79970662 Earthquake Resistant Design

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EARTHQUAKE RESISTANT DESIGN

DISSERTATION IN ARCHITECTURE

2010-2011

Coordinators :

Mr. Suptendu. P. Biswas

Mr. Leon Morenas

Submitted by

Aman Bhadauria 28/SSAA/B.ARCH/2006

SUSHANT SCHOOL OF ART AND ARCHITECTURE


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SUSHANT SCHOOL OF ART AND ARCHITECTURESECTOR-55, GURGAON-122 003, HARYANA

BONAFIDE CERTIFICATE

Dated: May 3rd 2011

Dissertation Title: Earthquake Resistant Design.

This following study is hereby approved as a creditable work on the approved

subject, carried out and presented in a manner sufficiently satisfactory to

warrant acceptance as a pre requisite to the degree for which it has been

submitted.

It is understood by this approval that the undersigned does not necessarily

endorse or approve any statement made, opinion expressed or conclusion drawn

therein but approves the study only for the purpose for which it had been

submitted and satisfies as per the requirements laid down by the seminar

committee.

Student: Aman Bhadauria

0371691606/SSAA/B.Arch./2006-11


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Table of ContentsIntroduction ......................................................................................................................................4

Design Philosophy ofEarthquake Resistant Structures .................................................................... 7

Design Philosophy .......................................................................................................................... 8

ImpactofEarthquakes ...................................................................................................................... 9

EffectsofEarthquakes .................................................................................................................... 10

Direct Effects ............................................................................................................................... 10

Indirect Effects ............................................................................................................................. 11

How Architecturalfeatures affects Buildingsduring Earthquakes ?................................................ 13

Importance of Architectural features ........................................................................................... 14

Size of the Buildings .............................................................................................................. 16

Construction Materials ................................................................................................................... 19

Brittle and Ductile Building Materials ........................................................................................... 20

Masonry ................................................................................................................................ 20

Concrete ............................................................................................................................... 20

Steel ...................................................................................................................................... 21

Earthquake Construction Typologies ............................................................................................ 22

Howtoreduce Earthquakeeffectson Buildings.............................................................................. 24

Base Isolation .............................................................................................................................. 25

Guidelinesforthe Earthquakeresistant Buildings .......................................................................... 27

The Seismic zonesinIndia............................................................................................................... 29

Basic Geography and Tectonic features ....................................................................................... 30

Prominent past Earthquakes in India ........................................................................................... 31

Indianstandardson Earthquake Engineering.................................................................................. 34

The Affectsofthe Earthquakeonthe Reinforcedconcrete Buildings............................................. 43

Roles of Floor slabs and Masonry walls ........................................................................................ 44

Howtomakestone Masonry Buildings Earthquakeresistant? ..................................................... 44

Howtoreduce Earthquake Effectson Buildings? .......................................................................... 49

Various Methods ......................................................................................................................... 51


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INTRODUCTION

Earthquakes are major geological phenomena. Man has been terrified of

this phenomenon for ages, as little has been known about the causes of

earthquakes, but it leaves behind a trail of destruction. There are

hundreds of small earthquakes around the world every day. Some of

them are so minor that humans cannot feel them, but seismographs and

other sensitive machines can record them. Earthquakes occur when

tectonic plates move and rub against each other. Sometimes, due to this

movement, they snap and rebound to their original position. This might

cause a large earthquakes as the tectonic plates try to settle down. This is

known as the Elastic Rebound Theory.

Every year, earthquakes take the lives of thousands of people, and

destroy property worth billions. The 2010 Haiti Earthquake killed over

1,50,000 people and destroyed entire cities and villages. Designing

Earthquake Resistant Structures is indispensable. It is imperative that

structures are designed to resist earthquake forces, in order to reduce the

loss of life. The science ofEarthquake Engineering and Structural Design

has improved tremendously, and thus, today, we can design safe

structures which can safely withstand earthquakes of reasonable

magnitude.

The most destructive of all earthquake hazards is caused by seismic

waves reaching the ground surface at places where human-built

structures, such as buildings and bridges, are located. When seismic

waves reach the surface of the earth at such places, they give rise to

what is known as strong ground motion. Strong ground motions causes

buildings and other structures to move and shake in a variety of complex

ways.


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Many buildings cannot withstand this movement and suffer damages of

various kinds and degrees. Most deaths, injuries, damages and economic

losses caused by earthquake result from ground motion acting on

buildings and other manmade structures not capable of withstanding such

movement.

Experience in past earthquakes has demonstrated that many common

buildings and typical methods of construction lack basic resistance to

earthquake forces. In most cases this resistance can be achieved by

following simple, inexpensive principles of good building construction

practice. Adherence to these simple rules will not prevent all damage in

moderate or large earthquakes, but life threatening collapses should be

prevented, and damage limited to repairable proportions. These principles

fall into several broad categories:

y Planning and layout of the building involving consideration of thelocation of rooms and walls, openings such as doors and

windows, the number of storeys, etc. At this stage, site and

foundation aspects should also be considered.

y Lay out and general design of the structural framing system withspecial attention to furnishing lateral resistance, and

y Consideration of highly loaded and critical sections with provisionof reinforcement as required.

Earthquakes cause massive vibrations in the Earths crust. This can cause

a number of problems in the ground, which in turn becomes a hazard to

all life and property. The effect depends on the geology of soil and

topography of the land.


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For categorising the buildings with the purpose of achieving seismicresistance at economical cost, three parameters turn out to be significant:

1. Seismic intensity zone where the building is located,2. How important the building is,3. How stiff is the foundation soil.

A combination of these parameters will determine the extent ofappropriate seismic strengthening of the building.

The importance of the building should be a factor in grading it forstrengthening purposes. Therefore, following is the classification of thebuildings.

IMPORTANT: Hospitals, clinics, communication buildings, fire and policestations, water supply facilities, meeting halls, schools, dormitories,cultural treasures such as museums, monuments and temples, etc.

ORDINARY: Housings, hostels, offices, warehouses, factories, etc.Severity of ground shaking at a given location during an earthquake canbe minor, moderate and strong. Thus relatively speaking, minor shakingoccurs frequently; moderate shaking occasionally and strong shakingrarely. For instance, on average annually about 800 earthquakes ofmagnitude 5.0-5.9 occur in the world while about 18 for magnitude range7.0-7.9. So we should design and construct a building to resist that rareearthquake shaking that may come only once in 500 years or even once


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in 2000 years, even though the life of the building may be 50 or 100years

Design Philosophy ofEarthquake

ResistantStructures

Engineers do not attempt to make earthquake proof buildings that will notget damaged even during the rare but strong earthquake; such buildingswill be too robust and also too expensive. Instead the engineeringintention is to make buildings earthquake-resistant; such buildings resistthe effects of ground shaking, although they may get damaged severelybut would not collapse during the strong earthquake. Thus, safety ofpeople and contents is assured in earthquake-resistant buildings, andthereby a disaster is avoided. This is a major objective of seismic design

codes throughout the world.


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Design Philosophy1

1. Under minor but frequent shaking, the main members of the buildingsthat carry vertical and horizontal forces should not be damaged;

however buildings parts that do not carry load may sustain repairabledamage.

2. Under moderate but occasional shaking, the main members maysustain repairable damage, while the other parts that do not carry

load may sustain repairable damage.

3. Under strong but rare shaking, the main members may sustain severedamage, but the building should not collapse.

1Naeim, F., Ed., (2001), The Seismic Design Handbook, Kluwer Academic Publishers, Boston, USA.


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Earthquake resistant design is therefore concerned about ensuring thatthe damages in buildings during earthquakes are of acceptable variety,and also that they occur at the right places and in right amounts. Thisapproach of earthquake resistant design is much like the use of electricalfuses in houses; to protect the entire electrical wiring and appliances inthe house, you sacrifice some small parts of electrical circuit, called fuses;these fuses are easily replaced after the electrical over-current. Likewiseto save the building from collapsing you need to allow some pre-determined parts to undergo the acceptable type and level of damage.

Earthquake resistant buildings, particularly their main elements, need tobe built with ductility in them. Such buildings have the ability to swayback-and-forth during an earthquake, and to withstand the earthquakeeffects with some damage, but without collapse.

ImpactofEarthquakes

Earthquakes do not kill people, but buildings do. We are heavilydependent upon the civic amenities or life-lines like water supply, electricpower supply, drainage. Earthquake can disturb civic amenities in a majorway. Lifeline like hospitals, health care centers have major role in naturalcatastrophe like earthquake. Hence additional care while designing thesestructures is needed. A severe earthquake can have very damagingconsequences upon a regions development and economy.

Its has its impacts on

y Lifeline and societyy Affects a Large number ofPeople.y Losses to Lives, Livelihoods, Property.y Civic amenitiesy Heritagey Loss of housing.y Damage to infrastructurey Disruption of transport and communication.y Disruption of marketing systems.y Breakdown of social order.y Loss of business.y Loss of industrial output.Among all disasters that can take place, earthquake has the maximumloss of life and limbs. Tremendous loss of property, especially buildingsis caused, leaving a large mass of population shelter less. Buildings asbadly damaged as this require demolition.


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Our heritage connects us with our ancestors and gives a sense of prideand belongings. The new structures can be often rebuilt but the loss ofheritage is a huge loss. Since the reconstruction is difficult as well asthe very sense of it being built historically is lost forever.

The healthcare center where everyone looks for healing, itself lookingfor health touch is the sad scene during earthquakes. These type offacility needs to be given extra level of earthquake protection. Sincehealthcare buildings have to play a major role in case of catastrophe,additional care is needed in their design. Seismic code provisionsrequire these buildings to be designed for higher levels of earthquakeloads.

EffectsofEarthquakes

In a comprehensive design approach, it should be recognized thatdamage to structures and facilities may result from different seismiceffects. These effects can be classified as Direct and Indirect (orConsequential) as follows:

y Direct Effects:1.Ground failures (or instabilities due to ground failures)

Surface faulting surface or fault rupture).

Vibration of soil (or effects of seismic waves).Ground cracking.

a. Liquefaction.I.

Ground lurching.b.

Differential settlement.c.

Lateral spreading.d.

Landslides.

2. Vibrations transmitted from the ground to the structure.


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y Indirect Effects (or Consequential Phenomena):(3) Tsunamis

(4) Seiches

(5) Landslides

(6) Floods

(7) Fires

The seismic effect or damage that usually concerns the Architect, and

which is taken into account by code seismic-resistant design provisions, isthe vibration of the structure in response to ground shaking at itsfoundation. Although damage due to other effects may exceed that dueto vibration, procedures for gauging the probability of these effects andfor coping with them are outside the scope of the structural engineeringdiscipline and so are usually not included in seismic-resistant codes.Nonetheless, the structural engineer should be aware of the differentseismic hazards and should advise the client of potential damage involvedin locating structures at certain sites. Thus the first step in the designprocedure of a future structure should be the analysis of the suitability ofthe site selected with proper consideration for the potential of any one of

the above types of damage.

The effects of earthquakes include, but are not limited to, the following:

y Shaking and ground ruptureShaking and ground rupture are the main effects created byearthquakes, principally resulting in more or less severe damage tobuildings and other rigid structures. The severity of the local effectsdepends on the complex combination of the earthquake magnitude,

the distance from the epicenter, and the local geological andgeomorphologic conditions, which may amplify or reduce wavepropagation. The ground-shaking is measured by ground acceleration.

Specific local geological, geomorphologic, and geostructural featurescan induce high levels of shaking on the ground surface even from low-intensity earthquakes. This effect is called site or local amplification. Itis principally due to the transfer of the seismic motion from hard deep


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soils to soft superficial soils and to effects of seismic energyfocalization owing to typical geometrical setting of the deposits.

Ground rupture is a visible breaking and displacement of the Earth'ssurface along the trace of the fault, which may be of the order ofseveral metres in the case of major earthquakes. Ground rupture is amajor risk for large engineering structures such as dams, bridges andnuclear power stations and requires careful mapping of existing faultsto identify any likely to break the ground surface within the life of thestructure.2

y Landslides and avalanchesEarthquakes, along with severe storms, volcanic activity, and coastal

wave attack, and wildfires, can produce slope instability leading tolandslides, a major geological hazard. Landslide danger may persistwhile emergency personnel are attempting rescue.

y FiresEarthquakes can cause fires by damaging electrical power or gas lines.In the event of water mains rupturing and a loss of pressure, it may

also become difficult to stop the spread of a fire once it has started.

y Soil liquefactionSoil liquefaction occurs when, because of the shaking, water-saturatedgranular material (such as sand) temporarily loses its strength andtransforms from a solid to a liquid. Soil liquefaction may cause rigidstructures, like buildings and bridges, to tilt or sink into the liquefieddeposits. This can be a devastating effect of earthquakes.

y TsunamiTsunamis are long-wavelength, long-period sea waves produced by thesudden or abrupt movement of large volumes of water. In the openocean the distance between wave crests can surpass 100 kilometers(62 miles), and the wave periods can vary from five minutes to one

2http://nisee.berkeley.edu/bertero/html/earthquake-resistant_construction.html


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hour. Such tsunamis travel 600-800 kilometers per hour (373497miles per hour), depending on water depth. Large waves produced byan earthquake or a submarine landslide can overrun nearby coastalareas in a matter of minutes. Tsunamis can also travel thousands ofkilometers across Open Ocean and wreak destruction on far shoreshours after the earthquake that generated them.

Ordinarily, subduction earthquakes under magnitude 7.5 on the Richterscale do not cause tsunamis, although some instances of this havebeen recorded. Most destructive tsunamis are caused by earthquakesof magnitude 7.5 or more.

y Floods

A flood is an overflow of any amount of water that reaches land.[34]Floods occur usually when the volume of water within a body of water,such as a river or lake, exceeds the total capacity of the formation, andas a result some of the water flows or sits outside of the normalperimeter of the body. However, floods may be secondary effects ofearthquakes, if dams are damaged. Earthquakes may cause landslipsto dam rivers, which then collapse and cause floods.

yHuman impacts

Earthquakes may lead to disease, lack of basic necessities, loss of life,higher insurance premiums, general property damage, road and bridgedamage, and collapse or destabilization (potentially leading to futurecollapse) of buildings. Earthquakes can also precede volcaniceruptions, which cause further problems.

HOW ARCHITECTURAL FEATURESAFFECTS BUILDINGS DURING

EARTHQUAKES?

Importance of Architectural features:


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The behaviour of a building during earthquakes depends critically on itsoverall shape, size and geometry, in addition to how the earthquakeforces are carried to the ground. Hence, at the planning stage itself,architects and structural engineers must work together to ensure that theunfavourable features are avoided and a good building configuration ischosen. The importance of the configuration of a building was aptlysummarised by Late Henry Degenkolb, a noted Earthquake Engineer ofUSA, as:

If we have a poor configuration to start with, all the engineer can do is to

provide a band-aid - improve a basically poor solution as best as he can.

Conversely, if we start-off with a good configuration and reasonable framing

system, even a poor engineer cannot harm its ultimate performance too

much.

Architectural Features:

A desire to create an aesthetic and functionally efficient structure drivesarchitects to conceive wonderful and imaginative structures. Sometimesthe shapeof the building catches the eye of the visitor, sometimes thestructural systemappeals, and in other occasions both shapeandstructural systemwork together to make the structure a marvel.However, each of these choices of shapes and structure has significant

bearing on the performance of the building during strong earthquakes.The wide range of structural damages observed during past earthquakesacross the world is very educative in identifying structural configurationsThat is desirable versus those which must be avoided.3

Size of Buildings:

In tall buildings with large height-to-base size ratio (Following figure a),the horizontal movement of the floors during ground shaking is large. In

short but very long buildings (Following figure b), the damaging effectsduring earthquake shaking are many. And, in buildings with large planarea like warehouses (Figure c); the horizontal seismic forces can beexcessive to be carried by columns and walls.

3Arnold., and Reitherman,R., (1982), Building Configuration and Seismic Design, John Wiley, USA.


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Horizontal Layout of Buildings:

In general, buildings with simple geometry in plan (Following figure a)have performed well during strong earthquakes. Buildings with re-entrantcorners, like those U, V, H and + shaped in plan (Figure b), havesustained significant damage. Many times, the bad effects of theseinterior corners in the plan of buildings are avoided by making the

buildings in two parts. For example, an L-shaped plan can be broken upinto two rectangular plan shapes using a separation joint at the junction(Figure c). Often, the plan is simple, but the columns/walls are notequally distributed in plan. Buildings with such features tend to twistduring earthquake shaking.4

4Lagorio,H,J, (1990), EARTHQUAKES An Architects Guide to Non- Structural Seismic Hazard, John Wiley & Sons, Inc., USA.


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Vertical Layout of Buildings:

The earthquake forces developed at different floor levels in a buildingneed to be brought down along the height to the ground by the shortestpath; any deviation or discontinuity in this load transfer path results in

poor performance of the building. Buildings with vertical setbacks (like thehotel buildings with a few storeys wider than the rest) cause a suddenjump in earthquake forces at the level of discontinuity (Following figurea). Buildings that have fewer columns or walls in a particular storey orwith unusually tall storey (Figure b), tend to damage or collapse which isinitiated in that storey. Many buildings with an open ground storeyintended for parking collapsed or were severely damaged in Gujaratduring the 2001 Bhuj earthquake. Buildings on sloppy ground have


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unequal height columns along the slope, which causes ill effects liketwisting and damage in shorter columns (Figure c). Buildings withcolumns that hang or float on beams at an intermediate storey and do notgo all the way to the foundation, have discontinuities in the load transferpath (Figure d). Some buildings have reinforced concrete walls to carrythe earthquake loads to the foundation. Buildings, in which these walls donot go all the way to the ground but stop at an upper level, are liable toget severely damaged during earthquakes.5

5Arnold,C., and Reitherman,R., (1982), Building Configuration and Seismic Design, John Wiley, USA.


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Adjacency of Buildings:

When two buildings are too close to each other, they may pound on eachother during strong shaking. With increase in building height, this collisioncan be a greater problem. When building heights do not match (Figure 4),the roof of the shorter building may pound at the mid-height of thecolumn of the taller one; this can be very dangerous.

Building Design and Codes:

Looking ahead, of course, one will continue to make buildings interestingrather than monotonous. However, this need not be done at the cost ofpoor behaviour and earthquake safety of buildings. Architectural featuresthat are detrimental to earthquake response of buildings should beavoided. If not, they must be minimised. When irregular features areincluded in buildings, a considerably higher level of engineering effort isrequired in the structural design and yet the building may not be as goodas one with simple architectural features. Decisions made at the planningstage on building configuration are more important, or are known to havemade greater difference, than accurate determination of code specifieddesign forces.


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CONSTRUCTION MATERIALS:

In India, most non-urban buildings are made in masonry. In the plains,masonry is generally made of burnt clay bricks and cement mortar.However in hilly areas, stone masonry with mud mortar is moreprevalent. But now a day we are very familiar with R.C.C. buildings,and a variety of new composite constructions materials.

Brittle and Ductile Building Materials

I. Masonry

Masonry is made up of burnt clay bricks and cement or mud mortar.

Masonry can carry loads that cause compression (i.e. pressing together)but can hardly take load that causes tension (i.e. pulling apart). Masonryis a brittle material, these walls develop cracks once their ability to carryhorizontal load is exceeded. Thus infill walls act like sacrificial fuses inbuildings: they develop cracks under severe ground shaking but theyshare the load of the beams and columns until cracking.


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II. Concrete

Concrete is another material that has been popularly used in buildingconstruction particularly over the last four decades. Cement concrete ismade of crushed stone pieces (called aggregate), sand, cement and water

mixed in appropriate proportions. Concrete is much stronger thanmasonry under compressive loads, but again its behavior in tension ispoor. The properties of concrete critically depend on the amount of waterused in making concrete, too much and too little water both can causehavoc.

III. Steel

Steel is used in masonry and concrete buildings as reinforcement bars ofdiameter ranging from 6mm to 40mm. reinforcing steel can carry bothtensile and compressive loads. Moreover steel is a ductile material. Thisimportant property of ductility enables steel bars to undergo largeelongation before breaking. Concrete is used with steel reinforcementbars. This composite material is called as reinforced cement concrete. Theamount and location of steel in a member should be such that the failureof the member is by steel reaching its strength in tension before concretereaches its strength in compression. This type of failure is ductile failure,and is preferred over a failure where concrete fails first in compression.Therefore, providing more steel in R.C.C. buildings can be harmful even

EARTHQUAKE CONSTRUCTION TYPOLOGIES:

Earthquake construction means implementation of seismic design toenable building and non-building structures to live through the anticipatedearthquake exposure up to the expectations and in compliance with theapplicable building codes.

Design and construction are intimately related. To achieve a goodworkmanship, detailing of the members and their connections should be,possibly, simple. As any construction in general, earthquake construction

is a process that consists of the building, retrofitting or assembling ofinfrastructure given the construction materials available. The destabilizingaction of an earthquake on constructions may be direct (seismic motion ofthe ground) or indirect (earthquake-induced landslides, soil liquefactionand waves of tsunami).

A structure might have all the appearances of stability, yet offer nothingbut danger when an earthquake occurs. The crucial fact is that, for safety,


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earthquake-resistant construction techniques are as important as qualitycontrol and using correct materials.

To minimize possible losses, construction process should be organizedwith keeping in mind that earthquake may strike any time prior to theend of construction. Each construction project requires a qualified team ofprofessionals who understand the basic features of seismic performanceof different structures as well as construction management.

Adobe structures

One half of the world's population lives or works in the buildings made ofearth. Adobe type of mud bricks is one of the oldest and most widely usedbuilding materials. The use of adobe is very common in some of theworld's most hazard-prone regions, traditionally across Latin America,Africa, Indian subcontinent and other parts of Asia, Middle East and

Southern Europe.

Adobe buildings are considered very vulnerable at strong quakes.However, multiple ways of seismic strengthening of new and existingadobe buildings are available.

Key factors for the improved seismic performance of adobe constructionare:

y Quality of construction.y Compact, box-type layout.y Seismic reinforcement.

Limestone and sandstone structures

Limestone is very common in architecture. Many landmarks across theworld, including the pyramids in Egypt, are made of limestone. Manymedieval churches and castles in Europe are made of limestone andsandstone masonry. They are the long-lasting materials but their rather

heavy weight is not beneficial for adequate seismic performance.

Application of modern technology to seismic retrofitting can enhance thesurvivability of unreinforced masonry structures.


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Timber frame structures

Timber framing dates back thousands of years, and has been used inmany parts of the world during various periods such as ancient Japan,Europe and medieval England in localities where timber was in good

supply and building stone and the skills to work it were not.

The use of timber framing in buildings provides their complete skeletalframing which offers some structural benefits as the timber frame, ifproperly engineered, lends it to better seismicsurvivability.

Light-frame structures

Light-frame structures usually gain seismic resistance from rigid plywood

shear walls and wood structural panel diaphragms. Special provisions forseismic load-resisting systems for all engineered wood structures requiresconsideration of diaphragm ratios, horizontal and vertical diaphragmshears, and connector/fastener values. In addition, collectors, or dragstruts, to distribute shear along a diaphragm length are required.

Reinforced masonry structures

A construction system is used where steel reinforcement is embedded inthe mortar joints of masonry or placed in holes and after filled withconcrete or grout is called reinforced masonry.

There are various practices and techniques to achieve reinforcedmasonry. The most common type is the reinforced hollow unit masonry.The effectiveness of both vertical and horizontal reinforcement stronglydepends on the type and quality of the masonry, i.e. masonry units andmortar.

To achieve a ductile behavior of masonry, it is necessary that the shearstrength of the wall is greater than the tensile strength of reinforcementto ensure a kind of bending failure.

Reinforced concrete structures

Reinforced concrete is concrete in which steel reinforcement bars (rebars)or fibres have been incorporated to strengthen a material that would


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otherwise be brittle. It can be used to produce beams, columns, floors orbridges.

Prestressed concrete is a kind of reinforced concrete used for overcomingconcrete's natural weakness in tension. It can be applied to beams, floorsor bridges with a longer span than is practical with ordinary reinforcedconcrete. Prestressing tendons (generally of high tensile steel cable orrods) are used to provide a clamping load which produces a compressivestress that offsets the tensile stress that the concrete compressionmember would, otherwise, experience due to a bending load.

To prevent catastrophic collapse in response earth shaking (in the interestof life safety), a traditional reinforced concrete frame should have ductilejoints. Depending upon the methods used and the imposed seismicforces, such buildings may be immediately usable, require extensiverepair, or may have to be demolished.

Prestressed structures

Prestressed structure is the one whose overall integrity, stability andsecurity depend, primarily, on aprestressing. Prestressing means theintentional creation of permanent stresses in a structure for the purposeof improving its performance under various service conditions. Naturallypre-compression is used in the exterior wall ofColosseum, Rome.

There are the following basic types of prestressing:

y Pre-compression (mostly, with the own weight of a structure)y Pretensioning with high-strength embedded tendonsy Post-tensioning with high-strength bonded or unbonded tendons

Today, the concept of prestressed structure is widely engaged in design ofbuildings, underground structures, TV towers, power stations, floatingstorage and offshore facilities, nuclear reactor vessels, and numerouskinds of bridge system.A beneficial idea ofprestressing was, apparently,familiar to the ancient Rome architects; look, e.g., at the tall attic wall of

Colosseum working as a press for the wall piers beneath.


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Steel structures

Steel structures are considered mostly earthquake resistant but theirresistance should never be taken for granted. A great number of weldedsteel moment frame buildings, which looked earthquake-proof,

surprisingly experienced brittle behavior and were hazardously damagedin the 1994 Northridge earthquake. After that, the Federal EmergencyManagement Agency (FEMA) initiated development of repair techniquesand new design approaches to minimize damage to steel moment framebuildings in future earthquakes.

For structural steel seismic design based on Load and Resistance FactorDesign (LRFD) approach, it is very important to assess ability of astructure to develop and maintain its bearing resistance in the inelasticrange. A measure of this ability is ductility, which may be observed in amaterial itself, in a structural element, or to a whole structure.

All pre-qualified connection details and design methods contained in thebuilding codes of that time have been rescinded. The new provisionsstipulated that new designs be substantiated by testing or by use of test-verified calculations.6

HOW TO REDUCE EARTHQUAKE

EFFECTS ON BUILDINGS:

Why Earthquake Effects are to be reduced?

Conventional seismic design attempts to make buildings that do notcollapse under strong earthquake shaking, but may sustain damage tonon-structural elements (like glass facades) and to some structuralmembers in the building. This may render the building non-functionalafter the earthquake, which may be problematic in some structures, likehospitals, which need to remain functional in the aftermath of theearthquake. Special techniques are required to design buildings such thatthey remain practically undamaged even in a severe earthquake.Buildings with such improved seismic performance usually cost more thannormal buildings do. However, this cost is justified through improvedearthquake performance. Two basic technologies are used to protect

6http://www.hazardmapping.com/


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buildings from damaging earthquake effects. These are Base IsolationDevices and Seismic Dampers. The idea behind base isolation is to detach(isolate) the building from the ground in such a way that earthquakemotions are not transmitted up through the building, or at least greatlyreduced. Seismic dampers are special devices introduced in the buildingto absorb the energy provided by the ground motion to the building(much like the way shock absorbers in motor vehicles absorb the impactsdue to undulations of the road).

Base Isolation7

The concept of base isolation is explained through an example buildingresting on frictionless rollers (Following figure a). When the groundshakes, the rollers freely roll, but the building above does not move.Thus, no force is transferred to the building due to shaking of the ground;

simply, the building does not experience the earthquake. Now, if thesame building is rested on flexible pads that offer resistance againstlateral movements (Figure b), then some effect of the ground shaking willbe transferred to the building above. If the flexible pads are properlychosen, the forces induced by ground shaking can be a few times smallerthan that experienced by the building built directly on ground, namely afixed base building (Figure c). The flexible pads are called base-isolators,whereas the structures protected by means of these devices are calledbase-isolated buildings. The main feature of the base isolation technologyis that it introduces flexibility in the structure. As a result, a robustmedium-rise masonry or reinforced concrete building becomes extremely

flexible. The isolators are often designed to absorb energy and thus adddamping to the system. This helps in further reducing the seismicresponse of the building. Several commercial brands of base isolators areavailable in the market, and many of them look like large rubber pads,although there are other types that are based on sliding of one part of thebuilding relative to the other. A careful study is required to identify themost suitable type of device for a particular building. Also, base isolationis not suitable for all buildings. Most suitable candidates for base-isolationare low to medium-rise buildings rested on hard soil underneath; high-rise buildings or buildings rested on soft soil are not suitable for baseisolation.

7EERI, (1999), Lessons Learnt Over Time Learning from Earthquakes Series: Volume II Innovative Recovery in India


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8

Base Isolation in Real Buildings

Seismic isolation is a relatively recent and evolving technology. It hasbeen in increased use since the 1980s, and has been well evaluated andreviewed internationally. Base isolation has now been used in numerousbuildings in countries like Italy, Japan, New Zealand, and USA. Baseisolation is also useful for retrofitting important buildings (like hospitalsand historic buildings). By now, over 1000 buildings across the world have

8Earthquake Research Institute, Oakland (CA), USA; alsoavailable at http://www.nicee.org/readings/EERI_Report.htm .


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been equipped with seismic base isolation. In India, base isolationtechnique was first demonstrated after the 1993 Killari (Maharashtra)Earthquake [EERI, 1999]. Two single storey buildings (one school buildingand another shopping complex building) in newly relocated Killari townwere built with rubber base isolators resting on hard ground. Both werebrick masonry buildings with concrete roof. After the 2001 Bhuj (Gujarat)earthquake, the four-storey Bhuj Hospital building was built with baseisolation technique (Following figure).9

GUIDELINES FOR THE EARTHQUAKE

RESISTANT BUILDINGS:

One of the most critical decisions influencing the ability of asuperstructure to withstand earthquake ground shaking is the choice of its

basic plan shape and configuration. The importance of a proper selectionof the superstructure configuration will be discussed and illustrated for thecase of building structures.

Building structures may be of many types and configurations andthere is, of course, no universal ideal configuration for any particular type

9Hanson,R.D., and Soong,T.T., (2001), Seismic Design with Supplemental Energy Dissipation Devices, Earthquake Engineering Research Institute,

Oakland (CA), USA.


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of building. However, there are certain basic or guiding principlesof seismic-resistant design that can be used as guidelines in selecting anadequate building configuration structural layout, structural system,structural material and the non-structural components. These basicguidelines are as follows:

1.Building (superstructure and non-structural components) should be lightand avoid unnecessary masses.

2. Building and its superstructure should be simple, symmetric, and regularin plan and elevation to prevent significant torsional forces, avoidinglarge height-width ratio and large plan area.

3. Building and its superstructure should have a uniform and

continuous distribution of mass, stiffness, strength and ductility, avoidingformation of soft stories.

4. Superstructure should have relatively shorter spans than non-seismic-resistant structure and avoid use of long cantilevers.

5. The non-structural components should either be well separated so thatthey will not interact with the rest of the structure, or they should beintegrated with the structure. On the latter case, it is desirable that the

structure should have sufficient lateral stiffness to avoid significantdamage under minor and moderate earthquake shaking, and toughnesswith stable hysteric behaviour (that is, stability of strength, stiffness anddeformability) under the repeated reversal of deformations which couldbe induced by severe earthquake ground motion. The stiffer thestructure, the less sensitive it will be to the effects of the interacting non-structural components, and the tougher it is, the less sensitive it will beto effect of sudden failure of the interacting non-structural elements.

6. Superstructure should be detailed so that the inelastic deformations can

be constrained (controlled) to develop in desired regions and according toa desirable hierarchy.


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7. Superstructure should have the largest possible number of defence lines,that is, it should be composed of different tough structural subsystemswhich interact or are interconnected by very tough structural elements(structural fuses) whose inelastic behaviour would permit the wholestructure to find its way out from a critical stage of dynamic response.

8. Superstructure should be provided with balanced stiffness and strengthbetween its members, connections and supports.

9. The stiffness and strength of the entire building should be compatiblewith the stiffness and strength of the soil.

THE SEISMIC ZONES IN INDIA

Basic Geography and Tectonic Features10

10BMTPC, (1997), Vulnerability Atlas of India, Building Materials and Technology Promotion Council, Ministry of Urban Development,

Government of India, New Delhi.


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India lies at the north-western end of the Indo- Australian Plate, whichencompasses India, Australia, amajor portion of the Indian Ocean andother smallercountries. This plate is colliding against the huge EurasianPlate (Figure 1) and going under the EurasianPlate; this process of onetectonic plate getting underanother is called subduction. A sea, Tethys,

separatedthese plates before they collided. Part of thelithosphere, theEarths Crust, is covered by oceansand the rest by the continents. Theformer can undergosubduction at great depths when it converges againstanother plate, but the latter is buoyant and so tends toremain close tothe surface. When continents converge,large amounts of shortening andthickening takesplace, like at the Himalayas and the Tibet.Three chieftectonic sub-regions of India are themighty Himalayasalong the north,the plains of theGanges and other rivers, and the peninsula. TheHimalayas consist primarily of sediments accumulatedover longgeological time in the Tethys. The Indo-Gangetic basin with deepalluvium is a greatdepression caused by the load of the Himalayas on the

Continent. The peninsular part of the country consists of ancient rocksdeformed in the past Himalayan-like collisions. Erosion has exposed theroots of the old mountains and removed most of the topography. Therocks are very hard, but are softened by weathering near the surface.Before the Himalayan collision, several tens of millions of years ago, lavaflowed across the central part of peninsular India leaving layers of basaltrock. Coastal areas like Kachchh show marine deposits testifying tosubmergence under the sea millions of years ago.

Prominent Past Earthquakes in India11

11Dasgupta,S., et al, (2000), Seismotectonic Atlas of Indian and itsEnvirons, Geological Survey of India.


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A number of significant earthquakes occurred in and around India overthe past century (Figure 2). Some of these occurred in populated andurbanized areas and hence caused great damage. Many went unnoticed,as they occurred deep under the Earths 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 havealso occurred in the peninsular region (these are intra-plate earthquakes).Four Great earthquakes (M>8) occurred in a span of 53 years from 1897to 1950; the January 2001 Bhuj earthquake (M7.7) is almost as large.Each of these caused disasters, but also allowed us to learn aboutearthquakes and to advance earthquake engineering. For instance, 1819Cutch Earthquake produced an unprecedented ~3m high uplift of theground over 100km (calledAllahBund). The 1897 Assam Earthquakecaused severe damage up to 500km radial distances; the type of damagesustained led to improvements in the intensity scale from I-X to I-XII.

Extensive liquefaction of the ground took place over a length of300km(called the Slump Belt) during 1934 Bihar-Nepal earthquake in whichmany structures went afloat.


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The timing of the earthquake during the day and during the year criticallydetermines the number of casualties. Casualties are expected to be highfor earthquakes that strike during cold winter nights, when most of thepopulation is indoors.

Seismic Zones of India

The varying geology at different locations in the country implies that thelikelihood of damaging earthquakes taking place at different locations isdifferent. Thus, a seismic zone map is required to identify these regions.Based on the levels of intensities sustained during damaging pastearthquakes, the 1970 version of the zone map subdivided India into fivezones I, II, III, IV and V (Following figure). The maximum Modified

Mercalli (MM) intensity of seismic shaking expected in these zones were Vor less, VI, VII, VIII, and IXandhigher, respectively. Parts of Himalayanboundary in the north and northeast, and the Kachchh area in the westwere classified as zone V.


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The seismic zone maps are revised from time to time as moreunderstanding is gained on the geology, the seism tectonics and theseismic activity in the country. The Indian Standards provided the firstseismic zone map in 1962, which was later revised in 1967 and again in

1970. The map has been revised again in 2002 (Following Figure), and itnow has only four seismic zones II, III, IV and V. The areas falling inseismic zone I in the 1970 version of the map are merged with those ofseismic zone II. Also, the seismic zone map in the peninsular region hasbeen modified. Madras now comes in seismic zone III as against in zoneII in the 1970 version of the map. This 2002 seismic zone map is not thefinal word on the seismic hazard of the country, and hence there can beno sense of complacency in this regard.


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The national Seismic Zone Map presents a large-scale view of the seismiczones in the country. Local variations in soil type and geology cannot berepresented at that scale. Therefore, for important projects, such as amajor dam or a nuclear power plant, the seismic hazard is evaluatedspecifically for that site. Also, for the purposes of urban planning,metropolitan areas are micro zoned. Seismic microzonation accounts forlocal variations in geology, local soil profile, etc.

INDIAN STANDARDS ON EARTHQUAKE

ENGINEERING12:

Bureau of Indian standards, the National Standard Body of India, is aStatutory Organization under the Bureau of Indian Standards Act 1986.One of the activities is formulation of Indian Standards on differentsubjects of Engineering through various Division Councils. The Civil

12Bureau of Indian Standards (http://www.bis.org.in/other/quake.htm)


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Engineering Division Council is responsible for standardization in the fieldof Civil Engineering including Structural Engineering, Building materialsand components, Planning Design, Construction and Maintenance ofCivilEngineering Structures, Construction Practices, Safety in Buildingetc. These standards are evolved based on consensus principle through anet work of technical committee comprising representatives fromResearch and Development Organizations, Consumers, Industry, TestingLabs and Govt. Organizations etc.

The Civil Engineering Division Council is working towards to achieve theabove goal through 34 Sectional Committees covering wide range ofsubjects and one of the Sectional Committee is Earthquake EngineeringSectional Committee, CED 39.

India is one of the most disaster prone countries, vulnerable to almost allnatural and manmade disasters. About 85% area is vulnerable to one ormultiple disasters and about 57% area is in high seismic zone including

the capital of the country. Disaster prevention involves engineeringintervention in buildings and structures to make them strong enough towithstand the impact of natural hazard or to impose restrictions on landuse so that the exposure of the society to the hazard situation is avoidedor minimized.

Bureau of Indian Standards has rendered invaluable services by producinglarge number of national standards, which are of direct relevance to theconstruction industry and some of them particular to the mitigation ofdisasters. A detail of Indian Standards in the area of mitigation of natural

hazard of earthquake is given underneath.

Earthquake Engineering

Himalayan-Nagalushai region, Indo-Gangetic plain, Western India and

Cutch and Kathiawar regions are geologically unstable parts of the

country and some devastating earthquakes of the world

have occurred there. A major part of peninsular India has also been

visited by strong earthquakes, but these were relatively few in number

and had considerably lesser intensity. It has been a long felt need to

rationalize the earthquake resistant design and construction of structures

taking into account seismic data from studies of these earthquakes.


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It is to serve this purpose that standards have been formulated in the

field of Design and Construction ofEarthquake Resistant Structures and

also in the field of measurement and tests connected therewith by the

Earthquake Engineering Sectional Committee, CED 39. Following

standards have been formulated under this Committee:

IS 1893:1984 Criteria for Earthquake Resistant Design of Structures

This standard deals with earthquake resistant design of structures and is

applicable to buildings; elevated structures; bridges; dams etc. It also

gives a map which divides the country into five seismic zones based onthe seismic intensity.

IS 1893 was initially published in 1962 as `Recommendations forEarthquake Resistant Design of Structures and then revised in 1966. Asa result of additional seismic data collected in India and furtherknowledge and experience gained the standard was revised in 1970, 1975and then in 1984.

Consequent to the publication of this standard on account of earthquakesin various parts of the country including that in Uttar-

Kashi, Latur and Bhuj and technological advancement in the field, the

Sectional Committee decided to revise the standard into five parts which

deals with different types of structures:

Part 1: General provisions and Buildings

Part 2: Liquid Retaining Tanks Elevated and Ground

Supported

Part 3 : Bridges and Retaining Walls


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Part 4 : Industrial Structures Including Stack Like Structures

Part 5 : Dams and Embankments

IS 1893(Part 1):2002 `Criteria for Earthquake Resistant Design

of Structures: Part 1 General provisions and Buildings

This standard contains provisions that are general in nature and

applicable to all structures. Also, it contains provisions that are specific to

buildings only. It covers general principles and design criteria,

combinations, design spectrum, main attributes of buildings, dynamic

analysis, apart from seismic zoning map and seismic coefficients of

important towns, map showing epicentres, map showing tectonic features

and litho logical map of India.

Following are the major and important modifications made in thisrevision:

a) The seismic zone map is revised with only four zones, instead

of five.Erstwhile Zone I has been merged to Zone II and henceZone I does not appear in the new zoning; only Zones II, III, IV

and V do. The killari area has been included in Zone III andnecessary modifications made, keeping in view the probabilisticHazard Evaluation. The Bellary isolated zone has beenremoved. The parts of eastern coast area have shown similarhazard to that of the killari area, the level of Zone II has beenenhanced to Zone III and connected with Zone IIIofGodavari Graben area.

b) This revision adopts the procedure of first calculating theactual force that may be experienced by the structure during theprobable maximum earthquake, if it were to remainelastic. Then the concept of response reduction due to ductiledeformation or frictional energy dissipation in the cracks isbrought into the code explicitly, by introducing the `responsereduction factor in place of the earlier performance factor.

c) The values of seismic zone factors have been changed; thesenow reflect more realistic values of effective peak ground


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acceleration considering Maximum Considered Earthquake (MCE)and service life of structure in each seismic zone.

d) A clause has been introduced to restrict the use of foundationsvulnerable to differential settlements in severe seismic zones.

Here it is worthwhile to mention that it is not intended in this standard tolay down regulation so that no structure shall suffer any damage duringearthquake of all magnitudes. It has been endeavoured to ensure that asfar as, possible structures are able to respond, without structural damageto shocks of moderate intensities and without total collapse to shocks ofheavy intensities.

IS 1893(Part 4):2005 `Criteria for Earthquake Resistant Design of

Structures: Part 4 Industrial Structures Including Stack Like

Structures

This standard deals with earthquake resistant design of the industrialstructures (plant and auxiliary structures) including stack-like structuressuch as process industries, power plants, textile industries, off-shorestructures and marine/port/harbour structures.

In addition to the above, stack-like structures covered by this standardare such as transmission and communication towers, chimneys and stack-like structures and silos (including parabolic silos used for urea storage).

The characteristics (intensity, duration, etc) of seismic ground vibrationsexpected at any location depends upon the magnitude of earthquake, itsdepth of focus, distance from the epicentre, characteristics of the paththrough which the seismic waves travel, and the soil strata on which thestructure stands.

The response of a structure to ground vibrations is a function of thenature of foundations, soil, materials, form, size and mode of constructionof structures; and the duration and characteristics of ground motion. Thisstandard specifies design forces for structures standing on rocks or soils,

which do not settle, liquefy or slide due to loss of strength duringvibrations.

The design approach adopted in this standard is to ensure that structurespossess minimum strength to withstand minor earthquakes (


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Formulation of revised codes for other parts of IS 1893 are in advance

stages.

IS 4326:1993 Earthquake Resistant Design and Construction of

Buildings - Code of Practice

This standard provides guidance in selection of materials, special featuresof design and construction for earthquake resistant buildings includingmasonry construction, timber construction, prefabricated constructionetc. In this standard, it is intended to cover the specified features ofdesign and construction for earthquake resistance of buildings ofconventional types. The general principles to be observed in theconstruction of such earthquake resistant buildings as specified in thisstandard are Lightness, Continuity of Construction, avoiding/reinforcingProjecting and suspended parts, Building configuration, strength invarious directions, stable foundations, Ductility of structure, Connection tonon-structural parts and fire safety of structures.

Special Construction Features like Separation of Adjoining Structures,Crumple Section, and Foundation design, Roofs and Floors and Staircaseshave been elaborated in the standard. It also covers the detailspertaining to the type of construction, masonry construction withrectangular masonry units, masonry bearing walls, openings in bearingwalls, seismic strengthening arrangements, framing of thin load bearing

walls, reinforcing details for hollow block masonry, flooring/roofingwith precast components and timber construction.

IS 13827:1993 Improving Earthquake Resistance of Earthen Buildings

Guidelines

The guidelines covered in this standard deal with the design andconstruction aspects for improving earthquake resistance of earthenhouses, without the use of stabilizers such as lime, cement, asphalt, etc.

The provisions of this standard are applicable for seismic zones III, IV andV. No special provisions are considered necessary in Zone II. However,considering inherently weak against water and earthquake, earthenbuildings should preferably be avoided in flood prone, high rainfall areasand seismic zones IV and V.


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It has been recommended that such buildings should be light,single storied and of simple rectangular plan. Qualitative tests for thesuitability of soil have been suggested.

Guidelines for Block or Adobe Construction, Rammed earth construction,Seismic strengthening of bearing wall buildings, Internal bracing inearthen houses and earthen constructions with wood or canestructures have been elaborated in this standard.

IS 13828:1993 Improving Earthquake Resistance of Low Strength

Masonry Buildings Guidelines

This standard covers the special features of design and construction forimproving earthquake resistance of buildings of low-strength masonry.

The provisions of this standard are applicable in all seismic zones. Nospecial provisions are considered necessary for buildings in seismic zoneII if cement-sand mortar not leaner than 1:6 is used in masonry andthrough stones or bonding elements is used in stone walls.

The various provisions of IS 4326:1993 regarding general principles,special construction features, types of construction, categories ofbuildings and masonry construction with rectangular masonry buildings oflow strength dealt with in this standard. There are however certainrestrictions, exceptions and additional details which are specificallyincluded herein.

IS 13920:1993 Ductile Detailing of Reinforced Concrete Structures

Subjected to Seismic Forces Code of Practice

This standard covers the requirements for designing and detailing ofmonolithic reinforced concrete buildings so as to give them adequatetoughness and ductility to resist severe earthquake shocks withoutcollapse.

The provisions for reinforced concrete construction given in this standardapply specifically to monolithic reinforced concreteconstruction. Precast and/or prestressed concrete members may be usedonly if they can provide the same level of ductility as that of a monolithicreinforced concrete construction during or after an earthquake.


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Provisions on minimum and maximum reinforcement have beenelaborated which includes the requirements for beams at longitudinalreinforcement in beams at joint face, splices and anchoragerequirements. Provisions have been included for calculation of designshear force and for detailing of transverse reinforcement in beams.

Material specifications are indicated for lateral force resisting elements offrames. The provisions are also given for detailing of reinforcement in thewall web, boundary elements, coupling beams, around openings, atconstruction joints, and for the development, splicing and anchorage ofreinforcement.

IS 13935:1993 Repair and Seismic Strengthening of Buildings

Guidelines

This standard covers the selection of materials and techniques to be usedfor repair and seismic strengthening of damaged buildings duringearthquakes and retrofitting for upgrading of seismic resistance ofexisting buildings.

The provisions of this standard are applicable for buildings in seismiczones III to V of IS 1893:1984, which are based on damaging seismicintensities VII and more on MSK Scales.

The buildings affected by earthquake may suffer both non-structural andstructural damages. This standard lays down guidelines for non-structural/architectural as well as structural repairs, seismic strengtheningand seismic retrofitting of existing buildings. Guidelines have been givenfor selection of materials for repair work such as cement, steel, epoxyresins, epoxy mortar, quick setting cement mortar and special techniquessuch as shotcrete, mechanical anchorage etc. Seismic strengtheningtechniques for the modification of roofs or floors, inserting new walls,strengthening existing walls, masonry arches, random rubble masonrywalls, strengthening long walls, strengthening reinforced concretemembers and strengthening of foundations have been elaborated in

detail.

IS 6922:1973 Criteria for Safety and Design of Structures Subject to

Underground Blasts


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This standard deals with the safety of structures during undergroundblasting and is applicable to normal structures like buildings, elevatedstructures, bridges, retaining walls, concrete and masonry damsconstructed in materials like brickwork, stone masonry and concrete.

As underground blasting operations have become almost a must forexcavation purposes, this standard lays down criteria for safety of suchstructures from cracking and also specifies the effective accelerations fortheir design in certain cases.

IS 4991:1968 Criteria for Blast Resistant Design of Structures for

Explosions above Ground

This standard covers the criteria for design of structures for blast effectsof explosions above ground excluding blast effects of nuclear explosions.

IS 4967:1968 Recommendations for Seismic Instrumentation

for River Valley Projects

This standard covers recommendations for instrumentation forinvestigation of seismicity, study of micro tremors and predominantperiod of a dam site and permanent installation of instruments in the damand appurtenant structures and in surrounding areas.

These standards endeavour to provide a guideline in designing and

repairing of buildings under seismic forces.


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THE AFFECT OF THE EARTHQUAKE ON

THE REINFORCED CONCRETE

BUILDINGS:In recent times, reinforced concrete buildings have become common in

India, particularly in towns and cities. Reinforced concrete consists of two

primary materials, namely concrete with reinforcing steel bars. Concrete

is made of sand, crushed stone and cement, all mixed with pre-

determined amount of water. Concrete can be moulded into any desired

shape, and steel bars can be bent into many shapes. Thus, structures of

complex shapes are possible with RC.

A typical RC building is made of horizontal members (beams and slabs)and vertical members (columns and walls), and supported by foundations

that rest on ground. The system comprising of RC columns and

connecting beams is called a RC frame. The RC frame participates in

resisting the earthquake forces. Earthquake shaking generates inertia

forces in the building, which are proportional to the building mass. Since

most of the building mass is present at floor levels, earthquake induced

inertia forces primarily develop at the floor levels. These forces travel

downwards through slab and beams to columns and walls, and then to

the foundations from where they are dispersed to the ground. As inertia

forces accumulate downwards from the top of the building, the columnsand walls at lower stories experience higher earthquake induced forces

(Following figure) and are therefore designed to be stronger than those in

stories above.


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ROLES OF FLOOR SLABS AND MASONRYWALLS:

Floor slabs are horizontal plate like elements, which facilitate functional

use of buildings. Usually, beams and slabs at one storey level are cast

together. In residential multi storey buildings, thickness of slabs is only

about 110-150mm. When beams bend in the vertical direction during

earthquakes, these thin slabs bend along with them (Following figure a).And, when beams move with columns in the horizontal direction, the slab

usually forces the beams to move together with it. In most buildings, the

geometric distortion of the slab in negligible in the horizontal plane; this

behaviour is known as the rigid diaphragm action (following figure b).

Structural engineers must consider this during design.

Howto Make Stone Masonry Buildings

Earthquake Resistant?

Stone has been used in building construction in India since ancient timessince it is durable and locally available. There are huge numbers of stonebuildings in the country, ranging from rural houses to royal palaces andtemples. In a typical rural stone house, there is thick stone masonry walls(thickness ranges from 600 to 1200 mm) built using rounded stones fromriverbeds bound with mud mortar. These walls are constructed withstones placed in a random manner, and hence do not have the usuallayers (or courses) seen in brick walls. These uncoursed walls have twoexterior vertical layers (called Wythes) of large stones, filled in between


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with loose stone rubble and mud mortar. A typical uncoursed random(UCR) stone masonry wall is illustrated in Figure 1. In many cases, thesewalls support heavy roofs (for example, timber roof with thick mudoverlay).13

Laypersons may consider such stone masonry buildings robust due to thelarge wall thickness and robust appearance of stone construction. But,these buildings are one of the most deficient building systems fromearthquake-resistance point of view. The main deficiencies includeexcessive wall thickness, absence of any connection between the two

wythes of the wall, and use of roundstones (instead of shaped ones).Such dwellings have shown very poor performance during pastearthquakes in India and other countries (e.g., Greece, Iran, Turkey,former Yugoslavia). In the 1993 Killari (Maharashtra) earthquake alone,over 8,000 people died, most of them buried under the rubble oftraditional stone masonry dwellings. Likewise, a majority of the over13,800 deaths during 2001 Bhuj (Gujarat) earthquake is attributed to thecollapse of this type of construction. The main patterns of earthquakedamage include: (a) bulging/separation of walls in the horizontal directioninto two distinct wythes(Figure 2a), (b) separation of walls at corners andT-junctions (Figure 2b)14, (c) separation of poorly constructed roof from

walls, and eventual collapse of roof, and (d) disintegration of walls andeventual collapse of the whole dwelling.

13Brzev,S., Greene,M. and Sinha,R. (2001), Rubble stone masonry walls with timber walls and timber roof, World

HousingEncyclopedia (www.world-housing.net), India/Report 18, published by EERI and IAEE.14IAEE, (1986), Guidelines forEarthquake Resistant Non-Engineered Construction, The ACC Limited, Thane, 2001 (Seewww.niceee.org).


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Earthquake resistant features:

Low strength stone masonry buildings are weak against earthquakes, andshould be avoided in high seismic zones. The Indian Standard IS: 13828-1993 states that inclusion of special earthquake-resistant design and

construction features may raise the earthquake resistance of thesebuildings and reduce the loss of life. However, in spite of the seismicfeatures these buildings may not become totally free from heavy damageand even collapse in case of a major earthquake. The contribution of theeach of these features is difficult to quantify, but qualitatively thesefeatures have been observed to improve the performance of stonemasonry dwellings during past earthquakes. These features include:

(a) Ensure proper wall construction: The wall thicknessshould not exceed450mm. Round stone bouldersshould not be used in the construction!Instead, the stones should be shaped using chisels andhammers. Use of

mud mortar should be avoided in higher seismic zones. Instead, cement-sand mortar should be 1:6 (or richer) and lime-sand mortar 1:3 (orricher) should be used.

(b) Ensure proper bond in masonry courses: The masonry walls should bebuilt in construction lifts not exceeding 600mm. Through-stones(eachextending over full thickness of wall) or a pair of overlapping bond-stones(each extending over at least ths thickness of wall) must be used at


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every 600mm along the height and at a maximum spacing of 1.2m alongthe length (Figure 3).

(c) Provide horizontal reinforcing elements: The stone masonry dwellingsmust have horizontal bands (See IITK-BMTPCEarthquake Tip 14 forplinth, lintel, roofand gable bands). These bands can be constructed outof wood or reinforced concrete, and chosen based on economy. It isimportant to provide at least one band (either lintelband or roofband) instone masonry construction (Figure 4).15

15Publications of Building Materials and Technology Promotion Council,New Delhi (www.bmtpc.org):

(a) Retrofitting of Stone Houses in Marathwada Area of Maharashtra(b) Guidelines For Improving Earthquake Resistance ofHousing(c)Manual forRepair and Reconstruction ofHouses Damaged in Earthquake in October 1991 in the Garhwal Region ofUP


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(d) Control on overall dimensions and heights: The unsupported length of

wallsbetween cross-walls should be limited to 5m; for longer walls, crosssupports raised from the ground level called buttressesshould beprovided at spacing not more than 4m. The height of each storey shouldnot exceed 3.0m. In general, stone masonry buildings should not be tallerthan 2 storeys when built in cement mortar, and 1 storey when built inlime or mud mortar. The wall should have a thickness of at least one-sixthits height. Although, this type of stone masonry construction practice isdeficient with regards to earthquake resistance, its extensive use is likelyto continue due to tradition and low cost. But, to protect human lives andproperty in future earthquakes, it is necessary to follow proper stonemasonry construction as described above (especially features (a) and (b)in seismic zones III and higher). Also, the use of seismic bands is highlyrecommended (as described in feature (c) above.


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HOW TO REDUCE EARTHQUAKE

EFFECTS ON BUILDINGS:

Why Earthquake Effects are to be reduced?

Conventional seismic design attempts to make buildings that do notcollapse under strong earthquake shaking, but may sustain damage tonon-structural elements (like glass facades) and to some structural

members in the building. This may render the building non-functionalafter the earthquake, which may be problematic in some structures, likehospitals, which need to remain functional in the aftermath of theearthquake. Special techniques are required to design buildings such thatthey remain practically undamaged even in a severe earthquake.Buildings with such improved seismic performance usually cost more thannormal buildings do. However, this cost is justified through improvedearthquake performance. Two basic technologies are used to protectbuildings from damaging earthquake effects. These are Base IsolationDevices and Seismic Dampers. The idea behind base isolation is to detach(isolate) the building from the ground in such a way that earthquakemotions are not transmitted up through the building, or at least greatlyreduced. Seismic dampers are special devices introduced in the buildingto absorb the energy provided by the ground motion to the building(much like the way shock absorbers in motor vehicles absorb the impactsdue to undulations of the road).

Base Isolation (Discussed earlier)

After development of passive devices such as base isolation and TMD. The

next logical step is to control the action of these devices in an optimalmanner by an external energy source the resulting system is known

as active control device system. Active control has been very widely used

in aerospace structures. In recent years significant progress has been

made on the analytical side of active control for civil engineering

structures. Also a few models explains as shown that there is great

promise in the technology and that one may expect to see in the


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foreseeable future several dynamicDynamic Intelligent Buildings the

term itself seems to have been joined by the Kajima Corporation in Japan.

In one of their pamphlet the concept of Active control had been explained

in every simple manner and it is worth quoting here.

People standing in swaying train or bus try to maintain balance by

unintentionally bracing their legs or by relaying on the mussels of their spine

and stomach. By providing a similar function to a building it can dampen

immensely the vibrations when confronted with an earthquake. This is the

concept of Dynamic Intelligent Building (DIB).

16

The philosophy of the past conventional a seismic structure is to respond

passively to an earthquake. In contrast in the DIB which we propose the

building it functions actively against earthquakes and attempts to controlthe vibrations. The sensor distributed inside and outside of the building

transmits information to the computer installed in the building which can

make analyses and judgment, and as if the buildings possess intelligence

16http://articles.architectjaved.com/earthquake_resistant_structures/active-control-devices-for-earthquake-

resistance/


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pertaining to the earthquake amends its own structural characteristics

minutes by minute.

Active Control System

The basic configuration of an active control system is schematically shownin figure. The system consists of three basic elements:

1. Sensors to measure external excitation and/or structural response.2. Computer hardware and software to compute control forces on the

basis of observed excitation and/or structural response.

3. Actuators to provide the necessary control forces.Thus in active system has to necessarily have an external energy input todrive the actuators. On the other hand passive systems do not required

external energy and their efficiency depends on tunings of system toexpected excitation and structural behaviour. As a result, the passivesystems are effective only for the modes of the vibrations for which theseare tuned. Thus the advantage of an active system lies in its much widerrange of applicability since the control forces are worked out on the basisof actual excitation and structural behaviour. In the active system whenonly external excitation is measured system is said to be in open-looped.However when the structural response is used as input, the system is inclosed loop control. In certain instances the excitation and response bothare used and it is termed as open-closed loop control.

Control Force Devices

Many ways have been proposed to apply control forces to a structure.Some of these have been tested in laboratory on scaled down models.Some of the ideas have been put forward for applications of active forcesare briefly described in the following:

Active-tuned Mass Dampers (TMD)

These are in passive mode have been used in a number of structures as

mentioned earlier. Hence active TMD is a natural extension. In thissystem 1% of the total building mass is directly excited by an actuatorwith no spring and dash pot. The system has been termed as Active MassDriver (AMD). The experiments indicated that the building vibrations arereduced about 25% by the use of AMD.


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Tendon Control

Various analytical studies have been done using tendons for activecontrol. At low excitations, even with the active control system off, thetendon will act in passive modes by resisting deformations in thestructures though resulting tension in the tendon. At higher excitations

one may switch over to Active mode where an actuator applies therequired tension in tendons.

Other Methods

The liquid sloshing during earthquakes has assumed significanceimportance in view of over flow of petroleum products from storage tankin post earthquakes. One of the important consideration with sloshing isthat is associated with a very low damping. The wave height wascontrolled through force applied to the side wall by a hydraulic actuator.The active control successfully reduced wave heights to the level of 6% ofthose without control, for harmonic excitations at sloshing frequency. Forearthquake type excitation the wave heights were reduced to 19% level.


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4.Lagorio,H,J, (1990), EARTHQUAKES An Architects Guide to Non- Structural Seismic

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5. Arnold,C., and Reitherman,R., (1982), Building Configuration and Seismic Design,John Wiley, USA.

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7.EERI, (1999), Lessons Learnt Over Time Learning from Earthquakes Series: Volume II

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8.Hanson,R.D., and Soong,T.T., (2001), Seismic Design with Supplemental Energy DissipationDevices, Earthquake Engineering Research Institute, Oakland (CA), USA.

9. BMTPC, (1997), Vulnerability Atlas of India, Building Materials and Technology PromotionCouncil, Ministry of Urban Development,

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10.Dasgupta,S., et al, (2000), Seismotectonic Atlas of Indian and itsEnvirons, Geological

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11.Bureau of Indian Standards (http://www.bis.org.in/other/quake.htm)

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timber walls and timber roof, World HousingEncyclopedia (www.world-housing.net),

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13.IAEE, (1986), Guidelines forEarthquakeResistant Non-Engineered Construction, The ACCLimited, Thane, 2001 (See www.niceee.org).

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(a) Retrofitting of Stone Houses in Marathwada Area of Maharashtra(b) Guidelines For Improving Earthquake Resistance ofHousing(c)Manual forRepair and Reconstruction ofHouses Damaged in Earthquake in October 1991 in theGarhwal Region ofUP

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79970662 Earthquake Resistant Design

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