,rock)UHS
Can avoid with modification of site term in hazard integral (OpenSHA)
Unconservative bias
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2. Soil-Foundation-Structure Interaction (SFSI)
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SFSI for MCE Linear springs and
dashpots model soil-foundation interaction
Input motion same at all points along foundation
Input can be reduced for kinematic effects
See FEMA 440 & ASCE 41-06 for details
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3. Ground Motion Selection and Modification Identify controlling earthquakes
Select representative ground motions
Modify accelerograms to match target spectrum
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Identify Controlling Earthquakes Specify natural period band – SE decision Deaggregation Plots
T = 1 sec T = 5 sec
M1 – R1 M2 – R2
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Issues with Ground Motion Selection Number of ground motion sets Multiple controlling earthquakes Near-fault effects Effects poorly represented in ground
motion database: Basin Effects M > ~ 8, long-duration motion
Use of simulations
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Number of Accelerograms - N No less than three (use maximum
responses)
Use average responses if 7 or more motions used
More needed if multiple controlling earthquakes
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Near Fault Effects
Select a(t) for both cases
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Transform FN & FP a(t) into X & Y a(t)
Fault
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Simulated Ground Motions (e.g., ShakeOut)Sa (T = 3 sec, 5 = 5%)
gGraves et al. (2008)
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Simulated Ground Motions (e.g., ShakeOut)
Can produce realistic-appearing wave forms
Need for calibration
Most broadband methods are inadequately validated or have biases
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Issues with Ground Motion Modification
Target Sa Site-specific Sa Conditional mean Sa (CMS)
Modification procedures constant scaling spectral matching
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Target Sa
UHS encompasses many events Not achievable in a given event Scenerio spectra (CMS) more realistic; need > 1
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Accelerogram Modification Constant Scaling
Spectral Matching
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Accelerogram Modification Constant Scaling
Spectral Matching
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Spectral Matching
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Selection and Scaling Recommendations N > 7 (N limited by $ and time) Use hazard deaggregations ® controlling EQs CMS – use several ® different Sa shapes Scaling (constant or spectral matching)
SE’s decision Simulated accelerograms (M > ~ 8)
- ADV: long duration and basin effects - DISADV: verification issues, access to quality simulations
Peer Review – Important