Earthquake engineering introduction

8/18/2019 Earthquake engineering introduction

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What are Earthquakes? An earthquake is a series of shockwaves generated following the brittle failure of rocks within

the earth's crust or upper mantle as a result of a build up ofstress.

How are they generated? The topmost part of the earth ismade up of the various types of soil and rock I.e. crust, therock in this crust fails due to the movements of the plates andthe strain energy stored in the rock before failure stored in it

prepared by: Siddharth G.Shah. 1

during the deformation periods is released , and this energyflows in the form of the seismic waves and they causesvibrations.

What is plate? Plate is the part of the upper mantle and thecrust in the surface of the earth which is in motion due toconvection taking place due to temperature and pressuregradient between the earth surface and the center of the earth.


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The view of various plates on the earth surfaces


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Figure : Inside the Earth; Crust,

Mantle, Outer Core, Inner Core



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finitions

•Focus: That point within the Earth from which originates the first motion

of an earthquake and its elastic waves.•Hypocenter: The calculated location of the focus of an earthquake.

•Epicenter : That point on the Earth's surface directly above the

hypocenter of an earthquake.



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•Foreshock A small tremor that commonly precedes a larger earthquake

or main shock by seconds to weeks and that originates at or near the

focus of the larger earthquake.

• Tsunami A huge sea wave caused by earthquakes. (Referred to by many

as a tidal wave.)


• Aftershock: An earthquake which follows a larger earthquake or main

shock and originates at or near the focus of the larger earthquake.

Generally, major earthquakes are followed by a larger number of

aftershocks, decreasing in frequency with time.


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Earthquake Intensity The effects of earthquake waves at the surface can be measured usingan intensity scale. This is an arbitrary scale based on observations of phenomena such as: the typeand extent of damage, whether sleeping people were woken, whether items fell fromshelves,whether the event was felt or heard. Mercalli intensity is based upon observations ofthe resulting earthquake damage and not actually measured on instruments. Invented by

Italian seismologist Giuseppe Mercalli

Modified Mercalli Intensity Scale

1 Not felt. Recorded by seismographs.

2 Rarely felt, usually only on top floors of high buildings.

3 Felt indoors, like a passing light truck.

4 Windows, dishes, doors rattle. Like passing train.


e y a . ma o ec s upse .

6 Books off shelves. Trees shake. Isolated damage.

7 Difficult to stand. Many poor buildings damaged.

8 Significant damage. Branches broken from trees.

9 General panic. Serious damage. Ground cracking.10 Most buildings destroyed. Rails bent slightly.

11 Rails bent greatly. Pipelines destroyed.

12Near total damage. Objects thrown into the air.


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Magnitude: A measure of the strength/ Force of an earthquake orstrain energy released by it, as determined by seismographicobservations. The local body- and surface-wave magnitudes will

have approximately the same numerical value. The Richter scalewas created in 1935 by the American seismologist Charles F.Richter. It measures how much the ground shakes 60 miles from theearthquakes epicenter. Richter magnitudes increase logarithmically,meaning the energy increases 10 times for each magnitudenumber.i.e. 4.0 is 10 times greater than 3.0, 5.0 is 100 times greater

than a 3.0 6.0 is 1000 times greater than 3.0• Measurement recorded on a scale of 1.0 to 12.0


• 1.0 = 30 lbs of TNT explosion on a construction site

• 4.0 = small nuclear weapon (1000 tons of TNT)

• 7.0 = Largest Nuclear weapon available (32,000 tons of TNT)

• 8.0 = SF Quake 1906 (1 billion tons of TNT)

• 10.0 = San Andreas fault around the earth (75,000 largest nuclear bombs)

• 12.0 = Earth splits in half


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Richter

Magnitude

Mercalli

Intensity

Description

2 I Usually not felt, but detected by instruments.

II Felt by very few people.

3 III Felt by many, often mistaken for a passing vehicle.

IV Felt by many indoors, dishes and doors disturbed.

4 V Felt by nearly everyone. People awakened. Cracked walls, trees disturbed.

5 VI Felt by all. Many run outdoors. Furniture moves. Slight damage occurs.

VII Everyone runs outdoors. Poorly built buildings suffer severe damage. Slight

damage every where else.


6 VIII Everyone runs outdoors. Moderate to major damage. Minor damage to

specially designed buildings. Chimneys and walls collapse.

7 IX All buildings suffer major damage. Ground cracks, pipes break, foundations

shift.

X Major damage. Structures destroyed. Ground is badly cracked. Landslides

occur.

8 XI Almost all structures fall. Bridges wrecked. Very wide cracks in ground.

XII Total destruction. Ground surface waves seen. Objects thrown into the air.

All construction destroyed.


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Unlike the Richter Scale, it does not measure the absolute strength of the earthquake, but howstrongly it is felt at a particular place.

The Scale is useful in comparing damage from different quakes and in plotting damagepatterns from a given quake to figure out the factors that contribute to earthquake damage.

A map showing the Mercalli intensity at different locations for the same quake can be quiterevealing.

These maps can be related to geological maps to see what effect the underlying rocks have onthe intensity of the quake. They show that softer porous soils shake very violently, whilebedrock is less affected.

Intensity values can be correlated with other measures of ground motion, such asdisplacement, velocity and acceleration. For example, MMI 6 corresponds to a peak groundvelocity of about 50 mm/s.


Intensity is easy to use, even for historic earthquakes.

Other intensity scales are used in some countries.

1. The Rossi-Forel scale is relatively old, has values from 1 to 10, and is still used in somecountries.

2. The JMA (Japan Meteorological Agency) scale from 1 to 7 is used in Japan and Taiwan.3. The MSK scale is the most recent, has 12 values that approximate but are not the same as theModified Mercalli values, and is used extensively in Europe.

4. A Modified Mercalli Intensity of six is abbreviated as MMI 6.


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Intensity Variability•Maximum intensity normally occurs near the earthquake epicentre, withintensity values then decreasing with distance. An earthquake has a singlemagnitude, but intensity varies with distance.

•Many factors affect surface ground motion, including topography andnear-surface geology, especially soft surface sediments. These variations canbe considerable, even over short distances.

•It is common to find intensities ranging by ±1 unit in a neighborhood, and

not unusual to find values ±2 or more.


o e erca sca e: erca sca e mo e or ort mer can con t ons.


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•Benioff Zone: A dipping zone containing earthquake hypocentres

lying along the top of a subducting plate.

•Focal zone The rupture zone of an earthquake. In the case of a great

earth , the focal zone may extend several hundred kilometers in

length

•Subduction zone An elongated region along which a block of crusts descends

relative to another crustal block, for example, the descent of the Pacific plate beneaththe Andean late alon the Andean trench.


•Teleseism An earthquake that is distant from the recording station.

•Travel time The time required for a wave train to travel from its source to a point of

observation.


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Seismic Zones of India The varying geology at different locations in the country

implies that the likelihood of damaging earthquakes taking place at different locations

is different. Thus, a seismic zone map is required to identify these regions. Based on

the levels of intensities sustained during damaging past earthquakes, the 1970

version of the zone map subdivided India into five zones – I, II, III, IV and V. Themaximum Modified Mercalli (MM) intensity of seismic shaking expected in these zones

were V or less, VI , VII , VIII , and IX and higher , respectively. Parts of Himalayan

boundary in the north and northeast, and the Kachcha area in the west were classified

as zone V. The seismic zone maps are revised from time to time as more

understanding is gained on the geology, the seismotectonics and the seismic activity

in the country. The Indian Standards provided the first seismic zone map in 1962,which was later revised in1967 and again in 1970. The map has been revised again in


2002 (Figure 4), and it now has only four seismic zones – II, III, IV and V. The areas

falling in seismic zone I in the 1970 version of the map are merged with those of

seismic zone II. Also, the seismic zone map in the peninsular region has been

modified. Madras now comes in seismic zone III as against in zone II in the 1970

version of the map. This 2002 seismic zone map is not the final word on the seismic

hazard of the country, and hence there can be no senseof complacency in this

regard.


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Prominent Past Earthquakes in India

A number of significant earthquakes occurred in and around India over the past

century (Figure 2). Some of these occurred in populated and urbanized areas and

hence caused great damage. Many went unnoticed, as they occurred deepunder the Earth’s surface or in relatively un-inhabited places. Some of the

damaging and recent earthquakes are listed in Table 1. Most earthquakes occur

along the Himalayan plate boundary (these are inter-plate earthquakes), but a

number of earthquakes have also occurred in the peninsular region (these are

intra-plate earthquakes). Four Great earthquakes (M>8) occurred in a span of 53

years from 1897 to 1950; the January 2001 Bhuj earthquake (M7.7) is almost as


large. Each of these caused disasters, but also allowed us to learn about

earthquakes and to advance earthquake engineering. For instance, 1819 Cutch

Earthquake produced an unprecedented ~3m high uplift of the ground over 100km

(called Allah Bund ). The 1897 Assam Earthquake caused severe damage up to

500km radial distances; the type of damage sustained led to improvements in the

intensity scale from I-X to I-XII. Extensive liquefaction of the ground took place

over a length of 300km (called the Slump Belt ) during 1934 Bihar-Nepal

earthquake in which many structures went afloat.


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LIST OF SOME SIGNIFICANT EARTHQUAKES IN INDIA

SINCE 1991

DATE EPICENTRE LOCATION MAGNITUDE

Lat( Deg N ) Long( Deg E

)

1991 OCT 20 30.75 78.86 UTTARKASHI, UP HILLS 6.6

1993 SEP 30 18.07 76.62 LATUR-OSMANABAD, MAHARASHTRA 6.3

1997 MAY 22 23.08 80.06 JABALPUR,MP 6.0


1999 MAR 29 30.41 79.42 CHAMOLI DIST, UP 6.8

2001 JAN 26 23.40 70.28 BHUJ ,GUJRAT 6.9


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Seismic waves

The sudden movement of rocks along a fault causes vibrations that

transmit energy through the earth in the form of waves.Body waves- moves through the interior of the earth starting at thefocus They are known as body waves because they travel though thebody of a material in all directions and not just at the surface, as waterwaves do.

Surface waves-travel out from the epicenter

along the surface of the earth


wave type particle motion name

body waveslongitudinal P wave

transverse S wave

surface waves

horizontal transverse Love wave

vertical ellipticalRayleigh wave


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1.Body waves:-

(a) P-waves: Primary/ push & pull / Longitudinal Compression wave, particle moving back

and forth along the direction of wave propagation. Velocity 5 to 7 km/sec (fastest seismic waves)

in the crust (more than seven times the speed of sound).

k is the bulk modulus

µ is shear modulus

ρ is density of the elastic medium

They travels hrough fluids, and solids. They are compression waves and rely on the compressional

strength and elasticity of the materials to propagate. For P waves, the motion of the meterial particlesthat transmit the energy move parallel to the direction of propagation. P waves travel the same way


as sound waves in air. The transmission of compressional waves is due to the strong electronic

between atoms that get squeezed together too tightly.


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Body waves:-

(b) S-waves-Shear wave / secondary or transverse waves. Particle oscillating in the direction normal

to the direction of wave propagation Velocity 3 to 4 km/sec

µ is shear modulus

ρ is density of the elastic medium

S waves depends on the shear strength of the material. Imagine a very long and narrow block of Jello,

and then imagine shaking the end of it and then imagine shaking the end of it from side to side. A shear

wave will propagate down the long length of it. You shake it from side to side but the wave travels

forward and perpendicular to the direction of shaking. You can try this with a long spring or a Slinky

suspended from strings also. If you give it a sudden sideways deflection and a transverse or shear wavewill travel both lengths of the spring. Now try to imagine doing the same thing with water in a tank. No


shear wave will propagate through fluids because gases and fluids have no shear strength. They give

too easily. However, the strength of atomic bonds in solids allows them to transmit tranverse motions. S

waves do not travel as fast as P waves and have a velocity of about 3.5 km/s in the crust.


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Surface waves are very similar to ocean waves as they only occur at the

surface of the earth and do not penetrate into the interior deeply. There are two

types of surface waves: Love waves and Rayleigh waves. Typically, it thesurface waves that do the most damage during an earthquake, especially at

distances far from the epicenter. Most of the damage in the 1985 Mexico City

earthquake was from surface waves that had traveled over 200 kilometers from

the epicenter located near the west coast of Mexico. The velocity of surface

waves varies with their wavelength but always travel slower than P and S

waves.



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Buckled steel column

that

supports 6 stories

above it.

Compare the column

to the

adjacent masonry

Collapsed garage –

limited crossbracing

between columns

probably was a

significant factor

Corner of triangular

building

narrow corner has

little lateral

support


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Earthquake Engineering Design Factors

• Shape of building – square or rectangular buildings generally perform better

• Height of building – different heights shake at different frequencies and are most

affected by waves of different wavelengths

• Number and size of openings (e.g. windows, doors)

• Building materials: wood, steel, masonry, concrete, etc. (concrete is the most

widely used) more ductile materials (e.g. metals) perform better than less ductile

materials (e.g. unreinforced concrete)


• Importance of joints between structural elements and cross-bracing

• Risk from heavy roof materials

• Hazards from non-structural failure (e.g. falling glass, interior contents)


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1. Understand the hazard

• long-term prediction

• mapping

• damage scenarios

2. Make plans to deal with the hazard and reduce the risk to people

• Implement and enforce building codes, zoning, and land-use planning

which take into account the earthquake hazard so that new construction

will not add to the risk

• Retrofit (strengthen) or replace older, high-risk buildings, especially

Earthquake Preparation and Engineering


critical facilities (e.g. hospitals, fire stations, schools)

• Prepare/retrofit critical infrastructure (lifelines)

(e.g. water, energy, transportation, communication)

• Prepare and practice public emergency plans – include emergency

management personnel: fire, police, hospitals, etc.• Educate public about the hazard and what they should do

• Prepare yourself: e.g. food, water, light, heat, reducing the risk in your

home, personal emergency plan


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Precaution during Earthquakeswhen buildings collapse, the weight of the ceilings falling upon the

objects or furniture inside crushes these objects, leaving a space or voidnext to them.

• The larger the object, the stronger, the less it will compact. The lessthe

object compacts, the larger the void, the greater the probability that theperson who is using this void for safety will not be injured.

• TEN TIPS FOR EARTHQUAKE SAFETY

1. Most everyone who simply "ducks and covers" WHEN BUILDINGS

COLLAPSE are crushed to death. People who get under objects, likedesks or cars are crushed


2. It is a natural safety/survival instinct. You can survive in a smallervoid. Get next to an object, next to a sofa, get down next to a largebulky object that will compress slightly but leave a void next to it.

3. As Wood is flexible and moves with the force of the earthquake & evenIf the wooden building does collapse, large survival voids are created &

has less concentrated, crushing weight thus Wooden buildings are thesafest type of construction to be in during an earthquake... While Brickbuildings will break into individual bricks. Bricks will cause many injuriesbut less squashed bodies than concrete slabs.

Contd…


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Precaution during Earthquakes Contd..5.If you are in bed during the night and an earthquake occurs, simply rolloff the bed. A safe void will exist around the bed. lie down on the floor,next to the bottom of the bed during an earthquake.

6. If an earthquake happens and you cannot easily escape by getting out thedoor or window, then lie down and curl up in the fetal position next to asofa, or large chair ,or stand near the corner6. If you stand under a doorway and the doorjamb falls forward or backwardyou will be crushed by the ceiling above. If the door jam falls sideways youwill be cut in half by the doorway.7.Never go to the stairs. (they swing separately from the main part of thebuilding). The stairs and remainder of the building continuously bump into each

other until structural failure of the stairs takes place. Even if the building doesn'tcollapse, stay away from the stairs. Even if the stairs are not collapsed by the


ear qua e, ey may co apse a er w en over oa e y ee ng peop e. eyshould always be checked for safety, even when the rest of the building is notdamaged.8.It is much better to be near the outside of the building rather than theinterior. The farther inside you are from the outside perimeter of thebuilding the greater the probability that your escape route will be blocked Peopleinside of their vehicles are crushed when the road above falls in

an earthquake and crushes their vehicles you can survive by getting out andsitting or lying next to your vehicles.9.paper does not compact. Large voids are found surrounding stacks of paper so if possible stay near such place if you find near by like;newspaper offices and otheroffices with a lot of paper .

Earthquake engineering introduction

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