base isolation - final report

Base Isolation – A Technique Earthquake Engineering

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SEMINAR REPORT

ON

BASE ISOLATION – A TECHNIQUE

Prepared by:

Charmy Modi 100420106034

Yati Tank 100420106035

Sanket Solanki 100420106036

Henish Patel 100420106037

Darshika Patel 100420106038

Ronak Jariwala 100420106039

Ruchika Patel 100420106040

Under the Guidance of:

Prof. DHARMESH BHAGAT

Prof. JIGAR SEVALIA

FEBRUARY 2013

B.E. (3RD

YEAR) 6TH

SEMESTER

Department of Civil Engineering

Sarvajanik College of Engineering & Technology

Athwalines, Surat, Gujarat


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SARVAJANIK COLLEGE OF ENGINEERING & TECHNOLOGY

Athwalines, Surat, Gujarat

DEPARTMENT OF CIVIL ENGINEERING

CERTIFICATE

This is to certify that the seminar report entitled Base Isolation-A Technique is prepared &

presented by Charmy Modi (34), Yati Tank (35), Sanket Solanki (36), Henish Patel (37),

Darshika Patel (38), Ronak Jariwala (39), Ruchika Patel (40) of B.E. III year, VI Semester

Civil Engineering during year 2011-12.

His / Her work is satisfactory.

Signature of Supervisors


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CONTENTS

1. Literature review…………………………………………………………………...4

2. Introduction………………………………………………………………………...5

3. Response of Base Isolated Buildings……………………………………………....8

4. Spherical sliding isolation system…………………………………………………10

5. Types of bearings………………………………………………………………….10

5.1 Lead rubber bearings………………………………………………………….10

5.2 Elastomeric bearings…………………………………………………………..11

5.3 High-damping rubber bearings (HDRBs)……………………………………..12

5.4 Hybrid type: lead high-damping rubber bearings (LHDRBs)…………………13

6. Maintenance and management of the isolation system……………………………15

7. Factors which enable the use of the base isolation………………………………...16

8. Practical application of base isolation……………………………………………...17

8.1 The first seismically isolated building…………………………………………17

8.2 The first seismically isolated bridge…………………………………………...18

9. The future of seismic isolation……………………………………………………..18

10. Limitations………………………………………………………………………….19

11. Case study……………………………………………………………………….….20

12. Conclusion………………………………………………………………………….22

13. Reference……………………………………………………………………………23


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Base Isolation – A Technique

1. Literature Review:

P. Komodromos in his book explains the basics of earthquakes and the detailed

description on the seismic isolation. He has described the need of this technique, its main

objective, principle on which it works, what exactly seismic isolation is?, the

mathematical calculations required to be done during the design of the seismically

isolated structures, the various factors affecting the building, etc.

Henry J. Lagorio has mainly written this book for the civil engineers and members of

the architectural profession as a means of transferring to them some of the latest

developments in earthquake hazards reduction. His interest in earthquake engineering

began in 1947 when he joined the faculty of architecture at the University of California at

Berkeley. He states that being the engineer, the main aim should be the safety of the

people. Now earthquake proof structures could not be built but at least earthquake

resistant structures could be built by using various techniques. One of those techniques is

the Base isolation technique.

David Dowrick has written this book to help professionals of a wide range of disciplines

in their attempts to reduce the social and economic risks of earthquakes. Earthquake risk

reduction involves so many issues in planning, design, regulation, quality control and

finance that are difficult for any individual to gain a full perspective on the issue, or for

any society to move forward in the quest at their desired speed. The general principles of

this book apply to the whole built environment. This book was written from the

standpoint of a designer trying to keep a broad perspective on the total process starting

from the nature of the loading through the details of the construction.

George G. Penelis and Andreas J. Kappos have started the book with the physics of

earthquake generation and finished it with the social aspects of mitigating the effect of

earthquakes. He has described about the various methods so that the structures which are

earthquake resistant.


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2. Introduction:

What is base isolation?

Base isolation is one of the most popular means of protecting a structure against

earthquake forces. It is one of most powerful tools of earthquake engineering pertaining

to the passive structural vibration control technologies. It is easiest to see the principle at

work by referring directly to the most widely used of these advanced techniques, known

as base isolation. A base isolated structure is supported by a series of bearing pads, which

are placed between the buildings and building foundation.

The concept of base isolation is explained through an example of 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 the shaking of

the ground; simply, the building does not experience the earthquake.


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Now, if the same building is rested on the flexible pads that offer resistance

against lateral movements, then some effect of the ground shaking will be transferred to

the building. If the flexible pads are properly chosen, the forces induced by ground

shaking can be a few times smaller than that experienced by the building built directly on

ground, namely a fixed base building. 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. As a result, a robust medium-rise masonry or reinforced concrete building

becomes extremely flexible. The isolators are often designed, to absorb energy and thus

add damping to the system. This helps in further reducing the seismic response of the

building. Many of the base isolators look like large rubber pads, although there are other

types that are based on sliding of one part of the building relative to other.

Base isolation is not suitable for all buildings. Mostly 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.


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Concept of Base Isolation

3. Response of Base Isolated Buildings:

The base-isolated building retains its original, rectangular shape. The base isolated

building itself escapes the deformation and damage-which implies that the inertial forces

acting on the base isolated building have been reduced. Experiments and observations of

base-isolated buildings in earthquakes to as little as ¼ of the acceleration of comparable

fixed-base buildings.

Acceleration is decreased because the base isolation system lengthens a buildings

period of vibration, the time it takes for a building to rock back and forth and then back

again. And in general, structures with longer periods of vibration tend to reduce

acceleration, while those with shorter periods tend to increase or amplify acceleration.


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Spherical Sliding Base Isolation


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4. Spherical sliding isolation system:

Spherical sliding isolation systems are another type of base isolation. The building

is supported by bearing pads that have a curved surface and low friction. During an

earthquake the building is free to slide on the bearings. Since the bearings have a curved

surface, the building slides both horizontally and vertically. The forces needed to move

the building upwards limits the horizontal or lateral forces which would otherwise cause

building deformations. Also by adjusting the radius of the bearings curved surface, this

property can be used to design bearings that also lengthen the buildings period of

vibration.

5. Types of bearings:

5.1 Lead-rubber bearings

These are the frequently-used types of base isolation bearings. A lead rubber

bearing is made from layers of rubber sandwiched together with layers of steel. In the

middle of the solid lead “plug”. On top and bottom, the bearing is fitted with steel plates

which are used to attach the bearing to the building and foundation. The bearing is very

stiff and strong in the vertical direction, but flexible in the horizontal direction.

Lead is a crystalline material which changes its structure temporarily, under

deformations beyond its yield point, and regains its original structure and elastic properties

as soon as the deformation is removed by the restoring force in the rubber. Note that lead

has good fatigue properties for subsequent cycles of loading beyond its yield point.


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How it works?

To get a basic idea of how base isolation works, first examine the above diagram.

This shows earthquake acting on base isolated building and a conventional, fixed-base

and building. As a result of an earthquake, the ground beneath each building begins to

move. Each building responds with movement which tends towards the right. The

buildings displacement in the direction opposite the ground motion is actually due to

inertia. The inertia forces acting on a building are the most important of all those

generated during an earthquake.

In addition to displacing towards right, the un-isolated building is also shown to

be changing its shape from a rectangle to a parallelogram. We say that the building is

deforming. The primary cause of earthquake damage to buildings is the deformation

which the building undergoes as a result of the inertial forces upon it.

5.2 Elastomeric isolation system

The most popular seismic isolation systems use elastomeric bearings which

consist of thin rubber sheets bonded onto thin steel plates and combine with an energy

dissipation mechanism. The rubber sheets are vulcanized and bonded to the thin steel

plates under pressure and heat.

The inner thin steel plates provide the vertical load capacity and Stiffness,

and prevent lateral bulging of the rubber.In particular the steel plates laterally constrain

the rubber sheets as vertical load is applied to the elastomeric bearing, providing the

vertical stiffness. Horizontal flexibility is provided by the shearing deformability of the


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rubber sheets which are not restrained from deform in that direction by the steel plates.

Thick mounting steel plates are bonded to the bottom and top surfaces allowing the

isolator to be firmly connected to the foundation below and the superstructure above.

The energy dissipation mechanism is based either on the plastic deformation of a

metal or on the inherent damping properties of the rubber. In the first case either lead

plugs are inserted in the elastomeric bearings or auxiliary dampers based on deformations

of lead or steel are used.

Lead rubber bearings (LRBs) and high-damping rubber bearings (HDRBs)

are most useful in seismic isolation since they provide the following in a single unit:

Vertical support due to the high vertical stiffness, which is usually several

hundred times the horizontal stiffness .Sufficient vertical stiffness is necessary to

avoid rocking of the structure.

Horizontal flexibility which shifts the fundamental frequency of the structure out

of the dangerous for resonance frequency range.

An energy dissipation mechanism, either via the plastic deformation of the lead

plug or through the inherent damping properties of high damping rubber.

Finally, there are also some systems that use natural rubber bearings (NRBs) with

additional steel or lead damper; in this case energy dissipation results from the plastic

deformations of the damper.

5.3 High-damping rubber bearings (HDRBs):

This type of bearing consists of thin layers of high damping rubber sandwiched

between steel plates. The same manufacturing methods for vulcanization and bonding

that are used for LRBs are also used to construct HDRBs. The only difference is the

composition of the rubber compound, which provides increased damping.

High-damping rubber is actually a filled rubber compound with inherent damping

properties due to the addition of special fillers, such as carbon and resins. The addition of

fillers increases the inherent damping properties of rubber without affecting its

mechanical properties. When shear stresses are applied to high-damping rubber, a sliding


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of molecules generates frictional heat which is a mechanism of energy dissipation. In

unfilled natural rubber, used for LRBs, frictional heat is negligible because the molecular

attraction in physical cross links is very weak. The energy dissipation mechanism of an

HDRB is available for both small and large strains, is constant and is characterized by

smooth elliptical hysteresis loops. Experimental studies of high damping rubber bearings

verified the anticipated energy dissipation capacity, which is, typically, equivalent to

about 15% damping ratio of equivalent linear elastic models.

However, HDRBs may not provide the necessary initial rigidity under service

loads and minor lateral loads, although some initial rigidity is provided by high-damping

rubber compounds which exhibit higher stiffness under small strains. A structure isolated

with HDRBs essentially has a constant, large fundamental period due to the flexibility of

the isolation system, which makes the structure vulnerable to wind action with dominant

frequencies close to the fundamental frequency. In addition, the damping and mechanical

properties of the HDRB appear to be temperature dependent while the hysteretic energy

dissipation mechanism of the LRB is not. HDRBs are not as widely used in seismic

isolation as LRBs.

5.4 Hybrid type: lead high-damping rubber bearing (LHDRBs)

A hybrid type of lead high-damping rubber bearing (LHDRBs) may consist of

layers of high-damping rubber sandwiched between steel plates and a smaller diameter

lead cylinder plug firmly in a hole at its center. This hybrid bearing may have the

advantages of both isolation systems discussed above. The LHDRBs has both an initial

rigidity, due to the presence of the lead plug, and a continuous energy dissipation

mechanism, due to the damping properties of the high-damping rubber. Therefore, the

isolation system is expected to perform well in both weak and extreme earthquakes as

well as under minor lateral loads. In addition, the properties of this bearing will be less

dependent on temperature changes and shear strains than those of HDRB. Also, the use of

LHDRBs allows the reduction of the initial stiffness of the isolation system which is

responsible for higher frequency effects. A final advantage is that the effectiveness of

such a device and its compact size make it suitable for cases where installation space is


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limited. LHDRBs offer a practical and cost-effective trade-off between the advantages

and limitations of LRBs and HDRBs.

When practically possible, it may be more effective to place the high-damping

rubber bearings with lead plugs at the perimeter of the building, preferably under the

columns situated as far away as possible from the center of stiffness and mass of the

isolated structure. High-damping rubber bearings with no lead plugs may be used under

the internal columns. This configuration will allow lower prior-to-yielding stiffness of the

lead plugs and consequently, a smoother change of the stiffness during yielding and

reverse loading. The high initial stiffness and its sudden changes are responsible for

higher mode effects and acceleration increases, which may be avoided by reducing the

initial stiffness. In addition, lower initial stiffness will provide a higher degree of

isolation at the prior-to-yielding stage. Placing the bearings with the lead plugs under the

external columns away from the center of stiffness will provide higher resistance against

torsion, due to the larger diameter between the points of application of forces and the

center of stiffness of the isolation system. Note that this configuration may be used only

when there is a rigid diaphragm at the isolation level to redistribute the inertia forces to

the LHDRBs.

An effective isolation system must have both viscous and hysteretic damping and

must ensure a continuous energy dissipation mechanism. The viscous damping, which is

velocity dependent, will ensure a continuous energy dissipation mechanism for both

severe earthquakes and micro tremors. Viscous damping may be provided by actual

viscous dampers or by rubber with inherent damping properties. The latter does not

actually provide such damping, which may, however, be assumed due to the rubber’s

smooth elliptical hysteresis loops during cyclic loading. The optimum viscous damping

ratio lies within 20 to 30%; higher values lead to increase in floor acceleration.


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6. Maintenance and Management of the Isolation System:

The isolation system must remain operational for the world expected lifetime of

the structure under all possible environmental effects. These may corrode the metallic

parts of the isolation system and deteriorate the elastomer; such effects may be reduced

by using a protective rubber cover. The maintenance of the isolation system, and

especially that of the seismic gap, must be frequent to ensure the vertical loads which

they must sustain. When the construction of a diaphragm is not possible, the bearings

must be designed in proportion to the magnitude of the lateral forces carried by the

members above them. It is difficult to take into account such design issues due to the

uncertainties involved; therefore, the construction of a diaphragm above the isolation

level must be anticipated. Note that here, the existence of a rigid diaphragm is assumed.

Finally, the selected location of the bearings must be such that access to them is enabled

for inspection and possible replacement purposes.


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7. Factors which enable the use of base isolation:

Several factors favor the development and practical application of seismic

isolation. First of all there are increased requirements for the performance of structure in

severe earthquakes which are not met by current conventional design philosophy. These

are crucial to structures containing sensitive and expensive equipment vulnerable even to

micro tremors.

Second, the advance of computer technology and modern structural analysis

method enables the development of reliable software to stimulate the response of

structures. The development of seismic engineering and earthquake engineering to level

where reliable prediction can be made for expected earthquakes, is another important

factor.

In addition, the construction of shaking tables which can stimulate actual

earthquake excitations makes possible the experiment validation of the behavior of

seismic isolation system. The study of other minor loads such as wind load and the

reliable quantifications of their expected intensities and frequency of occurrence, also

enables the use of seismic isolation.

Finally, development, manufacture and extensive research in the area of structural

material enable the reliable use of modern material for seismic isolation device. The

development of device which decipate energy and provide the restoring force to avoid

permanent displacement also allow the practical implementation of the seismic isolation

concept.


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8. Practical application of Base Isolation:

Researchers on the subject of seismically isolated buildings state that proper

application of this technology leads to better performing structures that will remain

essentially elastic during large earthquakes. There are more than 3000 seismically

isolated structures around the world. The number includes not only buildings but also

bridges and tanks. Of these approximately 150 are in United States. Literature on seismic

isolation repeatedly quotes the University of Southern California Hospital in Los Angeles

as a significant example of a seismically isolated, seven-story-plus-basement structure

that survived the Northridge earthquake and remained operational. The technology of

seismic isolation has recently made remarkable advancements since the concept was first

put into practice with two buildings constructed on rollers: one in Mexico, the other in

Sevastopol, Ukraine.

8.1 The first seismically isolated building

The first seismically isolated building with a rubber isolation system emerged in

1969 in Skopje, in former Yugoslavia. It is a three-story school building that rests on

solid blocks of rubber without the inner horizontal steel-reinforcing plates as is done

today.

The first seismically isolated building in the United States was the Foothill

Communities Law and Justice Center in Rancho Cucamonga, California, completed in

1985. It took some time until another isolated building was built in the United States. The

reason for the reluctance was quite simple: Seismic isolated structures did not find their

way into the building codes. Design professionals, on the other hand, were not able to

show any appreciable savings to their clients by using this system.

Theoretically, in a perfectly functioning seismic isolation there will be no lateral

seismic force acting on the isolated superstructure. Yet if the building codes and building

officials persist in designing such superstructures to the same lateral forces pertaining to a

fixed-base structure, there will be no savings. This is because the superstructure will end

up with equally heavy steel sections or massive reinforced-concrete sizes to counter

relatively light lateral seismic design forces.


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8.2 The first seismically isolated bridge

The first bridge structure that utilized an isolation system, with added damping,

was the Te Teko viaduct in New Zealand, built in 1988. The isolation system contains a

sandwich of laminated steel and rubber bearing layers with a central lead core for energy

dissipation. This type of isolation system, referred to as lead-rubber bearing, is now widely

used. The first building supplied with LRB isolation was the William Clayton Building in

Wellington, New Zealand, in 1981.

9. The future of seismic isolation:

Seismic isolation is an effective design scheme which successfully addresses

earthquake loadings, and not only provides safety but also prevents damage. Seismic

isolation is particularly useful for low-to medium rise buildings which happen to have

their fundamental frequencies within the dangerous-for-resonance range of dominant

earthquake frequencies. Critical facilities, such as emergency response centers, hospitals,

fire-stations, utilizes and communication centers, should remain operational of such

essential, for the public interest, facilities may be prevented by using seismic isolation to

enhance their earthquake capacity by reducing the seismic loads that they may experience

during a powerful earthquake.

It is very useful, and is probably the only currently available technology that can

be used, to seismically upgrade historic structures or to protect very sensitive equipment

and the valuable contents of a building.


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10. Limitations:

Although seismic isolation is a very promising design method for dealing with

earthquake loads, it cannot be used for all structures and at all sites.

10.1 Superstructure characteristics

A seismic isolation is generally suitable for low- to medium-rise buildings which

have their fundamental frequency in the range of the usual dominant frequencies of

earthquakes. Super structure characteristics such as height, width, aspect ratio and

stiffness are related to the applicability and effectiveness of seismic isolation.

10.2 Site characteristics

The seismicity of the particular region must be considered in order to determine

the necessity of seismically isolated structure in that region. Base isolation is not suitable

for all buildings. Mostly 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.

10.3 Surrounding structures

All adjacent structures or facilities which may impose restriction on the seismic

isolation system must be taken into account, especially in order to estimate the

maximum allowable displacement. There are cases where the location of adjacent

structures does not allow the use of seismic isolation. Finally, the surrounding ground

may also impose restriction and special design details may be needed to enable the use

of seismic isolation.


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11. Case Study:

NEW BHUJ HOSPITAL COMPLETED WITH EARTHQUAKE ENGINEERING

NZ TECHNOLOGY

7 May 2003

The completion of the new earthquake resistant Bhuj District Hospital in India’s earthquake

prone Gujarat State is a particularly satisfying achievement for members of the Wellington-based

Earthquake Engineering NZ business cluster, says EENZ Chairman David Hopkins.

The 300-bed Bhuj hospital replaces the building that claimed 176 lives when it collapsed during

the major January 2001 Gujarat earthquake. This is the first new building in India to be fitted

with the earthquake-resistant NZ developed base-isolation technology. The hospital’s base

isolation design and bearings have been provided with the assistance of Earthquake Engineering

NZ members.


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A bi-lingual Hindi and English video has been made by Story!inc and Execam of the Creative

Capital Cluster of the achievement of rebuilding the new Bhuj hospital within two years of the

earthquake by India’s leading architects, engineers and construction firm working with the

assistance of New Zealand’s specialist earthquake engineering expertise. Trade NZ, Industry NZ

and Cluster members have sponsored the making of this video for use in India.

The Indian design team for the hospital has been led by architect Uday Pattanayak of EFN

Ribeiro Associates, New Delhi, and Structural Engineer Kamal Sabharwal. The construction

company is India’s largest, Larsen & Toubro. Cluster member Beca’s internationally renowned

seismic expert Richard Sharpe was working on a project in India at the time of the earthquake.

With the assistance of the New Zealand Government and support of the Earthquake Engineering

NZ cluster he was able to identify the reconstruction of the Bhuj hospital as a suitable project for

New Zealand’s earthquake engineering assistance. He recommended that the replacement

hospital be fitted with New Zealand developed base isolation lead rubber bearings. This robust

technology is well-suited to construction styles in India.

The specialist computer-based earthquake-resistant base-isolation building design work was

undertaken in Wellington by fellow cluster members Holmes Consulting Group and Dunning

Thornton Consultants, with the bearings manufactured and supplied by Robinson Seismic Ltd.

The lead rubber bearing technology was invented by Cluster member Bill Robinson.

The New Zealand Government contributed $ 150,000 to the cost of the project base-isolation

feasibility study and design work as part of the initial disaster recovery stage. The Indian Prime

Minister’s Relief Fund funded the hospital construction, including the cost of the Robinson

Seismic Ltd bearings.

Other follow-up project opportunities In India worth several millions of dollars are being

pursued by members of the Earthquake Engineering NZ and associated Natural Hazards NZ

business clusters. These include further base-isolated building projects as well as several World


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Bank funded disaster risk management projects.

Mack Morum

Project Manager

Bhuj Hospital Project

Earthquake Engineering NZ India Export Network

Robinson Seismic Ltd.

12. Conclusion:

We can use base isolation technique to construct the earthquake resistant building.

Proper materials and design should be selected to get the best result. The safety of people

should be the main aim.


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13. References:

P. Komodromos

Seismic isolation for earthquake resistance structure.

Henry J. Lagorio

Earthquake – An architect’s guide to nonstructural seismic hazards.

Charles K. Erdey

Earthquake Engineering.

Yousef Bozorgnia & Vitelmo V. Bertero

Earthquake Engineering.

Wai – Fan Chen & Charles Scawthorn

Earthquake Engineering Hand Book

George G. Penelis & Andreas J. Kappas

Earthquake resistant concrete structure

David Dowrick

Earthquake risk reduction

base isolation - final report

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