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Base isolation of structures

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Base isolation of structures

  1. 1. PRESENTED BY, 1
  2. 2.  INTRODUCTION TO EARTHQUAKE  CONCEPT OF BASE ISOLATION  TYPES OF BASE ISOLATION SYSTEMS  IMPLIMENTATION OF BASE ISOLATION  CASE STUDY  CONCLUSIONS 2CONTENTS
  3. 3. INTRODUCTION  An earthquake is the perceptible shaking of the surface of the Earth due to underground movement along a fault plane or from volcanic activity  The severity of the shaking can range from barely felt to violent enough to toss people around  An Earthquake is the result of Sudden release of energy in the Earth’s crust creates seismic waves, which causes vibration of the ground and structures resting on it  Depending on the characteristics of these vibrations, the ground may develop cracks, fissures and settlements.  Shaking and ground rupture are the main effects, principally resulting in more or less severe damage to buildings and other rigid structures. 3
  4. 4. INTRODUCTION  The possible risk of loss of life adds a very serious dimension to seismic design, putting a moral responsibility on structural engineers.  Objective of Earthquake Resistant Design is to make such buildings that can resist effect of ground motion and would not collapse during the strong Earthquake.  In recent times, many new systems 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 researched, implemented and accepted seismic protection systems is base isolation. 4
  5. 5. Earthquake Protective Systems Passive Protective Systems Hybrid Protective Systems Active Protective Systems Tuned Mass Damper Energy Dissipation Base Isolation Active Isolation Semi-Active Isolation Semi-Active Mass Damping Active Mass Damping Active Bracing Adaptive Control 5 Base Isolation is the most common System EARTHQUAKE PROTECTIVE SYSTEMS
  6. 6. What is Base Isolation?  It is a system that may be defined as a flexible or sliding interface positioned between a structure and its foundation, for the purpose of decoupling the horizontal motions of the ground from the horizontal motions of the structure, thereby reducing earthquake damage to the structure and its contents. Base isolation system absorbs and deflects the energy released from the earthquake before it is transferred to the structure 6 BASE-ISOLATED BUILDING
  7. 7.  The term isolation refers to reduced interaction between structure and the ground.  Since the seismic isolation system is located under the structure, it is referred as ‘Base isolation’.  Base isolation is a passive control system meaning thereby that it does not require any external force or energy for its activation.  The base isolators used in this system mitigate the effect of an earthquake by decoupling the components of the buildings from direct contact with the ground essentially isolating the structure from potentially dangerous ground motions.  The base-isolation techniques prove to be very effective for the seismic protection of new framed buildings as well as for the seismic retrofitting of existing ones. 7
  8. 8. OBJECTIVES OF SEISMIC ISOLATION SYSTEM  Minimizing interruption of use of facility (Immediate Occupancy Performance Level)  Reducing damaging deformations in structural and non-structural component  Reducing acceleration response to minimize contents related damage  preventing plastic deformation of structural elements  Protection of Building Frame  Protection of Non-structural Components & Contents  Provide for an Operational facility after the Earthquake  Protection of Life - Safety of occupants  Improvement for Safety of Building 8 Enhance performance of structures at all hazard levels by,
  9. 9. The concept of separating the structure from the ground to avoid earthquake damage is quite simple to grasp. After all, in an earthquake the ground moves and it is this ground movement which causes most of the damage to structures. An airplane flying over an earthquake is not affected. So, the principle is simple. Separate the structure from the ground. The ground will move but the building will not move. 9 Rigid attachment of a building to its foundation -strengthen to resist damage ? Decouple the structure from its foundation -flexibility to resist damage ?
  10. 10. How can the structure be separated from the ground for earthquake loads but still resist gravity? Ideal separation would be good. Perhaps an air gap, frictionless rollers, a well- oiled sliding surface, sky hooks, magnetic levitation have practical restraints. An air gap would not provide vertical support; a sky-hook needs to hang from something; frictionless rollers, sliders or magnetic levitation would allow the building to move for blocks under a gust of wind. 10PRINCIPLE OF BASE ISOLATION
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  13. 13. ANIMATION SHOWING BASE ISOLATION 13 FIXED BASE ISOLATED BASE
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  15. 15. RESPONSE OF BASE ISOLATED BUILDINGS VERSUS FIXED BASE RESPONSE Drift on Isolation Interface Reduced Superstructure Deformations and acceleration response for Base Isolated Structure 15
  16. 16. Earthquake Resistant House Base Isolated House 'Earthquake-resistant' technology enables the building to counteract the earthquake load by making its strength and resilience great enough to resist shakings. Although it can protect the building safely, it is ac-companied by a risk that the furniture inside could fall or drop. Seismic Isolation system turns destructive seismic shakings into slower and softer ones preventing possible damage. This structure can evade the tremors, taking them in stride and safeguarding the building, human lives and property
  17. 17. CONSIDERATION FOR SEISMIC ISOLATION The benefits of using seismic isolation for earthquake resistant design are:  Isolation leads to a simpler structure with much less complicated seismic analysis as compared with conventional structures  Isolated designs are less sensitive to uncertainties in ground motion  Minor damage at the design level event means immediate reoccupation  The performance of the isolators is highly predictable, so they are much more reliable than conventional structural components  Even in case of larger-than-expected seismic events, damage will concentrate in the isolation system, where elements can be easily substituted to restore the complete functionality of the structure  Base Isolation minimizes the need for strengthening measures like adding shear walls, frames, and bracing by reducing the earthquake forces imparted to the building.  Base Isolated building are capable of resisting GSA blasts loads and their ability to move reduces the overall impact of the blast force on the building. 17
  18. 18. CONCEPT OF BASE ISOLATION 18
  19. 19. 19 Base isolation, as a strategy to protect structure from earthquake, revolves around a few basic elements of understanding 1. Period-shifting of structure 2. Mode of vibration 3. Damping and cutting of load transmission path 4. Minimum rigidity CONCEPT OF BASE ISOLATION Isolators have large deformation potential allowing for large drift on the Isolation Interface
  20. 20. BASIC ELEMENTS OF SEISMIC ISOLATION The three basic elements in seismic isolation systems are,  A vertical-load carrying device that provides lateral flexibility so that the period of vibration of the total system is lengthened sufficiently to reduce the force response  A damper or energy dissipater so that the relative deflections across the flexible mounting can be limited to a practical design level  A means of providing rigidity under low (service) load  Steel plates, vulcanized rubber, and a lead plug in the center of the design create these functional contrasting directional components. 20
  21. 21. PRINCIPLE OF BASE ISOLATION  The fundamental principle of base isolation is to modify the response of the building so that the ground can move below the building without transmitting these motions into the building.  A building that is perfectly rigid will have a zero period.  When the ground moves the acceleration induced in the structure will be equal to the ground acceleration and there will be zero relative displacement between the structure and the ground.  The structure and ground move by same amount.  A building that is perfectly flexible will have an infinite period.  For this type of structure, when the ground beneath the structure moves there will be zero acceleration induced in the structure and the relative displacement between the structure and ground will be equal to the ground displacement  So in flexible structures the structure will not move, the ground will. 21
  22. 22. INSTALLATION OF SEISMIC ISOLATION 22 Layout and installation details for the isolation system - depends on the site constraints, Type of structure, Construction and other related factors. SEISMIC-ISOLATION CONFIGURATIONS
  23. 23. ISOLATOR LOCATIONS  The requirement for installation of a base isolation system is that the building be able to move horizontally relative to the ground, usually at least 100 mm.  The most common configuration is to install a diaphragm immediately above the isolators.  If the building has a basement then the options are to install the isolators at the top, bottom or mid-height of the basements columns and walls. 23
  24. 24. SUITABILITY OF BASE ISOLATION  The subsoil does not produce a predominance of long period ground motion.  The structure is fairly jointed with sufficiently high column load.  The site permits horizontal displacements at the base of the order of 200 mm or more.  Lateral loads due to wind are less than approximately 10% of the weight of the structure.  The structure has two stories or more(heavy)  The structure is fairly squat. 24 MOST EFFECTIVE Structure on Stiff Soil Structure with Low Fundamental Period (Low-Rise Building) LEAST EFFECTIVE Structure on Soft Soil Structure with High Fundamental Period (High-Rise Building) Each project must be assessed individually and early in design phase to determine the suitability for seismic isolation.
  25. 25. IS IT AN ECONOMIC SOLUTION?  Base isolation allows for a reduction in structural elements of the building with less ductile detailing needed  Widely held misconception is that seismic isolation is expensive  E.g. Union House built in Auckland in 1983 with base isolation produced an estimated 7% cost saving in the total construction cost of $6.6million which included a construction time saving of 3 months due to the structural form requiring less seismic force, ductility demands and structural deformations  As a general rule the inclusion of all aspects of seismic isolation in a new structure will add no more than 3% to total construction cost and considerably less when assessed against the benefits of isolation  Seismic isolation devices require no maintenance during the life of the building  Following any significant event they should be inspected to ensure bolts and load plates are still in place.  Devices do not need replacing after an earthquake unless the event was in excess of their design specification in which case removal of some devices for testing is recommended.  Because the building is protected from major damage, repair costs following an earthquake will be lower to non-existent 25
  26. 26. SLIDING SYSTEM 1. Resilient friction system 2. Friction pendulum system ELASTOMERIC BEARING 1. Natural rubber bearing 2. Low damping rubber bearings 3. Lead plug bearings 4. High damping rubber bearing 26 TYPES OF BASE ISOLATION SYSTEMS
  27. 27. SLIDING SYSTEM  Uses sliding elements between the foundation and base of the structure.  The sliding displacements are controlled by high-tension springs or laminated rubber bearings, or by making the sliding surface curved.  These mechanisms provide a restoring force to return the structure to its equilibrium position. 27 TYPES OF BASE ISOLATION SYSTEMS
  28. 28. SLIDING ISOLATOR WITHOUT RECENTERING CAPACITY  This consists of a horizontal sliding surface, allowing a displacement and thus dissipating energy by means of defined friction between both sliding components and stainless steel.  One particular problem with a sliding structure is the residual displacements that occur after major earthquakes. 28 TYPES OF BASE ISOLATION SYSTEMS
  29. 29. SLIDING ISOLATOR WITH RECENTERING CAPACITY  Consists of a concave sliding plate.  Due to geometry, each horizontal displacement results in a vertical movement of the isolator.  They remain horizontally flexible, dissipate energy and recenter the superstructure into neutral position 29 TYPES OF BASE ISOLATION SYSTEMS
  30. 30. 30 Flat Sliding Bearing TYPES OF BASE ISOLATION SYSTEMS
  31. 31. FRICTION PENDULUM SYSTEM  The Friction pendulum system (FPS) is a sliding isolation system wherein the weight of the structure is supported on spherical sliding surfaces that slide relative to each other when the ground motion exceeds a threshold level. 31 TYPES OF BASE ISOLATION SYSTEMS
  32. 32. ELASTOMERIC ISOLATORS  These are formed of thin horizontal layers of natural or synthetic rubber bonded between steel plates.  The steel plates prevent the rubber layers from bulging and so the bearing is able to support higher vertical loads with only small deformations.  Plain elastomeric bearings provide flexibility but no significant damping and will move under service loads. 32 TYPES OF BASE ISOLATION SYSTEMS
  33. 33. 33 ELASTOMERIC ISOLATORS
  34. 34. LOW DAMPING NATURAL OR SYNTHETIC RUBBER BEARINGS  Elastomeric bearings use either natural rubber or synthetic rubber (such as neoprene), which have little inherent damping.  For isolation they are generally used with special elastomer compounds (high damping rubber bearings) or in combination with other devices (lead rubber bearings). 34 TYPES OF BASE ISOLATION SYSTEMS
  35. 35. LEAD RUBBER BEARINGS  A lead-rubber bearing is formed of a lead plug force-fitted into a pre- formed hole in an elastomeric bearing.  The lead core provides rigidity under service loads and energy dissipation under high lateral loads.  The entire bearing is encased in cover rubber to provide environmental protection. 35 TYPES OF BASE ISOLATION SYSTEMS
  36. 36.  When subjected to low lateral loads (such as minor earthquake) the lead rubber bearing is stiff both laterally and vertically. 36 TYPES OF BASE ISOLATION SYSTEMS
  37. 37. Seismic isolation is a relatively recent and evolving technology. It has been in increased use since the 1980s, and has been well evaluated and reviewed internationally. 1st application in New Zealand in 1974 1st US application in 1984 1st Japanese application in 1985 1st Indian application in 2001 Traditionally, the application of the system is seen in larger buildings and bridges. Additionally, engineers have made an effort to apply the system at a lower cost in residential areas. Base isolation techniques have been utilized worldwide for retrofitting historical structures and monuments to reduce any possible destruction. Also on a smaller scale, museums have started to use the system to ensure the security of artifacts  Base Isolators need not be placed only at foundation level to resist earthquake ground motions. They can even be placed at any floor level to isolate vibrations of machine also 37
  38. 38. Coal is burnt in the furnace of the steam generator and converts water to steam. Steam will be conveyed to the turbine blades which will rotate the turbine. Turbine is coupled to the generator and electricity is generated. 38 TURBO GENERATOR SUPPORTS
  39. 39. The resulting vibrations can be significant and therefore, the generator must be isolated in such a way that the vibration is damped and not transmitted to the foundation This is accomplished by mechanical dampers or spring-mounting the core The turbo generator foundation is dynamically uncoupled from the substructure In order to protect the machine against earthquakes and to avoid resonance amplitudes, the spring units are partially combined with viscodamping, 39
  40. 40. 40 Frame stiffness and natural frequencies of vibration are important parameters due to the once per revolution (60/50 Hz) and twice per revolution (120/100 Hz) characteristics of the generators in conjunction with the stimulus from the power system frequency Therefore great care is taken to ensure the natural frequencies of the core and the frame are not near 60/50 or 120/100 Hz
  41. 41. BASE ISOLATION SYSTEM IN NEW BHUJ HOSPITAL The 300 bed Bhuj Hospital that claimed 176 lives when it collapsed during the major January 2001 Gujarat Earthquake is studied  This was the first new building in India to be fitted with earthquake – resistant NZ developed base isolation technology Structural engineers Dunning Thornton Consultants from Wellington were part of the New Zealand design team and supervised installation of the first bearings on site in late 2001.  280 lead rubber bearings were installed in the structure 41
  42. 42. Armenia is the one of the world leaders in development and application of base isolation technologies The last applications of seismic isolation took place in design and construction of 10-20-story multifunctional buildings  The soil conditions in all cases are good and the soils here are of category II with the predominant period of vibrations of not more than 0.5 sec 42
  43. 43.  ‘Cascade’ building, as one of the most complicated building 43
  44. 44.  The first mode vibrations’ period in longitudinal direction is equal to 1.90 sec and in transverse direction - 1.91 sec, while the corresponding periods for the non-isolated structure would be 0.83 sec and 0.86 sec  This means that seismic isolation has reduced the maximum spectral acceleration by a factor larger than 2  These figures prove the high effectiveness of seismic isolation and reliability of the buildings during strong seismic actions with the PGA equal to 0.4 g and even more. 44
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  47. 47. 47 Seismic base isolation system has proved to be a reliable method of earthquake resistant design. The main significance of the system is to protect people and infrastructures from the danger of seismic activity. The success of this method is largely attributed to the development of isolation devices and proper planning. Noting that the advances in earthquake engineering and the construction practice are as dynamic as the world we live in. In order to use the latest technology and ensure highest level of safety in the built environment, it is imperative that the design and construction communities utilize the most current technologies available like the Base isolation system.
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