Base isolation
- 1. Technical Seminar Report on BASE ISOLATION CONCEPT AND TECHNIQUES BY M. PREM SAI 11H61A0135 DEPARTMENT OF CIVIL ENGINEERING ANURAG GROUP OF INSTITUTIONS, VENKATAPUR, HYDERABAD
- 2. CONTENTS INTRODUCTION BASE ISOLATION CONCEPT OF BASE ISOLATION PRINCIPLE OF BASE ISOLATION SUITABILITY OF BASE ISOLATION FIXED BASE STRUCTURE VS ISOLATED BASE STRUCTURE ISOLATION COMPONENTS ISOLATOR LOCATIONS IN REAL SCENARIO APPLICATIONS CONCLUSION
- 3. 1. INTRODUCTION Earthquake by itself, is not a disaster, it is natural phenomenon result from ground movement, sometimes violent. These 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.
- 4. The possible risk of loss of life adds a very serious dimension to seismic design, putting a moral responsibility on structural engineers. 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 implemented and accepted seismic protection systems is base isolation. 1. INTRODUCTION
- 5. 2. 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. Seismic isolation is a design strategy, which uncouples the structure for the damaging effects of the ground motion.
- 6. 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”. The other purpose of an isolation system is to provide an additional means of energy dissipation, thereby reducing the transmitted acceleration into the superstructure. The decoupling allows the building to behave more flexibly which improves its response to an earthquake. 2. BASE ISOLATION
- 7. 3. CONCEPT OF BASE ISOLATION The concept of base isolation is explained through an example building resting on frictionless rollers. When the ground shakes, the rollers freely roll, but the building above does not move. Thus, no force is transferred to the building due to shaking of the ground; simply, the building does not experience the earthquake.
- 8. 3. CONCEPT OF BASE ISOLATION
- 9. Now, if the same building is rested on flexible pads that offer resistance against lateral movements, then some effect of the ground shaking will be transferred to the building above. The flexible pads are called base-isolators, whereas the structures protected by means of these devices are called base-isolated buildings. The main feature of the base isolation technology is that it introduces flexibility in the structure. 3. CONCEPT OF BASE ISOLATION
- 10. 3. CONCEPT OF BASE ISOLATION
- 11. A careful study is required to identify the most suitable type of device for a particular building. Also, base isolation is not suitable for all buildings. Most suitable structures for base-isolation are Low to medium-rise buildings rested on hard soil underneath High-rise buildings or buildings rested on soft soil are not suitable for base isolation. 3. CONCEPT OF BASE ISOLATION
- 12. 4. 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.
- 13. The structure and ground move the 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. 4. PRINCIPLE OF BASE ISOLATION
- 14. So in flexible structures the structure will not move, the ground will. 4. PRINCIPLE OF BASE ISOLATION
- 15. 5. SUITABILITY OF BASE ISOLATION Earthquake protection of structures using base isolation technique is generally suitable if the following conditions are fulfilled 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.
- 16. 6. Fixed Base Structure vs Isolated Base Structure During earthquakes, the conventional structure without seismic isolation is subjected to substantial story drifts, which may lead to damage or even collapse of the building. Whereas the isolated structure vibrates almost like a rigid body with large deformations or displacements restricted by the isolation bearings. The decoupling effect gives this extra advantange to isolated structures.
- 17. The lateral forces of the isolated building are not only reduced in magnitude but also fairly redistributed over the floors, which further mitigates the overturning moment of the structure. 6. Fixed Base Structure vs Isolated Base Structure
- 18. 6. Fixed Base Structure vs Isolated Base Structure
- 19. 7. ISOLATION COMPONENTS Elastomeric Isolators Natural Rubber Bearings Low-Damping Rubber Bearings Lead-Rubber Bearings High-Damping Rubber Bearings Sliding Isolators Resilient Friction System Friction Pendulum System
- 20. Elastomeric Isolators These 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 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. 7. ISOLATION COMPONENTS
- 21. Methods used to overcome these deficits include lead cores in the bearing, specially formulated elastomers with high damping and stiffness for small strains or other devices in parallel. 7. ISOLATION COMPONENTS
- 22. Low Damping Natural or Synthetic Rubber Bearings Elastomeric bearings use either natural rubber or synthetic rubber (such as neoprene), which have little inherent damping, usually 2% to 3% of critical viscous damping. For isolation they are generally used with special elastomer compounds (high damping rubber bearings) or in combination with other devices (lead rubber bearings). They are also flexible at all strain levels. 7. ISOLATION COMPONENTS
- 23. 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. Top and bottom steel plates, thicker than the internal shims, are used to accommodate mounting hardware. The entire bearing is encased in cover rubber to provide environmental protection. 7. ISOLATION COMPONENTS
- 24. When subjected to low lateral loads (such as minor earthquake, wind or traffic loads) the lead rubber bearing is stiff both laterally and vertically. The lateral stiffness results from the high elastic stiffness of the lead plug and the vertical rigidity (which remains at all load levels) results from the steel-rubber construction of the bearing. 7. ISOLATION COMPONENTS
- 25. Isolation Systems based on Sliding The second most common type of isolation system uses sliding elements between the foundation and base of the structure. In this type of Isolation system, 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. 7. ISOLATION COMPONENTS
- 26. 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. 7. ISOLATION COMPONENTS
- 27. Sliding Isolator with Recentering Capacity Compared with sliding isolators, sliding isolation pendula (SIPs) with recentering capacity have a concave sliding plate. Due to geometry, each horizontal displacement results in a vertical movement of the isolator. The potential energy, stored by the superstructure, which has been pushed to the top, automatically results in recentering the bearing into neutral position. 7. ISOLATION COMPONENTS
- 28. They remain horizontally flexible, dissipate energy and recenter the superstructure into neutral position. 7. ISOLATION COMPONENTS
- 29. 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. 7. ISOLATION COMPONENTS
- 30. 8. 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.
- 32. 9. BASE ISOLATION IN REAL BUILDINGS 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. Base isolation is also useful for retrofitting important buildings (like hospitals and historic buildings). By now, over 1000 buildings across the world have been equipped with seismic base isolation.
- 33. In India, base isolation technique was first demonstrated after the 1993 Killari (Maharashtra) Earthquake. Two single storey buildings in newly relocated Killari town were built with rubber base isolators resting on hard ground. After the 2001 Bhuj (Gujarat) earthquake, the four-storey Bhuj Hospital building was built with base isolation technique. All were brick masonry buildings with concrete roof. 9. BASE ISOLATION IN REAL BUILDINGS
- 34. The figure shows the base isolation technique used in the Bhuj Hospital building with the help of Base Isolators. 9. BASE ISOLATION IN REAL BUILDINGS
- 35. 10. APPLICATIONS Base isolation provides an alternative to the conventional, fixed base design of structures. Base Isolation minimizes the need for strengthening measures of adding shear walls, frames, and bracing by reducing the earthquake forces imparted to the building. Base isolation had the effect of reducing the earthquake force demands on the superstructure to 30% of the demands for a fixed-base structure.
- 36. 11. 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. 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.