earthquake engineering unit-v

8/4/2019 Earthquake Engineering Unit-V

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UNIIT-V

EARTHQUAKE

ENGINEERING


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EARTHQUAKE ENGINEERING

Earthquake engineering deals with the study ofearthquake effects on people and their builtenvironment and device ways and means to reducethese effects.

Inputs from seismology are required to understandground motion characteristics and ground shakinghazards associated with the occurrence of moderate tolarge-sized earthquakes.

Earthquakes usually strike without warning.


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Earthquakes have been an integral component

of the geological evolution of planet Earth.

First attempt to study and document

earthquake effects dates back to 1755 after the

occurrence of Lisbon Earthquake in Portugal.

Scientific earthquake research is mainly a

product of 20th century.


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Earthquake mechanics.

Earthquake prediction and control.

The prompt detection of tsunamis

(seismic sea waves).

Earthquake resistant construction.

Seismic building code improvements.

RESEARCH ACTIVITY CENTERS ON

SEVERAL DIVERSIFIED TOPICS


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Land use zoning.

Earthquake risk and hazard perception.

Disaster preparedness.

Study of concern and fears of people who haveexperienced the effect of an earthquake.

Data from above investigations help to form anintegrated picture of most complex field of studythat is termed as Urban seismology.


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Seismology: The science of earthquakes and

related phenomena.

Seismology: The science of earthquakes plus thephysics ofearths interior (essentially with regard

to seismic wave propagation and the knowledge it

provides regarding the internal structure of

earth). Engineering Seismology: The science of elastic

waves or seismic waves:

a) Their origin (earthquake, explosion etc.),

b) Their propagation through the earths interior,

c) Their recording including the interpretation of

the records.


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Applied seismology

Seismic prospecting

Engineering seismology

Strong ground motion seismology

OTHERS BRANCHES OF SEISMOLOGY


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EFFECTS OF EARTHQUAKE

Earthquakes are capable of producing a number ofdirect and induced effects.

Direct effects: Fault displacements, tectonic uplift and

ground shaking are direct effects. Induced effects: Various types of ground failure,

tsunamis and fire represent principal induced effects.

Most large earthquakes generate multiple hazards,

and their impact is most pronounced when they strikeurban environments.


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CAUSES OF EARTHQUAKE

An earthquake is produced when the elastic strainenergy stored in the rocks is suddenly released.

A rock sample under slow varying stresses wouldexperience a change in shape and size or deform.

Under slow varying stresses the rocks behave in anelastic manner (Theory of elasticity is applied to studytheir behavior).

When the rocks deform, the strain energy is stored in

them like it is stored in a compressed spring.


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DEFORMARION OF EARTH CRUST

Microcracks are formed parallel to the axis of

maximum compression when brittle rocks aresubjected to triaxial compression.

This happens when applied stress reaches onethird to two third the fracture stress at a givenconfining pressure.

Following factors participate in the deformation ofearth crust /lithosphere:

Time over which the stresses are applied,

Rate at which the stresses are applied

Compressive and tensile strength of rocks.

Temperature, confining pressure and presenceof liquids at crustal depths.


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Brittle Deformation: When the applied stressesexceed the compressive or tensile strength ofrocks, the rocks fail by rupturing. This is alsocalled brittle deformation.

This sudden failure will produce a fracturesurface and stored elastic strain energy in therocks is suddenly released producing anearthquake


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TERMINOLOGY


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EARTHQUAKE: An earthquake is a sudden transientmotion or series of motions of the ground originating in a

limited region and spreading from there in all directions.This motion occurs on account of sudden release of elasticstrain energy stored along active geological features.

MACROSEISMIC EFFECTS: Effects of earthquake thatcan be observed on the large scale in the field without theaid of instruments.

INTENSITY: It refers to the degree of shaking at aspecified locality. It is rating assigned to a locality using adescriptive scale, with grades indicated by Romannumerals from I to XII.


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TELESEISM: An earthquake recorded by aseismograph at a great distance is termed teleseism.By international convention this distance is morethan 2000 kilometers from the epicenter.

MAGNITUDE: It is a measure of the size of anearthquake. It is a rating independent of the place

of observation. Normally It is calculated frommeasurement of amplitude of seismic wavesobserved on the seismograms.

MICROSEISMIC EFFECTS: These are smallscale effects observable with the help ofinstruments.


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ISOSEISMALS: Curves connectinglocalities where equal intensity is observed

in an earthquake. They are now morecommonly mapped as boundaries betweenregions of successive intensity ratings.

MEIZOSEISMAL AREA: The areawithin the isoseismal of highest intensity iscalled the meizoseismal area.


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HYPOCENTER OR FOCUS: The point of origin of elasticdisturbance constituting an earthquake. This point seems torepresent the position of the initial rupture.

EPICENTER: The point on the surface of the earth verticallyabove hypocenter is called an epicenter.

EPICENTRAL DISTANCE (

): It is the distance fromrecording station to the epicenter . It is expressed in terms ofangle subtended at the center of the earth between the tworadii joining the epicenter and station.

HYPOCENTRAL DISTANCE: The distance (d+

) fromhypocenter to the recording station is called hypocentraldistance.


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ORIGIN TIME: The onset time of anearthquake at the hypocenter.

TRANSIT OR TRAVEL TIME: The timebetween the origin time and the arrival time ofa given seismic wave at a specified point

(usually at a seismograph station).


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SEISMOGRAPH:An instrument that writes a permanentcontinuous record of the earths motion following anearthquake.

SEISMOGRAM: The time-wise record of the groundmotion written by a seismograph.

ACCELEROGRAPH: An instrument that records theground motion in terms of acceleration in the epicentralregion of strong shaking. It produces the time wise recordof ground acceleration at a specific locality.

ACCELEROGRAM: The ground motion record producedby an accelerograph.


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MICROEARTHQUAKES: Very smallearthquakes with magnitude less than threeon Richter scale are calledMicroearthquakes.

High gain high frequency ultra- sensitiveseismographs are used to monitor these forseismological and engineering applications.


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LITHOSPHERE: The outer rigid shell of the earth, situatedabove the asthenosphere and containing, the crust,continents and plates.

ASTHENOSPHERE: The world wide layer below thelithosphere which is marked by low seismic velocities andhigh seismic attenuation. It is a soft layer, probably molten.It may be site of convection.

PLATE: One of the dozen or more segments of thelithosphere that are internally rigid and moveindependently over the interior . The plates meet atconvergent boundaries and separate at divergentboundaries.

PLATE TECTONICS: Theory and study of plateformation, movement and destruction: the attempt toexplain seismicity, volcanism, mountain-building andpalemagnetic evidence in terms of plate motion.


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SEISMIC WAVES

The strain energy released from an earthquake sourcepropagates in the form of elastic waves also called

seismic waves.

These waves are recorded by the seismographs placed on

the surface of the earth.

The seismic waves propagate through the whole of theearth interior, or along its surface layers.


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Seismogram of an earthquake in EL Salvador as recorded near New York

City. P, S, SS, LR and LQ indicate the first arrival of different kinds of seismic

waves


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TYPES OF SEISMIC WAVES

The seismic waves are of two main types:

Body waves

Surface waves.

The body waves propagate through the interior or

body of the earth. These are of two types:i) Longitudinal waves or P-waves

ii) Transverse or S-waves.

The surface waves propagate along the surface of the

earth. These are also of two main typesi) Love waves (LQ)

ii) Rayleigh (LR) waves.


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The total sequence of seismic waves istermed the wave train.

The beginning of each new burst of energyis called a phase.

The amplitude is one half the trace height.

The time between the first wave arrival andthe drop off to background noise is called

earthquake duration or coda length.


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LONGITUDINAL WAVES OR P-WAVES

Possess longitudinal motion similar to soundwaves.

Propagate through all material types (solid,liquids and gases).

First to arrive at a recording station (called

primary waves).

Create a push and pull effect in materials throughwhich they propagate.

Can be perceived, if the earthquake is largeenough, as a sudden shaking emanating frombelow the surface.


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TRANSVERSE WAVES OR S-WAVES

These are similar to electromagnetic waves.

Vertical (SV) and horizontal (SH) components atright angles to the direction of wave propagation.

Propagate only through solid substances becausegases can not be sheared.

Shear and twist crustal material.

Often more damaging to the works of construction

than the P-waves.


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P WAVES AND S WAVES


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SURFACE WAVES

Generally having 10-20 sec wave periods, 20-80 kmwavelengths and 3 km/ sec velocities.

Love waves contain substantial energy but their long

periods have smoothening effect on the imparted

ground motion. This greatly reducing their damagepotential.

Wave amplitudes are largest at the surface and

decrease with depth. The largest amplitudes are associated with shallow

focus earthquakes.


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LOVE WAVES

Love waves result from the interaction of SH-waveswith the earths surface layer.

They are similar to SH waves.

Ground motion is in the horizontal plane, andparticles vibrate at right angle to the direction of wavepropagation. These waves cannot pass through waterbodies.

Usually faster than Rayleigh waves.


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RAYLEIGH WAVES

These waves introduce a retrograde elliptical motion

similar to ocean waves.


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LOVE WAVES AND RAYLEIGH WAVES


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PLATE TECTONICS

According to the theory of plate tectonics, the

lithosphere is broken into a number of moderately rigidplates. The plates slide over a partially molten plastic

asthenosphere.

According to the relative motions of adjacent plates wecan define three kinds of plate boundaries:

(i ) Divergent Plate Boundaries or Spreading Centers:

Typically Mid-Ocean Ridges.

(ii ) Convergent Plate Boundaries: Typically Subduction

Zones.

(iii) Transform plate boundaries: Typically SA Fault.


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Along divergent plate boundaries plates separate.

In the process of plate separation, partially molten mantlematerial upwells along linear ocean ridges and new

lithosphere is created along the trailing edges of thediverging plates.

Along convergent plate boundaries the leading edge of oneplate overrides another plate. The overridden plate issubducted into the mantle where lithosphere is resorbed.

This subduction process produces deep-sea trenches,shallow intermediate and deep focus earthquakes,adjacent mountain ranges, and both basaltic and andesiticvolcanism.

Along transform plate boundaries plates slide past each

other, with neither creation nor destruction of lithosphere.Occasionally marked by scarps, transform plateboundaries are characterized by shallow focusearthquakes with horizontal slips.


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Map of major and some minor tectonic plates. Arrows indicate the relative

direction of motion of tectonic plates (after Bolt, 1978 ).


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Selected global earthquake locations from 1977 to 1994 showing that

earthquakes occur along well defined belts of seismicity


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Map showing plate boundaries that are delineated based on the

earthquake locations given in previous slide


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FROM THE STUDY OF GLOBAL DISTRIBUTION OF EARTHQUAKESFOLLOWING IMPORTANT CONCLUSIONS CAN BE DRAWN:

(1) Earthquakes are global but their present geographical distribution isstructured and follows defined patterns. There are extensive aseismicregions and belts of high seismicity.

(2) Earthquakes occur in continents as well as in oceans, and clusterstrongly in both space and time.

(3) The earthquakes occur up to 700 km depth.

(4) Major and great earthquakes (where field observations are possible)are usually accompanied by ground deformation,

fault rupture, subsidence and surface uplift.

(5) Tsunamis are large sea waves and are caused by large submarineearthquakes.


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EARTHQUAKE INTENSITY AND

INTENSITY SCALESI

Earthquake Intensity represents macroseismic effectsof an earthquake on humans and their products andfeatures on the earth surface at some locality.

Intensity is determined by observations of themacroseismicity effects of earthquakes in the field.

Intensity is a measure of the severity of an

earthquake.


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Magnitude of the earthquake

Distance from the Epicenter

Focal depth of the earthquake

Geologic and soil conditions of the locality

Type of construction ( age and workmanship)

Expertise of the observer

FACTORS EFFECTING EARTHQUAKE INTENSITY


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ISOSEISMAL MAPS

A map is constructed to show the variation ofintensity with distance from epicenter for a

particular earthquake. The procedure is to plot the intensity values at

their respective localities on a base map. Thelines connecting localities having equal value

of intensity are called isoseismals. The map is termed the isoseismal map.


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EARTHQUAKE MAGNITUDE C. F. Richter introduced the magnitude scale in 1930s,

while issuing the listing of earthquakes in Southern

California.

Richter started with two assumptions:

Two earthquakes having the same hypocenter and recorded

at the same station, the larger earthquake will have largeramplitude. If the hypocenters differ, the smaller earthquakemay occur closer to a given station and have largeramplitude.

If similar Seismographs placed at varying distances areused to record these two earthquakes; and a plot betweenmaximum ground motion (Y-axis) and epicentral distances(X-axis) is made, we should get two curves for twoearthquakes. The higher curve represents the larger

earthquake.


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EARTHQUAKE GROUND MOTION

The ground motion due to seismic waves can be

divided into two general levels:

Small amplitude motion from distant or smallearthquakes.

Large amplitude motion due to nearby or largeearthquakes.

Ground motion at a particular site is influenced bythree main factors:

Characteristics of source of seismic wavesPath through which the seismic waves travel

Local geological structure below the recording site


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The estimation of large amplitude ground motion,called the strong ground motion, is of interest toengineers and those concerned with mitigating theeffects of earthquakes on society. Strong motionaccelerographs, used to record strong ground motion,provide records of acceleration time history.

The peak of the maximum acceleration, normallyexpressed in terms of acceleration due to gravity, ismeasured in three orthogonal directions from theserecords.

These records are uncorrected and needs someprocessing for the purpose of removing ambient noiseand instrument response.

SEISMIC ZONING MAP


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SEISMIC ZONING MAP

A seismic zoning map specifies the level of forces or

motions for earthquake resistant design. Such a mapdiffers from seismicity map that provides informationabout the occurrence of earthquakes.

To apply building codes in regions having differingseismicity and earthquake potential, it is necessary toprepare seismic zoning map indicating areas ofprobable maximum earthquake ground motion interms of peak acceleration or peak velocities or any

other measure of strong shaking.

S S C O G O


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SEISMIC ZONING MAP OF

INDIA

In order to evaluate strong ground motioncharacteristics and basic seismic coefficients indifferent parts of the country for earthquakeresistant design of ordinary structures the firstIndian standard seismic zoning map waspublished in 1962 (IS:1893-1962).

This map was prepared based on availableearthquake intensity data and locations ofmoderate, large and great earthquakes.


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The correlation between probable maximum magnitudeand corresponding MM intensities in each of the seismic

zones of 1970 IS map are as follows: Seismic zone I II III IV V

Probable maximum 7

magnitude

Likely maximum


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2002: GENERAL PROVISIONS AND BUILDINGS)


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The seismic zoning map (Fig.10) is the latest Indian StandardCriteria for Earthquake Resistant design of Structures

(IS:1893-(Part I) 2002: General Provisions and Buildings)that has been revised recently and indicates four seismiczones. In this map the zones I and II of earlier zoning maphave been merged together. The zone factor, Z for these zonesis given below:

Seismic Zone II III IV V

Seismic Intensity Low Moderate Severe Very Severe

Z 0.10 0.16 0.24 0.36

From above seismic zoning maps the design seismic forces are computedon the basis of importance of the structure and its soil foundation system.

earthquake engineering unit-v

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