1.0INTRODUCTION

1.1 Fundamental Concepts Of Base Isolation

The term base isolation uses the word a) isolation in itsmeaning of the state of being separated and b) base as a partthat supports from beneath or serves as a foundation for anobject or structure. As suggested in the literal sense, thestructure (a building, bridge or piece of equipment) isseparated from its foundation. The original terminology ofbase isolation is more commonly replaced with seismicisolation nowadays, reflecting that in some cases theseparation is somewhere above the base – for example, in abuilding the superstructure may be separated from substructurecolumns. In another sense, the term seismic isolation is moreaccurate anyway in that the structure is separated from theeffects of the seism, or earthquake. The only way a structurecan be supported under gravity is to rest on the ground.Isolation conflicts with this fundamental structuralengineering requirement. How can the structure be separatedfrom the ground for earthquake loads but still resist gravity?It is practical isolation systems that provide a compromisebetween attachment to the ground to resist gravity andseparation from the ground to resist earthquakes. Seismicisolation is a means of reducing the seismic demand on thestructure.

1

Figure 1: concept of base isolation

1.2 Basic Elements Of Base Isolation

Seismic Isolation increases the fundamental period ofvibration so that the structure is subjected to lowerearthquake forces. However, the reduction in force isaccompanied by an increase in displacement demand which mustbe accommodated within the flexible mount, Furthermore, longerperiod buildings can be lively under service loads. Thefollowing are three basic elements in any practical isolationsystem, they are:

1. A flexible mounting so that the period of vibration of thebuilding is lengthened sufficiently to reduce the forceresponse.2. A damper of energy dissipater so that the relativedeflections across the flexible mounting can be limited to apractical design level.3. A means of providing rigidity under low (service) loadlevels such as wind and braking force.

2

Flexibility: Due to additional flexibility the period ofstructure is elongated. From the acceleration response curveshown in it may be observed that reductions in base shearoccur as the period of vibration of the structure islengthened. The extent to which these forces are reduced isprimarily dependent on the nature of the earthquake groundmotion and the period of the non-isolated structure.

Figure 1.1: acceleration response curve

Energy dissipation: Additional flexibility needed tolengthen the period of the structure will give rise to largerelative displacement across the flexible mount. Largerelative displacements can be controlled if substantialadditional damping is introduced into the structure at theisolation level. One of the most effective means of providinga substantial level of damping is through hysteric energydissipation. The hysteric refers to the offset between theloading and unloading curves under cyclic loading.

Rigidity under low lateral loads: While lateralflexibility is highly desirable for high seismic loads, it is

3

clearly undesirable to have a structural system which willvibrate perceptibly under frequently occurring loads such aswind Loads or braking loads. Mechanical energy dissipaters maybe used to provide rigidity at these service loads by virtueof their high initial elastic stiffness.

1.3 Action Of Base Isolation

Hence, “base isolation” or, seismic isolation separates upperstructure from base or, from down structure by changing of fixjoint with flexible one. Increasing of flexibility is done bythe insertion of additional elements in structure, known asisolators. Usually, these isolators are inserted between upperstructure and foundation. Seismic isolation system absorbslarger part of seismic energy. Therefore, vibration effects ofsoil to upper structure are drastically reduced. For baseisolated structure the situation is quite different. In suchcases, the whole upper structure gets a displacement (whichnaturally remains in limits) and the relative displacement ofdifferent stories is so small that the structure can withstanda comparatively high seismic tremor with a low seismic loadingin a safe, efficient and economic manner.

1.4 Goal Of Base Isolation

A high proportion of the world is subjected to earthquakes andsociety expects that structural engineers will design ourbuildings so that they can survive the effects of theseearthquakes. As for all the load cases encountered in thedesign process, such as gravity and wind, should work to meeta single basic equation: CAPACITY > DEMAND.

Earthquakes happen and are uncontrollable. So, in that sense,we have to accept the demand and make sure that the capacity

4

exceeds it. The earthquake causes inertia forces proportionalto the product of the building mass and the earthquake groundaccelerations. As the ground accelerations increases, thestrength of the building, the capacity, must be increased toavoid structural damage. But it is not practical to continueto increase the strength of the building indefinitely. In highseismic zones the accelerations causing forces in the buildingmay exceed one or even two times the acceleration due togravity, g. It is easy to visualize the strength needed forthis level of load – strength to resist 1 g means than thebuilding could resist gravity applied sideways, which meansthat the building could be tipped on its side and heldhorizontal without damage. Designing for this level ofstrength is not easy, nor cheap. So, most codes allowengineers to use ductility to achieve the capacity. Ductilityis a concept of allowing the structural elements to deformbeyond their elastic limit in a controlled manner. Beyond thislimit, the structural elements soften and the displacementsincrease with only a small increase in force. The elasticlimit is the load point up to which the effects of loads arenon- permanent; that is, when the load is removed the materialreturns to its initial condition. Once this elastic limit isexceeded changes occur. These changes are permanent and nonreversible when the load is removed. A design philosophyfocused on capacity leads to a choice of two evils:

1. Continue to increase the elastic strength. This isexpensive and for buildings leads to higher flooraccelerations. Mitigation of structural damage by furtherstrengthening may cause more damage to the contents than wouldoccur in a building with less strength.2. Limit the elastic strength and detail for ductility. Thisapproach accepts damage to structural components, which maynot be repairable. Base isolation takes the opposite approach, it attempts toreduce the demand rather than increase the capacity. We cannotcontrol the earthquake itself but we can modify the demand itmakes on the structure by preventing the motions beingtransmitted from the foundation into the structure above. So,the primary reason to use isolation is to mitigate earthquakeeffects. Naturally, there is a cost associated with isolationand so it only makes sense to use it when the benefits exceed

5

this cost. And, of course, the cost benefit ratio must be moreattractive than that available from alternative measures ofproviding earthquake resistance. Nowadays Base Isolation isthe most powerful tool of the earthquake engineeringpertaining to the passive structural vibration controltechnologies. It is meant to enable a building or non-buildingstructure to survive a potentially devastating seismic impactthrough a proper initial design or subsequent modifications.In some cases, application of Base Isolation can raise both astructure's seismic performance and its seismic sustainabilityconsiderably

1.5 Comparative Study Of Base Isolation And Fixed Base:A large proportion of world’s population lives in regions ofseismic hazards, at risk from earthquakes of varying severityand frequency of occurrence. Earthquake causes significantloss of life and damage of property every year. So, tomitigate the effect of earthquake on building the baseisolation technique one of the best solutions. Seismicisolation consists of essentially the installation ofmechanisms which decouple the structure from base by providingseismic isolators. The seismic isolation system is mountedbeneath the superstructure and is referred as ‘BaseIsolation’. The main purpose of the base isolation device isto minimize the horizontal acceleration transmitted to thesuperstructure. Base isolation is very promising technology toprotect different structures like building, bridges, airportterminals and nuclear power plants etc. from seismicexcitation.

The earthquakes in the recent past have provided enoughevidence of performance of different type of structures underdifferent earthquake conditions and at different foundation

6

conditions as a food for thought to the engineers andscientists. This has given birth to different type oftechniques to save the structures from the earthquakeseffects. Conventional seismic design attempts to makebuildings that do not collapse under strong earthquakeshaking, but may sustain damage to non-structural elements(like glass facades) and to some structural members in thebuilding. This may render the building non-functional afterthe earthquake, which may be problematic in some structures,like hospitals, which need to remain functional in theaftermath of the earthquake. Two basic technologies are usedto protect buildings from damaging earthquake effects. Theseare Base Isolation Devices and Seismic Dampers. Base isolationis also known as ‘seismic base isolation’ or ‘base isolationsystem’. Seismic isolation separates the structure from theharmful motions of the ground by providing flexibility andenergy dissipation capability through the insertion of theisolated device so called isolators between the foundation andthe building structure.

7

Figure 2: fixed base andisolated base

2. EARTHQUAKE PROTECTIVE SYSTEMS

Structural control for seismic loads is a rapidly expandingfield and the family of control systems, also known asearthquake protective systems, now embraces passive, activeand hybrid systems. Applications to buildings, bridges andindustrial plant have been made in many of the seismicallyactive countries of the world. Structural control provides analternative to conventional design methods, which are based onductile (yielding) response. In many applications, elasticperformance during large earthquake events is economicallyfeasible and the methodology permits performance-based designcriteria, now required in many modern seismic design codes, tobe satisfied more readily than conventional methods.Applications to the retrofit of existing structures have been

8

particularly attractive, especially to the upgrading ofhistorical buildings.

As noted above, the family of earthquake protective systemshas grown to include passive, active and hybrid (semi-active)systems as shown in Figure 1. Passive systems are perhaps thebest known and these include seismic (base) isolation andpassive (mechanical) energy dissipation. Certainly isolationis the most developed member of the family at the present timewith continuing developments in hardware, applications, designcodes and retrofit manuals. But there has also been continuinggrowth in the passive energy dissipation field with anincreasing number of different kinds of dissipation devicesbeing developed, accompanied by technically complexapplications such as those found in near-field sites.

Figure3: family of earthquake protective systems

2.1 Passive Protective Systems

9

Research and development on passive energy dissipation systemsbegan about the same time as modern isolation, motivated atthe time, by a need to control isolator displacements. Thusthe early passive energy dissipaters were used in associationwith elastomeric and sliding isolators and were principally ofthe metallic type (steel cones and tapered plates). Today thestate-of-the-art has evolved to the point where they are usedas an alternate to isolation and may be incorporated withinthe structural frame of a building or between the super andsubstructures of a bridge. Their function is to dissipateenergy during an earthquake and reduce the demand on thestructural frame. However, these dampers will also addstrength and stiffness to the frame and if these contributionsare large compared to the frame itself, they may offset theadvantages generated by the increase in damping.Nevertheless, significant reductions in damage to the frame(ductile yielding) are possible but not to the same extent asthat provided by an isolation system. However, unlikeisolation, passive energy dissipators also reduce windresponse and are applicable to a wider range of structures,especially those sensitive to long period ground motions. Inretrofit situations they may also be less intrusive than acomparable isolation system. Passive energy dissipators may besimply classified as hysteretic or viscoelastic [Constantinou,Soong, and Dargush, 1998]. Hysteretic dissipators include theyielding of metals due to flexure, shear, torsion, orextrusion (metallic dampers) and sliding (friction dampers).They are all essentially displacement-dependent devices.

Viscoelastic systems include viscoelastic solids, fluidorificing (fluid dampers), and viscoelastic fluids. They areessentially velocity-dependent devices (viscous in nature) andmany are also frequency dependent. Some passive energydissipators are modifications of the above set and may includeelastic springs or pressurized cylinders to develop pre-loadand re-centering capabilities. In a subset of passive energydissipators are tuned mass dampers (TMD) and tuned liquiddampers (TLD). Used principally for controlling windvibrations in elastic structures these vibration absorbersdampen motion by transferring kinetic energy between variousmodes of vibration. Tuned to a particular dominant mode, sucha device may enhance the damping in tall structures by about

10

5%, which is usually sufficient for improved comfort levelsduring wind storms. Seismic applications are however rare andthese devices are not discussed further in this paper. It isperhaps worth noting that TMDs are frequently used during theconstruction of tall slender structures, such as the towers ofsuspension bridges, for protection against wind loads. Theworld’s longest suspension bridge, the Akashi-Kaikyo bridgenear Kobe, Japan, was under construction at the time of theGreat Hanshin Earthquake in 1995. The epicenter was on theNojima fault which passes between the two towers at a shallowangle. Although at their full height at the time of theearthquake, the towers were not damaged, possibly due (inpart) to the presence of the TMDs. Common passive energydissipators in use today include hysteretic flexural,torsional and extrusion devices, friction dampers, andviscoelastic solid and fluid dampers. In 1998, the number ofapplications in North America (Mexico, US and Canada) was ofthe order of 80 [Constantinou, Soong and Dargush, 1998]. Ofthese about onehalf were viscoelastic fluid dampers, one-thirdwere friction dampers, and the remaining one-sixth includedhysteretic, viscoelastic solid, and tuned mass dampers.

2.2 Active, Hybrid And Semi-Active ControlSystems

Active, semi-active and hybrid structural control systems area natural evolution of passive control technologies such asbase isolation and passive energy dissipation. The possibleuse of active control systems and some combinations of passiveand active systems, so called hybrid systems, as a means ofstructural protection against wind and seismic loads hasreceived considerable attention in recent years. Active/hybridcontrol systems are force delivery devices integrated withreal-time processing evaluators/controllers and sensors withinthe structure. They act simultaneously with the hazardousexcitation to provide enhanced structural behavior forimproved service and safety. Remarkable progress has been made

11

over the last twenty years. The First and Second WorldConferences on Structural Control held in 1994 [Housner et al,1994b] and 1998 [Kobori et al, 1998], respectively, attractedover 700 participants from 17 countries and demonstrated theworldwide interest instructural control. Research to date has reached the stagewhere active systems have been installed in full-scalestructures. Active systems have also been used temporarily inconstruction of bridges or large span structures (e.g.,lifelines, roofs) where no other means can provide adequateprotection.

12

Figure 6: structure with active control

13

14

3. ISOLATION SYSTEMSDespite wide variation in detail base isolation techniquefollow two basic approaches with certain common features. Inthe first approach the isolation system introduces the layerof low lateral stiffness between the structure and thefoundation. With this isolation layer the structure has anatural period that is much longer than its fixed base naturalperiod. as shown by the elastic diagram spectrum, thislengthening of period can reduce the pseudo acceleration andhence the earthquake induced forces in the structure but thedeformation is increased. This deformation is concentrated inthe isolation system, however, accompanied by only smalldeformations in the structure .this type of structure iseffective even if the system is linear and undamped. Dampingis beneficial, however, in further reducing the forces in thestructure and the deformation in the isolation system.

The most common system of this uses short, cylindricalbearings with one or more holes and alternating layers ofsteel plates and hard rubber. Interposed between the base ofthe structure and the foundation these laminated bearings arestrong and stiff under vertical loads yet very flexible underlateral forces. because the natural damping of the structureis low. Additional damping is provided in the form ofmechanical damper.

The second most common type of isolation system uses slidingelements between the foundation and the base of the structure.The shear force transmitted to the structure across theisolation interface is limited by keeping the coefficient offriction as low as practical. However, the friction must besufficiently high to sustain strong winds and earthquakeswithout sliding, a requirement that reduces the isolationeffect. In this type of isolation system the slidingdisplacements are controlled by high tension springs and

15

laminated rubber bearings,or by making the sliding systemscurved;these mechanisms prvide a restoring force, otherwiseunavailable in this type of system, to return the structure toits equilibrium position.the friction pendulum system FPS issliding isolation system wherein the weight of the structureis supposed on spherical sliding surfaces that slide relativeto eachother when the ground motion exceeds a thresholdlevel.the restoring action is caused by raising the buildingslightly when sliding occurs on the spherical surface. Thedynamics of structures on slider type isolation systems iscomplicated because the slip process is intrinsically nonlienear.

3.1 Recognition Of Isolation Types

16

Many types of isolation system have been proposed and havebeen developed to varying stages, with some remaining no morethan concepts and others having a long list of installedprojects. A discussion of generic types of system and focus ondifferent types (especially rubber bearing) of isolators alongwith their characteristics is provided subsequently.

4.0 TYPES OF ISOLATORThe development of isolators ensured the properties required for the achievement of perfect base isolation. The chart details the various types of Isolators used through out the world. A brief description along with their basic functions and advantages is also included just after the chart. As the present research is mainly highlighting the use of LRB and HDRB type of isolator, so special attention is given to their characteristics.

17

4.1 Bearing Type

4.1.1 Elastomeric (Rubber) Bearings

Rubber bearings are formed of horizontal layers of natural orsynthetic rubber in thin layers bonded between steel plates.The steel plates prevent the rubber layers from blown up orbusting. In such mechanism the bearing is capable to supporthigher vertical loads with only smaller deflection (typically1 to 3 mm under full gravity load). The internal steel layersdo not restrict horizontal deformations of the rubber layersin shear. So, the bearings are much more flexible underlateral loads than vertical loads. This is why; the bearingworks as a flexible unit.

Characteristics Of Rubber Isolator

As described earlier, isolation system works with theprinciple that a rigid mass is isolated from a flexiblesupporting structure. Optimum isolation of a building fromground may be achieved by choosing a rubber bearing isolatorbased on the knowledge of its static and dynamiccharacteristics determined from laboratory experiments. Forthis reason, understanding the properties of rubber isolatorsis necessary for the vibration analysis. Some of the importantproperties are as follows.

(a) Load Capacity And Size Of Rubber Bearings

18

For most bearing types the plan size required increases asvertical load increases but the height or radius is constantregardless of vertical loads. This is because all bearings atsuch stages are subjected to the same displacement. Therefore,the bearing can be sized according to the vertical loads theysupport. Three types of curves are seen in the plot. All thecurves are showing that the lower the vertical load, the lowerthe required bearing diameter.

(b) Absorption

Shocks originating due to the occurrence of an earthquake canbe controlled if substantial additional damping is introducedinto the isolation system. A high damping rubber isolatorprovides a substantial level of damping through hystereticenergy dissipation. Hysteretic refers to the offset betweenthe loading and unloading curves under cyclic loading. Figureshows an idealized force- displacement loop where the enclosed

19

area is measure of the energy dissipated during one cycle ofmotion.

(c) Durability Under Cyclic Loading

Rubber isolator remains more or less stable under cyclicloading. Results of cyclic displacement test applied, to arubber isolator shows that at a speed equivalent to an actualseismic event the friction factor remains stable. Figure showsa typical friction factor versus number of cycles in a cyclicloading test of rubber isolator. The figure represents that

20

rubber isolator is durable. There are mainly two types ofRubber Bearing. They are LRB and HDRB.

Lead rubber bearing (LRB)

This type of elastomeric bearings consist of thin layers oflow damping natural rubber and steel plates built in alternatelayers and a lead cylinder plug firmly fitted in a hole at itscentre to deform in pure shear as shown in Figure 14. The LRBwas invented in New Zealand in 1975 and has been usedextensively in New Zealand, Japan and United States. The steelplates in the bearing force the lead plug to deform in shear.This bearing provides an elastic restoring force and also, byselection of the appropriate size of lead plug, producesrequired amount of damping. The force deformation behavior ofthe bearing is shown in Figure. Performance of LRB ismaintained during repeated strong earthquakes, with properdurability and reliability.

21

Basic functions of LRB

(1) Load supporting function: Rubber reinforced with steelplates provides stable support for structures. Multilayerconstruction rather than single layer rubber pads providesbetter vertical rigidity for supporting a building.(2) Horizontal elasticity function (prolongedoscillation period): With the help of LRB, earthquakevibration is converted to low speed motion. As horizontalstiffness of the multi- layer rubber bearing is low, strongearthquake vibration is lightened and the oscillation periodof the building is increased.(3) Restoration function: Horizontal elasticity of LRBreturns the building to its original position. In a LRB,elasticity mainly comes from restoring force of the rubberlayers. After an earthquake this restoring force returns thebuilding to the original position.(4) Damping function: Provides required amount of dampingnecessary. LRB mainly are of two shapes. One is conventionalround and the other type is square. Though their basicfunction remains same, yet changes in shapes are advantageous.In many occasions as economy concern, reduced size, stabilityand capacity for large deformation.

22

High Damping Rubber Bearing (HDRB)

HDRB is one type of elastomeric bearing. This type of bearingconsist of thin layers of high damping rubber and steel platesbuilt in alternate layers as shown in Figure. The verticalstiffness of the bearing is several hundred times thehorizontal stiffness due to the presence of internal steelplates. Horizontal stiffness of the bearing is controlled bythe low shear modulus of elastomer while steel plates provideshigh vertical stiffness as well as prevent bulging of rubber.High vertical stiffness of the bearing has no effect on thehorizontal stiffness. The damping in the bearing is increasedby adding extra-fine carbon block, oils or resins and otherproprietary fillers. The dominant features of HDRB system arethe parallel action of linear spring and viscous damping. Thedamping in the isolator is neither viscous nor hysteretic, butsomewhat in between. The ideal force deformation behaviour ofthe bearing is shown in Figure.

23

Figure 15: high damping rubber bearings

Figure 16: force deformation behaviour

Basic functions of HDRB

(1) Vertical load bearing function: Rubber reinforced withsteel plates provides stable support for structures.Multilayer construction rather than single layer rubber padsprovides better vertical rigidity for supporting a building.(2) Horizontal elasticity function (prolonged oscillationperiod): With the help of HDRB earthquake vibration isconverted to low speed motion. As horizontal stiffness of themulti- layer rubber bearing is low, strong earthquake

24

vibration is lightened and the oscillation period of thebuilding is increased.(3) Restoration function: Horizontal elasticity of HDRBreturns the building to its original position. In a HDRB,elasticity mainly comes from restoring force of the rubberlayers. After an earthquake this restoring force returns thebuilding to the original position.(4) Damping function: Provides required amount of damping upto a higher value.

4.1.2 Sliding Bearings

Sliding isolation system (Figure) is simple in concept and ithas a theoretical appeal. A layer with a defined coefficientof friction will limit the acceleration to this value and theforces, which can be transmitted, will also be limited to thecoefficient of friction multiplied by weight.

Figure 17: Sliding bearings

Following are some of the utilities of using sliders:

(1)Sliding movement provides flexibility and the forcedisplacement traces a rectangular shape that is the optimumfor equivalent viscous damping.(2) A pure sliding system will have unbounded displacements,with an upper limit equal to the maximum ground displacementsfor a coefficient of friction close to zero. Two types of

25

sliding systems are commonly used. A brief description withtheir basic functions, advantages and suitability is asfollows.

Sliding Support With Rubber-Pad (SSR)

When sliding base isolation system incorporates multilayernatural rubber pad then it is known as SSR. Advantages ofsuch bearings are:(1) SSR can provide vibration isolation for light loads aswell as large deformation performance like a large-scaleisolation system.(2) It provides protection against a wide range of tremorsfrom small vibrations to major earthquakes.(3) It can be used in conjunction with other isolation systemssuch as LRB and HDRB.

Basic functions of SSR

For small vibrations, shear deformation of the rubber layersprovides the same isolation effect as conventional multilayerrubber bearings. For large vibrations, sliding materials slideto provide the same deformation performance as large-scaleisolation systems.

Friction Pendulum System (FPS)

Sliding friction pendulum isolation system (Figure) is onetype of flexible isolation system suitable for small to large-scale buildings. It combines sliding a sliding action and arestoring force by geometry. Functions of FPS are same as SSRsystem.

Advantages of FPS include:

26

(1) It is possible to set the oscillation period of a buildingregardless of its weight.(2) This system can reduce costs not only because of the lowcost of its device but also due to the low cost ofinstallation.(3) The device is simple, works well and easy to install.Furthermore, it saves space and is practical for a seismicreinforcement.(4) Performance of such device is stable due to the highdurability of the device.(5) As it requires only a simple visual check to maintain thedevice, maintenance is very easy

Figure 18: friction pendulum system

4.2 Damping type

Damping provides sufficient resistance to structure againstservice loading. The effect of damping on dynamic response is

27

beneficial. Generally all structural systems exhibit dampingto various degrees. It is assumed that, structural damping isviscous by nature. Damping coefficient relates force tovelocity. If damping coefficient is sufficiently large, it ispossible to totally restrain the oscillatory motion. Dampingthat suppress totally the oscillatory motion is termed ascritical damping. Damping is usually neglected in frequencyand period calculations unless it exceeds about 20%.Normallytwo types of damping are used in building. A brief descriptionfollows.

4.2.1 Elementary Damping

This really means damping as a whole, i.e. the device (eachand every element) itself acts as a purely damping devicerather than an isolator. Purely damping devices can be used inlow weight buildings to restrain the oscillating motion of thebuilding. Another option may be using in conjunction withrubber bearing so-called as High Damping Rubber bearing. Insuch devices amount of damping is significantly high, usuallyfrom 8 to 15% of critical damping. Lead plug damper is one ofthe forms of elementary damping. The basic functions of suchdamper include:(a) Vibration damping Junction: Lead plug damper absorbs large vibration of the building. As the layers of rubber are distorted, the lead plug is plastically deformed and at such stage it absorbs the earthquake energy and quickly damps the vibration.(b) Trigger function: It also reduces vibration form sourceother than earthquake. For example, when vibration isgenerated by strong winds, the relative rigidity of the leadplug reduces the effect of such vibration.

4.2.2 Supplementary Damping

28

There are some types of isolators (discussed in bearing portion), which are capable of providing flexibility but not significant damping, or resistance to service loads. In order to strengthen the damping phenomena, supplementary devices areincluded with general Isolators. Damping of such type can be termed as supplementary damping. One of the most popular typesof supplementary damping is viscous damping. This device provides damping but not service load resistance. It does not have any elastic stiffness and for this reasons it adds less force to the system than other devices.

4.3 Other Types

Apart from bearing and sliding type, there are some other types of isolators, which are also used in building but rarely. Springs, rollers, sleeved piles are some examples of such isolators. A brief description of them is also included here.

4.3.1 Springs

Spring isolators are devices whose working mechanism is basedon steel springs. They are mostly used for machineryisolation. The main drawbacks of springs are two. Firstly,they are most flexible in both the vertical and horizontaldirections. Secondly, springs alone have little damping andwill move excessively under service loads.

4.3.2 Rollers

Cylindrical rollers and ball bearings are of this type. Likesprings they are commonly used for machinery isolation. The

29

resistance to movement and damping of rollers and ballbearings are sufficient under service loads.

4.3.3 Sleeved piles

The pin ended structural members, that is, piles inside asleeve provide flexibility and allow movement of the softfirst story in a building. This type of piles is knownassleeved piles. Sleeved piles provide flexibility but nodamping. Hence damping devices are required to use along withsleeved piles.

30

5.0 Applications:In recent years base isolation has become an increasinglyapplied structural design technique for buildings and bridgesin highly seismic areas. Many types of structures have beenbuilt using this approach, and many others are in the designphase or under construction. Most of the completed buildingsand those under construction use rubber isolation bearings insome way in the isolation system.

The ideas behind the concept of base isolation are quitesimple. There are two basic types of isolation systems. Thesystem that has been adopted most widely in recent years istypified by the use of elastomeric bearings, the elastomermade of either natural rubber or neoprene. In this approach,the building or structure is decoupled from the horizontalcomponents of the earthquake ground motion by interposing alayer with low horizontal stiffness between the structure andthe foundation. This layer gives the structure a fundamentalfrequency that is much lower than its fixed-base frequency andalso much lower than the predominant frequencies of the groundmotion. The first dynamic mode of the isolated structureinvolves deformation only in the isolation system, thestructure above being to all intents and purposes rigid. Thehigher modes that will produce deformation in the structureare orthogonal to the first mode and consequently also to theground motion. These higher modes do not participate in themotion, so that if there is high energy in the ground motionat these higher frequencies, this energy cannot be transmittedinto the structure. The isolation system does not absorb theearthquake energy, but rather deflects it through the dynamicsof the system. This type of isolation works when the system islinear and even when undamped; however, some damping is

31

beneficial to suppress any possible resonance at the isolationfrequency.

The second basic type of isolation system is typified by thesliding system. This works by limiting the transfer of shearacross the isolation interface. Many sliding systems have beenproposed and some have been used. In China there are at leastthree buildings on sliding systems that use a speciallyselected sand at the sliding interface. A type of isolationcontaining a lead-bronze plate sliding on stainless steel withan elastomeric bearing has been used for a nuclear power plantin South Africa. The friction-pendulum system is a slidingsystem using a special interfacial material sliding onstainless steel and has been used for several projects in theUnited States, both new and retrofit construction.

Research At EERC

Research on the development of natural rubber bearings forisolating buildings from earthquakes began in 1976 at theEarthquake Engineering Research Center (EERC) (now PEER, thePacific Engineering Research Center) of the University ofCalifornia at Berkeley. The initial research program was ajoint effort by EERC and the Malaysian Rubber ProducersResearch Association (MRPRA), U.K. The program was funded byMRPRA through a number of grants over several years, withlater funding provided by the National Science Foundation andthe Electric Power Research Institute. Professor James M.Kelly directed the research at EERC, which includedconsiderable theoretical and experimental contributions bygraduate students.

Although not an entirely new idea at the time—a few methodsusing rollers or sliders had been proposed—the concept of baseisolation was considered to be very impractical by most of thestructural engineering profession. The research project beganwith a set of hand-made bearings of extremely low-modulusrubber used with a simple three-story, single-bay, 20-tonmodel. Shaking table tests showed that isolation bearingscould bring about reductions in acceleration by factors of as

32

much as ten when compared to those of conventional design andthat, as predicted, the model would respond as a rigid bodywith all deformation concentrated in the isolation system. Itwas also clear that a certain degree of damping was needed inthe system and that the scale of the model was too small toallow more practical rubber compounds to be used.

In 1978, a more convincing demonstration of the isolationconcept was achieved with a more realistic five-story, three-bay model weighing 40 tons and by using damping-enhancedbearings made by commercial techniques. A strong interestthroughout the EERC research program was in the influence ofisolation on the response of equipment and contents in astructure, which tend to sustain more damage when conventionalmethods of seismic-resistant design are used and which, inmany buildings, are much more costly than the structureitself. An extensive series of tests on the five-story framedemonstrated that isolation with rubber bearings could providevery substantial reductions in the accelerations experiencedby internal equipment, exceeding the reductions experienced bythe structure. However, the same tests showed that whenadditional elements (such as steel energy-absorbing devices,frictional systems, or lead plugs in the bearings) were addedto the isolation system to increase damping, the reductions inacceleration to the equipment were not achieved because theadded elements also induced responses in the higher modes ofthe structure, affecting the equipment. It became clear thatthe optimum method of increasing damping was to provide it inthe rubber compound itself. This method was applied later inthe compound developed by MRPRA and used in the first base-isolated building in the United States, described below.

Rubber bearings are relatively easy to manufacture, have nomoving parts, are unaffected by time, and are very resistantto environmental degradation.

Base Isolation In US

The first base-isolated building in the United States is theFoothill Communities Law and Justice Centre, a $30 millionlegal services center in Rancho Cucamonga San BernardinoCounty, about 97 km (60 miles) east of downtown Los Angeles.Completed in 1985, the building is four stories high with a

33

full basement and sub-basement for the isolation system, whichconsists of 98 isolators of multilayered natural rubberbearings reinforced with steel plates. The superstructure ofthe building has a structural steel frame stiffened by bracedframes in some bays.

Foothill Communities Law and JusticeCenter.

The building is located 20 km (12miles) from the San Andreas fault. SanBernardino County, the first in theU.S. to have a thorough earthquakepreparedness program, asked that the building be designed fora Richter magnitude 8.3 earthquake, the maximum credibleearthquake for that site. The design selected for theisolation system, which accounted for possible torsion,incorporated a maximum horizontal displacement demand of 380mm (15 in.) in the isolators at the corners of the building.Tests of full-scale sample bearings verified this capacity.

The highly filled natural rubber from which the isolators aremade, developed as part of the EERC research program, hasmechanical properties that make it ideal for a base isolationsystem. The shear stiffness of this rubber is high for smallstrains but decreases by a factor of about four or five as thestrain increases, reaching a minimum value at a shear strainof 50 percent. For strains greater than 100 percent, thestiffness begins to increase again, providing a fail-safeaction under a very high load. The damping follows the samepattern but less dramatically, decreasing from an initialvalue of 20 percent to a minimum of 10 percent and thenincreasing again. The design of the system assumes minimumvalues of stiffness and damping and a linear response. Thehigh initial stiffness is invoked only for wind load designand the large strain response only for fail-safe action.

This high-damping rubber system was also adopted for the FireDepartment Command and Control Facility (FCCF) of Los AngelesCounty, completed in 1990. (The same type of high-dampingrubber bearing was also used for the Italian telephonecompany, S.I.P., Ancona, Italy, the first modern base-isolatedbuilding in Europe.) The FCCF building houses the computer

34

systems for the emergency services of the county and istherefore required to remain functional after an extremeevent.

Fire Department Command and Control Facility.

The decision to use base isolationfor this project was reached bycomparing conventional and isolationschemes designed to provide the samedegree of protection. In most projects, the isolation designcosts five percent more. Not only was the isolation designestimate 6 percent less in this case but is less for anybuilding when equivalent levels of protection are considered.Furthermore, these costs are first costs. Life-cycle costs areeven more favorable. Also noteworthy is that the conventionalcode design requires only a minimal level of protection, thatthe structure not collapse; whereas isolation design providesa higher level of protection.

The University of Southern California Teaching Hospital ineastern Los Angeles is an eight-story concentrically bracedsteel frame supported on 68 lead rubber isolators and 81elastomeric isolators. The building was instrumented by theCalifornia Strong Motion Instrumentation Program soon afterits completion in 1991. The foundation system consists ofspread footings and grade beams on rock. Because of functionalrequirements, both the building plan and elevation are highlyirregular with numerous setbacks over the height. Two wings ateither side of the building are connected through what isreferred to as the "necked-down" portion of the building, andin the original fixed-base design the irregular configurationled to both coupling between the lateral and torsionalvibration modes and very large shear force demands in theslender region between the two rings.

(Even in the isolated design steeltrusses are required to carry theshears in the necked-down region.)These were two of the main reasonsthat seismic isolation was eventuallychosen for this structure.

35

University of Southern California University Hospital.

The University of Southern California (USC) Teaching hospitalwas 36 km (23 miles) from the epicenter of the Mw 6.8 1994Northridge earthquake. The peak ground acceleration outsidethe building was 0.49 g, and the accelerations inside thebuilding were around 0.10 to 0.13 g. In this earthquake thestructure was effectively isolated from ground motions strongenough to cause significant damage to other buildings in themedical center. The records obtained from the USC hospital areparticularly encouraging in that they represent the mostsevere test of an isolated building to date.

Nuclear Applications

Isolation used in conventional nuclear plants greatlysimplifies the expensive and time-consuming design andqualification of the equipment, piping, and supports forseismic loading. In addition, when seismic design criteria areincreased due to the discovery of nearby faults, for example,the plant need not be redesigned; upgrading the isolationsystem is sufficient.

In an experimental program at EERC isolation bearings weredesigned, produced, and tested for two types of liquid metalreactor designs. The first, called PRISM, uses high-shapefactor isolation bearings designed to provide horizontalisolation only. In the other design, SAFR, the reactor issupported on low-shape bearings that provide both horizontaland vertical isolation. The results of this test seriesextended the range of the isolator types with well-understoodcharacteristics.

Base Isolation in Japan

After a slow start, base isolation research and development inJapan increased rapidly. The first large base-isolatedbuilding was completed in 1986. Although such buildings inJapan require special approval from the Ministry ofConstruction, as of June 30, 1998, 550 base-isolated buildingshad been approved.

36

Base isolation has advanced rapidly in Japan for severalreasons. The expenditure for research and development inengineering is high with a significant amount designatedspecifically for base isolation; the large constructioncompanies aggressively market the technology; the approvalprocess for constructing a base-isolated building is astraightforward and standardized process; and the highseismicity of Japan encourages the Japanese to favor the long-term benefits of life safety and building life-cycle costswhen making seismic design decisions. The system most commonlyused in the past has been natural rubber bearings withmechanical dampers or lead-rubber bearings. Recently, however,there has been an increasing use of high-damping naturalrubber isolators. There are now several large buildings thatuse these high-damping bearings: an outstanding example is thecomputer center for the Tohoku Electric Power Company inSendai, Miyako Province.

6.0 Conclusion

The obligations for practical isolation system to beincorporated in building structures are flexibility, Dampingand resistance to service loads. Additional requirements suchas durability, cost, ease of installation and specific projectrequirements influences device selection but all practicalsystems should contain these essential elements. The entiresuperstructure is to be supported on discrete isolators whosedynamic characteristics are chosen to uncouple the groundmotion. Displacement and yielding are concentrated at thelevel of the isolation devices, and the superstructure behavesvery much like a rigid body. A through revise has been doneperformed regarding the sequential development of seismicisolation systems. This study also addressed the detail cramon isolation system, properties, characteristics of variousdevice categories, recognition along with its effect onbuilding Structures.

37

7.0 References

1. Passive control of structures for seismic loads ian gbuckle.2. Active, semi-active and hybrid control of structures t tsoong1 and b f spencer, jr2.3. Seismic isolation in buildings to be a practical reality:Behavior of structure and installation technique A. B. M.Saiful Islam*, Mohammed Jameel and Mohd Zamin JumaatDepartment of Civil Engineering, University of Malaya, KualaLumpur, Malaysia.4. Basic Characteristics and Durability of Low-FrictionSliding Bearings for Base IsolationMasahiko Higashino.5. Cheng, F.Y., Jiang, H., and Lou, K., 2008, Smartstructures, innovative systems for seismic response control,CRC Press.6. Islam, ABMS., Ahmad, S.I., 2010, Isolation system designfor buildings in Dhaka: its feasibility and economicimplication., Proc., Int., Conf. on Engineering Research,Innovation and Education, Bangladesh, Sylhet, 99-104.7. Islam, ABMS., Ahmad, S. I., Jameel, M., Jumaat, M. Z.,2010a, Seismic base isolation for buildings in regions of lowto moderate seismicity: A practical alternative design,Practice Periodical on Structural Design and Construction,ASCE.[DOI: 10.1061/ (ASCE) SC.1943- 5576.0000093].8. Islam, ABMS., Ahmad, S. I., Al-Hussaini, T. M., 2010b,Effect of isolation on buildings in Dhaka. 3rd Int. EarthquakeSymposium. BES. Bangladesh, Dhaka, 465-472.9. Hussain, R. R., Islam, ABMS., Ahmad, S. I., 2010, Baseisolators as earthquake protection devices in buildings, VDMPublishing House Ltd. Benoit Novel, simultaneously publishedin USA & U.K.10. Providakis, C. P., 2008, Effect of LRB isolators andsupplemental viscous dampers on seismic isolated buildingsunder near-fault excitations, J. Eng. Struct., 30, 1187-1198.11. Dall'Asta, A., Ragni, L., 2006, Experimental tests andanalytical model of high damping rubber dissipating devices.,J. Eng. Struct., 28, 1874-1884.

38

12. Dall’Asta, A., Ragni, L., 2008, Nonlinear behavior ofdynamic systems with high damping rubber devices. J. Eng.Struct., 30, 3610-3618.13. Chandak, N. R., 2012, Response spectrum analysis ofreinforced concrete buildings. J. Inst. of Eng. India, Ser.,A. 93(2), 121-128.14. P. P. Thakre, and O. R. Jaiswal, 2011, Comparative studyof fixed base and isolated base building using seismicanalysis, J. Earth Sci. and Eng.,4(6), 520-525.15. J. J. Bommer, A. S. Elnashai, A. G. Weir, 2000,Compatible acceleration and displacement spectra for seismicdesign codes, Proc. 12th World Conf. Earthquake Eng.,Auckland.16. SAP2000, Integrated software for structural analysis anddesign: Computers and Struct. Inc. Berkeley, California.17. N. Torunbalci and G. Ozpalanlar, 2008, Earthquakeresponse analysis of mid-story buildings isolated with variousseismic isolation techniques, Proc. 14th World Conference onEarthquake Engineering, China.18. IS1893 (Part-1)-2002, Criteria for earthquake resistantdesign of structures-general provisions and buildings (fifthrevision), Bureau of Indian Standards, India.19. EC 8, 2004, Eurocode 8: Design of structures forearthquake resistance Part 1: general rules, seismic actionsand rules for buildings, European Norm. European Committee forStandardization, Central Secretariat, Rue De Stassart 36, B-1050 Brussels.

39

40