Base isolation of structures

 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

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

 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

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,

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

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

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
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

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 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.

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 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

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

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

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

30
Flat Sliding Bearing

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

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

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

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

 When subjected to low lateral loads (such as minor earthquake) the
lead rubber bearing is stiff both laterally and vertically.
36

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

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

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
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

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

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

 ‘Cascade’ building, as one of the most complicated building
43

 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

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.

Base isolation of structures

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Base isolation of structures