Seismic Base Isolation

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; chichi 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 Page | 2

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

Fig: inertial force due to ground motion

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

Fixed Base

Period

Base Isolated

Fig: Shift of period in base isolated structures

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Fig: Effect of damping on acceleration of the structure

H i stor y of seismi c base isolati on

The

first

evidence

of

architects using the principle of base

isolation

protection Pasargadae,

was a

for

earthquake

discovered city

in

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

foundation

which is

built

a of

second wide,

smoothed stones which are linked together, forming a plate that

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

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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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 innovation

first of

patent mechanical

for

the

isolators

recent 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. Fig: Use of timber for base isolation

After

the 2001 Bhuj (Gujarat) earthquake, the fourstorey Bhuj Hospital building was built with

base isolation Technique. The new 300-bed hospital was fitted with a New Zealand-developed leadrubber base-isolation system after the local hospital in Bhuji was collapsed claiming approx. 176 lives.

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TYPES OF SEISMIC ISOLATORS

3. TYPES OF SEISMIC ISOLATORS

Isolation Devices

Elastomeric Isolators

Natural Rubber Bearings

LowDamping Rubber Bearings

LeadPlug Bearings

Sliding Isolators

HighDamping Rubber Bearings

Resilient Friction System

Friction Pendulum System

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). El astomer ic bear i ng

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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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 Page | 9


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. Page | 10


Lead rubber bearing shows desirable hysteretic damping characteristics which enhances the structural response of the system

Fig: lead rubber bearing hysteresis



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.

Sli din g 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 Page | 11


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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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.

Fig: FPS assembly and h steresis behaviour

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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:



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; ẍ g g =absolute ground acceleration. Equation for frequency of system and base isolator

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NUMERICAL BACKGROUND

Ti me per iod T and stif fn ess of common isolators 

Friction Pendulum system

√ 

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 6m

3m

3m

4m

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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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. 2. 3. 4. 5. 6. 7. 8.

Wang, Yen-Po, Fundementals of seismic base isolation, International training programs for seismic structures, NCREE Lin Su, Goodarz Ahmadi, and Iradj G. Tadjbakhsh; Comparative Study Of Base Isolation Systems; Journal of Engineering Mechanics, Vol. 115,No. 9, September, 1989 Saurav Manarbek, Study of Base isolation system, Thesis work -M-tech, Massachusetts Institue of Technology Trevor E Kelly, S.E. Holmes Consulting Group Ltd. Base Isolation Of Structures; Design Guidelines, Revision 2001 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 2002 J.M. Ferritto,1 Member , Studies On Seismic Isolation Of Buildings Journal of Structural Engineering, Vol. 117, No. 11, November,1991 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. 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 T echnology 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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Seismic Base Isolation

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