Geotechnical Earthquake Engineering: Ground Motions Steve Kramer Department of Civil and Environmental Engineering University of Washington Seattle, WA

Geotechnical Earthquake Engineering:Geotechnical Earthquake Engineering:

Ground MotionsGround Motions

Steve Kramer

Department of Civil and Environmental Engineering

University of Washington

Seattle, WA


PSHA (or DSHA) provides uniform hazard spectrum

Codes generally produce design spectrum

For linear structural analysis, response spectrum is all you need – no need for individual ground motions

For nonlinear analyses time histories are required

Nonlinear structural analyses

Nonlinear geotechnical analyses

Nonlinear SSI analyses

All are becoming more common

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.

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.


Sources: PEER NGA Database (http://peer.berkeley.edu/nga/)


Sources: PEER NGA Database (http://peer.berkeley.edu/nga/search.html)

Can search for records with characteristics similar to those controlling hazard at site of interest

Can search for records with characteristics similar to those controlling hazard at site of interest


Sources: COSMOS Database

http://db.cosmos-eq.org/scripts/search.plx

Can search for records with characteristics similar to those controlling hazard

at site of interest

Can search for records with characteristics similar to those controlling hazard

at site of interest


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

Sa

TTo

Ground Motion ScalingGround 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.

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.


Simulation

Example:

After Norm Abrahamson COSMOS workshop presentation

Initial motion too strong

Initial motion too weak

Spectrum after

matching


Simulation

Example:


Original

Modified


Simulation

Example:


Original

Modified


Simulation

Example:


Original

Modified


Simulation

Example:



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.

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.


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

Candidate motions should be consistent with these characteristics


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


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 faultingSignificant

period range


Epsilon

To

To

To

Scaled negative motion is too strong

Scaled positive motion is too weak

Be careful – look for local peaks and valleys

in candidate motions prior to scaling

Be careful – look for local peaks and valleys

in candidate motions prior to scaling


Epsilon

Baker (2007) took 382 representative ground motions

Computed values for each, chose 20 highest and 20 lowest

Computed spectra after scaling to same Sa(T=0.8)

Hazard Analysis and Ground MotionsHazard 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

Results of site response analyses will be sensitive to ground motion inputs – need to pay careful attention to this issue

Documents

Geotechnical Earthquake Engineering: Ground Motions Steve Kramer Department of Civil and Environmental Engineering University of Washington Seattle, WA