08-16-2017, 08:48 PM
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INTRODUCTION
Earthquake by itself, is not a disaster, it is natural phenomenon result from ground movement, sometimes violent. Disaster occurs when man made buildings and other structures cannot cope up with these ground movements and fail causing death and destruction of property.
The energy that is released after an earthquake sets up waves called primary wave and secondary waves. These further produce surface waves, which cause vibration of the ground and structures standing on top. Depending on the characteristics of these vibrations, the ground may develop cracks, fissures and settlements.
Seismic events are not predictable and a major event can never be ruled out in the life of a structure located in a severe seismic zone. Both non-structural and structural damage may be expected in a major seismic event. The possible risk of loss of life adds a very serious dimension to seismic design, putting a moral responsibility on structural engineers to be comprehensive in their work.
In recent times, many new systems and devices using non conventional materials have been developed, either to reduce the earthquake forces acting on the structure or to absorb a part of seismic energy. One of the most widely implemented and accepted seismic protection systems is base isolation.
BASE ISOLATION
Base isolation is one of the most widely accepted seismic protection systems in earthquake prone areas. It mitigates the effect of an earthquake by essentially isolating the structure from potentially dangerous ground motions, especially in frequency range where building is mostly affected.
Seismic isolation is a design a strategy, which uncouples the structure for the damaging effects of the ground motion. The term isolation refers to reduced interaction between structure and the ground. When the seismic isolation system is located under the structure, it is referred as base isolation.
History:
The concept of seismic isolation appears to be over a century old. John Milne at the university of Tokyo first reported it. Imperial Hotel in Tokyo, completed in 1921, was founded on a shallow layer of firm soil supported by an underlying layer of soft mud. Cushioned from the devastating ground motion, the hotel survived the 1923 Tokyo earthquake. There have been some instances of unintentional isolated structures where the structures survived earthquake by sliding on their foundation. One of the early uses of elastomeric bearings as isolation devices was reported in 1969 for an elementary school in Yugoslavia. Now, the concept of seismic isolation has matured into a practical really over the last 20 years.
Need:
The need for hospitals or emergency facilities to be functioning post earthquake is clear. Expected performance level for such buildings should be fully occupational-performance level of immediate occupancy. There is ever increasing necessity of protecting nonstructural components and highly sensitive equipments. The key objective of base isolation is mitigating damage to contents of structure.
The basic dilemma in providing superior seismic resistance of a building is the difficulty in minimizing the interstorey drift and floor accelerations simultaneously. Large interstorey drifts cause damage to non-structural components. Interstorey drifts can be minimized by stiffening the structure, but this leads to amplification of the ground motion, which leads to high floor acceleration, which can damage sensitive internal equipment. Making the system more flexible can reduce floor acceleration, but this leads to large interstorey drifts. The only practical way of reducing interstorey drift and floor acceleration simultaneously is to use base isolation, which provides the necessary flexibility, with the displacements concentrated at the isolation level.
Concept of base isolation:
During a seismic event, a finite quantity of energy is input into the structure. This input energy is transformed into both kinetic and potential (strain) energy, which must either absorbed or dissipated through heat. Considering the energy conservation of a structure.
E = Ek + Es + En + Ed
Where E- Absolute energy input from earthquake motion i.e., represents the work done by total base shear force at the foundation, Ek Absolute kinetic energy, Es recoverable elastic strain energy, En-irrecoverable energy dissipated by structural system through inelastic or other forms of action and Ed-Energy dissipated by supplemental damping devices.
Conventional Earthquake resistant structural systems:
In conventional earthquake resistant structural systems, acceptable performance is achieved by the occurrence of inelastic deformation i.e., En. In such systems, specially selected ductile components are designed to with stand several cycles well beyond yield under reverse loading, with yield level chosen so that forces are transmitted to other compounds of the structure are limited to their elastic range. The yielding lengthens the fundamental period of the structure, detuning the response away from the energy frequency of earthquake ground motion. The hysteric behavior of these selected components provides energy dissipation and ductile behavior ensures sufficient deformation capacity and structure rides out of earthquake attack. They are fixed base systems.
The dissipation of energy due to earthquake forces occurs only after the structure is partly damaged. Thus structural damage is prerequisite for force reduction and hysteric energy dissipation.
Base isolated systems:
Base isolated systems differ from conventional structural systems in the methods by which the lengthening of fundamental period and hysteric energy dissipating mechanisms provided and the degree of reduction in transmission of earthquake forces into the structure.
In base isolation systems, the fundamental aim is to substantially reduce the transmission of earthquake forces and energy into the structure. Moreover, the reduction in earthquake forces is achieved without damage to the structure. The reduction in transmission of forces and hysteric action originate from the isolation system and do not depend on the structural damage. This is achieved by mounting the structure on an isolating layer with considerable horizontal flexibility. The isolators are specifically designed to accommodate large horizontal movements while carrying the structural loads. Thus by interposing structural elements with low horizontal stiffness, the fundamental frequency of overall structure is much lower than the predominant frequencies of the ground motion.
Seismic isolation is characterized by flexibility and energy absorption capability. The flexibility alone is insufficient to defeat away a major portion of the earthquake energy so that inelastic action does not occur, i.e., En is minimized energy dissipation in the isolation system Ed is then useful in limiting the displacement response and in avoiding resonance.
Design considerations:
A number of factors need to be considered by an engineer, architect or owner to decide on seismic isolation for a project. Among the foremost is the evaluation of seismic hazard, which includes local geology, proximity to faults, soil conditions, characteristics of possible earthquakes such as period and severity. Subsequently, performance levels for different intensities of earthquakes need to be evaluated.
Since the isolators carry large vertical loads and deform to significant lateral displacement, the components of the structure above and below the isolator need to be designed appropriately. Plane of isolation may be chosen based on the practical aspects of installation and relative strengths of super and sub structure components. Specifically, for the isolation system to work properly, the structure should be free to move in any direction up the maximum specified displacement. Typically a seismic moat is provided around the structure to allow this movement. It is imperative that owners and occupiers of seismically isolated structures are aware of the functional importance of seismic gap and the need for this space to be left clear.
To maintain the functional purpose of the structure after a seismic event, all the utilities, electrical connections and waste pipelines should be designed to accommodate the maximum seismic displacement. The main connections between the building and the ground, such as stairs, entryways and elevators need to be unconnected across the isolation plane. In general, all the interaction between the structure and the ground need to be designed and detailed.
Seismic isolation provides immediate occupancy performance level following strong events. Costs and benefits of different approaches may be evaluated in determining the incorporation of seismic isolation.
Basic elements of base isolation system:
The following are the basic elements of a practical isolation system:
Flexibility:
A flexible mounting so that period of vibration of the whole system is lengthened, sufficiently to reduce the force. Substantial reduction in base shear is possible as the period of vibration is lengthened, but the degree of reduction is depends on the initial fixed base period and shape of the response spectra curve. However, the additional flexibility needed to lengthen the period will also result in large relative displacements across the flexible mount. These displacements can be reduced if additional damping is introduced at the level of isolators.
There are many possible ways of introduction of flexibility into the system. These include elastomeric bearings, sliders, rollers, sliding plates, cable suspensions, sleeved pits, rocking (stepping) foundations, air cushions and coil springs. Of these elastomeric bearings and sliding foundations are most practical ones.
Energy dissipation:
Damping or energy dissipation devices are provided in a base isolation system in order to control the relative deflections in between the building and ground to a practical design level. Damping can be achieved through viscous damping, hysteretic dissipation or friction. The term hysteretic refers to the offset in loading and unloading curves under cyclic loading. Work done during loading is not completely recovered during unloading and the difference is lost as heat.
The practical means of achieving energy dissipation include mechanical devices which use the plastic deformation of either mild steel or lead to achieve energy dissipation; mild steel bars in tension and cantilevers in flexure; friction between metallic and non metallic
Rigidity under service loads:
While lateral flexibility is required to isolate against seismic loads, it is undesirable to have structural system that will vibrate perceptibly under frequently occurring loads such as minor earthquakes or wind loads. Specially formulated elastomers take advantage of dependence of shear modulus on strain amplitude to provide initial resistance to wind. At low strains these elastomers exhibit high moduli that are typically 3-4 times greater than their moduli at higher strains.
ISOLATION DEVICES
Successful seismic isolation of a particular structure is strongly dependent on the appropriate choice of the isolation system. The isolation system should essentially be
Able to support the structure
Provide horizontal flexibility
Able to dissipate energy
These three functions could be concentrated into a single device or could be provided by means of different components. In addition to these basic requirements, it is desirable that isolation system should be rigid for low lateral loads so as to avoid perceptible vibration during frequent minor earthquakes or wind loads. Different types of devices have been developed to achieve these properties. Brief reviews of the development of some isolation systems are discussed here.
Laminated Rubber Bearings:
Rubber bearings offer the simplest method of isolation and are relatively easy to manufacture. The bearings are made of vulcanization bonding of sheets of rubber to thin steel reinforced plates. In this process, steel plates having adhesive coatings are placed in a mould precisely at equal spacing with unvulcanized elastomeric filling the space. The assembly is then treated at certain pressure and temperature condition. The bearings are very stiff in vertical direction and very flexible in horizontal direction. High vertical stiffness of these bearings is achieved through the laminated construction of the bearing using steel plates. The ratio of vertical stiffness and horizontal stiffness should be high and the desired value is in the range of 200 to 500.
The damping achieved from natural rubber is low, of the order of 2 to 4 percent and therefore these bearings are called as low damping rubber bearings. It is unusual to utilize it without some other element able to provide some increased damping. The force-displacement behavior of these bearing is generally linear.
The most common elastomeric used in elastomeric bearings are natural rubber, neoprene rubber, butyl rubber and nitrile rubber. The mechanical properties of natural rubber are superior to those of most synthetic elastomers used for seismic isolation bearings. Therefore, natural rubber is more frequently recommended material for use in elastomeric bearing followed by neoprene. Butyl rubbers are suitable for low temperature applications and nitrile rubber have limited application in offshore oil structures.
Many buildings in Europe have been built on rubber bearings to isolate them from vibration due to underground railways.
Lead Rubber Bearings:
Owing to low damping in rubber compounds that is in adequate to control the displacements of the isolation system, researchers developed a system that uses a cylinder
of lead enclosed is an elastomeric bearing. The function of lead plate is primarily to dissipate energy while the laminated rubber bearing provides the lateral flexibility.
The reason for choosing lead as the material for insert in isolators is that it is a crystalline structure under deformation, but almost instantly regains its original crystal structure when the deformation ceases. Also lead yield in shear at relatively low stress of about 9653 N/mm, the lead plug acts as an hysteretic danger and dissipates significant energy in ground motion.
Lead rubber bearings provide an economical and effective solution, incorporating period shifting, increased damping, high stiffness at low strains and providing vertical support in a single device. The lead plug produces substantial increase in damping, from about 3 percent of critical in the available rubber to 10-15 percent and also increases the resistance to the frequently occurring loads such as minor earthquakes or wind loads. However, the part of self-centering property of the laminated rubber bearings lost after insertion of lead plug. The lead plug generally reduces the system displacement but may cause increased higher mode response and thus influence the response of the equipment.
Necessity of smart base isolation
A smart base isolation strategy is proposed and shown to effectively protect structures against extreme earthquake with out sacrificing performance during the move frequent, moderate seismic events. The proposed smart base isolation system is composed of conventional low-damping elastomeric bearings and controlled (semi active) dampers such as magneto rheological fluid dampers. The main virtue of these semi active controllable systems arises from the combination of the adaptable nature of a fully active control system with in the stability characteristics of passive control systems, while maintaing low-power requirements. Smart dampers are to provide a superior base isolation system for a broad class of earthquakes including near-source events as well as for a broad range of input levels. Thus a smart damper system can protect a structure from extreme earthquakes with out sacrificing performance during the more frequent moderate seismic events.
Magneto rheological damper (MRD) Semi active control devices have received significant attention in recent years because they offer the adaptability of active control devices without requiring the associated large power sources. Magnetorheological dampers are semi-active control devices that use MR fluids to produce controllable dampers. They potentially offer highly reliable operation and can be viewed as fail-safe in that they become passive dampers should the control hardware malfunction
A semi-active control device is one that has properties that adjacent in real time but cannot input energy into the system being controlled. In the Magnetorheological damper, the main part is the Magnetorheological fluid, which has the main function of vibration absorption
MR fluids are the magnetic analogs of electrological fluids and typically consist of micron-sized, magnetically polarizible particles dispersed in a carrier medium such as mineral or silicon oil. When a magnetic field in applied to the fluids, particle chain form and the fluid becomes a semi-solid and exhibits vistoplastic behavior similar to that of ER fluid. Transition to rheological equilibrium can be achieved in a few milliseconds, allowing construction of devices with high bandwidth. MR fluid can cooperate at temperatures form 40-150degree Celsius with only slight variations of yield stress MR fluids are not sensitive to impurities such as are commonly encountered during manufacturing and usage, and title particle/carrier fluid separation takes place in MR fluids under common flow conditions. Further a wider choice of additives (surfactants, dispersants, friction modifiers, antiwear agents etc). Since electrochemistry does not effect the magnetic-polarization mechanism. The MR fluid can be readily controlled with a low voltage (e.g.: 12-24 V), current-driven power supply outputting only 1-2amps.
MR fluid consists of a suspension of small colloidal particles, each of which contains many tiny, randomly oriented magnetic grains. An external magnetic moment in applied in each particle. Each particle then becomes a magnet moment in applied in each particle. Each particle then becomes a magnet is controlled by the applied field strength. A sea of magnets interacts with each other and form chains, which further coalesce into large, scale structures within colloidal suspension, these fluid induced structures within colloidal suspension. These fluid induced structures within colloidal suspension. These fluid induced structures dramatically modify the viscosity, turning the fluid suspension into a solid similar plastic. The properties of a suspension into a solid similar plastic. The properties of an MR fluid can be switched on and off rapidly and repeatedly by the application of external magnetic fluid.
There are three classifications for dampening systems:
Passive:
This is an uncontrolled damper, which requires no input power to operate. They are simple and generally low in cost but unable to adapt to changing needs.
Active:
Active dampers are force generators that actively push on the structure to counteract a disturbance. They are fully controllable and require a great deal of power.
Semi-Active:
Combines features of passive and active damping. Rather than push on the structure they counteract motion with a controlled resistive force to reduce motion. They are fully controllable yet require little input power. Unlike active devices they do not have the potential to go out of control and destabilize the structure. MR fluid dampers are semi-active devices that change their damping level by varying the amount of current supplied to an internal electro-magnet that controls the flow of MR fluid.
Magnetorheological Effect
In the absence of a magnetic field, the MR fluids posses a relatively small apparent viscosity and therefore exhibit flow properties similar to those of common dispersions such as paints. However when an electric field is applied, the originally multi domain particles, which have little or no net magnetization, are transformed into particles with a net magnetic moment m . This introduces an additional inter particle force the magnetic dipole-dipole interaction. For Ferro magnetic particles, the magnetic dipole-dipole interaction energy is considerably stronger than the other inter particle forces.
Fig; 1
Synthesis of MR fluids MR fluids based on iron and iron oxide particles suspended in polar organic liquids, rather than mineral oil or silicone oil, were prepared. Additives were also incorporated in an effort to enhance redispersibility and yield stress of these suspensions. The process for preparing these MR fluids typical involved introducing the magnetic particles into the base liquid under low shear conditions, followed by ball milling with Zirconia (ZrO2 ) grinding media for 24 hours. Iron powders were used for the synthesis of iron based MR fluids and the iron oxide based MR fluids
MR FLUID DAMPERS
SCHEMATIC OF MR DAMPER
Inside the MR fluid damper, an electromagnetic coil is wrapped around three sections of the piston. Approximately 5 liters of MR fluid is used to fill the damper's main chamber. During an earthquake, sensors attached to the building will signal the computer to supply the dampers with an electrical charge. This electrical charge then magnetizes the coil, turning the MR fluid from a liquid to a near solid. Now, the electromagnet will likely pulse as the vibrations ripple through the building. This vibration will cause the MR fluid to change from liquid to solid thousands of times per second, and may cause the temperature of the fluid to rise.
A thermal expansion accumulator is fixed to the top of the damper housing to allow for the expansion of the fluid as it heats up. This accumulator prevents a dangerous rise in pressure as the fluid expands
MR 180KN DAMPER
A full-scale MR fluid damper that is 1-meter long and weighs 250 kilograms. This one damper can exert 20 tons (200,000 N) of force on a building
The MR 180 KN damper is a large-scale magnetically responsive (MR) fluid damper unsurpassed in its combination of controllability, responsiveness, and energy density. Real-time damping is controlled by the increase in yield stress of the MR fluid in response to magnetic field strength. The response time of the fluid damping is on average 60-milliseconds as the magnetic field is changed. Featuring straightforward controls, mounting configuration, simple design, and quiet operation, this MR damper is especially well suited for structural applications
CONTROL OF A STRUCTURE USING MR DAMPERS
Buildings equipped with MR fluid dampers will mitigate vibrations during an earthquake. These large dampers filled with MR fluid are used to stabilize the structures during earthquakes. This diagram shows how the dampers would work during an earthquake
Depending on the size of the building, there could be an array of possibly hundreds of dampers. Each damper would sit on the floor and be attached to the chevron braces that are welded into a steel crossbeam. As the building begins to shake, the dampers would move back and forth to compensate for the vibration of the shock. When it's magnetized, the MR fluid increases the amount of force that the dampers can exert.
EXPERIMENTAL STUDY OF MR DAMPERS FOR SIESMIC PROTECTION
To investigate the performance of the MR damper, a series of experiments was conducted with the MR damper, which is used to control a three-story test structure subjected to a one-dimensional ground excitation. The results indicate that the MR damper is quite effective for structural response reduction over a wide class of seismic excitations. The focus of this experiment is to demonstrate the ability of the MR damper to reduce structural responses over a wide range of loading conditions. In the experiments, the ability of the system to reduce the peak responses, in the case of the earthquake excitation. The results reported here indicate that this semi-active control system is quite effective for seismic response reduction over a wide range of seismic excitations.
Construction of the Model:
The building frame is constructed of steel, with a height of 158 cm. The floor masses of the model weigh a total of 227 kg, distributed evenly between the three floors. .In this experiment, a single magneto-rheological (MR) damper is installed between the ground and first floor shown in Fig. 1. The MR damper employed here is a prototype device, shown schematically in Fig. 2. The damper is 21.5cm long in its extended position and stroke. The main cylinder is 3.8cm in diameter and houses the piston, the magnetic circuit, an accumulator and 50ml of MR fluid. The magnetic field produced in the device is generated by a small electromagnet in the piston head. Figure 1 is a diagram of the semi-actively controlled, three-story, model building. The test structure used in this experiment is designed to be a scale model of the prototype building and is subjected to a one-dimensional ground motion. has a cm
The current for the electromagnet is supplied by a linear current driver, which generates a current that is proportional to the applied voltage. The peak power required is less than 10 watts. The system, including the damper and the current driver, has a response time of typically less than 10 msec. A number of sensors are installed in the model building for use in determining the control action. Accelerometers located on each of the three floors provide measurements of the absolute accelerations, an LVDT (linear variable differential transformer) measures the displacement of the MR damper, and a force transducer is placed in series with the MR damper to measure the control force f being applied to the structure.
By using the control force, the floors begin to vibrate; these vibrations are picked by the sensors and fed into the control computer. The computer then analyses these vibrations and gives signals to current driver to provide the necessary current to the damper so that the damper can act within msecs. Damper reduces the vibrations, the uncontrolled vibration of the floors and the controlled vibrations are recorded and evaluated. The experimental results are summarized in the following sections.
Figure 11 shows the uncontrolled and semi-actively controlled responses for the tested structure. There are two graphs showing the responses of the building during the time of excitation. The graphs show the displacement V/s time and acceleration V/s time readings for controlled structure using MR damper and uncontrolled structure without MR damper. The effectiveness of the proposed control strategy is clearly seen.
Here the dark line shows the amplitude and acceleration of the first and second graphs respectively for the controlled structure and blue line shows amplitude and acceleration of the uncontrolled structure. The result shows that there is a reduction peak third floor displacement by 74.5% and the peak third floor acceleration being reduced by 47.6% over the uncontrolled responses.
LIMITATIONS
Base isolation enables the reduction in earthquake-induced forces by lengthening the period of vibration of the structure. Benefits obtained from base isolation are in structures for which the fundamental period of vibration without base isolation is less than 1 seconds. The natural period of building increases with increasing height. Taller buildings reach a limit at which the natural period is long enough to attract low earthquake forces without isolation. Therefore, seismic isolation is most applicable to low and medium rise buildings and becomes less effective for tall ones. The cut off mainly depends on structural systems or type of framing system.
Cost involved in constructing a new building is higher than the cost of conventional earthquake resistant structural system. Seismic isolation bearings are expensive. Due to these economic considerations, these devices have so far been used for important buildings only, even in developed countries. To enable its use for common buildings, some low cost devices has to be developed.
Requirement of tests on prototype bearing of every type increases the cost of the project. Therefore, development and standardization of testing methods for evaluating the properties of isolation devices should be formulated.
CONCLUSION
Seismic base isolation method has proved to be a reliable method of earthquake resistant Design. The success of this method is largely attributed to the development of isolation devices and proper planning. Different types of isolation devices have been proposed and extensive research has been made on them. They can serve the purpose for almost all types of conditions. Adaptable isolation systems are required to be effective during a wide range of seismic events. Efforts are required to find the solutions for the situations like near fault regions where wide variety of earthquake motions may occur. Besides, the existing devices are expensive and to make isolation feasible for ordinary buildings, it is essential to develop cost effective devices.
REFERENCES
1. Chopra, A. K, Dynamics of structures, Pearson Education Asia, 2001
2. Hoskere, Vidyashankar, Some fundamental aspects of Seismic resistant design, Aug-Sept 2001, Vol: 2.
3. Jain, Sarvesh Kumar, Seismic isolation devices: A Review, Bridge and Structural Engineering Journal, June 2004, Vol: 7
4. Kasalanati, Amarnath, Base isolated structure, the master builder, Aug-Sept 2001, Vol: 2.
5. Rai, C, Durgesh., Future trends in Earthquake resistant design of structures, Current Science, Oct 2001, Vol: 79, No:9.
6. Ramallo, J. C, Johnson. E. A, Spenser. B. F, Smart base isolation systems, Journal of Engineering Mechanics, Oct 2002, Vol: 128.
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INTRODUCTION
Earthquake by itself, is not a disaster, it is natural phenomenon result from ground movement, sometimes violent. Disaster occurs when man made buildings and other structures cannot cope up with these ground movements and fail causing death and destruction of property.
The energy that is released after an earthquake sets up waves called primary wave and secondary waves. These further produce surface waves, which cause vibration of the ground and structures standing on top. Depending on the characteristics of these vibrations, the ground may develop cracks, fissures and settlements.
Seismic events are not predictable and a major event can never be ruled out in the life of a structure located in a severe seismic zone. Both non-structural and structural damage may be expected in a major seismic event. The possible risk of loss of life adds a very serious dimension to seismic design, putting a moral responsibility on structural engineers to be comprehensive in their work.
In recent times, many new systems and devices using non conventional materials have been developed, either to reduce the earthquake forces acting on the structure or to absorb a part of seismic energy. One of the most widely implemented and accepted seismic protection systems is base isolation.
BASE ISOLATION
Base isolation is one of the most widely accepted seismic protection systems in earthquake prone areas. It mitigates the effect of an earthquake by essentially isolating the structure from potentially dangerous ground motions, especially in frequency range where building is mostly affected.
Seismic isolation is a design a strategy, which uncouples the structure for the damaging effects of the ground motion. The term isolation refers to reduced interaction between structure and the ground. When the seismic isolation system is located under the structure, it is referred as base isolation.
History:
The concept of seismic isolation appears to be over a century old. John Milne at the university of Tokyo first reported it. Imperial Hotel in Tokyo, completed in 1921, was founded on a shallow layer of firm soil supported by an underlying layer of soft mud. Cushioned from the devastating ground motion, the hotel survived the 1923 Tokyo earthquake. There have been some instances of unintentional isolated structures where the structures survived earthquake by sliding on their foundation. One of the early uses of elastomeric bearings as isolation devices was reported in 1969 for an elementary school in Yugoslavia. Now, the concept of seismic isolation has matured into a practical really over the last 20 years.
Need:
The need for hospitals or emergency facilities to be functioning post earthquake is clear. Expected performance level for such buildings should be fully occupational-performance level of immediate occupancy. There is ever increasing necessity of protecting nonstructural components and highly sensitive equipments. The key objective of base isolation is mitigating damage to contents of structure.
The basic dilemma in providing superior seismic resistance of a building is the difficulty in minimizing the interstorey drift and floor accelerations simultaneously. Large interstorey drifts cause damage to non-structural components. Interstorey drifts can be minimized by stiffening the structure, but this leads to amplification of the ground motion, which leads to high floor acceleration, which can damage sensitive internal equipment. Making the system more flexible can reduce floor acceleration, but this leads to large interstorey drifts. The only practical way of reducing interstorey drift and floor acceleration simultaneously is to use base isolation, which provides the necessary flexibility, with the displacements concentrated at the isolation level.
Concept of base isolation:
During a seismic event, a finite quantity of energy is input into the structure. This input energy is transformed into both kinetic and potential (strain) energy, which must either absorbed or dissipated through heat. Considering the energy conservation of a structure.
E = Ek + Es + En + Ed
Where E- Absolute energy input from earthquake motion i.e., represents the work done by total base shear force at the foundation, Ek Absolute kinetic energy, Es recoverable elastic strain energy, En-irrecoverable energy dissipated by structural system through inelastic or other forms of action and Ed-Energy dissipated by supplemental damping devices.
Conventional Earthquake resistant structural systems:
In conventional earthquake resistant structural systems, acceptable performance is achieved by the occurrence of inelastic deformation i.e., En. In such systems, specially selected ductile components are designed to with stand several cycles well beyond yield under reverse loading, with yield level chosen so that forces are transmitted to other compounds of the structure are limited to their elastic range. The yielding lengthens the fundamental period of the structure, detuning the response away from the energy frequency of earthquake ground motion. The hysteric behavior of these selected components provides energy dissipation and ductile behavior ensures sufficient deformation capacity and structure rides out of earthquake attack. They are fixed base systems.
The dissipation of energy due to earthquake forces occurs only after the structure is partly damaged. Thus structural damage is prerequisite for force reduction and hysteric energy dissipation.
Base isolated systems:
Base isolated systems differ from conventional structural systems in the methods by which the lengthening of fundamental period and hysteric energy dissipating mechanisms provided and the degree of reduction in transmission of earthquake forces into the structure.
In base isolation systems, the fundamental aim is to substantially reduce the transmission of earthquake forces and energy into the structure. Moreover, the reduction in earthquake forces is achieved without damage to the structure. The reduction in transmission of forces and hysteric action originate from the isolation system and do not depend on the structural damage. This is achieved by mounting the structure on an isolating layer with considerable horizontal flexibility. The isolators are specifically designed to accommodate large horizontal movements while carrying the structural loads. Thus by interposing structural elements with low horizontal stiffness, the fundamental frequency of overall structure is much lower than the predominant frequencies of the ground motion.
Seismic isolation is characterized by flexibility and energy absorption capability. The flexibility alone is insufficient to defeat away a major portion of the earthquake energy so that inelastic action does not occur, i.e., En is minimized energy dissipation in the isolation system Ed is then useful in limiting the displacement response and in avoiding resonance.
Design considerations:
A number of factors need to be considered by an engineer, architect or owner to decide on seismic isolation for a project. Among the foremost is the evaluation of seismic hazard, which includes local geology, proximity to faults, soil conditions, characteristics of possible earthquakes such as period and severity. Subsequently, performance levels for different intensities of earthquakes need to be evaluated.
Since the isolators carry large vertical loads and deform to significant lateral displacement, the components of the structure above and below the isolator need to be designed appropriately. Plane of isolation may be chosen based on the practical aspects of installation and relative strengths of super and sub structure components. Specifically, for the isolation system to work properly, the structure should be free to move in any direction up the maximum specified displacement. Typically a seismic moat is provided around the structure to allow this movement. It is imperative that owners and occupiers of seismically isolated structures are aware of the functional importance of seismic gap and the need for this space to be left clear.
To maintain the functional purpose of the structure after a seismic event, all the utilities, electrical connections and waste pipelines should be designed to accommodate the maximum seismic displacement. The main connections between the building and the ground, such as stairs, entryways and elevators need to be unconnected across the isolation plane. In general, all the interaction between the structure and the ground need to be designed and detailed.
Seismic isolation provides immediate occupancy performance level following strong events. Costs and benefits of different approaches may be evaluated in determining the incorporation of seismic isolation.
Basic elements of base isolation system:
The following are the basic elements of a practical isolation system:
Flexibility:
A flexible mounting so that period of vibration of the whole system is lengthened, sufficiently to reduce the force. Substantial reduction in base shear is possible as the period of vibration is lengthened, but the degree of reduction is depends on the initial fixed base period and shape of the response spectra curve. However, the additional flexibility needed to lengthen the period will also result in large relative displacements across the flexible mount. These displacements can be reduced if additional damping is introduced at the level of isolators.
There are many possible ways of introduction of flexibility into the system. These include elastomeric bearings, sliders, rollers, sliding plates, cable suspensions, sleeved pits, rocking (stepping) foundations, air cushions and coil springs. Of these elastomeric bearings and sliding foundations are most practical ones.
Energy dissipation:
Damping or energy dissipation devices are provided in a base isolation system in order to control the relative deflections in between the building and ground to a practical design level. Damping can be achieved through viscous damping, hysteretic dissipation or friction. The term hysteretic refers to the offset in loading and unloading curves under cyclic loading. Work done during loading is not completely recovered during unloading and the difference is lost as heat.
The practical means of achieving energy dissipation include mechanical devices which use the plastic deformation of either mild steel or lead to achieve energy dissipation; mild steel bars in tension and cantilevers in flexure; friction between metallic and non metallic
Rigidity under service loads:
While lateral flexibility is required to isolate against seismic loads, it is undesirable to have structural system that will vibrate perceptibly under frequently occurring loads such as minor earthquakes or wind loads. Specially formulated elastomers take advantage of dependence of shear modulus on strain amplitude to provide initial resistance to wind. At low strains these elastomers exhibit high moduli that are typically 3-4 times greater than their moduli at higher strains.
ISOLATION DEVICES
Successful seismic isolation of a particular structure is strongly dependent on the appropriate choice of the isolation system. The isolation system should essentially be
Able to support the structure
Provide horizontal flexibility
Able to dissipate energy
These three functions could be concentrated into a single device or could be provided by means of different components. In addition to these basic requirements, it is desirable that isolation system should be rigid for low lateral loads so as to avoid perceptible vibration during frequent minor earthquakes or wind loads. Different types of devices have been developed to achieve these properties. Brief reviews of the development of some isolation systems are discussed here.
Laminated Rubber Bearings:
Rubber bearings offer the simplest method of isolation and are relatively easy to manufacture. The bearings are made of vulcanization bonding of sheets of rubber to thin steel reinforced plates. In this process, steel plates having adhesive coatings are placed in a mould precisely at equal spacing with unvulcanized elastomeric filling the space. The assembly is then treated at certain pressure and temperature condition. The bearings are very stiff in vertical direction and very flexible in horizontal direction. High vertical stiffness of these bearings is achieved through the laminated construction of the bearing using steel plates. The ratio of vertical stiffness and horizontal stiffness should be high and the desired value is in the range of 200 to 500.
The damping achieved from natural rubber is low, of the order of 2 to 4 percent and therefore these bearings are called as low damping rubber bearings. It is unusual to utilize it without some other element able to provide some increased damping. The force-displacement behavior of these bearing is generally linear.
The most common elastomeric used in elastomeric bearings are natural rubber, neoprene rubber, butyl rubber and nitrile rubber. The mechanical properties of natural rubber are superior to those of most synthetic elastomers used for seismic isolation bearings. Therefore, natural rubber is more frequently recommended material for use in elastomeric bearing followed by neoprene. Butyl rubbers are suitable for low temperature applications and nitrile rubber have limited application in offshore oil structures.
Many buildings in Europe have been built on rubber bearings to isolate them from vibration due to underground railways.
Lead Rubber Bearings:
Owing to low damping in rubber compounds that is in adequate to control the displacements of the isolation system, researchers developed a system that uses a cylinder
of lead enclosed is an elastomeric bearing. The function of lead plate is primarily to dissipate energy while the laminated rubber bearing provides the lateral flexibility.
The reason for choosing lead as the material for insert in isolators is that it is a crystalline structure under deformation, but almost instantly regains its original crystal structure when the deformation ceases. Also lead yield in shear at relatively low stress of about 9653 N/mm, the lead plug acts as an hysteretic danger and dissipates significant energy in ground motion.
Lead rubber bearings provide an economical and effective solution, incorporating period shifting, increased damping, high stiffness at low strains and providing vertical support in a single device. The lead plug produces substantial increase in damping, from about 3 percent of critical in the available rubber to 10-15 percent and also increases the resistance to the frequently occurring loads such as minor earthquakes or wind loads. However, the part of self-centering property of the laminated rubber bearings lost after insertion of lead plug. The lead plug generally reduces the system displacement but may cause increased higher mode response and thus influence the response of the equipment.
Necessity of smart base isolation
A smart base isolation strategy is proposed and shown to effectively protect structures against extreme earthquake with out sacrificing performance during the move frequent, moderate seismic events. The proposed smart base isolation system is composed of conventional low-damping elastomeric bearings and controlled (semi active) dampers such as magneto rheological fluid dampers. The main virtue of these semi active controllable systems arises from the combination of the adaptable nature of a fully active control system with in the stability characteristics of passive control systems, while maintaing low-power requirements. Smart dampers are to provide a superior base isolation system for a broad class of earthquakes including near-source events as well as for a broad range of input levels. Thus a smart damper system can protect a structure from extreme earthquakes with out sacrificing performance during the more frequent moderate seismic events.
Magneto rheological damper (MRD) Semi active control devices have received significant attention in recent years because they offer the adaptability of active control devices without requiring the associated large power sources. Magnetorheological dampers are semi-active control devices that use MR fluids to produce controllable dampers. They potentially offer highly reliable operation and can be viewed as fail-safe in that they become passive dampers should the control hardware malfunction
A semi-active control device is one that has properties that adjacent in real time but cannot input energy into the system being controlled. In the Magnetorheological damper, the main part is the Magnetorheological fluid, which has the main function of vibration absorption
MR fluids are the magnetic analogs of electrological fluids and typically consist of micron-sized, magnetically polarizible particles dispersed in a carrier medium such as mineral or silicon oil. When a magnetic field in applied to the fluids, particle chain form and the fluid becomes a semi-solid and exhibits vistoplastic behavior similar to that of ER fluid. Transition to rheological equilibrium can be achieved in a few milliseconds, allowing construction of devices with high bandwidth. MR fluid can cooperate at temperatures form 40-150degree Celsius with only slight variations of yield stress MR fluids are not sensitive to impurities such as are commonly encountered during manufacturing and usage, and title particle/carrier fluid separation takes place in MR fluids under common flow conditions. Further a wider choice of additives (surfactants, dispersants, friction modifiers, antiwear agents etc). Since electrochemistry does not effect the magnetic-polarization mechanism. The MR fluid can be readily controlled with a low voltage (e.g.: 12-24 V), current-driven power supply outputting only 1-2amps.
MR fluid consists of a suspension of small colloidal particles, each of which contains many tiny, randomly oriented magnetic grains. An external magnetic moment in applied in each particle. Each particle then becomes a magnet moment in applied in each particle. Each particle then becomes a magnet is controlled by the applied field strength. A sea of magnets interacts with each other and form chains, which further coalesce into large, scale structures within colloidal suspension, these fluid induced structures within colloidal suspension. These fluid induced structures within colloidal suspension. These fluid induced structures dramatically modify the viscosity, turning the fluid suspension into a solid similar plastic. The properties of a suspension into a solid similar plastic. The properties of an MR fluid can be switched on and off rapidly and repeatedly by the application of external magnetic fluid.
There are three classifications for dampening systems:
Passive:
This is an uncontrolled damper, which requires no input power to operate. They are simple and generally low in cost but unable to adapt to changing needs.
Active:
Active dampers are force generators that actively push on the structure to counteract a disturbance. They are fully controllable and require a great deal of power.
Semi-Active:
Combines features of passive and active damping. Rather than push on the structure they counteract motion with a controlled resistive force to reduce motion. They are fully controllable yet require little input power. Unlike active devices they do not have the potential to go out of control and destabilize the structure. MR fluid dampers are semi-active devices that change their damping level by varying the amount of current supplied to an internal electro-magnet that controls the flow of MR fluid.
Magnetorheological Effect
In the absence of a magnetic field, the MR fluids posses a relatively small apparent viscosity and therefore exhibit flow properties similar to those of common dispersions such as paints. However when an electric field is applied, the originally multi domain particles, which have little or no net magnetization, are transformed into particles with a net magnetic moment m . This introduces an additional inter particle force the magnetic dipole-dipole interaction. For Ferro magnetic particles, the magnetic dipole-dipole interaction energy is considerably stronger than the other inter particle forces.
Fig; 1
Synthesis of MR fluids MR fluids based on iron and iron oxide particles suspended in polar organic liquids, rather than mineral oil or silicone oil, were prepared. Additives were also incorporated in an effort to enhance redispersibility and yield stress of these suspensions. The process for preparing these MR fluids typical involved introducing the magnetic particles into the base liquid under low shear conditions, followed by ball milling with Zirconia (ZrO2 ) grinding media for 24 hours. Iron powders were used for the synthesis of iron based MR fluids and the iron oxide based MR fluids
MR FLUID DAMPERS
SCHEMATIC OF MR DAMPER
Inside the MR fluid damper, an electromagnetic coil is wrapped around three sections of the piston. Approximately 5 liters of MR fluid is used to fill the damper's main chamber. During an earthquake, sensors attached to the building will signal the computer to supply the dampers with an electrical charge. This electrical charge then magnetizes the coil, turning the MR fluid from a liquid to a near solid. Now, the electromagnet will likely pulse as the vibrations ripple through the building. This vibration will cause the MR fluid to change from liquid to solid thousands of times per second, and may cause the temperature of the fluid to rise.
A thermal expansion accumulator is fixed to the top of the damper housing to allow for the expansion of the fluid as it heats up. This accumulator prevents a dangerous rise in pressure as the fluid expands
MR 180KN DAMPER
A full-scale MR fluid damper that is 1-meter long and weighs 250 kilograms. This one damper can exert 20 tons (200,000 N) of force on a building
The MR 180 KN damper is a large-scale magnetically responsive (MR) fluid damper unsurpassed in its combination of controllability, responsiveness, and energy density. Real-time damping is controlled by the increase in yield stress of the MR fluid in response to magnetic field strength. The response time of the fluid damping is on average 60-milliseconds as the magnetic field is changed. Featuring straightforward controls, mounting configuration, simple design, and quiet operation, this MR damper is especially well suited for structural applications
CONTROL OF A STRUCTURE USING MR DAMPERS
Buildings equipped with MR fluid dampers will mitigate vibrations during an earthquake. These large dampers filled with MR fluid are used to stabilize the structures during earthquakes. This diagram shows how the dampers would work during an earthquake
Depending on the size of the building, there could be an array of possibly hundreds of dampers. Each damper would sit on the floor and be attached to the chevron braces that are welded into a steel crossbeam. As the building begins to shake, the dampers would move back and forth to compensate for the vibration of the shock. When it's magnetized, the MR fluid increases the amount of force that the dampers can exert.
EXPERIMENTAL STUDY OF MR DAMPERS FOR SIESMIC PROTECTION
To investigate the performance of the MR damper, a series of experiments was conducted with the MR damper, which is used to control a three-story test structure subjected to a one-dimensional ground excitation. The results indicate that the MR damper is quite effective for structural response reduction over a wide class of seismic excitations. The focus of this experiment is to demonstrate the ability of the MR damper to reduce structural responses over a wide range of loading conditions. In the experiments, the ability of the system to reduce the peak responses, in the case of the earthquake excitation. The results reported here indicate that this semi-active control system is quite effective for seismic response reduction over a wide range of seismic excitations.
Construction of the Model:
The building frame is constructed of steel, with a height of 158 cm. The floor masses of the model weigh a total of 227 kg, distributed evenly between the three floors. .In this experiment, a single magneto-rheological (MR) damper is installed between the ground and first floor shown in Fig. 1. The MR damper employed here is a prototype device, shown schematically in Fig. 2. The damper is 21.5cm long in its extended position and stroke. The main cylinder is 3.8cm in diameter and houses the piston, the magnetic circuit, an accumulator and 50ml of MR fluid. The magnetic field produced in the device is generated by a small electromagnet in the piston head. Figure 1 is a diagram of the semi-actively controlled, three-story, model building. The test structure used in this experiment is designed to be a scale model of the prototype building and is subjected to a one-dimensional ground motion. has a cm
The current for the electromagnet is supplied by a linear current driver, which generates a current that is proportional to the applied voltage. The peak power required is less than 10 watts. The system, including the damper and the current driver, has a response time of typically less than 10 msec. A number of sensors are installed in the model building for use in determining the control action. Accelerometers located on each of the three floors provide measurements of the absolute accelerations, an LVDT (linear variable differential transformer) measures the displacement of the MR damper, and a force transducer is placed in series with the MR damper to measure the control force f being applied to the structure.
By using the control force, the floors begin to vibrate; these vibrations are picked by the sensors and fed into the control computer. The computer then analyses these vibrations and gives signals to current driver to provide the necessary current to the damper so that the damper can act within msecs. Damper reduces the vibrations, the uncontrolled vibration of the floors and the controlled vibrations are recorded and evaluated. The experimental results are summarized in the following sections.
Figure 11 shows the uncontrolled and semi-actively controlled responses for the tested structure. There are two graphs showing the responses of the building during the time of excitation. The graphs show the displacement V/s time and acceleration V/s time readings for controlled structure using MR damper and uncontrolled structure without MR damper. The effectiveness of the proposed control strategy is clearly seen.
Here the dark line shows the amplitude and acceleration of the first and second graphs respectively for the controlled structure and blue line shows amplitude and acceleration of the uncontrolled structure. The result shows that there is a reduction peak third floor displacement by 74.5% and the peak third floor acceleration being reduced by 47.6% over the uncontrolled responses.
LIMITATIONS
Base isolation enables the reduction in earthquake-induced forces by lengthening the period of vibration of the structure. Benefits obtained from base isolation are in structures for which the fundamental period of vibration without base isolation is less than 1 seconds. The natural period of building increases with increasing height. Taller buildings reach a limit at which the natural period is long enough to attract low earthquake forces without isolation. Therefore, seismic isolation is most applicable to low and medium rise buildings and becomes less effective for tall ones. The cut off mainly depends on structural systems or type of framing system.
Cost involved in constructing a new building is higher than the cost of conventional earthquake resistant structural system. Seismic isolation bearings are expensive. Due to these economic considerations, these devices have so far been used for important buildings only, even in developed countries. To enable its use for common buildings, some low cost devices has to be developed.
Requirement of tests on prototype bearing of every type increases the cost of the project. Therefore, development and standardization of testing methods for evaluating the properties of isolation devices should be formulated.
CONCLUSION
Seismic base isolation method has proved to be a reliable method of earthquake resistant Design. The success of this method is largely attributed to the development of isolation devices and proper planning. Different types of isolation devices have been proposed and extensive research has been made on them. They can serve the purpose for almost all types of conditions. Adaptable isolation systems are required to be effective during a wide range of seismic events. Efforts are required to find the solutions for the situations like near fault regions where wide variety of earthquake motions may occur. Besides, the existing devices are expensive and to make isolation feasible for ordinary buildings, it is essential to develop cost effective devices.
REFERENCES
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4. Kasalanati, Amarnath, Base isolated structure, the master builder, Aug-Sept 2001, Vol: 2.
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6. Ramallo, J. C, Johnson. E. A, Spenser. B. F, Smart base isolation systems, Journal of Engineering Mechanics, Oct 2002, Vol: 128.