Seismic Input and SoilStructure Interaction
(Ch. 5 of TBI report, PEER 2010/05)
TBI Committee Members
Y. Bozorgnia
C.B. Crouse
J.P. Stewart
October 8, 2010
Outline
1. 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
Two SHA Approaches
Recommendation
Use General Procedure if geotechnical
engineer is inexperienced or unqualified to
perform site-specific probabilistic and
deterministic SHA.
Two SHA Approaches (cont.)
2. Site-Specific (Preferred)


Probabilistic
Deterministic
Probabilistic Seismic Hazard Analysis
(PSHA)

Source models
 Eqk locations
 M range
 Recurrence
Probabilistic Seismic Hazard Analysis
(PSHA)


Source models
Ground motion prediction
equations (GMPEs):
mSa, sSa | (M, r, S, …)
PSHA Output: Ground-Motion Hazard
Curves
Uniform Hazard Spectrum
Recommendations for PSHA

For experienced PSHA users only

Use QA-checked software

Account for alternate seismic source
parameters and GMPEs (epistemic
uncertainty)
Logic Tree
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)
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
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
Site-Specific Deterministic Method
ASCE 7, Sect. 21.2.2

Find Fault  largest median Sa

Compute 1.5 x median Sa (ASCE 7-05)

Compute Sa84th >1.5Samedian (ASCE 7-10)
Site Response Analysis
ASCE 7-05; Ch.21
Site-Specific Ground Motion
PSHA/DSHA – Vs30
PSHA/DSHA – Ref. Vs30
`
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:
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
2. Soil-Foundation-Structure Interaction (SFSI)
SFSI for MCE




Linear springs and
dashpots model soilfoundation interaction
Input motion same at all
points along foundation
Input can be reduced for
kinematic effects
See FEMA 440 & ASCE
41-06 for details
3. Ground Motion Selection and
Modification

Identify controlling earthquakes

Select representative ground motions

Modify accelerograms to match target
spectrum
Identify Controlling Earthquakes


Specify natural period band – SE decision
Deaggregation Plots
T = 1 sec
M1 – R1
T = 5 sec
M2 – R2
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
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
Near Fault Effects
Select a(t) for both cases
Transform FN & FP a(t) into X & Y a(t)
Fault
Simulated Ground Motions (e.g., ShakeOut)
Sa (T = 3 sec, 5 = 5%)
g
Graves et al. (2008)
Simulated Ground Motions (e.g., ShakeOut)



Can produce realistic-appearing wave
forms
Need for calibration
Most broadband methods are
inadequately validated or have biases
Issues with Ground Motion
Modification

Target Sa



Site-specific Sa
Conditional mean Sa (CMS)
Modification procedures


constant scaling
spectral matching
Target Sa



UHS encompasses many events
Not achievable in a given event
Scenerio spectra (CMS) more realistic; need > 1
Accelerogram Modification

Constant Scaling

Spectral Matching
Accelerogram Modification

Constant Scaling

Spectral Matching
Spectral Matching
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