base isolation of structures

BASE ISOLATION OF STRUCTURES

PRESENTED BY,

SYEDA NUSRATH FATHIMA

1

INTRODUCTION TO EARTHQUAKE

CONCEPT OF BASE ISOLATION

TYPES OF BASE ISOLATION SYSTEMS

IMPLIMENTATION OF BASE ISOLATION

CASE STUDY

CONCLUSIONS

2CONTENTS

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

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

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

BASE ISOLATIONWhat 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

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.

7BASE ISOLATION

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

8Enhance performance of structures at all hazard levels by,

PRINCIPLE OF BASE ISOLATION

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 ?

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

ANIMATION SHOWING BASE ISOLATION 13

FIXED BASE ISOLATED BASE

RESPONSE OF BASE ISOLATED BUILDINGS VERSUS FIXED BASE RESPONSE

Drift on Isolation Interface

Reduced Superstructure Deformations and acceleration response for Base Isolated Structure

15

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

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

CONCEPT OF

BASE ISOLATION

18

19

Base isolation, as a strategy to protect structure from earthquake, revolves around a few basic elements of understanding1. Period-shifting of structure2. Mode of vibration3. Damping and cutting of load

transmission path4. Minimum rigidity

CONCEPT OF BASE ISOLATION

Isolators have large deformation potential allowing for large drift on the Isolation Interface

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

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

INSTALLATION OF

SEISMIC ISOLATION

22Layout and installation details for the isolation system

- depends on the site constraints, Type of structure, Construction and other related factors.

SEISMIC-ISOLATION CONFIGURATIONS

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

SUITABILITY OF BASE ISOLATIONThe 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 SoilStructure with Low Fundamental Period (Low-Rise Building)

LEAST EFFECTIVEStructure on Soft SoilStructure 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.

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

SLIDING SYSTEM1. Resilient friction system2. Friction pendulum system

ELASTOMERIC BEARING1. Natural rubber bearing2. Low damping rubber bearings3. Lead plug bearings4. High damping rubber bearing

26TYPES OF BASE ISOLATION SYSTEMS

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.


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.


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


30

Flat Sliding Bearing

TYPES OF BASE ISOLATION SYSTEMS

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.


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.


33

ELASTOMERIC ISOLATORS

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).


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.


When subjected to low lateral loads (such as minor earthquake) the

lead rubber bearing is stiff both laterally and vertically.


APPLICATION OF BASE ISOLATIONSeismic 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 19741st US application in 19841st Japanese application in 19851st 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

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

APPLICATION OF BASE ISOLATION

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 foundationThis is accomplished by mechanical dampers or spring-mounting the coreThe turbo generator foundation is dynamically uncoupled from the substructureIn order to protect the machine against earthquakes and to avoid resonance amplitudes, the spring units are partially combined with viscodamping,

39APPLICATION OF BASE ISOLATION

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

APPLICATION OF BASE ISOLATION

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 technologyStructural 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

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


‘Cascade’ building, as one of the most complicated building


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.


CONCLUSION 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.

THANK YOU48

base isolation of structures

Engineering