lecture27-local site effetcs

8/12/2019 Lecture27-Local Site Effetcs

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Local Site Effects

Lecture-27

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Important Aspects of Earthquake Motion

Earthquake damage is influenced by ground motion:

Amplitude

Frequency content

Duration

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Important Aspects of Earthquake Motion

Earthquake damage is influenced by ground motion:

Amplitude

Frequency content

Duration

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Local site effects

Ground surface motions are affected by local site conditions

Site effects can influence:

Amplitude - may amplify or de-amplify motion

Frequency content - may shift to higher or lower

Duration - may extend duration of strong shaking

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Local site effects

Softer soil at site A will amplify low-frequency input motions much more strongly than will

the stiffer soils of site B. At higher frequencies, the opposite behavior would be expected.6

Source: Kramer (1996)


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Local site effects

1999 Kocaeli, Turkey earthquake

Some sites are totally damaged and some sites are safe in same locality8

Source: google images


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Local site effects

1957 San Francisco Earthquake (M = 5.3)

9Source: Kramer (1996)


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1957 San Francisco Earthquake (M = 5.3)

Recordings of ground motion at several locations in San Francisco were madeduring 1957 earthquake.

Variations in ground motion in terms of peak horizontal accelerations and

response spectra with the variation in soil conditions along a 4-mile section is

observed.

Ground surface motions at the rock outcrops were quite similar, but the

amplitude and frequency content of the motions at sites underlain by thick soil

deposits were markedly different .

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Mexico Earthquake, 1985

Fig: Strong motion instruments and geotechnical conditions in Mexico city

(a) Locations of strong motion instruments relative to foothill, transition and lake

zones (b) Contours of soft soil thickness (After Stone et al., 1987) 11



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Mexico Earthquake, 1985 (Ms=8.1)

The earthquake of 1985 was quite large but its great distance from Mexico cityproduced accelerations at the UNAM (Foothill zone) site of only 0.03 g to 0.04 g. In

the transition zone, peak accelerations were slightly greater than those at UNAM

but still quite low. In the Lake zone, peak accelerations were almost 5 times greater

than those at UNAM.

The ground motion in Lake zone was characterized by strong levels of shaking overlonger durations compared to the transition zone and foothill zone.

The frequency content of the motion in lake zone was also different from the other

two zones.

Spectral accelerations in lake zone were about 10 times more than those at foothill

zone.

Hence the damage was severe in the lake zone compared to the other zones.

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Local site effects

1985, Mexico Earthquake

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San Francisco Bay Area, 1989 Loma Preita Earthquake

Fig: Measured peak horizontal accelerations (in gs) in the San Francisco Bay area

during 1989 Loma Preita earthquake.

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1989 Loma Preita Earthquake (Ms= 7.1)

The epicenter was located at about 100 km south of San Francisco and Oakland,California. The Loma Preita earthquake produced less intense ground shaking

around the epicenter but caused higher damages in certain areas in San Francisco

and Oakland.

The San Francisco Bay basin in largely filled with alluvial deposits of clays and silty

to sandy clays with some layers of sandy and gravelly soils.

The deeper deposits were overconsolidated by historical glacial sea-level

drawdown, but the upper unit was deposited after the last drawdown episode.

Both the epicentral region and the Bay area were well instrumented with

seismographs and accelerometers.

The response of two instruments, those located at Yerba Buena Island and Treasure

Island are useful in understanding the effect of local site conditions on the ground

motion.

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San Francisco Bay Area, 1989 Loma Preita

Earthquake

Fig: Ground surface motions at Yerba Buena Island and Treasure Island

(a) Time histories (b) Response Spectra (After Seed et al., 1990)

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Local site effects

Data from multiple events

Fig: Approximate relationship between peak acceleration on rock and other local

site conditions (After Seed et al., 1976)

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Topographical effects

Triangular infinite wedge

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Basin effects

Alluvial basins common in many populated areas

Many depths, widths, shapes

Generally filled with softer materials

Can refract waves to focus energy

Can lead to development of surface waves

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Basin effects

Significance of basin depends on shape

Wide, shallow basin

One-dimensional analysis OK in center

Two-dimensional effects may be important near edges

Deep, narrow basin

One-dimensional analysis may not be applicable25


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Presence of weathered rockCustomary to define seismic bedrock as:

Lithological bedrock, or

Material with vs> or 800 m/sec

Such conditions are used as reference sites

Weathering may produce vsgradient near top of rock

Can induce dynamic response in rock

Motion at top of rock may reflect specific vsprofile

Can produce errors when used as reference motion

Some sites of lithological bedrock have been shown to be more soil-

like than rock-like. 26


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Indian Code Specifications

Indian Standard IS 1893 (2002)

CRITERIA FOR EARTHQUAKE RESISTANT

DESIGN OF STRUCTURES

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S i i Z i f I di


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Seismic Zoning of India

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Indian Code Specifications

According to Seismic Zone Map of India, populations exceeding

half a million are in earthquake Zones III, IV, V.

Zone III :- Ahemdabad, Vadodara, Rajkot, Bhavnagar,

Surat,Mumbai, Agra, Bhiwandi, nashik, Kanpur Pune,

Bhubneshwar, Cuttack, Asansol, Kochi Kolkata, Varanasi, Bareilly,

Lucknow, Indore, Jabalpur, Vijaywada, Dhanwad, Chennai,

Coimbatore, Manglore, Kozhakode ,Trivandrum.

Zone IV:- Dehradun, New Delhi, Jamunanagar, Patna, Meerut,

Jammu, Amristar, Jalandhar.

Zone V:- Guwahati and Srinagar.

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Indian codal provisions

The design horizontal seismic coefficient ah for a structure

shall be determined by the following expression:

Z is zone factor for the Maximum Considered Earthquake

(MCE ) and service life of structure in a zone. The factor 2 in

the denominator of Z is used so as to reduce the MaximumConsidered Earthquake ( MCE ) zone factor to the factor for

Design Basis Earthquake ( DBE ).

Rg

ZISa ah 2

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I di d l i i


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I is importance factor, depending upon the functional use of

the structures, characterized by hazardous consequences of

its failure, post-earthquake functional needs, historical value,or economic importance .

R is Response reduction factor, depending on the perceived

seismic damage performance of the structure, characterized

by ductile or brittle deformations. However, the ratio (I/R )shall not be greater than 1.0

Sa/g is average response acceleration coefficient

Rg

ZIS

a

a

h 2

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Figure shows the effect of local soil conditions on response spectrum as given by

IS code.



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Figure shows the variation of spectral acceleration coefficient with natural period

given by IS code for stiff structures with fundamental period less than 0.1 sec


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Kramer (1996) Geotechnical Earthquake Engineering, Prentice Hall.

IS 1893:2002 . Indian Standard. Criteria For Earthquake Resistant Design of

Structures

Borcherdt, R.D. (1970) Effects of local geology on ground motion near San

Francisco Bay, Bulletin of Seismological Society of America, 60, 2961.

Baise, L. G., S. D. Glaser and D. Dreger (2003). Site Response at Treasure and

Yerba Buena Islands, California. Journal Geotechnical and Geoenvironmental

Engineering, 129(6), 415-426.

References

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