NATIONAL INSTITUTE OF TECHNOLOGY DURGAPUR

SEISMIC BASE ISOLATION

SEMINAR II

Submitted by: Miaaza Hussain

Rollno: 10/CE/61

CONTENT

SEISMIC BASE ISOLATION

CONTENTS

Introduction( Problem Statement)

Seismic base Isolation

Types of seismic isolators

Literature review

Numerical background

Case study

Conclusion

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INTRODUCTION

1. INTRODUCTION

Over the past few decades, earth quake resistant design of building has become a major topic

of interest among structural engineers. Major earthquakes (e.g. Northridge, 1994; Kobe, 1995; chi-

chi 1999 etc.) have caused destruction to many structures and also have cost the world many lives.

Hence different techniques have been proposed to release the earthquake forces subjected to a

structure.

Fig: The Kaiser Permanente Building after the Northridge Earthquake of January 17, 1994

Different techniques currently used to minimise the earthquake effect on structures are:

▫ Shear wall

▫ Braced frame

▫ Moment resisting frames

▫ Increasing ductility via extra reinforcement

▫ Use of Damping devices

A high proportion of the world is subjected to earthquakes and therefore structural engineers are

bound to a higher level of responsibility towards the public for survival against the effects of these

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INTRODUCTION

earthquakes. As per all the designs we encounter, most of them are based on the concept that the

capacity of the building should be greater than the demand. As

far as earthquakes are concerned, they are unpredictable and

demands for a high structural strength or capacity. Hence it is

necessary to make sure the capacity exceeds the demand. But

this is not an ideal situation. Earthquakes cause inertial forces

proportional to the product of building mass and the earthquake

ground accelerations. As the ground acceleration increases, the

strength of the building must be increased to avoid structural

damage. In high seismic zones, ground acceleration may exceed

the acceleration due to gravity causing huge amount of force on

the structure. Though this is the case it is not practical to increase

the building strength indefinitely.

Designing for such high seismic loads are not easy or practical, nor cheap. Hence most codes

follow ductility to achieve capacity. Ductility is the concept of allowing the structural elements to

deform beyond their elastic limit in a controlled manner. Beyond elastic limit, the structural

elements soften and the displacements increase with only a small increase in force. The deformation

which occurs beyond the elastic limit is non-reversible when the load is removed. These

deformations may cause dramatic structural damage, especially to parts made of materials like

concrete which will show cracking and spalling when the elastic limit is exceeded.

Fig: Ductility concept of design

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INTRODUCTION

For most structural materials, ductility equals structural damage, in that the effect of both is the

same in terms of the definition of damage as that which impairs the usefulness of the object.

Ductility will generally cause visible damage. The capacity of a structure to continue to resist loads

will be impaired.

Several uncertainties with the ductility design strategy is primarily attributed to:

(1) The desired “strong column weak beam” mechanism may not form in reality, due to

existence of walls

(2) Shear failure of columns due to inappropriate geometrical proportions or short- column

effect.

(3) Construction difficulty in grouting, especially at beam column joints due to complexity of

steel reinforcements

To enhance structural safety and integrity against severe earthquakes, more effective and reliable

techniques for aseismic design of structures based on structural control concept is desired. Among

the structural control schemes developed, seismic base isolation is one of the most promising

alternatives. It can be adopted for both new structures and as well as to retrofit existing building and

bridges.

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SEISMIC BASE ISOLATION

2. SEISMIC BASE ISOLATION

The term base isolation means separating or decoupling the superstructure from its base or

the foundation. The original terminology of base isolation is more commonly replaced with seismic

isolation as isolation is not always necessarily done at the base level. In case of bridges the

superstructure of the bridge is isolated from the substructure columns with isolators/bearings. In

another sense it is more accurate to express base isolation is separation of structure from seism or

earthquake.

Base isolation is thought of as an aseismic design approach in which the building is protected

from hazards of earthquake forces by a mechanism which reduces the transmission of horizontal

accelerations into the structure. The main strategies to achieve seismic isolation includes period shift

of the structure and cutting-off load transmission path. A base isolator reduces the fundamental

frequency of structural vibration to a value lower than the predominant energy containing

frequencies of the earthquake. Additional means of energy dissipation damping is provided by an

isolator so that the base accelerations are not transferred to the structure.

Advantages gained by a base isolation system include:

Reduced floor Acceleration and Inter-storey Drift

Less (or no) Damage to Structural Members

Better Protection of Secondary Systems

Fig: Shift of period in base isolated structures

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Base Isolated

Fixed Base

Response

Period

SEISMIC BASE ISOLATION

Fig: Effect of damping on acceleration of the structure

History of seismic base isolation

The first evidence of

architects using the principle of

base isolation for earthquake

protection was discovered in

Pasargadae, a city in ancient

Persia, now Iran: it goes back to

6th century BC. It works by

having a wide and deep stone and

mortar foundation, smoothed at the

top, upon which a second

foundation is built of wide,

smoothed stones which are linked

together, forming a plate that

slides back and forth over the

lower foundation in case of an earthquake, leaving the structure intact.

In ancient day base isolations technique was used in many structures. Such forms of base

isolation included pouring layers of soft sand or gravel under the foundation as well as construction

above a stack cut-out stones. Sometimes Timber was used under Bearing Walls which can roll on

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Fig: Mausoleum von Kyrus dem Großen(Tomb of cyrus): The first evidence of architects using the principle of base isolation

SEISMIC BASE ISOLATION

each other and dissipate the earthquake induced energy. They have been designed and constructed in

a way that allows the ground to move with the rolling movement of the building on the foundation.

The first patent for the recent

innovation of mechanical isolators was

released in 1980.

In India, base isolation technique was

first demonstrated after the 1993 Killari

(Maharashtra) Earthquake. Two single storey

building were built with rubber base isolators

resting on hard ground in Killari town. After

the 2001 Bhuj (Gujarat) earthquake, the four-

storey Bhuj Hospital building was built with

base isolation Technique. The new 300-bed hospital was fitted with a New Zealand-developed lead-

rubber base-isolation system after the local hospital in Bhuji was collapsed claiming approx. 176

lives.

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Fig: Use of timber for base isolation

TYPES OF SEISMIC ISOLATORS

3. TYPES OF SEISMIC ISOLATORS

The above network shows different categories of seismic isolating system and different types of

isolators under these categories. In this chapter a brief description of different types of isolators are

presented.

Acceptance isolator performance criteria of isolators are that they will:

• Remain stable for required design displacements.

• Provide energy dissipation with increasing displacement.

• Not suffer a loss in force-resisting capacity under repeated cyclic loading.

• Have quantifiable engineering parameters (e.g., force-deflection characteristics and

damping).

Elastomeric bearing

An elastomeric bearing consists of alternating layers of rubber and steel shims bonded together to

form a unit. Rubber layers are typically 8 mm to 20 mm thick, separated by 2 mm or 3 mm thick

steel shims. The steel shims prevent the rubber layers from bulging and so the unit can support high

vertical loads with small vertical deflections (typically 1 mm to 3 mm under full gravity load). The

internal shims do not restrict horizontal deformations of the rubber layers in shear and so the bearing

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Isolation Devices

Elastomeric Isolators

Natural Rubber

Bearings

Low-Damping Rubber

Bearings

Lead-Plug

Bearings

High-Damping Rubber

Bearings

Sliding Isolators

Resilient Friction System

Friction Pendulum

System

TYPES OF SEISMIC ISOLATORS

is much more flexible under lateral loads than vertical loads, typically by at least two orders of

magnitude.

Elastomeric bearings have been used extensively for many years, especially in bridges, and samples

have been shown to be functioning well after over 50 years of service. They provide a good means of

providing the flexibility required for base isolation. Elastomeric bearings use either natural rubber or

synthetic rubber (such as neoprene), which have little inherent damping, usually 2% to 3% of critical

viscous damping. They are also flexible at all strain levels and so do not provide resistance to

movement under service loads. Therefore, for isolation they are generally used with special

elastomer compounds (high damping rubber bearings) or in combination with other devices (lead

rubber bearings).

Natural Rubber Bearing

Natural rubber bearing also known as laminated rubber bearing are manufactured of either

natural rubber or neoprene, a synthetic rubber material famous for its toughness and

durability which has similar behaviour to natural rubber. A typical natural rubber bearing

arrangement is shown below.

Fig: Components of a natural rubber type laminated bearing

Natural rubber bearing comprises of alternating rubber and steel shim layers joined together to

produce a composite bearing by vulcanisation process under pressure. Steel shims add vertical

stiffness to the bearing and hence prevent rocking response of an isolated structure. Steel shims

prevent rubber from bulging out under high axial compressive loads. The shims do not contribute to

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TYPES OF SEISMIC ISOLATORS

lateral stiffness of bearing as it is controlled by the shear modulus of the elastic material. The bearing

is mounted between two thick endplates to facilitate the connection between the foundation and the

isolation mat.

Though natural rubber bearings are easy to install, the main drawback of this type of bearing are low

damping and it inability to handle service wind loads due to low stiffness. Natural rubber bearing

generally exhibit a critical damping value of 2-3%. Hence natural rubber bearings require additional

damping devices such as viscous or hysteretic dampers to cater for service and extreme seismic

loads.

Lead Rubber Bearing

Fig: Lead rubber bearing

Lead rubber bearings have a much better capability to provide adequate stiffness for lateral loads and

better damping characteristics than that of rubber bearings. The configuration of lead rubber bearing

is same as that of the natural rubber bearing except there is one or more cylindrical lead plugs in the

centre of the arrangement as shown in the figure above. For this reason lead rubber bearings are also

named as lead plug bearings. This arrangement of lead plug gives high stiffness to the structure

under low service and wind loads. Under extreme events, lead deforms plastically reducing the

stiffness of the whole isolation device to the stiffness of rubber alone. During the plastic deformation

of the lead plug energy is being dissipated in a hysteric manner. Lead plug deforms similar as rubber

but dissipates kinetic energy in the form of heat, thus reducing the energy absorbed by the building.

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TYPES OF SEISMIC ISOLATORS

Lead rubber bearing shows desirable hysteretic damping characteristics which enhances the

structural response of the system

High Damping Rubber Bearing (HDR)

High damping natural rubber bearing eliminates the use of supplementary damping devices in case of

natural rubber bearing. The component assembly of high damping natural rubber bearing is same as

that of the natural rubber bearing but the type of elastomeric material used is different. The increase

of damping up to 20-30% is achieved through addition of fillers (carbon, oil and resins) in high

damping natural rubber bearings. For most HDR used to date the effective damping is around 15% at

low strains reducing to 8%-12% for strains above 100%, although some synthetic compounds can

provide 15% or more damping at higher strains.

Sliding isolators

The primary advantage of sliding devices is their ability to eliminate torsional effect in

asymmetric structure. The frictional force utilised in sliding device is equal to the axial force on the

sliding device due to weight. Therefore the centre of gravity of a building coincides with the centre

of the stiffness of the isolation system thus eliminating the torsional effect in asymmetric structures.

The elastomeric bearings have a widespread application though sliding type bearings on the other

hand, are impractical due to lack of restoring capability. To overcome this drawback friction

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Fig: lead rubber bearing hysteresis

TYPES OF SEISMIC ISOLATORS

pendulum system (FPS) is introduced which utilises a sliding interface to provide restoring stiffness

and to dissipate energy.

Resilient friction system

Fig: Assembly of resilient friction system

As shown in figure the resilient friction base isolator are composed of a set of metal plates

which can slide on each other with a central rubber core and/or peripheral rubber cores. The

rings are enclosed in a very flexible rubber covering which protects the metal rings from

corrosion and dust. To reduce the friction the sliding plates are coated with Teflon. The

rubber core helps to distribute the lateral displacement and velocity along the height of the

isolator. The resilient friction base isolator is characterised by the coefficient of friction of the

sliding elements and the total lateral stiffness of the rubber core. Under seismic loads friction

damping plays the main role as the energy dissipater rather than the rubber material.

Friction pendulum bearing (FPB) system

Friction pendulum bearing combine sliding with pendulum action. The arrangement consists

of an articulated slider on a spherical concave chrome surface. The slider is covered with

polished bearing material such as Teflon. The friction coffiecent between the surface is in the

order of 0.1 for high velocity sliding and 0.05 for low velocity sliding. FPS is activated when

earthquake forces exceed the value of static friction. The restoring force in FPS is

proportional to weight supported by the bearing and inversely proportional to the radius of

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TYPES OF SEISMIC ISOLATORS

curvature of the concave surface. FPS is functionally equivalent to LRB and HDRB in

lengthening structures fundamental period with additional advantages such as period

invariance, torsional resistance, temperature insensitivity and durability. These bearing offer

versatile properties which can satisfy the diverse requirement of building bridges and

industrial facilities.

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Fig: FPS assembly and hysteresis behaviour

LITERATURE REVIEW

4.0 LITERATURE REVIEW

Seismically isolated bridges have been researched and investigated by academia and

engineers for many years. Due to this extensive effort, seismic isolation design has become a

practical option for earthquake resistant design. Results from numerous computer simulations

and shake table experiments have shown the advantages of seismically isolated bridges

compared to non-isolated bridges.

Several studies have proved the efficiency of base isolated structure in regard to

increasing the natural period of the structure as well as reducing the inertial force acting on

structure during seismic condition. For low-rise regular-frame Navy construction situated on

a rock or stiff site and housing-sensitive equipment like computers or costly contents, base

isolation of the columns offers the potential for significant damage reduction and also

possible initial cost savings. It is recommended that consideration be given to base isolation

in the early stages of design formulation. [6]

Types of isolators and its reliability at different earthquake strengths are studied by

many researchers. Lin Su [2] and his team have performed a comparative study on different

base isolators to find their effectiveness. It is shown that in general the base isolation systems

protect the structure from the effects of high amplitude and high frequency oscillations that

fall in the same range as the natural frequencies of the structure. It was found out that all base

isolators perform satisfactorily under common earthquakes. Also for earthquakes with low

frequency energy, NS system and LRB systems are not applicable as they may cause

undesirable amplification of ground excitation. B. C. Lin et al. [10] carried out studies on

different isolator system namely laminated rubber bearing system, the New Zealand system,

and the resilient-friction base isolator system. Results showed that friction plays an important

role in energy absorption and is therefore a key factor contributing to the effectiveness of a

base isolation and R-FBI base-isolator system was found to have the broadest range of

applicability.

Base isolators are sometimes used side by side with damping systems. J. C.

Ramallo[5] et al. proposed a smart isolated system and compared the effectiveness with lead

rubber bearing system. He concluded that a smart damper, due to its adaptive nature can

reduce base drifts as well, and sometimes better, than the LRB system while simultaneously

reducing structural accelerations, inter story drifts, and base shears. LRB system was found

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LITERATURE REVIEW

out to be partially effective towards seismic accelerations other than the design forces. N.

Wongprasert [8] carried out simulations of FPS and LDR system isolated models and the

results showed 20% reduction in inter-storey drift.

There are cases when the base isolated structure comes in contact with the adjacent

cases. Vasant A. Matsagar[11] concluded that Superstructure acceleration in base-isolated

building increases significantly due to its impact upon the adjacent structure during an

earthquake. Higher modes of vibration are excited when impact between the base-isolated

building and adjacent structure occurs. Also stiffness of the adjacent structure has significant

influence on the base isolated structure.

Base isolation can be installed in new structures as well for retrofitting of other

structures. It was confirmed by Matsutaro Seki [12] that the base isolation technology is the

feasible retrofitting method in order to conquer the limitation of the weak strength and the

architectural feature of the building. His studies were based on retrofitting on masonry

building.

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NUMERICAL BACKGROUND

5. NUMERICAL BACKGROUND

For the general representation of a seismically isolated SDOF system the equation of motion iis

given by:

Mẍ+cẋ+Kx=⩑ f −M 1 ẍ g

Where where f =supplemental force exerted by the damper or the LRB lead plug; ⩑=[1 0] TT gives

the position of the supplemental damper force; 1=vector whose elements are all unity; ẍg g =absolute

ground acceleration.

Equation for frequency of system and base isolator

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NUMERICAL BACKGROUND

Time period T and stiffness of common isolators

Friction Pendulum system

T=2 π √( Rg

)

R= Radius of curvature of the concave surface

g= gravitational acceleration

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CASE STUDY

6.0 CASE STUDY

Programme used: ABAQUS

Details of the model: Steel frame made of 50 x 50 mm box type steel beam and columns

Thicknesses of the hollow steel beams are 10mm

Bay length= 6 m in all 4 sides

Details of the isolator model: 600 mm dia steel plates (30mm thick)

500mm dia rubber shims.(12nos): 27 mm thick

Fig: [i] fixed base , [ii] base isolated structure

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3 m

3 m

4 m

6 m

CASE STUDY

Fig: Rubber bearng Isolator

RESULTS

Fixed based: frequency at different modes

Base isolated : frequency at different modes

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CASE STUDY

Fixed based: Displacement (top corner node x direction)

Base isolated: Displacement (top corner node x direction)

Results shows that under dynamic loading the frequency at different modes of fixed base model are

higher than that of base isolated structure. Also the deflection taken at the top end node is larger at

time intervals.

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CASE STUDY

CONCLUSION

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REFERENCES

REFERENCES

1. Wang, Yen-Po, Fundementals of seismic base isolation, International training programs for seismic structures, NCREE

2. Lin Su, Goodarz Ahmadi, and Iradj G. Tadjbakhsh; Comparative Study Of Base Isolation Systems; Journal of Engineering Mechanics, Vol. 115,No. 9, September, 1989

3. Saurav Manarbek, Study of Base isolation system, Thesis work-M-tech, Massachusetts Institue of Technology4. Trevor E Kelly, S.E. Holmes Consulting Group Ltd. Base Isolation Of Structures; Design Guidelines, Revision

20015. J. C. Ramallo; E. A. Johnson, A.M.Asce; And B. F. Spencer Jr., M.Asce, ‘‘Smart’’ Base Isolation Systems,

Journal Of Engineering Mechanics / October 20026. J.M. Ferritto,1 Member, Studies On Seismic Isolation Of Buildings Journal of Structural Engineering, Vol. 117,

No. 11, November,19917. H. W. Shenton , J Associate Member, A. N. Lin, Member, Relative Performance Of Fixed-Base And Base-

Isolated Concrete Frames, Journal of Structural Engineering, Vol. 119, No. 10, October,1993.8. N. Wongprasert, M. D. Symans, Numerical Evaluation of Adaptive Base-Isolated Structures Subjected to

Earthquake Ground Motions, Journal Of Engineering Mechanics ASCE/ February 2005

9. Satish Nagarajaiah, Andrei M. Reinhorn, Michalakis C. Constantinou, Nonlinear Dynamic Analysis Of 3-

Dbase-Isolated Structures, Journal of Structural Engineering, Vol. 117, No. 7, July, 1991

10. B. C. Lin, I. G. Tadjbakhsh, A. S. Papageorgiou,and G. Ahmadi, Performance Of Earthquakeisolation Systems,

Journal of Engineering Mechanics, Vol. 116, No.2, February, 1990.

11. Vasant A. Matsagar, R.S. Jangid, Seismic response of base-isolated structures during impact with adjacent

structures, Department of Civil Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076,

India

12. Matsutaro Seki, Masaaki Miyazaki, Yasuhiro Tsuneki And Kunio Kataoka, Masonry School Building

Retrofitted By Base Isolation Technology, 12WCEE2000

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