EQ ground motions.pdf

8/14/2019 EQ ground motions.pdf

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by

I. M. Idriss

Professor Emeritus

University of Californ ia at Davis

e-mail: [email protected]

EARTHQUAKE GROUND MOTIONS

Presented at the Session on

"Earthquake Effects on Soils and Foundations"

Part of the Refresher Course on

"Seismic Analysis and Retrofitting of Life line Buildings in Delhi, India"

May 26, 2005

Audi torium of the Delhi Secretariat Building

Delhi, India

PROCEDURES AVAILABLE FOR ESTIMATING

EARTHQUAKE GROUND MOTIONS

1) Accumulation of strong motion data & correlation with

identified local subsur face conditions -- empirical approach.

2) Use of analytical procedures to calculate ground motions

at a given site semi-empirical approach.

These analytical procedures include:

a) Source Path Site

b) Local Site Response


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EMPIRICAL RELATIONSHIPS

Instrumental records at a multitude of local site conditions

have clearly shown the influence of local site conditions

on the characteristics of earthquake ground motions.

Therefore, empirical relationships typically incorporate

the influence of local site conditions.

Some derive separate sets of equations for each generalized

site condition.

Some introduce a parameter (e.g., Vs30

) that is used to

reflect these local site conditions.


Geotechnical Subsurface Classification

Devised by Geomatrix

A -- Rock (Vs30 > 600 m/sec)

B Shallow sti ff soil (up to 20 m thick over rock)

C Deep narrow soil (more than 20 m over rock & narrow val ley)

D Deep broad soi l (more than 20 m over rock & broad valley)

E Soft deep soi l


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NEHRP Site Classification

One Letter: Geotechnical Subsurface Classification

Average shear wave veloci ty, Vs30, in top 30 m

A -- Vs > 1500 m/sec

B Vs = 760 to 1500 m/sec

C Vs = 360 to 760 m/sec

D Vs = 180 to 360 m/sec

E Vs < 180 m/sec

F Liquefiable Soils

EXAMPLES

RECORDED DATA


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1994 Northridge

Earthquake

EMPIRICAL APPROACH

Closest Distance to the Rupture Surface - km

3 4 5 6 8 20 30 40 50 60 80 200 30010 100

Peak

HorizontalAcceleration-g

0.02

0.03

0.04

0.05

0.06

0.08

0.2

0.3

0.4

0.5

0.6

0.8

0.01

0.1

1

Soil Sites

Rock Sites

1994 Northridg e Earthquake

Strong & Weak HorizontalComponents Recorded at

Rock Sites

Soil Sites

Derived MedianRelationships


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Period - sec

0.01 0.1 1 10Spec

tral

Acce

lera

tion

(Sa

)/Pea

kGroun

dAcce

lerati

on

(pga

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Motions Recorded at SoilSites -- Northridge Earthquake

Motions Recorded at RockSites -- Northridge Earthquake

1994 Northridge

Earthquake

Period sec0.01 0.1 1

Sp

aA

eaoz

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.3 g 0.7 g

Soil SitesNorthridge Earthquake


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Longitude120.0 120.5 121.0 121.5 122.0Lt

22.0

22.5

23.0

23.5

24.0

24.5

25.0

Approx RuptureSurfaceCategory B SitesCategory C SitesCategory D SitesCategory E Sites

1999 Chi-Chi

Earthquake

Distance - km1 10 100

Pe

akHor

izon

talAcce

lera

tion-g

0.01

0.1

1

Motions Recorded at

Category B SitesCategory C SitesCategory D SitesCategory E Sites

1999 Chi-Chi

Earthquake


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Peak Horizontal Acceleration at Category B Sites -g

0.0 0.1 0.2 0.3 0.4 0.5

P

eakHorizontalAccelerationatSoilSites-g

0.0

0.1

0.2

0.3

0.4

0.5

:

Deep Soil Sites(San Fernando )

Category D Sites

Category E Sites

Category B Sites

1999 Chi-Chi

Earthquake

Period - sec0.01 0.1 1

Spec

tra

lAcce

lera

tion

/zpa

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.3g 0.4g 0.7g

Category B SitesChi-Chi Earthquake


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OBSERVATIONS RE: EMPIRICAL APPROACH

1. Local site conditions do affect earthquake ground motions

2. The observed trends in zpa & in spectral shapes indicate

a strong nonlinear response of soil sites

3. These trends are reasonably well accommodated in the current

Code provisions

4. The on-going work (NGA) to derive updated attenuation

relationships shou ld shed more light on the influence oflocal site conditions on earthquake ground motions.

5. The available recorded data are not suffi cient to cover all site

conditions

6. Therefore, it is necessary to utilize analytical procedures to

augment the recorded data & to evaluate response at soil

sites for which little or no recorded data exist

OBSERVATIONS RE: EMPIRICAL APPROACH (Cont 'd)


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ANALYTICAL PROCEDURES

SEMI-EMPIRICAL APPROACHES

SOURCE PATH SITE

Excellent for assessing relative effects o f variousparameters on earthquake ground motions

Has proven to be a valuable tool for calculatingearthquake ground motions at rock & stiff so il sites.

Not well suited for calculating effects of local site

conditions on earthquake ground motions

LOCAL SITE RESPONSE

Has proven to be a valuable tool for calculatingeffects of local site conditions on earthquake

ground motions

Al lows for incorporat ing nonlinear soil behaviorunder dynamic loading conditions

Equivalent Linear Representation

Non Linear Representation

ANALYTICAL PROCEDURES

SEMI-EMPIRICAL APPROACHES


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The equivalent linear procedure has been & continues

to be the most widely used procedure in practice for

calculating site response & for developing si te specific

earthquake ground motions and design parameters.

There are several non-linear techniques that have been

and continue to be developed.

These procedures are needed to make reasonably

accurate estimates of displacement / deformationsof earth structures.

LOCAL SITE RESPONSE

Factors that Influence Site Response Calculations

Input Rock Motion

Soil Profile

Soil Properties

Method o f Analysis


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Treasure Island & Yerba Buena Island Sites

SF-Oakland Bay Area

Example No. 1


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Shear Wave Velocity - ft/sec

0 500 1000 1500 2000 2500

DepthBelowG

roundSurface-ft

0

40

80

120

160

200

240

280

320

Best Estiamte Velocities usedin the analyses

Measured Velocities:

Downhole (Redpath, 1991)

SCPT (Rollins et al, 1992)

Downhole (Gibbs et al, 1992)

Treasure

Island Site

Wave Velocity - ft/sec0 1000 2000 3000 4000

Dept

hBe

low

Groun

dSur

face-

ft

0

20

40

60

80

Yerba Buena Island


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Time - sec

0 5 10 15 20 25

Acceleratio

n-g

-0.16

-0.08

0.00

0.08

0.16EW Component of Recording

at Treasure Island

Time - sec

0 5 10 15 20 25

Acceleration-g

-0.16

-0.08

0.00

0.08

0.16EW Component of Recording

at Yerba Buena Island

Period - sec

0.1 1 10

Spec

tra

lAcce

lera

tion

-g

0.0

0.2

0.4

0.6

0.8

Spectrum for MotionRecorded at TI

Spectrum for motionRecorded at YB


14/27


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Period - sec

0.1 1 10

Spec

tra

lAcce

lera

tion

-g

0.0

0.2

0.4

0.6

0.8Spectrum for Motion

Recorded at TI

Spectrum for motioncalculated at TI usingas input motion:recording at YB

Spectrum for motioncalculated at TI usingas input motion:ALL excep t YB

EFFECTS OF SOIL PROPERTIES

ON SITE RESPONSE

Treasure Island Site


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Period - sec

0.01 0.1 1 10

Spec

tra

lAcce

lera

tion-g

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

damping = 5 percent

Spectrum for MotionRecorded at TI

Spectrum for motioncalculated at TI usingas input motion recordingat YB & best estimate v

s

Spectrum for motioncalculated at TI usingas input motion recordingat YB & range of vs

San Francisco Ai rport & Sierra Point Sites

Example No. 2


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Shear Wave Velocity - ft/sec

0 500 1000 1500 2000 2500 3000

DepthBelowG

roundSurface-ft

0

50

100

150

200

250

300

350

400

450

500

Best Estimate VelocitiesUsed in the Analyses

Gibbs et al (1992)Redpath (1991)

Shear Wave VelocityMeasurements b y:

San Francisco

Airpor t Site


18/27

3535

Period - sec

0.01 0.1 1 10

Spec

tra

lAcce

lera

tion-g

0.0

0.2

0.4

0.6

0.8

1.0

1.2

damping = 5 percent

Spectrum for motionrecorded at Sierra Point

Spectrum for motionrecorded at SFO

3636

Period - sec

0.01 0.1 1 10

Spec

tra

lAcce

lera

tion-g

0.0

0.2

0.4

0.6

0.8

1.0

1.2

damping = 5 percent

Spectrum for motion calculatedat SFO using motion recordedat Sierra Point as input motion

Spectrum for motionrecorded at SFO


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EFFECTS OF INPUT ROCK MOTION

ON SITE RESPONSE

San Francisco Airport Site

3838

Period - sec

0.01 0.1 1 10

Spec

tra

lAcce

lera

tion-g

0.0

0.2

0.4

0.6

0.8

1.0

1.2

damping = 5 percent

Spectrum for MotionRecorded at SFO

Spectrum for motioncalculated at SFO usingas input motion:recording at Sierra Point

Spectrum for motioncalculated at TI usingas input motion:

ALL excep t Sier ra Poi nt


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ON SITE RESPONSE

San Francisco Airport Site

4040

Period - sec

0.01 0.1 1 10

Spec

tra

lAcce

lera

tion-g

0.0

0.2

0.4

0.6

0.8

1.0

1.2

damping = 5 percent

Spectrum for MotionRecorded at SFO

Spectrum for motioncalculated at SFO usingas input motion recordingat SP & best estimate v

s

Spectrum for motioncalculated at SFO usingas input motion recordingat SP & range of v

s


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La Cienega Downhole Array

Los Angeles

Example No. 3

4242


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4343

4444


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4545


ON SITE RESPONSE

La Cienega Site

4646

Shear Wave Velocit ies

La Cienega Site

Los Angeles

Shear Wave Velocity (ft/sec)

0 500 1000 1500 2000 2500 3000

Depth

Belowt

heGroundSurface(ft)

0

100

200

300

400

500

600

700

800

900

measured vsvalues

vsvalues used in b oth analyses

below a depth of 80 ft

vsvalues used in analysis 1

from ground surface to 80 ft

vsvalues used in analysis 2

from ground surface to 80 ft


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4747

Peak Horizontal A cceleration (g)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

DepthBelowt

heGroundSurface(ft)

0

100

200

300

400

500

600

700

800

900

Peak Recorded Values

Input: at depth = 830 ftAnaly sis 1

Input: at depth = 830 ftAnaly sis 2

La Cienega Site

Los Angeles

4848

Peak Horizontal A cceleration (g)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Depth

Belowt

heGroundSurface(ft)

0

10

20

30

40

50

60

70

80

90

100

Peak Recorded Values

Input: at depth = 830 ftAnalysi s 1

Input: at depth = 830 ftAnalysi s 2

La Cienega Site

Los Angeles


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4949

Factors that Influence Site Response Calculations

Input Rock Motion

Soil Profile

Soil Properties

Method of Analysis

5050

ANALYTICAL APPROACH

Observations re: Analytical Approach

1. The input motion has a very significant influence on

calculated site response

2. The soil profile also has a very significant influence on

calculated site response

3. The soil properties also influence the calculated site response,but to a lesser extent than input motion or soil profile

4. The method of analysis can have a profound influence if

inappropriately applied


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5151

CLOSING COMMENTS

Equivalent Linear Procedure:

Limit depth of soil pro file to about 500 ft

Limit level of shaking to that resulting in no more than about

1 percent shear strain at any depth within the soil pro file

5252

CLOSING COMMENTS

Non Linear Procedure:

Select only those procedures & corresponding computer

programs that have been calibrated with a sufficient number

Of case histories

Limit level of shaking to that resulting in no more than about

A few percent shear strain at any depth within the soi l profi le


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5353

CLOSING COMMENTS

Al l Procedures:

Selection of input motion is a critical step

use at least 7 reasonably uncorrelated input motions

Al low for appropriate var iat ion in the soi l properties

& the soil profile

5454

THANK YOU

EQ ground motions.pdf

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