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

5-2

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

5-48

AASHTO Ground Motion CD

5-49

Ground Motion CD – 2nd Screen

5-50

Ground Motion CD – 3rd Screen

5-51

Ground Motion CD – 4rd Screen

5-52

AASHTO Ground Motion – 5th

Screen

5-53

UHS and AASHTO Spectra Comparison

5-54

The AASHTO CD also corrects the design spectrum for the designated site classspectrum for the designated site class.

5-55

Ground Motion CD – 3rd Screen

5-56

Ground Motion CD - 4th Screen

5-57

Ground Motion CD – 5th Screen

5-58

Ground Motion CD – 6th Screen

5-59

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)

5-60

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?