1.0 INTRODUCTION
1.1
Fundamental Fundamental Concepts Of Base Isolation
The term base isolation uses the word a) isolation in its meaning of the state of being separated and b) base as a part that supports from beneath or serves as a foundation for an object or structure. As suggested in the literal sense, the structure (a building, bridge or piece of equipment) is separated from its foundation. The original terminology of base isolation is more commonly replaced with seismic isolation nowadays, reflecting that in some cases the separation is somewhere above the base – for for example, in a building the superstructure may be separated sep arated from substructure columns. In another ano ther sense, sen se, the th e term te rm seismic seism ic isolation isol ation is more mor e accurate anyway in that the structure is separated from the effects of the seism, or earthquake. The only way a structure can be supported under gravity is to rest on the ground. Isolation conflicts with this fundamental structural engineering requirement. How can the structure be separated from the ground for earthquake loads but still resist gravity? It is practical isolation systems that provide a compromise between attachment to the ground to resist gravity and separation from the ground to resist earthquakes. Seismic isolation is a means of reducing the seismic demand on the structure.
Figure 1: concept of base isolation
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1.2
Basic Elements Of Base Isolation
Seismic Isolation increases the fundamental period of vibration so that the structure is subjected to lower earthquake forces. However, the reduction in force is accompanied by an increase in displacement demand which must be accommodated within the flexible mount, Furthermore, longer period buildings can be lively under service loads. The following are three basic elements in any practical isolation system, they are: 1. A flexible mounting so that the period of vibration of the building is lengthened sufficiently to reduce the force response. 2. A damper of energy dissipater so that the relative deflections across the flexible mounting can be limited to a practical design level. 3. A means of providing rigidity under low (service) load levels such as wind and braking force.
Flexibility: Due to additional flexibility the period of structure is elongated. From the acceleration response curve shown in it may be observed that reductions in base shear occur as the period of vibration of the structure is lengthened. The extent to which these forces are reduced is primarily dependent on the nature of the earthquake ground motion and the period of the non-isolated structure.
Figure 1.1: acceleration response curve
Energy dissipation : Additional flexibility needed to lengthen the period of the structure will give rise to large relative displacement across the flexible mount. Large relative displacements can be controlled if substantial additional damping is introduced into the structure at the isolation level. One of the most effective means of providing a substantial level of damping is through hysteric energy dissipation. The hysteric refers to the offset between the loading and unloading curves under und er cyclic loading.
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Rigidity under low lateral loads : While lateral flexibility is highly desirable for high seismic loads, it is clearly undesirable to have a structural system which will vibrate perceptibly under frequently occurring loads such as wind Loads or braking loads. Mechanical energy dissipaters may be used to provide rigidity at these service loads by virtue of their high initial elastic stiffness.
1.3
Action Of Base Isolation
Hence, ―base isolation‖ or, seismic isolation separates upper structure from base or, from down structure by changing of fix joint with flexible one. Increasing of flexibility is done by the insertion of additional elements in structure, known as isolators. Usually, these isolators are inserted between upper structure and foundation. Seismic isolation system absorbs larger part of seismic energy. Therefore, vibration effects of soil to upper structure are drastically reduced. For base isolated structure the situation is quite different. In such cases, the whole upper structure gets a displacement (which naturally remains in limits) and the relative displacement of different stories is so small that the structure can withstand a comparatively high seismic tremor with a low seismic loading in a safe, efficient and economic manner.
1.4
Goal Of Base Isolation
A high proportion of the world is subjected to earthquakes and society expects that structural engineers will design our buildings so that they can survive the effects of these earthquakes. As for all the load cases encountered in the design process, such as gravity and wind, should work to meet a 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 exceeds it. The earthquake causes inertia forces proportional to the product of the building mass and the earthquake ground accelerations. As the ground accelerations increases, the strength of the building, the capacity, must be increased to avoid structural damage. But it is not practical to continue to increase the strength of the building indefinitely. In high seismic zones the accelerations causing forces in the building may exceed one or even two times the acceleration due to gravity, g. It is easy to visualize the strength needed for this level of load – strength to resist 1 g means than the building could resist gravity applied sideways, which means that the building could be tipped on its side and held horizontal without damage. Designing for this level of strength is not easy, nor cheap. So, most codes allow engineers to use ductility to achieve the capacity. Ductility is a concept of allowing the structural elements to deform beyond their elastic limit in a controlled manner. Beyond this limit, the structural elements soften and the displacements increase with only a small increase in force. The elastic limit is the load point up to which the effects of loads are non- permanent; that is, when the load is removed the material returns to its initial 3
condition. Once this elastic limit is exceeded changes occur. These changes are permanent and non reversible when the load is removed. A design philosophy focused on capacity leads to a choice of two evils: 1. Continue to increase the elastic strength. This is expensive and for buildings leads to higher floor accelerations. Mitigation of structural damage by further strengthening may cause more damage to the contents than would occur in a building with less strength. 2. Limit the elastic strength and detail for ductility. This approach accepts damage to structural components, which may not be repairable. Base isolation takes the opposite approach, it attempts to reduce the demand rather than increase the capacity. We cannot control the earthquake itself but we can modify the demand it makes on the structure by preventing the motions being transmitted from the foundation into the structure above. So, the primary reason to use isolation is to mitigate earthquake effects. Naturally, there is a cost associated with isolation and so it only makes sense to use it when the benefits exceed this cost. And, of course, the cost benefit ratio must be more attractive than that available from alternative measures of providing earthquake resistance. Nowadays Base Isolation is the most powerful tool of the t he earthquake engineering pertaining to the passive structural vibration control technologies. It is meant to enable a building or non-building structure to survive a potentially devastating seismic impact through a proper initial design or subsequent modifications. In some cases, application of Base Isolation can raise both a structure's seismic performance and its seismic sustainability considerably
1.5 Comparative Study Of Base Isolation And Fixed Base: A large proportion of world‘s population lives in regions of seismic hazards, at risk from earthquakes of varying severity and frequency of occurrence. Earthquake causes significant loss of life and damage of property every year. So, to mitigate the effect of earthquake on building the base isolation technique one of the best solutions. Seismic isolation consists of essentially the installation of mechanisms which decouple the structure from base by providing seismic isolators. The seismic isolation system is mounted beneath the superstructure and is referred as ‗Base Isolation‘. The main purpose of the base isolation device is to minimize the horizontal acceleration transmitted to the superstructure. Base isolation is very promising technology to protect different structures like building, bridges, airport terminals and nuclear power plants etc. from seismic excitation.
The earthquakes in the recent past have provided enough evidence of performance of different type of structures under different earthquake conditions and at different foundation conditions as a food for thought to the engineers and scientists. This has given birth to different type of techniques to save the structures from the earthquakes effects. Conventional seismic design attempts to make buildings that do not collapse under strong earthquake shaking, but may sustain damage to non-structural elements (like glass facades) and to some structural members in the building. This may render the building non-functional after the 4
earthquake, which may be problematic in some structures, like hospitals, which need to remain functional in the aftermath of the earthquake. Two basic technologies are used to protect buildings from damaging earthquake effects. These are Base Isolation Devices and Seismic Dampers. Base isolation is also known as ‗seismic base isolation‘ or ‗base isolation system‘. Seismic isolation separates the structure from the harmful motions of the ground by providing flexibility and energy dissipation capability capabilit y through the insertion of the isolated device so called isolators between the foundation and the building structure.
Figure 2: fixed base and isolated base
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2. EARTHQUAKE PROTECTIVE SYSTEMS Structural control for seismic loads is a rapidly expanding field and the family of control systems, also known as earthquake protective systems, now embraces passive, active and hybrid systems. Applications to buildings, bridges and industrial plant have been made in many of the seismically active countries of the world. Structural control provides an alternative to conventional design methods, which are based on ductile (yielding) response. In many applications, elastic performance during large earthquake events is economically feasible and the methodology permits performance-based design criteria, now required in many modern seismic design codes, to be satisfied more readily than conventional methods. Applications to the retrofit of existing structures have been particularly attractive, especially to the upgrading of historical buildings. As noted above, the family of earthquake protective systems has grown to include passive, active and hybrid (semi-active) systems as shown in Figure 1. Passive systems are perhaps the best known and these include seismic (base) isolation and passive (mechanical) energy dissipation. Certainly isolation is the most developed member of the family at the present time with continuing developments in hardware, applications, design codes and retrofit manuals. But there has also been continuing growth in the passive energy dissipation field with an increasing number of different kinds of dissipation devices being developed, accompanied by technically complex applications such as those found in near-field sites.
Figure3: family of earthquake protective systems
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2.1 Passive Protective Systems Research and development on passive energy dissipation systems began about the same time as modern isolation, motivated at the time, by a need to control isolator displacements. Thus the early passive energy dissipaters were used in association with elastomeric and sliding isolators and were principally of the metallic type (steel cones and tapered plates). Today the state-of-the-art has evolved to the point where they are used as an alternate to isolation and may be incorporated within the structural frame of a building or between the super and substructures of a bridge. Their function is to dissipate energy during an earthquake and reduce the demand on the structural frame. However, these dampers will also add strength and stiffness to the frame and if these contributions are large compared to the frame itself, they may offset the advantages generated by the increase in damping. Nevertheless, significant signi ficant reductions in damage to the t he frame (ductile (ductil e yielding) are possible but not to the same extent as that provided by an isolation system. However, unlike isolation, passive energy dissipators also reduce wind response and are applicable to a wider range of structures, especially those sensitive to long period ground motions. In retrofit situations they may also be less intrusive than a comparable isolation system. Passive energy dissipators may be simply classified as hysteretic or viscoelastic [Constantinou, Soong, and Dargush, 1998]. Hysteretic dissipators include the yielding of metals due to flexure, shear, torsion, or extrusion (metallic dampers) and sliding (friction dampers). They are all essentially displacement-dependent devices. Viscoelastic systems include viscoelastic solids, fluid orificing (fluid dampers), and viscoelastic fluids. They are essentially velocity-dependent devices (viscous in nature) and many are also frequency dependent. Some passive energy dissipators are modifications of the above set and may include elastic springs or pressurized cylinders to develop pre-load and recentering capabilities. In a subset of passive energy dissipators are tuned mass dampers (TMD) and tuned liquid dampers (TLD). Used principally for controlling wind vibrations in elastic structures these vibration absorbers dampen motion by transferring kinetic energy between various modes of o f vibration. Tuned to a particular p articular dominant mode, mod e, such a device devic e may enhance the damping in tall structures by about 5%, which is usually sufficient for improved comfort levels during wind storms. Seismic applications are however rare and these devices are not discussed further in this paper. It is perhaps worth noting that TMDs are frequently used during the construction of tall slender structures, such as the towers of suspension bridges, for protection against wind loads. The world‘s longest suspension bridge, the Akashi-Kaikyo bridge near Kobe, Japan, was under construction at the time of the Great Hanshin Earthquake in 1995. The epicenter was on the Nojima fault which passes between the two towers at a shallow angle. Although at their full height at the time of the earthquake, the towers were not damaged, possibly due (in part) to the presence of the TMDs. Common passive energy dissipators in use today include hysteretic flexural, torsional and extrusion devices, friction dampers, and viscoelastic solid and fluid dampers. In 1998, the number of applications in North America (Mexico, US and Canada) was of the order of 80 [Constantinou, Soong and Dargush, 1998]. Of these about onehalf were viscoelastic fluid dampers, one-third were friction dampers, and the remaining one-sixth included hysteretic, viscoelastic solid, and tuned mass dampers.
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2.2 Active, Hybrid And Semi-Active Control Systems Active, semi-active and hybrid structural control systems are a natural evolution of passive control technologies such as base isolation and passive energy dissipation. The possible use of active control systems and some combinations of passive and active systems, so called hybrid systems, as a means of structural protection against wind and seismic loads has received considerable attention in recent years. Active/hybrid control systems are force delivery devices integrated with real-time processing evaluators/controllers and sensors within the structure. They act simultaneously with the hazardous excitation to provide enhanced structural behavior for improved service and safety. Remarkable progress has been made over the last twenty years. The First and Second World Conferences on Structural Control held in 1994 [Housner et al, 1994b] and 1998 [Kobori et al, 1998], respectively, attracted over 700 participants from 17 countries and demonstrated the worldwide interest in structural control. Research to date has reached the stage where active systems have been installed in full-scale structures. Active systems have also been used temporarily in construction of bridges or large span structures (e.g., lifelines, roofs) where no other means can provide adequate protection.
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Figure 6: structure with active control
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3. ISOLATION SYSTEMS Despite wide variation in detail base isolation technique follow two basic approaches with certain common features. In the first approach the isolation system introduces the layer of low lateral stiffness between the structure and the foundation. With this isolation layer the structure has a natural period that is much longer than its fixed base natural period. as shown by the elastic diagram spectrum, this lengthening of period can reduce the pseudo acceleration and hence the earthquake induced forces in the structure but the deformation is increased. This deformation is concentrated in the isolation system, however, accompanied by only small deformations in the structure .this type of structure is effective even if the system is linear and undamped. Damping is beneficial, however, in further reducing the forces in the structure and the deformation in the isolation system. The most common system of this uses short, cylindrical bearings with one or more holes and alternating layers of steel plates and hard rubber. Interposed between the base of the structure and the foundation these laminated bearings are strong and stiff under vertical loads yet very
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flexible under lateral forces. because the natural damping of the structure is low. Additional damping is provided in the form of mechanical damper. The second most common type of isolation system uses sliding elements between the foundation and the base of the structure. The shear force transmitted to the structure across the isolation interface is limited by keeping the coefficient of friction as low as practical. However, the friction must be sufficiently high to sustain strong winds and earthquakes without sliding, a requirement that reduces the isolation effect. In this type of isolation system the sliding displacements are controlled by high tension springs and laminated rubber bearings,or by making makin g the sliding systems s ystems curved;these cur ved;these mechanisms prvide p rvide a restoring force, otherwise unavailable in this type of system, to return the structure to its equilibrium position.the friction pendulum system FPS is sliding isolation system wherein the weight of the structure is supposed on spherical sliding surfaces that slide relative to eachother when the ground motion exceeds a threshold level.the restoring action is caused by raising the building slightly s lightly when sliding occurs o ccurs on the spherical spher ical surface. su rface. The dynamics of structures structur es on slider type isolation systems is complicated because the slip process is intrinsically non lienear.
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3.1
Recognition Of Isolation Types
Many types of isolation system have been proposed and have been developed to varying stages, with some remaining no more than concepts and others having a long list of installed projects. A discussion of generic types of system and focus on different types (especially rubber bearing) of isolators along with their characteristics is provided subsequently.
4.0
TYPES OF ISOLATOR
The development of isolators ensured the properties required for the achievement of perfect base isolation. The chart details the various types of Isolators used through throu gh out the world. A brief description along alo ng with their basic functions functi ons and advantages is also als o included just after the th e 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.
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4.1 Bearing Type 4.1.1 Elastomeric (Rubber) Bearings Rubber bearings are formed of horizontal layers of natural or synthetic rubber in thin layers bonded between steel plates. The steel plates prevent the rubber layers from blown up or busting. In such su ch mechanism mechan ism the bearing bea ring is capable cap able to support su pport higher vertical loads with only o nly smaller deflection (typically 1 to 3 mm under full gravity load). The internal steel layers do not restrict horizontal deformations of the rubber layers in shear. So, the bearings are much more flexible under lateral loads than vertical loads. This is why; the bearing works as a flexible unit.
Characteristics Of Rubber Isolator
As described earlier, isolation system works with the principle that a rigid mass is isolated from a flexible supporting structure. Optimum isolation of a building from ground may be achieved by choosing a rubber bearing isolator based on the knowledge of its static and dynamic characteristics determined from laboratory experiments. For this reason, understanding the properties of rubber isolators is necessary for the vibration analysis. Some of the important properties are as follows.
(a) Load Capacity And Size Of Rubber Bearings For most bearing types the plan size required increases as vertical load increases but the height or radius is constant regardless of vertical loads. This is because all bearings at such stages are subjected to the same displacement. Therefore, the bearing can be sized according to the vertical loads they support. Three types of curves are seen in the plot. All the curves are showing that the lower the vertical load, the lower the required bearing diameter.
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(b) Absorption Shocks originating due to the occurrence of an earthquake can be controlled if substantial additional damping is introduced into the isolation system. A high damping rubber isolator provides a substantial level of damping through hysteretic energy dissipation. Hysteretic refers to the offset between the loading and unloading curves under cyclic loading. Figure shows an idealized force- displacement loop where the enclosed area is measure of the energy dissipated during one cycle of motion.
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(c) Durability Under Cyclic Loading Rubber isolator remains more or less stable under cyclic loading. Results of cyclic displacement test applied, to a rubber isolator shows that at a speed equivalent to an actual seismic event the friction factor remains stable. Figure shows a typical friction factor versus number of cycles in a cyclic loading test of rubber isolator. The figure represents that rubber isolator is durable. There are mainly two types of Rubber Bearing. They are LRB and HDRB.
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Lead rubber bearing (LRB)
This type of elastomeric bearings consist of thin layers of low damping natural rubber and steel plates built in alternate layers and a lead cylinder plug firmly fitted in a hole at its centre to deform in pure shear as shown in Figure 14. The LRB was invented in New Zealand in 1975 and has been used extensively in New Zealand, Japan and United States. The steel plates in the bearing force the lead plug to deform in shear. This bearing provides an elastic restoring force and also, by selection of the appropriate size of lead plug, produces required amount of damping. The force deformation behavior of the bearing is shown in Figure. Performance of LRB is maintained during repeated strong earthquakes, with proper durability and reliability.
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Basic functions of LRB
(1) Load supporting function : Rubber reinforced with steel plates provides stable support for structures. Multilayer construction rather than single layer rubber pads provides better vertical rigidity rigidi ty for supporting a building. build ing. (2) Horizontal elasticity function (prolonged oscillation period): With the help of LRB, earthquake vibration is converted to low speed motion. As horizontal stiffness of the multi- layer rubber bearing is low, strong earthquake vibration is lightened and the oscillation period of the building buildin g is increased. (3) Restoration function : Horizontal elasticity of LRB returns the building to its original position. In a LRB, elasticity elasticit y mainly comes from restoring force of the rubber layers. l ayers. After an earthquake this restoring force returns the building to the original position. (4) Damping function : Provides required amount of damping necessary. LRB mainly are of two shapes. One is conventional round and the other type is square. Though their basic function remains same, yet changes in shapes are advantageous. In many occasions as economy concern, reduced size, stability and capacity for large deformation.
High Damping Rubber Bearing (HDRB)
HDRB is one type of elastomeric bearing. This type of bearing consist of thin layers of high damping rubber and steel plates built in alternate layers as shown in Figure. The vertical stiffness of the bearing is several hundred times the horizontal stiffness due to the presence of internal steel plates. Horizontal stiffness of the bearing is controlled by the low shear modulus of elastomer while steel plates provides high vertical stiffness as well as prevent bulging of rubber. High vertical stiffness of the bearing has no effect on the horizontal stiffness. The damping in the bearing is increased by adding extra-fine carbon block, oils or resins and other proprietary fillers. The dominant features of HDRB system are the parallel action of linear spring and viscous damping. The damping in the isolator is neither viscous nor hysteretic, but somewhat in between. The ideal force deformation behaviour of the bearing is shown in Figure. 17
Figure 15: high damping damping rubber bearings
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Figure 16: force deformation behaviour
Basic functions of HDRB
(1) Vertical load bearing function: Rubber reinforced with steel plates provides stable support for structures. Multilayer construction rather than single layer rubber pads provides better vertical rigidity for supporting a building. (2) Horizontal elasticity function (prolonged oscillation period): With the help of HDRB earthquake vibration is converted to low speed motion. As horizontal stiffness of the multilayer rubber bearing is low, strong earthquake vibration is lightened and the oscillation period oscillation period of the building is increased. (3) Restoration function: Horizontal elasticity of HDRB returns the building to its original position. In a HDRB, elasticity ela sticity mainly mainl y comes from fro m restoring force of the rubber rub ber layers. la yers. After an earthquake this restoring force returns the building to the original position. (4) Damping function: Provides required amount of damping up to a higher value.
4.1.2 Sliding Bearings Sliding isolation system (Figure) is simple in concept and it has a theoretical appeal. A layer with a defined coefficient of friction will limit the acceleration to this value and the forces, which can be transmitted, will also be limited to the coefficient of friction multiplied by weight.
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Figure 17: Sliding bearings Following are some of the utilities of using sliders:
(1)Sliding movement provides flexibility and the force displacement traces a rectangular shape that is the optimum for equivalent viscous damping. (2) A pure sliding system will have unbounded displacements, with an upper limit equal to the maximum ground displacements for a coefficient of friction close to zero. Two types of sliding systems are commonly used. A brief description with their basic functions, advantages and suitability is as follows.
Sliding Support With Rubber-Pad (SSR)
When sliding base isolation system incorporates multilayer natural rubber pad then it is known as SSR. Advantages of such bearings are: (1) SSR can provide vibration isolation for light loads as well as large deformation performance like a large-scale large -scale isolation system. s ystem. (2) It provides protection against a wide range of tremors from small vibrations to major earthquakes. (3) It can be used in conjunction with other isolation systems such as LRB and HDRB.
Basic functions of SSR
For small vibrations, shear deformation of the rubber layers provides the same isolation effect as conventional multilayer rubber bearings. For large vibrations, sliding materials slide to provide the same deformation deformat ion performance as large-scale larg e-scale isolation systems. s ystems.
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Friction Pendulum System (FPS)
Sliding friction pendulum isolation system (Figure) is one type of flexible isolation system suitable for small to large-scale buildings. It combines sliding a sliding action and a restoring force by geometry. Functions of FPS are same as SSR system. Advantages of FPS include : (1) It is possible to set the oscillation period of a building regardless of its weight. (2) This system can reduce costs not only because of the low cost of its device but also due to the low cost of installation. (3) The device is simple, works well and easy to install. Furthermore, it saves space and is practical for a seismic reinforcement. rei nforcement. (4) Performance of such device is stable due to the high durability of the device. (5) As it requires only a simple visual check to maintain the device, maintenance is very easy
Figure 18: friction pendulum system
4.2 Damping type Damping provides sufficient resistance to structure against service loading. The effect of damping on dynamic response is beneficial. Generally all structural systems exhibit damping to various degrees. It is assumed that, structural damping is viscous by nature. Damping coefficient relates force to velocity. If damping coefficient is sufficiently large, it is possible to totally restrain the oscillatory motion. Damping that suppress totally the oscillatory motion is termed as critical damping. Damping is usually neglected in frequency and period calculations unless it exceeds about 20%.Normally two types of damping are used in building. A brief description descript ion follows.
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4.2.1 Elementary Damping This really means damping as a whole, i.e. the device (each and every element) itself acts as a purely damping device rather than an isolator. Purely damping devices can be used in low weight buildings to restrain the oscillating motion of the building. Another option may be using in conjunction with rubber bearing so-called as High Damping Rubber bearing. In such devices amount of damping is significantly high, usually from 8 to 15% of critical damping. Lead plug damper is one of the forms of elementary damping. The basic functions of such damper include: (a) Vibration damping Junction : Lead plug damper absorbs large vibration of the building. As the layers of rubber ru bber are distorted, the lead le ad plug is plastically plasticall y deformed and at such stage it absorbs the earthquake energy and quickly damps the vibration.
( b) Trigger function :
It also reduces vibration form source other than earthquake. For example, when vibration is generated by strong winds, the relative rigidity of the lead plug reduces the effect of such vibration.
4.2.2 Supplementary Damping There are some types of isolators (discussed in bearing portion), which are capable of providing flexibility flexibi lity but not significant damping, dampi ng, or resistance to service servi ce loads. In order to strengthen the damping phenomena, supplementary devices are included with general Isolators. Damping of such type can be termed as supplementary damping. One of the most popular types of supplementary supp lementary damping is viscous visco us damping. This device provides p rovides damping but not service load resistance. res istance. It does not have any an y elastic stiffness and for f or 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 based on steel springs. They are mostly used for machinery isolation. The main drawbacks of springs are two. Firstly, they are most flexible in both the vertical and horizontal directions. Secondly, springs alone have little damping and will move excessively under service loads.
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4.3.2 Rollers Cylindrical rollers and ball bearings are of this type. Like springs they are commonly used for machinery isolation. The resistance to movement and damping of rollers and ball bearings are sufficient under service loads.
4.3.3 Sleeved piles The pin ended structural members, that is, piles inside a sleeve provide flexibility and allow movement of the soft first story in a building. This type of piles is known assleeved piles. Sleeved piles provide flexibility but no damping. Hence damping devices are required to use along with sleeved piles.
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5.0 Applications: In recent years base isolation has become an increasingly applied structural design technique for buildings and bridges in highly seismic areas. Many types of structures have been built using this approach, and many others are in the design phase or under construction. Most of the completed buildings and those under construction use rubber isolation bearings in some way in the isolation system. The ideas behind the concept of base isolation are quite simple. There are two basic types of isolation systems. The system that has been adopted most widely in recent years is typified by the use of elastomeric bearings, the elastomer made of either natural rubber or neoprene. In this approach, the building or structure is decoupled from the horizontal components of the earthquake ground motion by interposing a layer with low horizontal stiffness between the structure and the foundation. This layer gives the structure a fundamental frequency that is much lower than its fixed-base frequency and also much lower than the predominant frequencies of the ground motion. The first dynamic mode of the isolated structure involves deformation only in the isolation system, the structure above being to all intents and purposes rigid. The higher modes that will produce deformation in the structure are orthogonal to the first mode and consequently also to the ground motion. These higher modes do not participate in the th e motion, motio n, so that if there the re is high energy in the t he ground g round motion at these th ese higher hi gher frequencies, this energy cannot be transmitted into the structure. The isolation system does not absorb the earthquake energy, but rather deflects it through the dynamics of the system. This type of isolation works when the system is linear and even when undamped; however, some damping is beneficial to suppress any possible resonance at the isolation frequency. The second basic type of isolation system is typified by the sliding system. This works by limiting the transfer of shear across the isolation interface. Many sliding systems have been proposed and some have been used. In China there are at least three buildings on sliding systems that use a specially selected sand at the sliding interface. A type of isolation containing a lead-bronze plate sliding on stainless steel with an elastomeric bearing has been used for a nuclear power plant in South Africa. The friction-pendulum system is a sliding system using a special interfacial material sliding on stainless steel and has been used for several projects in the United States, both new and retrofit construction.
Research At EERC Research on the development of natural rubber bearings for isolating buildings from earthquakes began in 1976 at the Earthquake Engineering Research Center (EERC) (now PEER, the Pacific Engineering Research Center) of the University of California at Berkeley. The initial research program was a joint effort by EERC and the Malaysian Rubber Producers Research Association (MRPRA), U.K. The program was funded by MRPRA through a number of grants over several years, with later funding provided by the National Science Foundation and the Electric Power Research Institute. Professor James M. Kelly directed the research at EERC, which included considerable theoretical and experimental contributions by graduate students. 24
Although not an entirely new idea at the time — a few methods using rollers or sliders had been proposed p roposed — the the concept of base isolation was considered to be very impractical by most of the structural engineering profession. The research project began with a set of hand-made bearings of extremely extremel y low-modulus rubber used use d with a simple three-story, single-bay, 20-ton 2 0-ton model. Shaking table tests showed that isolation bearings could bring about reductions in acceleration by factors of as much as ten when compared to those of conventional design and that, as predicted, the model would respond as a rigid body with all deformation concentrated in the isolation system. It was also clear that a certain degree of damping was needed in the system and that the scale of the model was too small to allow more practical rubber compounds to be used. In 1978, a more convincing demonstration of the isolation concept was achieved with a more realistic five-story, three-bay model weighing 40 tons and by using damping-enhanced bearings made by commercial techniques. A strong interest throughout the EERC research program was in the influence of isolation on the response of equipment and contents in a structure, which tend to sustain more damage when conventional methods of seismic-resistant design are used and which, in many buildings, are much more costly than the structure itself. An extensive series of tests on the five-story frame demonstrated that isolation with rubber bearings could provide very substantial reductions in the accelerations experienced by internal equipment, exceeding the reductions experienced by the structure. However, the same tests showed that when additional elements (such as steel energy-absorbing devices, frictional systems, or lead plugs in the bearings) were added to the isolation system to increase damping, the reductions in acceleration to the equipment were not achieved because the added elements also induced responses in the higher modes of the structure, affecting the equipment. It became clear that the optimum method of increasing damping was to provide it in the rubber compound itself. This method was applied later in the 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 no moving parts, are unaffected by time, and are very resistant to environmental degradation.
Base Isolation In US The first base-isolated building in the United States is the Foothill Communities Law and Justice Centre, a $30 million legal services center in Rancho Cucamonga San Bernardino County, about 97 km (60 miles) east of downtown Los Angeles. Completed in 1985, the building is four stories high with a full basement and sub-basement for the isolation system, which consists of 98 isolators of multilayered natural rubber bearings reinforced with steel plates. The superstructure of the building has a structural steel frame stiffened by braced frames in some bays. Foothill Communities Law and Justice Center. The building is located 20 km (12 miles) from the San Andreas fault. San Bernardino County, the first in the U.S. to have a thorough earthquake preparedness program, asked that the building be designed for a Richter magnitude 8.3 earthquake, the maximum credible earthquake for that site. The design selected for the isolation system, which accounted for possible torsion, 25
incorporated a maximum horizontal displacement demand of 380 mm (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 are made, developed as part of the EERC research program, has mechanical properties that make it ideal for a base isolation system. The shear stiffness of this rubber is high for small strains but decreases by a factor of about four or five as the strain increases, reaching a minimum value at a shear strain of 50 percent. For strains greater than tha n 100 percent, the th e stiffness stiffnes s begins to increase incr ease again, again , providin g a fail-safe action under a very high load. The damping follows the same pattern but less dramatically, decreasing from an initial value of 20 percent to a minimum of 10 percent and then increasing again. The design of the system assumes minimum values of stiffness and damping and a linear response. The high initial stiffness is invoked only for wind load design and the large strain response only for fail-safe action. This high-damping rubber system was also adopted for the Fire Department Command and Control Facility (FCCF) of Los Angeles County, completed in 1990. (The same type of highdamping rubber bearing was also used for the Italian telephone company, S.I.P., Ancona, Italy, the first modern base-isolated building in Europe.) The FCCF building houses the computer systems for the emergency services of the county and is therefore required to remain functional after an extreme event. Fire Department Command and Control Facility.
The decision to use base isolation for this project was reached by comparing conventional and isolation schemes designed to provide the same degree of protection. In most projects, the isolation design costs five percent more. Not only was the isolation design estimate 6 percent less in this case but is less for any building when equivalent levels of protection are considered. Furthermore, these costs are first costs. Life-cycle costs are even more favorable. Also noteworthy is that the conventional code design requires only a minimal level of protection, that t hat the structure not collapse; coll apse; whereas wherea s isolation isolatio n design provides a higher level of protection. The University of Southern California Teaching Hospital in eastern Los Angeles is an eightstory concentrically braced steel frame supported on 68 lead rubber isolators and 81 elastomeric isolators. The building was instrumented by the California Strong Motion Instrumentation Program soon after its completion in 1991. The foundation system consists of spread footings and grade beams on rock. Because of functional requirements, both the building plan and elevation elevati on are highl y irregular with wit h numerous setbacks s etbacks over the height. Two wings at either side of the building are connected through what is referred to as the "neckeddown" portion of the building, and in the original fixed-base design the irregular configuration led to both coupling between the lateral and torsional vibration modes and very large shear force demands in the slender region between the two rings.
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(Even in the isolated design steel trusses are required to carry the shears in the necked-down region.) These were two of the main reasons that seismic isolation was eventually chosen for this structure. University of Southern California University Hospital.
The University of Southern California (USC) Teaching hospital was 36 km (23 miles) from the epicenter of the Mw 6.8 1994 Northridge earthquake. The peak ground acceleration outside the building was 0.49 g, and the accelerations inside the building were around 0.10 to 0.13 g. In this earthquake the structure was effectively isolated from ground motions strong enough to cause significant damage to other buildings in the medical center. The records obtained from the USC hospital are particularly encouraging in that they represent the most severe test of an isolated building to date.
Nuclear Applications Isolation used in conventional nuclear plants greatly simplifies the expensive and timeconsuming design and qualification of the equipment, piping, and supports for seismic loading. In addition, when seismic design criteria are increased due to the discovery of nearby faults, for example, the plant need not be redesigned; upgrading the isolation system is sufficient. In an experimental program at EERC isolation bearings were designed, produced, and tested for two types of liquid metal reactor designs. The first, called PRISM, uses high-shape factor isolation bearings designed to provide horizontal isolation only. In the other design, SAFR, the reactor is supported on low-shape bearings that provide both horizontal and vertical isolation. The results of this test series extended the range of the isolator types with wellunderstood characteristics.
Base Isolation in Japan After a slow start, base isolation research and development in Japan increased rapidly. The first large base-isolated building was completed in 1986. Although such buildings in Japan require special approval from the Ministry of Construction, as of June 30, 1998, 550 baseisolated buildings had been approved. Base isolation has advanced rapidly in Japan for several reasons. The expenditure for research and development in engineering is high with a significant amount designated specifically for base isolation; the large construction companies aggressively market the technology; the approval process for constructing a base-isolated building is a straightforward and standardized process; and the high seismicity of Japan encourages the Japanese to favor the long-term benefits of life safety and building life-cycle costs when making seismic design decisions. The system most commonly used in the past has been natural rubber bearings with mechanical dampers or lead-rubber bearings. Recently, however, there has been an increasing use of high-damping natural rubber isolators. There are now several large buildings that use these high-damping bearings: an outstanding example is the computer center for the Tohoku Electric Power Company in Sendai, Miyako Province. 27
6.0 Conclusion The obligations for practical isolation system to be incorporated in building structures are flexibility, Damping and resistance to service loads. Additional requirements such as durability, cost, ease of installation and specific project requirements influences device selection but all practical systems should contain these essential elements. The entire superstructure is to be supported on discrete isolators whose dynamic characteristics are chosen to uncouple the ground motion. Displacement and yielding are concentrated at the level of the isolation devices, and the superstructure behaves very much like a rigid body. A through revise has been done performed regarding the sequential development of seismic isolation systems. This study also addressed the detail cram on isolation system, properties, characteristics of various device categories, recognition along with its effect on building Structures.
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7.0 References 1. Passive control of structures for seismic loads ian g buckle. 2. Active, semi-active and hybrid control of structures t t soong1 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 Jumaat Department of Civil Engineering, University of Malaya, Kuala Lumpur, Malaysia. 4. Basic Characteristics and Durability of Low-Friction Sliding Bearings for Base Isolation Masahiko Higashino. 5. Cheng, F.Y., Jiang, H., and Lou, K., 2008, Smart structures, innovative systems for seismic response control, CRC Press. 6. Islam, ABMS., ABMS., Ahmad, S.I., 2010, Isolation system design for buildings in Dhaka: its feasibility and economic implication., Proc., Int., Conf. on Engineering Research, Innovation and Education, Bangladesh, Sylhet, 99-104. 7. Islam, ABMS., Ahmad, S. I., I., Jameel, M., Jumaat, M. Z., Z., 2010a, Seismic base isolation for buildings in regions of low to moderate seismicity: A practical alternative alternativ e design, Practice Periodical on Structural Design and Construction, ASCE.[DOI: 10.1061/ (ASCE) SC.19435576.0000093]. 8. Islam, ABMS., Ahmad, S. I., I., Al-Hussaini, T. M., M., 2010b, Effect of isolation on buildings in Dhaka. 3rd Int. Earthquake Symposium. BES. Bangladesh, Dhaka, 465-472. 9. Hussain, R. R., Islam, Islam, ABMS., Ahmad, Ahmad, S. I., 2010, Base isolators as earthquake protection devices in buildings, VDM Publishing House Ltd. Benoit Novel, simultaneously published in USA & U.K. 10. Providakis, C. P., 2008, Effect of LRB LRB isolators and supplemental viscous dampers on seismic isolated buildings under near-fault excitations, J. Eng. Struct., 30, 1187-1198. 11. Dall'Asta, A., Ragni, L., 2006, Experimental tests and analytical model of high damping rubber dissipating devices., J. Eng. Struct., 28, 1874-1884. 12. Dall‘Asta, A., Ragni, L., 2008, Nonlinear Nonlinear behavior of dynamic systems with high high damping rubber devices. J. Eng. Struct., 30, 3610-3618.
13. Chandak, N. R., 2012, Response spectrum analysis of reinforced concrete buildings. J. Inst. of Eng. India, Ser., A. 93(2), 121-128. 14. P. P. Thakre, and O. R. Jaiswal, 2011, Comparative study of fixed base and isolated base building using seismic analysis, an alysis, J. Earth Sci. and Eng.,4(6), 520-525. 520-5 25. 15. J. J. Bommer, A. S. Elnashai, A. G. Weir, 2000, Compatible acceleration and displacement spectra for seismic design codes, Proc. 12th World Conf. Earthquake Eng., Auckland. 16. SAP2000, Integrated software for structural analysis and design: Computers and Struct. Inc. Berkeley, California. 17. N. Torunbalci and G. Ozpalanlar, 2008, Earthquake response analysis of mid-story buildings isolated with various seismic seis mic isolation techniques, Proc. 14th World Conference on Earthquake Engineering, China. 18. IS1893 (Part-1)-2002, Criteria for earthquake resistant design of structures-general structures- general provisions and buildings bui ldings (fifth revision), Bureau Bure au of Indian Standards, India.
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19. EC 8, 2004, Eurocode 8: Design of structures for earthquake resistance Part 1: general rules, seismic actions and rules for buildings, European Norm. European Committee for Standardization, Central Secretariat, Rue De Stassart 36, B-1050 Brussels.
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