Base isolation

introduction, principle, concepts, types of base isolation, application

1. BASE ISOLATION1.What is base isolation system?Base Isolation Is One Of The Most Important Concepts For Earthquake Engineering Which Can Be Defined Is Separating Or Decoupling The Structure From Its Foundation.Base Isolation Technique developed to prevent or minimize damage to building during an earthquake.One of the most widely implemented seismic protection system is base isolation.The terms isolation refers to reduced interaction between structure and the ground.2.Concept of base isolation systemBase isolation is explained through an example of building resisting on frictionless rollers.When the ground shakes, the roller freely roll, but the building is not move.
Thus, no force transferred to the building due to shaking the ground. Because roller is absorbed whole shock. So building does not experience the earthquake.High rise buildings or building rested on soft soil are not suitable for base isolators.Most suitable structures for base isolation are low or medium rise building.3.Principle of base isolation systemThe fundamental principle of base isolation to modify the response of building so that the ground can move below the building without transmitting these motion into the building and other structure.The structure and ground moves the same amount.So in flexible structure the structure will not move with the ground.

2. The structure is perfectly rigid will have zero period.
Structure without base isolator is severe shaking with ground.Structure with base isolator is moving horizontally slow with isolators.4.When base isolation system is suitable?The sub soil does not produce a long period ground motion.When the structure is jointed sufficiently with high column load.Lateral load due to wind are less than approximately 10% of the structure weight.5. Difference between fixed and isolated base structureWhen the earthquake is affected on fixed base structure at that time structure is not defending against earthquake.But in base isolated structure the when earthquake is affected on the structure building is defending against earthquake very well.In fixed structure, structure is moves with ground motion.In isolated structure, structure is not moves with ground motion. But isolation bearing is moves with ground motion. So we can say the structure is safe.

3. Isolatorselastomeric isolators
Natural rubber bearinglow damping rubber bearinglead rubber bearingHigh damping rubber bearing
sliding isolators
flat sliding bearing(resilient friction system) spherical sliding bearing(friction pendulum system)
6. Types of base isolators

4. 7. Elastomeric base IsolatorsThese are formed of horizontal layers of natural oy synthetic rubber in thin layers bonded between steel plates.The steel plate prevent the rubber layers, so the bearing is able to support higher vertical loads with only small deformation.Plain elastomeric bearing provides flexibility and will move under service loads.
7(a). Low damping natural rubber bearingDamping ratio = 2% to 3%Manufacturing is easy. Response not strongly sensitive temperature, rate of loading and aging.Shear strain is exceed up to 100%.7(b). High damping natural rubber bearing
Damping is increased by adding extra-fine carbon black, oils or resins and other fillers.Maximum shear strain = 200 to 350%Damping ratio = 10 to 20% at 100% shear strainEffective damping depends on:

5. -velocity of loads-load history-temperature 7(c). Lead rubber bearing
Damping properties can be improve by plugging a lead core into bearing.Damping of the lead-plug bearing varies from 15% to 35%.The performance depends on the lateral force.Maximum shear strain = 125 to 200% 8. Sliding isolatorsThe second most common type of isolation system uses sliding elements between the foundation and base of the structure.By high tension springs or laminated rubber bearing by making sliding curved surface.These mechanisms provide a restoring force to return the structure to its equilibrium position.

6. 8(a). Flat sliding isolators (resilient friction sys.)Two types of flat sliding isolators : 1.With recentering capacity2.Without recentering capacity
8(b). Spherical sliding isolators (Rollers) (Friction pendulum)the friction pendulum system is sliding isolation system where in the weight of the structure is supported on spherical sliding surfaces that slide relative to each other when the ground motion exceed a threshold level.

7. 9. AdvantagesReduced the seismic demand of structure, thereby reducing the cost of structure.Less displacement during the earthquake.Improves safety of structure.Reduced damages caused during the structure.10. DisadvantagesChallenging to implement in an efficient manner.Allowance for building displacements.Inefficient for high rise buildingsNot suitable for buildings rested on soft soil. 11. ApplicationsBridgesBuildingsHistoric structure

8. 12. Real life applications(1). Tomb-of-Cyrus, Iran. World’s first base isolated structure.
(2). New hospital building, Bhuj Base isolated building