Geotechnical Earthquake Engineering:

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Geotechnical Earthquake Engineering Ground
Motions Steve Kramer Department of Civil and
Environmental Engineering University of
Washington Seattle, WA
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Ground Motions
PSHA (or DSHA) provides uniform hazard
spectrum Codes generally produce design spectrum
For nonlinear analyses time histories are
required Nonlinear structural analyses Nonlinear
geotechnical analyses Nonlinear SSI analyses
Goal is to identify / create one or more motions
that have amplitudes, frequency contents, and
durations that are consistent with the ground
shaking hazard at the site of interest.
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Ground Motions
PEER NGA Database (http//peer.berkeley.edu/nga/)
Sources
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Ground Motions
PEER NGA Database (http//peer.berkeley.edu/nga/se
arch.html)
Sources
Can search for records with characteristics
similar to those controlling hazard at site of
interest
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Ground Motions
COSMOS Database
Sources
http//db.cosmos-eq.org/scripts/search.plx
Can search for records with characteristics
similar to those controlling hazard at site of
interest
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Ground Motions
Problem Find ground motion(s) that match
target spectrum
Two common approaches Simulation single
spectrum-compatible ground motion Scaling suite
of motions with matching ensemble average
Required information Target spectrum Uniform
hazard spectrum (UHS) Code spectrum Fundamental
period of structure Intent of analyses for which
motions are to be used Mean or median
response Mean / median and indication of
variability of response
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Ground Motion Scaling
Simulation
Alter characteristics of motion to match target
spectrum Two common approaches Time domain
wavelets (actually, time and frequency
domains) Example RSPMATCH (Norm
Abrahamson) Frequency domain Fourier
analysis Example RASCAL (Walt Silva) Both
approaches start with some initial ground
motion Important that initial motion has
correct duration
Spectrum-compatible motions are useful for
determining the mean or median response of a
system. They do not provide direct insight into
the variability of that response.
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Ground Motion Scaling
Simulation
Example
Spectrum after matching
Initial motion too strong
Initial motion too weak
After Norm Abrahamson COSMOS workshop presentation
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Ground Motion Scaling
Simulation
Example
Original
Modified
After Norm Abrahamson COSMOS workshop presentation
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Ground Motion Scaling
Simulation
Example
Original
Modified
After Norm Abrahamson COSMOS workshop presentation
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Ground Motion Scaling
Simulation
Example
Original
Modified
After Norm Abrahamson COSMOS workshop presentation
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Ground Motion Scaling
Simulation
Example
After Norm Abrahamson COSMOS workshop presentation
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Ground Motion Scaling
Simulation
Example
Note In areas where seismic hazards come from
multiple sources, different parts of UHS may be
controlled by different sources single motion
producing entire UHS may not be physically
possible. In that case, use of
spectrum-compatible motion may be quite
conservative.
After Norm Abrahamson COSMOS workshop presentation
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Ground Motion Scaling
Scaling

• Alternatively, we can identify and scale actual
  recorded motions for (ensemble average)
  consistency with a target spectrum
• Use deaggregation to find representative (mean /
  modal) values of
• Magnitude
• Distance
• Style of faulting
• Select consistent motions from database (e.g.
  PEER NGA database) based on seismological
  properties
• Similar magnitude (within /- 0.5 provides
  reasonable duration)
• Similar distance range
• Similar spectral shape (or epsilon)
• Same style of faulting

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Ground Motion Scaling
Scaling
Scale motions by constant factor to match
target spectrum What constitutes a match? Match
is in average sense average of suite of
motions May be defined in terms of SRSS spectra
(multi-directional components) Usually need to
exceed target over significant period range For
structures, typically 0.2To 1.5To
Lower periods (higher frequencies) covers higher
mode response
Higher periods (lower frequencies) covers
damage-induced softening
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Ground Motion Scaling
Scaling

• Select large suite of ground motions (50 100 or
  so)
• Use deaggregation to find representative (mean /
  modal) values of
• Magnitude
• Distance
• Style of faulting
• Select consistent motions from database (e.g.
  PEER NGA database) based on seismological
  properties
• Similar magnitude (within /- 0.5 provides
  reasonable duration)
• Similar distance range
• Similar spectral shape (or epsilon)
• Same style of faulting

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Ground Motion Scaling
Epsilon
To
Scaled negative e motion is too strong
Be careful look for local peaks and valleys in
candidate motions prior to scaling
Scaled positive e motion is too weak
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Ground Motion Scaling
Epsilon
Baker (2007) took 382 representative ground
motions Computed e values for each, chose 20
highest and 20 lowest Computed spectra after
scaling to same Sa(T0.8)
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Hazard Analysis and Ground Motions
Summary Design levels of ground motion determined
by seismic hazard analysis DSHA deterministic
PSHA probabilistic Attenuation behavior is
critical Prediction of response spectrum may be
sufficient Ground motions may be
required Synthetic motions describe mean/median
level of shaking Scaled motions Reflect actual
earthquake characteristics Suite of motion
required can account for record-to-record
variability
Results of site response analyses will be
sensitive to ground motion inputs need to pay
careful attention to this issue