Base isolation and damping
- 1. INTRODUCTION TO EARTHQUAKE RESISTANCE MODERN TECHNIQUES Overview • Base Isolation • Elastomeric Isolation • Sliding Isolation • Combined Isolation • Seismic Dampers • Friction Dampers • Tune Mass Dampers • Visco Elastic Dampers
- 2. INTRODUCTION • Buildings respond to earthquake ground shaking in different ways. • Some alternatives to avoid significant damage in buildings in strong ground shaking are: – To provide the building with unreasonably high strength (which may not be economically justified). – To design the building to have a normal (economically justifiable) strength following damage resistant principles; in this case despite the seismic force being larger than the building strength damage will be minimal and restricted only to easily replaceable sacrificial components. – To alter the building’s characteristics through external intervention such that even in strong ground shaking the demand is less than the design strength of the building and its components
- 3. • Base Isolation: – Base isolation control is a passive control system – Base isolation enables buildings and non-buildings (such as bridges) to survive during sever earthquake. – base isolation can be used both for new as well as existing structures as retrofitting. – Base isolation doesn't make building earthquake proof but it increases earthquake resistance of buildings.
- 5. • Concept of base isolation – Isolation of superstructure from the foundation is known as base isolation. – It is the most powerful tool for passive structural vibration control technique. Fig.1 Base Isolated Structures
- 6. • Significantly Increase the Period of the Structure and the Damping so that the Response is Significantly Reduced. Fig 2 Spectral Response for a Typical Base Isolation System
- 7. TYPES OF BASE ISOLATIONS • Base isolation systems which uses Elastomeric Bearings Elastomeric Isolators
- 8. – This is the mostly widely used Base Isolator. – The elastomer is made of either Natural Rubber or Neoprene. – The structure is decoupled from the horizontal components of the earthquake ground motion. – A layer with low horizontal stiffness is introduced between the structure and the foundation.
- 13. • Sliding Base Isolation Systems – It is the second basic type of isolators. – This works by limiting the base shear across the isolator interface. Fig: Metallic Roller Bearing
- 14. • Spherical Sliding Base Isolators – The structure is supported by bearing pads that have curved surface and low friction. – During an earthquake, the building is free to slide on the bearings. Fig: Spherical Sliding Base Isolator
- 15. • Friction Pendulum Bearing – These are specially designed base isolators which works on the principle of simple pendulum. – It increases the natural time period of oscillation by causing the structure to slide along the concave inner surface through the frictional interface. – It also possesses a re-centering capability. Fig: Cross-section of Friction Pendulum Bearing
- 16. • Typically, bearings measure 3 feet in dia., 8 inches in height. • Benicia Martinez Bridge, California is one of the largest bridges to date to undertake a seismic isolation retrofit • Largest seismic isolation bearings, measuring 13 feet in diameter, and weighing 18145 kg. They have a lateral displacement capacity of 53 inches, and a 5 second period. Fig: Bearing used in Benicia Martinez Bridge (left) and Benicia Martinez Bridge (right)
- 17. 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.
- 18. • 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
- 19. • 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
- 20. • Advantages of Base Isolation – Isolates Building from ground motion • Lesser seismic loads, hence lesser damage to the structure. • Minimal repair of superstructure. – Building can remain serviceable throughout construction. – Does not involve major intrusion upon existing superstructure.
- 21. • Disadvantages of Base Isolation – Expensive – Cannot be applied partially to structures unlike other retrofitting – Challenging to implement in an efficient manner – Allowance for building displacements – Inefficient for high rise buildings – Not suitable for buildings rested on soft soil.
- 22. Seismic Dampers • Seismic Dampers are used in place of structural elements, like diagonal braces, for controlling seismic damage in structures. • It partly absorbs the seismic energy and reduces the motion of buildings. • Types:- – Viscous Dampers (energy is absorbed by silicone-based fluid passing between piston-cylinder arrangement), – Tune mass dampers – Friction Dampers (energy is absorbed by surfaces with friction between them rubbing against each other), and
- 25. • Tuned Mass Damper(TMD) – It is also known as an active mass damper (AMD) or harmonic absorber. – It is a device mounted in structures to reduce the amplitude of mechanical vibrations. – Their application can prevent discomfort, damage, or outright structural failure. – They are frequently used in power transmission, automobiles and tall buildings. TMD in Taipei 101
- 26. • Taipei 101 has the largest TMD sphere in the world and weighs 660 metric tonnes with a diameter of 5.5 metre and costs US$4 million (total structure costsUS$ 1.80 billion). TMD in Taipei 101
- 28. • Friction dampers: – These are the metal surface in contact with each others – Energy is absorbed by friction between metals – Typically it consists of several plates sliding against each others in opposite direction – It can be used in various moment resting connections, diagonal braces. Friction dampers