SEISMICGROUNDSEISMIC GROUND MOTIONHAZARDSMOTION … · 2010-10-26 · Peak ground motion values...
Transcript of SEISMICGROUNDSEISMIC GROUND MOTIONHAZARDSMOTION … · 2010-10-26 · Peak ground motion values...
SEISMIC GROUNDSEISMIC GROUND MOTION HAZARDSMOTION HAZARDS
Learning Outcomesg
Be able to: Develop AASHTO acceleration response
spectrum for rock (type B)spectrum for rock (type B) Adjust AASHTO spectrum for local site
conditions Interpret the results of a seismic hazard Interpret the results of a seismic hazard
deaggregation
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DESIGN GROUND MOTION PARAMETERS
Peak ground motion values Routinely evaluated for bedrock reference
site condition at the outset of the investigation
PGA most common, PGV, PGD also used Spectral values Most common input for force-based Most common input for force-based
structural design (usually acceleration) Magnitude and distance Magnitude and distance Used in many geotechnical analyses Used to develop representative time
histories
SEISMIC RETROFIT DESIGN GROUND MOTION LEVELS
Ground Motion Design Levelsg
Two Levels: Lower Level: 50% probability of Lower Level: 50% probability of
exceedance in 75 yrsU L l 7% b bilit f Upper Level: 7% probability of exceedance in 75 yrs
Based on Poisson Probability Model, Based on Poisson Probability Model, PE = 1 – e-vt
hwhere:“PE” is probability of exceedance of E p ygiven amplitude of ground motion“t” is lifetime of the bridget is lifetime of the bridge“v” annual frequency of exceedance of ythat ground motion amplitude
Equation can be rewritten as: Equation can be rewritten as:v = -ln(1 – PE)/t
Seismic Ground Motion HazardSeismic Ground Motion Hazard
For small probabilities (< 10%), annual frequency of exceedance v = P /tof exceedance, v = PE/t
e.g., for PE = 7% in 75 years, v = 0.07/75 = 0 0009 i 0 0010.0009, i.e., approx. = 0.001
Return period, Rp, is reciprocal of annual frequency of exceedance: Rp = 1/v
e.g., for v = 1/0.001, Rp = 1000 yearsg p y
Average Return Period for Ground Motion
1971 San Fernando Earthquake 1994 Northridge Earthquake
SEISMIC RESPONSE SPECTRUM
What’s a Response Spectrump pMax response of linear single degree of
freedom (SDOF) s stem to earthq akefreedom (SDOF) system to earthquake ground motion
• Depends upon fundamental period, damping ratio • Indicates the maximum response of structure to the ground motion
Have acceleration, velocity, and (relative) displacement spectrap p
• Acceleration response spectrum most common• Relative displacement spectra also used• Relative displacement spectra also used
How is Response Spectrum created?p pEl Centro (1942)
Actual Spectra vs. Smoothed Spectrump p
Range one structure may experienceRange one structure may experiencefor a variety of ground motions.
Amplification zone
SEISMIC HAZARD ANALYSIS
Types of Seismic Hazard Analysisyp y
Probabilistic (used for AASHTO Maps)Probabilistic (used for AASHTO Maps) Ground motion with specified probability of
occurrence for specified exposure periodoccurrence for specified exposure period Typically composed from many earthquakes of
diff t it d d di tdifferent magnitude and distanceDeterministic Specific design event(s) of well-defined magnitude and
source-to-site distance The temporal occurrence of the earthquakes and ground
motions are not accounted for
Seismic Hazard Analyses: Methodology
Probabilistic Analysis Resultsy
Seismic hazard curves
Uniform Hazard Spectrum (UHS)p ( )
SEISMIC HAZARD MAPS
(PGA,1 sec & 0.2 sec-1000 year)
PGA, 0.2 sec & 1.0 sec (1000 year)
T=1-second
T=0.0 secondPGA
T=0.2 second
rock rock rock
AASHTO Spectrum Constructionp
Plot (and connect linearly) Sa = PGA, T=0a
Sa = SDS, T = 0.2TS
Sa = SDS, T = TS Sa SDS, T TS
1/T d t T T 1/T decay at T>TS Sa = SD1, T = 1 sec
AASHTO Design Spectrumg pSpectral
Acceleration,Sa (g)
SDS: Sa @ 0.2 sec
Decays as 1/T
SD1: Sa @ 1.0 sec
A = PGA
1/T
sec
0 2 (TS) TS = SD1 / SDS
0.2 1.0 Spectral Period, T (Sec)
0.2 (TS) TS SD1 / SDS
LOCAL SITE CONDITIONS
Local Site Conditions
Significantly affect strong ground motions Effects observed in many earthquakesy
Observed impacts include: Amplitude Amplitude Duration
F t t Frequency content Coherence
1957 Daly City Earthquake
PGA Amplification in Mexico City (1985)
Building Damage at the SCT Site
Building Damage at the CAO Site
SA Amplification in Mexico City (1985)
Damage to I-880 in the Loma Prieta EQ
Impact of Local Site Conditionsp
Modification (amplification) of peak and spectral accelerations
Modification of frequency content (shape of q y ( presponse spectra)
Increase in duration
Increase in incoherence (spatial variability)
Spectral Shape and Amplificationp p p
Spectral shape accounted for since 1980sNew specifications account for shape and p pamplificationSpectral amplificationSpectral amplification
• Always occurs around natural frequency of site T0
= 4H/VS
• Is greatest when T0 is near predominant period of earthquake motion
SITE CLASSIFICATION
Site-Dependent Spectrump p Must adjust reference site spectra for local
site conditions Based upon (VS)30, shear wave velocity in top 100
ft (30 meters) Assumes no sharp transitions (impedance
t t ) i it filcontrasts) in site profile
For reference site, (VS)30 = 2500 ft/s Referred to as B/C boundary (boundary between
Site Class B and Site Class C)
Site class based upon (VS)30 (or other p ( S)30 (geotechnical characteristics) in top 100 ft (30 m)m)
Site Cl (VS)30 SPT SClass (VS)30 SPT Su
A > 5000 ft/s N.A. N.A.B 2500 - 5000 ft/s N.A. N.A.C 1200 - 2500 ft/s > 50 > 2 ksfD 600 - 1200 ft/s 15 - 50 1 -2 ksfE < 600 ft/s <15 < 1 ksfE < 600 ft/s <15 < 1 ksfF (Special Study Sites)
Average Shear Wave Velocityg y
Two layer system:1h h
Layer 1 travel time: Layer 2 travel time:1
1
Vh
2
2
Vh
Average velocity: 21
avgs hhhhV
Average velocity:
2
2
1
1
SS
g
Vh
Vh
M lti L S t
n
ihMulti-Layer System:
n
i
iavgs h
V 1
i iV1
AASHTO Site-Dependent Modificationsp
Spectral accelerations adjusted using site factors FA, FV, and FPGA based upon Site GClass (A-E) SDS = (SDS)Site Class B x FADS ( DS)Site Class B A
SD1 = (SD1)Site Class B x Fv
A = (A) x F A = (A)Site Class B x FPGA
Note: TS may also change as (in general) FA Note: TS may also change as (in general) FA= FV
SITE FACTORS
PGA Site Factor, FPGAPGAMapped Spectral Response Acceleration at Short
PeriodsSite Class
e odsPGA≤ 0.10 g
PGA = 0.20 g
PGA = 0.30 g
PGA = 0.40 g
PGA ≥ 0.50 g
A 0.8 0.8 0.8 0.8 0.8B 1.0 1.0 1.0 1.0 1.0C 1.2 1.2 1.1 1.0 1.0D 1.6 1.4 1.2 1.1 1.0E 2.5 1.7 1.2 0.9 0.9F a a a a a
Table notes:Use straight line interpolation for intermediate values of PGA, where PGA is the peak ground acceleration obtained from the ground motion mapsfrom the ground motion maps.
a Site-specific geotechnical investigation and dynamic site response analyses shall be performed.
Short Period Site Factor, FAA
Si Cl
Mapped Spectral Response Acceleration at Short Periods
Site ClassSs ≤ 0.25
gSs = 0.5
gSs = 0.75
gSs = 1.00
gSs ≥ 1.25
gA 0.8 0.8 0.8 0.8 0.8B 1.0 1.0 1.0 1.0 1.0C 1.2 1.2 1.1 1.0 1.0D 1.6 1.4 1.2 1.1 1.0E 2.5 1.7 1.2 0.9 0.9F a a a a a
Table notes:Use straight line interpolation for intermediate values of Ss, where Ss is the spectral acceleration at 0.2 seconds obtained from the ground motion mapsobtained from the ground motion maps.
a Site-specific geotechnical investigation and dynamic site response analyses shall be performed.
Long Period Site Factor, FVg V
SiteMapped Spectral Response Acceleration at 1 Second
PeriodsSite Class
e ods
S1 ≤ 0.1 g S1 = 0.2 g
S1 = 0.3 g
S1 = 0.4 g S1 ≥ 0.5 g
A 0.8 0.8 0.8 0.8 0.8B 1.0 1.0 1.0 1.0 1.0C 1.7 1.6 1.5 1.4 1.3D 2.4 2.0 1.8 1.6 1.5E 3.5 3.2 2.8 2.4 2.4F a a a a a
Table notes:
Use straight line interpolation for intermediate values of S1, where S1 is the spectral acceleration at 1.0 seconds obtained from the ground motion mapsfrom the ground motion maps.
a Site-specific geotechnical investigation and dynamic site response analyses shall be performed.
AASHTO Three Point Response Spectrum p pSDS=SsFa
ratio
n, S
a
S =S
PG
A
se A
ccel
e Sa=SD1T
SD1=S1Fv
F PG
AP
Res
pons
PGA FPGA
1.0Ts=SD1SDS
To=0.2Ts 0.2
Period (seconds)
100 Year Return Period
USGS web site does not provide hazard maps for the 100 year return period Need to use USGS hazard map data for 500,
1000 and 2500 year returns and extrapolate for 100 year return period.
Seismic Hazard Mapsp
S2/50
Si
Ln(S2/50)
Ln(Si)
S10/50 Ln(S10/50)
475 P 2475Return Period
Ln(475) Ln(P) Ln(2475)Natural Log (Return Period)
Linear Interpolation (ASCE 41-06)
))1(()475()2475()()()( 50/1050/2
PLnLnLn
SLnSLnSLn i
AASHTO GROUND MOTION CD
AASHTO f d d USGS t d l 1000 AASHTO funded USGS to develop 1000 yr Rp implementation CD Existing USGS maps were for 500 and 2500 yr
CD provides PGA, SA at T = 0.2 and 1 secCD provides PGA, SA at T 0.2 and 1 sec from UHS for reference site condition, plots spectrumspectrum Reference site condition, Site Class B, is rock at
surfacesurface Values provided by latitude and longitude or zip
codecode
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AASHTO Ground Motion CD
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Ground Motion CD – 2nd Screen
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Ground Motion CD – 3rd Screen
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Ground Motion CD – 4rd Screen
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AASHTO Ground Motion – 5th
Screen
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UHS and AASHTO Spectra Comparison
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The AASHTO CD also corrects the design spectrum for the designated site classspectrum for the designated site class.
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Ground Motion CD – 3rd Screen
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Ground Motion CD - 4th Screen
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Ground Motion CD – 5th Screen
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Ground Motion CD – 6th Screen
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AASHTO Ground Motions AASHTO CD does not provide hazard maps S O C does ot p o de a a d aps
for the 100 year return period AASHTO CD also does not provide AASHTO CD also does not provide
deaggregationU USGS d i Use USGS deaggregation at http://eqint.cr.usgs.gov/deaggint/
Use 1996 or 1998 for continental US, Use 1998 for Alaska and Hawaii (defaults to 1996
page)
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SEISMIC HAZARD DEAGGREGATION
Deaggregation of UHSgg g
Primary Goal:Primary Goal:
Break specified hazard parameter into p pcontributions by magnitude and distance
Important Issue to Note:
Di t ib ti d d h dDistribution may depend upon hazard parameter, spectral periodp , p p
Deaggregation TableDeaggregated Seismic Hazard PE = 2% in 50 years pga Bakersfield CA 35.373 deg N 119.018 deg W PGA=0.42440 g M<= 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0
d<= 25. 0.000 18.239 18.339 32.288 18.484 0.000 0.000 0.000 0.000 50. 0.000 0.019 0.024 0.086 4.200 7.059 0.000 0.000 0.000 75. 0.000 0.000 0.001 0.006 0.033 0.019 1.154 0.000 0.000 100. 0.000 0.000 0.000 0.000 0.015 0.008 0.000 0.000 0.000
125 0 000 0 000 0 000 0 000 0 009 0 002 0 000 0 000 0 000 125. 0.000 0.000 0.000 0.000 0.009 0.002 0.000 0.000 0.000 150. 0.000 0.000 0.000 0.000 0.001 0.003 0.000 0.000 0.000 175. 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 200. 0.000 0.000 0.000 0.000 0.007 0.002 0.000 0.000 0.000
Deaggregated Seismic Hazard PE = 2% in 50 years 1hz Bakersfield CA 35.373 deg N 119.018 deg W SA= 0.38360 g M<= 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0
d<= 25. 0.000 0.957 2.329 17.096 16.216 0.000 0.000 0.000 0.000 50. 0.000 0.004 0.019 0.272 12.374 23.708 0.000 0.000 0.000 75. 0.000 0.000 0.001 0.019 0.254 0.208 26.228 0.000 0.000
100. 0.000 0.000 0.000 0.001 0.074 0.097 0.000 0.000 0.000 100. 0.000 0.000 0.000 0.001 0.074 0.097 0.000 0.000 0.000 125. 0.000 0.000 0.000 0.001 0.037 0.016 0.000 0.000 0.000 150. 0.000 0.000 0.000 0.000 0.006 0.029 0.008 0.000 0.000 175. 0.000 0.000 0.000 0.000 0.002 0.006 0.000 0.000 0.000
200 0 000 0 000 0 000 0 000 0 015 0 025 0 000 0 000 0 000 200. 0.000 0.000 0.000 0.000 0.015 0.025 0.000 0.000 0.000
Deaggregation of PSHA
Geographic Deaggregationg p gg g
Consistency CheckyStep 1: Select a predominant event (magnitude and distance)
from deaggregated PSHA data
Step 2: Plot response spectrum for representative event against
UHS
Step 3: Compare plots. They should be in general agreement.
Learning Outcomesg
Be able to: Develop AASHTO acceleration response
spectrum for rock (type B)spectrum for rock (type B) Adjust AASHTO spectrum for local site
conditions Interpret the results of a seismic hazard Interpret the results of a seismic hazard
deaggregation
What questions do you have?