Earthquake Resistence structures

EARTHQUAKERESISTANCESTRUCTURES

ByD.UDAY KUMAR &J.PRASADDNBKRIST,Vidynagar4/12/2014

EARTHQUAKE RESISTANCE STRUCTURES

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ABTRACT:

Due to mining ,drilling of bore holes and manyother activities, earthquakes are most common in someplaces of india and many more countries. And they causemore damage for life and property. Most of our civilengineering constructions are failed by forces causeddue to earthquake. Even now also we don’t havethorough knowledge about the forces caused byearthquakes, due to which most of our civil engineeringstructures are failed. Most of our civil engineeringstructures are constructed by concrete it has more selfweight, so it is not possible to reduce the damage causedby the structures. Due to restless work of scientists wehave a solution to reduce the damage i.e by ‘’EarthquakeResistance Structure’’. The design of earthquakeresistance structures is called earthquake resistancedesign. By providing Base Isolation Devices we canseparate building from ground by some rubber devicesand also by introducing Seismic dampers and specialdevices for absorbing the energy caused by earthquakes .By this additional installations structures can attainstability . so safety can be possible by this designmethods .

In this article, we are going to discuss about somedesign methods which are relating to earthquakeresistance of structures.

INTRODUCTION

Earthquake-resistant structures are structuresdesigned to withstand earthquakes. While no structurecan be entirely immune to damage from earthquakes, thegoal of earthquake-resistant construction is to erectstructures that fare better during seismic activity thantheir conventional counterparts.

According to building codes, earthquake-resistantstructures are intended to withstand the largestearthquake of a certain probability that is likely to occurat their location. This means the loss of life should beminimized by preventing collapse of the buildings forrare earthquakes while the loss of functionality should belimited for more frequent ones.

Before designing the buildings to resistearthquakes we have thorough knowledge about thebehaviour of the earthquakes and how they effect thestability of buildings.

Earthquakes are induced to movement oftectonic plates slides over or push one another. In thisprocess lot of energy was released and the energy isdistributed in the form of waves around the epicenterand finally reach the ground surface. The magnitude ofearthquake depends upon the movement of plates, typeof soil along which the waves are moved i.e soft soil orhard rock. In hard rock movement is less but energy istransferred more quickly than soft soil.

An earthquake (also known as a quake, tremor ortemblor) is the result of a sudden release of energy in theEarth's crust that creates seismic waves. The seismicity,seismism or seismic activity of an area refers to thefrequency, type and size of earthquakes experiencedover a period of time.

Earthquakes are measured using observationsfrom seismometers. The moment magnitude is the mostcommon scale on which earthquakes larger thanapproximately 5 are reported for the entire globe. Themore numerous earthquakes smaller than magnitude 5reported by national seismological observatories aremeasured mostly on the local magnitude scale, alsoreferred to as the Richter scale. These two scales arenumerically similar over their range of validity.Magnitude 3 or lower earthquakes are mostly almostimperceptible or weak and magnitude 7 and overpotentially cause serious damage over larger areas,depending on their depth. The largest earthquakes inhistoric times have been of magnitude slightly over 9,although there is no limit to the possible magnitude. Themost recent large earthquake of magnitude 9.0 or largerwas a 9.0 magnitude earthquake in Japan in 2011 (as ofMarch 2014), and it was the largest Japanese earthquakesince records began. Intensity of shaking is measured onthe modified Mercalli scale. The shallower anearthquake, the more damage to structures it causes, allelse being equal.

WHY DO EARTHQUAKES HAPPEN ?

Earthquakes are usually caused when rockunderground suddenly breaks along a fault. This suddenrelease of energy causes the seismic waves that make theground shake. When two blocks of rock or two plates arerubbing against each other, they stick a little. They don'tjust slide smoothly; the rocks catch on each other. The


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rocks are still pushing against each other, but notmoving. After a while, the rocks break because of all thepressure that's built up. When the rocks break, theearthquake occurs. During the earthquake and afterward,the plates or blocks of rock start moving, and theycontinue to move until they get stuck again. The spotunderground where the rock breaks is called the focus ofthe earthquake. The place right above the focus (on topof the ground) is called the epicenter of the earthquake.

TRY THIS LITTLE EXPERIMENT:

1. Break a block of foam rubber in half.2. Put the pieces on a smooth table.3. Put the rough edges of the foam rubber pieces

together.4. While pushing the two pieces together lightly,

push one piece away from you along the tabletop while pulling the other piece toward you.See how they stick?

5. Keep pushing and pulling smoothly.

Soon a little bit of foam rubber along the crack(the fault) will break and the two pieces willsuddenly slip past each other. That suddenbreaking of the foam rubber is the earthquake.That's just what happens along a strike-slipfault

Fig.1

HUMAN ACTIVITIES :

DAMS AND RESERVOIRS:

It's just water, but, man, water is heavy. Largereservoirs of water created by dams have a long history

of inducing earthquakes, from Zambia to Greece toIndia. The 2008 earthquake in Sichuan, China, thatkilled nearly 70,000 people was one of the mostdevastating in recent memory, and some scientists thinkit was triggered by the construction of the Zipingpu Damnearby.

The Zipingpu Dam sits just a third of a mileaway from the fault, and the added weight of millions oftons of water could have hastened the fault's rupture."No geological process can come up with such aconcentration of mass in such as small area other than avolcano," said geologist Christian Klose about thebuildup of water.

GROUND WATER EXTRACTION:

Taking water out of ground, which causes thewater table to drop, can also destabilize an existing fault.

A 2011 earthquake in Lorca, Spain caused agreat amount of destruction for its 5.1 magnitudebecause its epicenter was located so close to the surface.Its shallow epicenter may be related to groundwaterextraction near Lorca, according to research published inNature Geoscience. Since 1960, water extraction hascaused the region's water table to drop by an incredible250 meters.

GEOTHERMAL POWER PLANTS:

At the end of the infamous San Andreas Fault inCalifornia lies the Salton Sea—and, along its southernshore, is the Salton Sea Geothermal Field. The powerplant extracts hot, high-pressured water out of theground and turns it into steam to run turbines generatingpower. If it seems like a bad idea to be extracting waterso close to the San Andreas Fault, well, you'd be right.

In a 2011 study published in Science,researchers at the University of California, Santa Cruz,found that seismic activity has increased around theSalton Sea as geothermal field operations have rampedup. Earthquake swarms—bursts of dozens of smallquakes below magnitude 6 or so—happen regularlyalong the Salton Sea. While these earthquake swarms bythemselves may not cause much damage, they couldinteract with other bigger faults, like the San Andreaslocated so conveniently nearby, to induce far moredamaging quakes.

Several hundred miles north in California isalso the Geysers Geothermal Field, the largestgeothermal field in the world. Because the Geysers don'tsit near a large fault, induced seismicity there is lesslikely to be majorly destructive. However, researchershave found that seismicity in the region has increased


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from basically zero in the 1960s to 20 or 30 smallseismic events per year.

Fig.2

FRACKING AND INJECTION WELLS:

When it comes to hydraulic fracturing, orfracking, it's actually not the extraction of oil or gasthat's the problem. . It's what happens to those afterward,when waste fracking fluid is injected back undergroundinto deep wells. The fluid can seep out and lubricatefaults, causing them to slip more easily. A study inGeology links a 2011 Oklahoma earthquake towastewater injection wells.

Seismic activity in Oklahoma has shot up alongwith the rise of fracking: The number of earthquakeswent from about a dozen in 2008 to over 1,000 in 2010.Earthquake swarms are now regular occurrences in theregion. With fracking in the United States steadilyincreasing, the dangers of wastewater injection wellsbecome more imminent.

SKYSCRAPERS:

The tallest building in the world when it wasbuilt, the Taipei 101 tower in Taiwan was supposed tobe the city's crown jewel, capable of withstandingtyphoon winds and earthquakes. Ironically, the verythings supposed to make it earthquake-resilient maybe… causing earthquakes.

According to a Taiwanese geologist Cheng-Horn Lin, Taipei 101's especially huge mass of 770,000tons is putting too much pressure on the soft sedimentaryrock below. This stress is due to all the extra steel andconcrete used to make the skyscraper solid enough towithstand earthquakes. Since construction began onTaipei 101, according to Lin, the region has seen severalmicro-earthquakes and two larger earthquakes directlyunderneath the building.

During its construction in March 2002, a 6.8earthquake did knock two cranes to the ground. Taipei101 itself, though, was undamaged. So, if the skyscraperdoes cause earthquakes in the future, at least you knowwhere to go?

Fig.3

NATURALLY OCCURING EARTHQUAKES:

Tectonic earthquakes occur anywhere in theearth where there is sufficient stored elastic strain energyto drive fracture propagation along a fault plane. Thesides of a fault move past each other smoothly and aseismically only if there are no irregularities or asperitiesalong the fault surface that increase the frictionalresistance. Most fault surfaces do have such asperitiesand this leads to a form of stick-slip behavior. Once thefault has locked, continued relative motion between theplates leads to increasing stress and therefore, storedstrain energy in the volume around the fault surface.This continues until the stress has risen sufficiently tobreak through the asperity, suddenly allowing slidingover the locked portion of the fault, releasing the storedenergy. This energy is released as a combination ofradiated elastic strain seismic waves, frictional heatingof the fault surface, and cracking of the rock, thuscausing an earthquake. This process of gradual build-upof strain and stress punctuated by occasional suddenearthquake failure is referred to as the elastic-reboundtheory. It is estimated that only 10 percent or less of anearthquake's total energy is radiated as seismic energy.Most of the earthquake's energy is used to power theearthquake fracture growth or is converted into heatgenerated by friction. Therefore, earthquakes lower theEarth's available elastic potential energy and raise itstemperature, though these changes are negligiblecompared to the conductive and convective flow of heatout from the Earth's deep interior.


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EARTHQUAKE FAULT TYPES

There are three main types of fault, all of whichmay cause an earthquake:

NORMAL FAULTS AND REVERSE FAULTS:

Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in thedirection of dip and movement on them involves avertical component. Normal faults occur mainly in areaswhere the crust is being extended such as a divergentboundary. Reverse faults occur in areas where the crustis being shortened such as at a convergent boundary.

Reverse faults, particularly those alongconvergent plate boundaries are associated with the mostpowerful earthquakes, including almost all of those ofmagnitude 8 or more. Strike-slip faults, particularlycontinental transforms can produce major earthquakesup to about magnitude 8. Earthquakes associated withnormal faults are generally less than magnitude 7.

STRIKE SLIP:

Strike-slip faults are steep structures where thetwo sides of the fault slip horizontally past each other;transform boundaries are a particular type of strike-slipfault. Many earthquakes are caused by movement onfaults that have components of both dip-slip and strike-slip; this is known as oblique slip.

Fig.4

Strike-slip faults tend to be oriented near vertically,resulting in an approximate width of 10 km within the

brittle crust thus earthquakes with magnitudes muchlarger than 8 are not possible. Maximum magnitudesalong many normal faults are even more limited becausemany of them are located along spreading centers, as inIceland, where the thickness of the brittle layer is onlyabout 6 km

HOW EARTHQUAKES EFFECTBUILDINGS?

Earthquakes cause damage as a result of thedifferent waves that they produce as the earthquakeenergy moves through and on the Earth. The way thatthe ground responds to the energy of earthquake wavesas they pass through depends on the geology of the area.

A hard rock, like granite or limestone, mayvibrate very quickly with short movements, but notbreak apart significantly. A wet sand or silt, on the otherhand, could be shaken enough that the pressure of thewater in the soil builds up enough to make the soilbehave like a liquid. This is called liquefaction, and isresponsible for much earthquake damage in low-lyingwet areas.

Damage to the ground during an earthquakeusually takes place in one of the following ways:

SHAKING:Moves the ground in place. This does not

usually cause significant damage to the ground itself, butoften results in major damage to structures in or on theground. This can include, not only buildings, but water,gas and sewer lines, train tracks, androads.

LANDSLIDES:Ground is moved (displaced) to somewhere

else.

LIQUEFACTION:Strength of the ground is removed, causing the

ground and objects on it to sink. Any heavy objectssitting on liquefied ground will rapidly sink. Thisincludes all types of natural features as well asstructures. Liquefaction can result in depressions, a typeof landslide called a lateral spread, and the formation ofsand blows. Sand blows are geysers or volcanoes of sandexpelled from cracks or holes in the ground due to highwater pressure in the saturated sand during earthquakeshaking. Sand blows have been known to open largefissures, create large depressions, and cover large areasof land with several inches of sand. This can impactroads and infrastructure, as well as bury large areas offarmland, making it unable to sustain crops.

The damage to structures can depend on thematerial that the structure is made out of, the type ofearthquake wave (motion) that is affecting the structure,


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and the ground on which the structure is built. Woodstructures respond to earthquakes differently than brickor masonry structures, because wood can bend, andmasonry tends to shatter. Likewise, buildings withreinforced steel in their walls tend to stand better thanunsupported buildings during shaking. The taller abuilding is the more the top of the building movesrelative to the bottom of the building; however allbuildings sway during an earthquake.

Fig.5

Structures tend to respond to earthquakes in oneof the following ways: bending, breaking, sinking andshaking. Buildings are complex structures though. Theyare made of multiple elements and components that arestressed and interact with one another when shaken byan earthquake. Buildings vary widely in size, geometry,structural system, construction material, and foundationcharacteristics. These attributes influence how a buildingperforms when the ground shakes.

The 1989 Loma Prieta earthquake set SanFrancisco’s Transamerica Pyramid swaying and rocking.An array of 22 sensors (small arrows) installed by theU.S. Geological Survey in the steel-frame structuredocumented that the horizontal displacement on the 49thfloor of the building was five times the inches measuredin the basement, as indicated by the recordings (redlines). No significant twisting of the building wasmeasured due to the symmetry of the building about itsvertical axis.

Permanent ground deformations can tear astructure apart. Some foundation types are better able toresist these permanent ground deformations than others.For example, the use of pile foundations, with the pilesextending beneath the anticipated zone of soilliquefaction, can be effective in mitigating the hazard’seffects. The use of heavily reinforced mats can also beeffective in resisting moderate ground deformation dueto fault rupture or lateral spreading. Most earthquake-induced building damage, however, is a result of groundshaking. When the ground shakes at a building site, the

building’s foundations vibrate in a manner that’s similarto the surrounding ground.

Brittle elements tend to break and lose strength.(Examples of brittle elements include unreinforcedmasonry walls that crack when overstressed in shear,and unconfined concrete elements that crush undercompressive overloads.) Ductile elements are able todeform beyond their elastic strength limit and continueto carry load. (Examples of ductile elements includetension braces and adequately braced beams in momentframes .

For economic reasons, building codes permitbuildings to be damaged by the infrequent severeearthquakes that may affect them, but prevent collapseand endangerment of life safety. For buildings that houseimportant functions essential to post-earthquakerecovery, including hospitals, fire stations, emergencycommunications centers, etc., codes adopt moreconservative criteria that’s intended to minimize the riskthat the buildings would be so severely damaged theycould not be used for their intended function.

Fig.6

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HOW BUILDINGS RESPONED TOEARTHQUAKES:

GROUND ACCELERATION AND BUILDINGDAMAGE:

Comparatively speaking, the absolutemovement of the ground and buildings during anearthquake is not actually all that large, even during amajor earthquake. That is, they do not usually undergodisplacements that are large relative to the building'sown dimensions. So, it is not the distance that a buildingmoves which alone causes damage.

Rather, it is because a building is suddenlyforced to move very quickly that it suffers damageduring an earthquake. Think of someone pulling a rugfrom beneath you. If they pull it quickly (i.e., accelerateit a great deal), then they needn't pull it very far to throw


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you off balance. On the other hand, if they pull the rugslowly and only gradually increase the speed of the rug,they can move (displace) it a great distance without thatsame unfortunate result.

Fig.7

In other words, the damage that a buildingsuffers primarily depends not upon its displacement, butupon acceleration. Whereas displacement is the actualdistance the ground and the building may move duringan earthquake, acceleration is a measure of how quicklythey change speed as they move. During an earthquake,the speed at which both the ground and building aremoving will reach some maximum. The more quicklythey reach this maximum, the greater their acceleration.

It's worthwhile mentioning here that in order tostudy the earthquake responses of buildings, manybuildings in earthquake-prone regions of the world havebeen equipped with strong motion accelerometers. Theseare special instruments which are capable of recordingthe accelerations of either the ground or building,depending upon their placement.

The recording of the motion itself is known as anaccelerogram. Shows an accelerogram recorded in ahospital building parking lot during the Northridge,California earthquake of January 17, 1994.

Fig.8

In addition to providing valuable information about thecharacteristics of the particular earthquake recorded orthe building where the accelerogram was recorded,accelerograms recorded in the past are also often used inthe earthquake response analysis and earthquake designof buildings yet to be constructed.

NEWTON’S LAW:Acceleration has this important influence on

damage, because, as an object in movement, the buildingobeys Newton' famous Second Law of Dynamics. Thesimplest form of the equation which expresses theSecond Law of Motion is

F = MA 1.1.

This states the Force acting on the building isequal to the Mass of the building times the Acceleration.So, as the acceleration of the ground, and in turn, of thebuilding, increase, so does the force which affects thebuilding, since the mass of the building doesn't change.

Of course, the greater the force affecting a building, themore damage it will suffer; decreasing F is an importantgoal of earthquake resistant design. When designing anew building, for example, it is desirable to make it aslight as possible, which means, of course, that M, and inturn, F will be lessened. As we've seen in the discussionof Advanced Earthquake Resistant Techniques, varioustechniques are now also available for reducing A

It is important to note that F is actually what'sknown as an inertial force, that is, the force is createdby the building's tendency to remain at rest, and in itsoriginal position, even though the ground beneath it ismoving. This is in accordance with another importantphysical law known as D'Alembert's Principle, whichstates that a mass acted upon by an acceleration tends tooppose that acceleration in an opposite direction andproportionally to the magnitude of the. This inertialforce F imposes strains upon the building's structuralelements. These structural elements primarily includethe building's beams, columns, load-bearing walls,floors, as well as the connecting elements that tie thesevarious structural elements together. If these strains are


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large enough, the building's structural elements sufferdamage of various kinds.

Fig.9

To illustrate the process of inertia generatedstrains within a structure, we can consider the simplestkind of structure imaginable--a simple, perfectly rigidblock of stone. During an earthquake, if this block issimply sitting on the ground without any attachment toit, the block will move freely in a direction opposite tothat of the ground motion, and with a force proportionalto the mass and acceleration of the block.

If the same block, however, is solidly founded inthe ground and no longer able to move freely, it must insome way absorb the inertial force internally. In Figure3, this internal uptake of force is shown to result incracking near the base of the block.

Fig.10

Of course, real buildings do not respond assimply as described above. There are a number ofimportant characteristics common to all buildings whichfurther affect and complicate a building's response interms of the accelerations it undergoes, and thedeformations and damages that it suffers.

BUILDING FREQUENCY PERIOD:

To begin with, as we discussed in the HowEarthquakes Affect Buildings, the magnitude of thebuilding response – that is, the accelerations which itundergoes – depends primarily upon the frequencies ofthe input ground motion and the building's naturalfrequency. When these are near or equal to one another,the building's response reaches a peak level.

In some circumstances, this dynamicamplification effect can increase the buildingacceleration to a value two times or more that of theground acceleration at the base of the building.Generally, buildings with higher natural frequencies, anda short natural period, tend to suffer higher accelerationsbut smaller displacement. In the case of buildings withlower natural frequencies, and a long natural period, thisis reversed as the buildings will experience loweraccelerations but larger displacements.

BUIDING STIFFNESS:The taller a building, the longer its natural

period tends to be. But the height of a building is alsorelated to another important structural characteristic: thebuilding flexibility. Taller buildings tend to be moreflexible than short buildings. (Only consider a thin metalrod. If it is very short, it is difficulty to bend it in yourhand.

If the rod is somewhat longer, and of the same diameter,it becomes much easier to bend. Buildings behavesimilarly.) We say that a short building is stiff, while ataller building is flexible. (Obviously, flexibility andstiffness are really just the two sides of the same coin. Ifsomething is stiff, it isn't flexible and vice-versa.)

Stiffness greatly affects the building's uptake ofearthquake generated force. Reconsider our firstexample above, of the rigid stone block deeply foundedin the soil. The rigid block of stone is very stiff; as aresult it responds in a simple, dramatic manner. Realbuildings, of course, are more inherently flexible, beingcomposed of many different parts.

Furthermore, not only is the block stiff, it is brittle; andbecause of this, it cracks during the earthquake. Thisleads us to the next important structural characteristicaffecting a building's earthquake response andperformance ductility.

DUCTILITY:

Ductility is the ability to undergo distortion ordeformation – bending, for example – without resultingin complete breakage or failure. To take once again the


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example of the rigid block, the block is an example of astructure with extremely low ductility. To see howductility can improve a building's performance during anearthquake.

For the block, we have substituted acombination of a metal rod and a weight. In response tothe ground motion, the rod bends but does not break. (Ofcourse, metals in general are more ductile than materialssuch as stone, brick and concrete.) Obviously, it is farmore desirable for a building to sustain a limited amountof deformation than for it to suffer a completebreakagefailure.

The ductility of a structure is in fact one of themost important factors affecting its earthquakeperformance. One of the primary tasks of an engineerdesigning a building to be earthquake resistant is toensure that the building will possess enough ductility towithstand the size and types of earthquakes it is likely toexperience during its lifetime.

Fig.11

DAMPING:

The last of the important structuralcharacteristics, or parameters, which we'll discuss here isdamping. As we noted earlier, ground and buildingmotion during an earthquake has a complex, vibratorynature. Rather than undergoing a single "yank" in onedirection, the building actually moves back and forth inmany different horizontal directions.

All vibrating objects, including buildings,tend to eventually stop vibrating as time goes on. Moreprecisely, the amplitude of vibration decays with time.Without damping, a vibrating object would never stop

vibrating, once it had been set in motion. Obviously,different objects possess differing degrees of damping. Abean bag, for example, has high damping; a trampolinehas low damping.

In a building undergoing an earthquake, damping – thedecay of the amplitude of a building's vibrations – is dueto internal friction and the absorption of energy by thebuilding's structural and nonstructural elements. Allbuildings possess some intrinsic damping.

The more damping a building possesses, thesooner it will stop vibrating--which of course is highlydesirable from the standpoint of earthquakeperformance. Today, some of the more advancedtechniques of earthquake resistant design andconstruction employ added damping devices like shockabsorbers to increase artificially the intrinsic damping ofa building and so improve its earthquake performance.

HOW DO YOU MADE AN EARTHQUAKEPROOF BUILDING?

The conventional approach to earthquakeresistant design of buildings depends upon providing thebuilding with strength, stiffness and inelasticdeformation capacity which are great enough towithstand a given level of earthquake–generated force.This is generally accomplished through the selection ofan appropriate structural configuration and the carefuldetailing of structural members, such as beams andcolumns, and the connections between them.

It can be achieved by two ways:

1. Base isolation devices2. Viscous dampers

BASE ISOLATION DEVICES:

It is easiest to see this principle at work byreferring directly to the most widely used of theseadvanced techniques, which is known as base isolation.A base isolated structure is supported by a series ofbearing pads which are placed between the building andthe building's foundation. A variety of different types ofbase isolation bearing pads have now been developed.For our example, we'll discuss lead–rubber bearings.These are among the frequently–used types of baseisolation bearings. A lead–rubber bearing is made fromlayers of rubber sandwiched together with layers ofsteel. In the middle of the bearing is a solid lead "plug."On top and bottom, the bearing is fitted with steel plateswhich are used to attach the bearing to the building and


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foundation. The bearing is very stiff and strong in thevertical direction, but flexible in the horizontal direction

Fig.12

EARTHQUAKE GENERATED FORCES:

To get a basic idea of how base isolation works.This shows an earthquake acting on both a base isolatedbuilding and a conventional, fixed–base, building. As aresult of an earthquake, the ground beneath eachbuilding begins to move. In Figure 13, it is shownmoving to the left.

Fig.13

Each building responds with movement whichtends toward the right. We say that the buildingundergoes displacement towards the right. The building'sdisplacement in the direction opposite the ground motionis actually due to inertia. The inertial forces acting on abuilding are the most important of all those generatedduring an earthquake.

It is important to know that the inertial forceswhich the building undergoes are proportional to thebuilding's acceleration during ground motion. It is alsoimportant to realize that buildings don't actually shift inonly one direction.

Fig.14

Because of the complex nature of earthquakeground motion, the building actually tends to vibrateback and forth in varying directions. So, Figure 3 isreally a kind of "snapshot" of the building at only oneparticular point of its earthquake response

In addition to displacing toward the right, theun–isolated building is also shown to be changing itsshape– from a rectangle to a parallelogram. We say thatthe building is deforming. The primary cause ofearthquake damage to buildings is the deformationwhich the building undergoes as a result of the inertialforces acting upon it.

The different types of damage which buildingscan suffer are quite varied and depend upon a largenumber of complicated factors. But to take one simpleexample, one can easily imagine what happens to twopieces of wood joined at a right angle by a few nails,when the very heavy building containing them suddenlystarts to move very quickly — the nails pull out and theconnection fails.

RESPONSE OF BASE ISOLATED BUILDING:

By contrast, even though it too is displacing,the base–isolated building retains its original,rectangular shape. It is the lead–rubber bearingssupporting the building that are deformed. The base–isolated building itself escapes the deformation anddamage—which implies that the inertial forces actingon the base–isolated building have been reduced.

Experiments and observations of base–isolatedbuildings in earthquakes have been shown to reducebuilding accelerations to as little as 1/4 of theacceleration of comparable fixed–base buildings, whicheach building undergoes as a percentage of gravity. As


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we noted above, inertial forces increase, and decrease,proportionally as acceleration increases or decreases.

Acceleration is decreased because the baseisolation system lengthens a building's period ofvibration, the time it takes for the building to rock backand forth and then back again. And in general, structureswith longer periods of vibration tend to reduceacceleration, while those with shorter periods tend toincrease or amplify acceleration.

Finally, since they are highly elastic, therubber isolation bearings don't suffer any damage. Butwhat about that lead plug in the middle of our examplebearing? It experiences the same deformation as therubber. However, it also generates heat as it does so.

In other words, the lead plug reduces, ordissipates, the energy of motion—i.e., kinetic energy—by converting that energy into heat. And by reducing theenergy entering the building, it helps to slow andeventually stop the building's vibrations sooner thanwould otherwise be the case —in other words, it dampsthe building's vibrations. (Damping is the fundamentalproperty of all vibrating bodies which tends to absorbthe body's energy of motion, and thus reduce theamplitude of vibrations until the body's motioneventually ceases.)

SPHERICAL SLIDING ISOLATION SYSTES:

As we said earlier, lead–rubber bearings are justone of a number of different types of base isolationbearings which have now been developed. SphericalSliding Isolation Systems are another type of baseisolation. The building is supported by bearing pads thathave a curved surface and low friction

During an earthquake, the building is free toslide on the bearings. Since the bearings have a curvedsurface, the building slides both horizontally andvertically. The force needed to move the buildingupwards limits the horizontal or lateral forces whichwould otherwise cause building deformations. Also, byadjusting the radius of the bearing's curved surface, thisproperty can be used to design bearings that alsolengthen the building's period of vibration.

For more information read this article titledProtective Systems for Buildings: Application ofSpherical Sliding Isolation Systems as it describes oneparticular type of spherical sliding isolation system, andits successful use in making some structures moreearthquake resistant.

Fig.15

DAMPERS:

The second of the major new techniques forimproving the earthquake resistance of buildings alsorelies upon damping and energy dissipation, but itgreatly extends the damping and energy dissipationprovided by lead–rubber bearings.

As we've said, a certain amount of vibrationenergy is transferred to the building by earthquakeground motion. Buildings themselves do possess aninherent ability to dissipate, or damp, this energy.However, the capacity of buildings to dissipate energybefore they begin to suffer deformation and damage isquite limited.

The building will dissipate energy either byundergoing large scale movement or sustainingincreased internal strains in elements such as thebuilding's columns and beams. Both of these eventuallyresult in varying degrees of damage. So, by equipping abuilding with additional devices which have highdamping capacity, we can greatly decrease the seismicenergy entering the building, and thus decrease buildingdamage.

Accordingly, a wide range of energydissipation devices have been developed and are nowbeing installed in real buildings. Energy dissipationdevices are also often called damping devices. The largenumber of damping devices that have been developedcan be grouped into three broad categories:

1. Friction Dampers– these utilize frictional forcesto dissipate energy

2. Metallic Dampers– utilize the deformation ofmetal elements within the damper

3. Viscoelastic Dampers– utilize the controlledshearing of solids

4. Viscous Dampers– utilized the forcedmovement (orificing) of fluids within thedamper


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FLUID VISCOUS DAMPERS:

Once again, to try to illustrate some of thegeneral principles of damping devices, we'll look moreclosely at one particular type of damping device, theFluid Viscous Damper, which is one variety of viscousdamper that has been widely utilized and has proven tobe very effective in a wide range of applications.

The article, titled Application of Fluid ViscousDampers to Earthquake Resistant Design, describes thebasic characteristics of fluid viscous dampers, theprocess of developing and testing them, and theinstallation of fluid viscous dampers in an actualbuilding to make it more earthquake resistant.

Damping devices are usually installed as part ofbracing systems. Figure 16 shows one type of damper–brace arrangement, with one end attached to a columnand one end attached to a floor beam. Primarily, thisarrangement provides the column with additionalsupport.

Most earthquake ground motion is in a horizontaldirection; so, it is a building's columns which normallyundergo the most displacement relative to the motion ofthe ground. Figure 16 also shows the damping deviceinstalled as part of the bracing system and gives someidea of its action.

Fig.16

FLUID DAMPER DESIGN:

The design elements of a fluid damper are relativelyfew. However, the detailing of these elements variesgreatly and can, in some cases, become both difficultand complex. Figure 17 depicts a typical fluid damperand its parts. It can be seen that by simply moving thepiston rod back and forth, fluid is orificed through thepiston head orifices, generating damping force.

Major part descriptions are as follows, usingFigure 17 as reference:

Piston Rod:

Highly polished on its outside diameter, the pistonrod slides through the seal and seal retainer. The externalend of the piston rod is affixed to one of the twomounting clevises. The internal end of the piston rodattaches to the piston head. In general, the piston rodmust react all damping forces, plus provide a sealinginterface with the seal. Since the piston rod is relativelyslender and must support column loading conditions, itis normally manufactured from high-strength steelmaterial. Stainless steel is preferred as a piston rodmaterial, since any type of rust or corrosion on the rodsurface can cause catastrophic seal failure. In addition,the design of the piston rod should be strain based, ratherthan stress based, since elastic flexing of the piston rodduring damper compression can cause binding or sealleakage.

Fig.17

Cylinder :

The damper cylinder contains the fluid mediumand must accept pressure vessel loading when thedamper is operating. Cylinders are usually manufacturedfrom seamless steel tubing. Welded or cast constructionis not permissible for damper cylinders, due to concernsabout fatigue life and stress cracking. Cylindersnormally are designed for a minimum proof pressureloading equal to 1.25 times the internal pressureexpected under a maximum credible seismic event. Bydefinition, the proof pressure loading must beaccommodated by the cylinder without yielding,damage, or leakage of any type.

Fluid :

Dampers used in structural engineeringapplications require a fluid that is fire-resistant,nontoxic, thermally stable, and will not degrade withage. This fluid must be classified as both nonflammableand non-combustible, with a fluid flashpoint above 90°C. At present, the only fluids possessing all of theseattributes are from the silicone family. Typical siliconefluids have a flashpoint in excess of 340° C, arecosmetically inert, completely non-toxic, and arethermally stable. The typical silicone fluid used in a


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damper is virtually identical to the silicone usedincommon hand and facial cream cosmetics.

Seal :The seals used in a fluid damper must be capable

of a long service life; at least 25 years without requiringperiodic replacement. The seal materials must becarefully chosen for this service life requirement and forcompatibility with the damper=s fluid. Since dampers instructures are often subject to long periods of infrequentuse, seals must not exhibit long-term sticking nor allowslow leakage of fluid. Most dampers use dynamic sealsat the piston rod interface, and static seals wherethe end caps or seal retainers are attached to the cylinder.For static seals, conventional elastomer oring seals haveproven to be acceptable. Dynamic seals for the pistonrod should be manufactured from high-strengthstructural polymers, to eliminate sticking or compressionset during long periods of inactivity. Typical dynamicseal materials include Teflon®, stabilized nylon, andmembers of the acetyl resin family. Dynamic sealsmanufactured from structural polymers do not age,degrade, or distort over time. In comparison,conventional elastomers will require periodicreplacement if used as dynamic seals in a damper.

Fig.18

Piston Head :

The piston head attaches to the piston rod, andeffectively divides the cylinder into two pressurechambers. As such, the piston head serves to sweep fluidthrough orifices located inside it, thus generatingdamping pressure. The piston head is usually a veryclose fit to the cylinder bore; in some cases the pistonhead may even incorporate a seal to the cylinder bore.

Piston heads are relatively simple in appearance.However, the orifice passages machined or built into thepiston head usually have very complex shapes,depending on the damping output equation selected.

Seal Retainer :

Used to close open ends of the cylinder, these areoften referred to as end caps, end plates, or stuffingboxes. It is preferable to use large diameter threadsturned on either the exterior or interior surface of thecylinder to engage the seal retainer. Alternate attachmentmeans, such as multiple bolts, studs, or cylinder tie rodsshould be avoided as these can be excited to resonanceby high frequency portions of either the earthquaketransient or the building response spectra.

Accumulator :

The simple damper depicted in Figure 4 utilizesan internal, in-line rod make-up accumulator. Theaccumulator consists of either a block of closed cellplastic foam, a moveable (and gas pressurized)accumulator piston, or a rubber bladder. The purpose ofthe accumulator is to allow for the volumetricdisplacement of the piston rod as it enters or exits thedamper during excitation. A second purpose is tocompensate for thermal expansion and contraction of thefluid. The damper in Figure 4 uses a control valve tometer the amount of fluid displaced into the accumulatorwhen the damper is being compressed. When the damperextends, the control valve opens to allow fluid from theaccumulator to freely enter the damper pressurechambers. Some types of dampers use a so-called“through rod,” where the piston rod goes entirelythrough the damper cylinder. These dampers do notrequire accumulators at all, but do require two sets ofseals.

Orifices :

The pressurized flow of the fluid through thepiston head is controlled by orifices. These can consistof complex modular machined passageways, oralternately, can use drilled holes, springloaded balls,poppets, or spools. Relatively complex orifices areneeded if the damper is to produce output with adamping exponent of less than two. Indeed, a simpledrilled hole orifice will follow Bernoulli’s equation, anddamper output will be limited to varying force with thesquare of the damper velocity. Since “velocity squared”damping is of limited use in seismic energy dissipation,more robust and sophisticated orifice methods areusually required. Depending on the damping outputequation desired, orifice passages may utilize


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converging or diverging flows, vortexes may be inducedto form at specific areas, or flow passages may bend ortwist radically.

DESIGN AND IMPLEMENTATION OF FLUIDDAMPERS:

A combination caisson/mat system was selectedfor the foundation of the tower. The reinforced concretemat system connects a series of caissons of up to 1.2mdiameter, reaching down only 40m into a rubble layerbelow the soft surface soil.

The concrete mat thickness varies from 1m-2m inthickness and ties together the caissons and the 0.8mthick foundation walls. The seismic code requirementsfor Mexico City involve the use of shock responsespectra, with the associated site transients. This iscombined with a limitation on allowable soil-bearingstress. The design team evaluated more than 25 differentstructural systems, but was unable to find a structuralconfiguration allowing a 55-floor building to beconstructed at the site. The best configurations yielded adesign with 35-38 floors maximum. The engineers notedthat it was probably no coincidence that the tallestexisting structures in Mexico City are roughly thisheight.

Fig.19

The potential of adding viscous damping to thestructure was evaluated as a means to reduce structuralstress during seismic loadings. The underlying designconcept was to use the dampers to reduce stress, thenlighten the building frame by removing steel until thestress crept up to the code allowables. Conceptually, thesteel that had been “removed” by this process could thenbe used to add additional floors.

Fig.20

For the Torre Mayor, inherent structuraldamping in the frame was assumed to be 1% of critical.Multiple computer runs were made with added fluiddamping in 2% increments. The approach used was toadd damping until a lightweight 55-plus story buildingwould result or until damping reached a value of 30%critical, at which point Constantinou and Symans’research indicated that peak stresses would begin toincrease.When the added damping in the structure reached 10%critical the resulting maximum height structure wascalculated to be 57 floors. The structural detailing of the.

Fig.21

new tower could begin, having achieved the goals of thebuilding’s owner for a 55-plus story structure. Figure 21is the architectural drawing of the building

TUNED MASS DAMPERS:

A tuned mass damper, also known as a harmonicabsorber, is a device mounted in structures to reduce the


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amplitude of mechanical vibrations. Their applicationcan prevent discomfort, damage, or outright structuralfailure. They are frequently used in power transmission,automobiles, and buildings.

Fig.22

. A schematic of a simple spring–mass–damper

system used to demonstrate the tuned mass dampersystem.

Tuned mass dampers stabilize against violentmotion caused by harmonic vibration. A tuned damperreduces the vibration of a system with a comparativelylightweight component so that the worst-case vibrationsare less intense. Roughly speaking, practical systems aretuned to either move the main mode away from atroubling excitation frequency, or to add damping to aresonance that is difficult or expensive to damp directly.

Fig.23

Wide span structures (bridges, spectatorstands, large stairs, stadium roofs) as well as slender tallstructures (chimneys, high rises) tend to be easilyexcited to high vibration amplitudes in one of their basicmode shapes, for example by wind or marching andjumping people. Low natural frequencies are typical forthis type of structures, due to their dimensions, as is theirlow damping. With GERB Tuned Mass Dampers(TMD), these vibrations can be reduced very effectively.

The TMD may consist of:

Spring Oscillating Mass Viscodamper

Fig. Tuned Mass Damper

as main components, or may be designed as a pendulum,also in combination with a Viscodamper.

Each TMD is tuned exactly to the structure anda certain natural frequency of it. Such TMD have beendesigned and built with an oscillating mass of 40 to10.000 kg (90 to 22.000 lbs) and natural frequenciesfrom 0.3 to 30 Hz. Vertical TMD are typically acombination of coil springs and Viscodampers®, whilein case of horizontal and torsional excitation in thecorresponding horizontal TMD the coil springs arereplaced by leaf springs or pendulum suspensions.

CONCLUSION:

In the modern world man has to face challengingproblems due to natural calamities. There is a need toovercome those problems to survive his life. It isdifficult asses these problems, earthquakes are comes inthat order. If an earthquake occurs it can cause moredamage than other calamities due collapse of structures.As a civil engineer we have the responsibility to givehealthy full environment to the people. We stated abovesome of the techniques to reduce the damage of thestructures due to earthquakes.

‘’We are here only to provide safety to public to leadtheir life’’,


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Fig.22




Fig.23







CONCLUSION:




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Fig.22




Fig.23







CONCLUSION:




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REFERENCES:

www.multyscience.co.uk

www.institute of structuralengineer.com

www.faddoengineers.com

Technological advance in Japanese building designand construction

http://www.taipei-101.com.tw/en/Tower/buildind_13-1.html

http://www.esm-gmbh.de/EN/Products/Tuned_mass_dampers

Engineering

Earthquake Resistence structures