BASE ISOLATION DESIGN FOR CIVIL COMPONENTS.pdf

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PROCEEDINGS, STRUCTURAL ENGINEERS WORLD CONGRESS, SAN FRANCISCO, CALIFORNIA,JULY (1998)

BASE ISOLATION DESIGN FOR CIVIL COMPONENTSAND CIVIL STRUCTURES

F. F. Tajirian

Seismic Isolation Engineering, Inc.

P. O. Box 11243, Oakland, California 94611, USA

ABSTRACT

Seismic isolation is being used worldwide to protect buildings, new and old, and their contents

from the destructive effects of earthquakes. This paper reviews applications of seismic isolation

to civil components, tanks, and industrial facilities. The benefits of seismic isolation to such

applications as well as differences in design requirements between building and non-building

isolation are illustrated through the examples described. Seismic isolation of individual

components is very beneficial in situations where existing components and their supports have to

be requalified for higher seismic loads. By using seismic isolation, it may be possible to avoid

expensive retrofitting of the supporting facility and the foundation. Three examples of this type of

retrofit are given in the paper.

INTRODUCTION

It has long been recognized that power plant vessels, computers, sensitive equipment, and tanks

typically found in industrial facilities are more vulnerable to earthquake damage than buildings.

During the Northridge Earthquake, there was significant damage to buildings, especially hospitals,

attributed to failure of contents such as tanks and pipes. Seismic isolation is a practical approach

for providing seismic protection for such systems and components. This is demonstrated in this

paper by reviewing several examples of seismic isolation where the primary purpose of using

isolation was the protection of components. Although the acceptance of this technology forisolation of components and tanks has been slower than for buildings, future applications should

increase as owners of industrial facilities realize that conventional seismic design techniques may

not be adequate in protecting such equipment.

In general, the rules developed for the design of isolators for buildings are also applicable to

components. Major design issues that differentiate the design of seismic isolators for components

from buildings include the following:


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• Components usually do not have sufficient mass. Consequently, the isolation system usually

consists of four bearings. These systems have less redundancy than isolation systems used in

buildings. It is therefore important to use high quality isolators to minimize chances of failure.

The development of new isolation techniques, including softer elastomers and low friction

rollers, would make it easier to isolate lighter components and possibly further lengthen the

isolation period. This will make isolation of components more appropriate for soft sites withfundamental site periods between one and two seconds.

• In existing facilities, isolated components may have to be located in confined spaces.

Consequently, a sufficient gap may not be available around the isolated structure.

• For isolated components that are located in the upper stories of a building, the displacements

may be larger than the design displacements used in base isolation of buildings due to

amplification of floor response.

SEISMIC ISOLATION OF TANKS

Seismic Vulnerabil ity of Tank Structures

Tanks have not performed well in recent earthquakes. Both concrete and steel tanks have been

seriously damaged. Several types of tank failure have been observed. Tanks may be damaged for

different reasons. Large shell hoop tensile stresses resulting from a combination of hydrostatic

pressure and hydrodynamic pressure due to horizontal and vertical ground motions could fail the

tank. A more common type of failure is known as “elephant’s foot buckling”. This is caused by

the large overturning base moments resulting from the impulsive and convective liquid loading on

the tank wall during an earthquake. The high vertical compressive stresses, which develop in the

tank’s shell, may cause buckling of the shell. These forces are generally greater in tanks that are

held down, thus tanks which are bolted down require thicker walls. High stresses near the hold-

down bolts may results in tearing of the tank shell. If the tanks are unanchored, the tanks

experience partial base uplifting which results in increased axial compressive stresses in the tank

due to the reduced contact area. A new approach developed by Malhotra (1997) proposes to

support the tank wall on a ring of vertically flexible rubber bearings, and the tank base plate is

supported directly on the soil to limit the compressive stresses resulting from uplift. Analysis

results by Malhotra have shown that overturning base moments are significantly reduced while

still maintaining acceptable levels of base uplift. Unanchored tanks can be damaged if the

horizontal seismic forces exceed the frictional resistance between the tank and its supporting base.

Finally, tanks roofs may be damaged or the contained liquid may be spilled due to sloshing waves

caused by the long period components of earthquakes.

Seismic I solation of LNG Tanks

Tanks are being built in increasing numbers to store Liquefied Natural Gas (LNG). These tanks

are very large and have capacities around 150,000 m3. They pose a great risk if they fail during an

earthquake. The important design issues associated with LNG tanks are summarized in a paper by

Bomhard and Stempniewski (1993).


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LNG tanks consist of an inner steel tank, which contains the LNG, and an outer concrete tank

encasing and protecting the inner tank, with insulation placed between the two structures. The

concrete tank is supported on a common concrete mat. The tank is supported by a group of

closely spaced columns to allow air to circulate below. Gas leakage from the containment system

can result in explosions and fires, and can cause catastrophic disaster for the environment as well

as human life. Thus, the tank structures are subjected to very stringent seismic safetyrequirements, which can have a major impact on the design of the tank.

Large tanks have a fundamental frequency between 2 and 10 Hz, placing them in the range of

resonance of most earthquake ground motions. In the design of buildings, seismic energy

absorption due to inelastic response is tolerated. In the case of tanks however, the requirement of

tightness and the containment function stand in the way of using this approach. Normally, in the

design of LNG tanks, it is preferable to avoid the use of anchor straps to prevent the inner tank

from uplifting during an earthquake in order to minimize the welded attachments to the cryogenic

steel. The mechanism of tank uplift is complex and not completely understood. To describe it

fully, the effects of large displacements, yielding of the base plate, membrane forces in the base,

phase relationship between the horizontal and vertical components and the effect of these parameters on the period of the system need to be considered. One approach to minimize the

potential for tank uplift is to use tanks with large diameter to height ratios. Ratios of four have

been used in some projects. This option may be a costly alternative resulting in undesirable tank

shapes and inefficient use of the site area.

Recently, two LNG tank projects have adopted seismic isolation to reduce seismic loads. The

seismic lateral loads were significantly reduced, allowing the use of more reasonable tank

dimensions, and diameter-to-height ratios as low as 2.3. Furthermore, economical tank designs

developed for areas of low and moderate seismicity may be used in areas of high seismicity.

Additionally, safety is enhanced by insuring that the containment capabilities of the outer tank are

not impacted during a strong earthquake. Figure 1 shows a schematic section through an isolatedLNG tank. The first project, is in Inchon on the west coast of Korea where three storage tanks,

each having a capacity of 100,000 m3, are being constructed, Koh (1997). The inner tank height

and diameter are 30 m and 68 m, respectively. Due to poor soil conditions at the site, a pile

foundation system is used. The isolation system consists of steel-laminated rubber bearings with a

diameter of 600mm and an overall height of 228 mm. The design isolation period is around 3

seconds. The design safe shutdown earthquake (SSE) has a maximum horizontal acceleration of

0.2 g.

The second project is located on Revithoussa Island in Greece where two tanks, each with a

capacity of 65,000 m3 are, being constructed. The inner tank, consisting of nickel steel, is

unanchored and has a diameter of 65.7 m and a height of 22.5 m. The outer tank is made of

prestressed concrete. The tanks are partially buried for reasons of aesthetics. Two preliminary

designs were developed for this project. One non-isolated and the other isolated. The non-

isolated alternative used the same inner tank geometry and massive anchors attached to both the

outer and inner tanks. The inner tank had a thicker shell and special detailing was provided to

minimize thermal effects. While it was determined to be the alternative with the least initial cost,

the owner opted for the isolated alternative because it was perceived to be a safer design. In the


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design selected, each tank is supported on 212 isolators. The isolators consist of Friction

Pendulum System (FPS) bearings with a radius of curvature of 1,880 mm and displacement

capacity of 300 mm. The earthquake was defined in terms of elastic 5-percent damped spectra.

Two levels were specified, the operating basis earthquake (OBE) and the safe shutdown

earthquake (SSE). The SSE earthquake had a peak ground acceleration of 0.48g and spectral

values of 0.61g and 0.29g at periods of 1.0 and 3.0 seconds respectively. The design called forelastic response of the tanks up to the SSE level event. It is expected that the tanks will be

operational by 1999.

CONCRETE

FOUNDATION

OUTER

PRESTRESSED

CONCRETETANK

CONCRETE

UPPER MAT

INNER

STEEL

TANK

CONCRETE

PEDESTAL

INSULATION

SEISMIC

ISOLATOR

LIQUEFIED

NATURAL GAS

(LNG)

Figure 1: Schematic of Isolated LNG Tank


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Figure 2: LNG Tank in Revithoussa Island in Greece during construction (Constantinou 1997)

SEISMIC ISOLATION OF ADVANCED NUCLEAR REACTORS

Several countries have initiated programs to develop seismic isolation systems for advanced

nuclear applications, Tajirian et al. (1990). The benefits of seismic isolation in power plant

applications can be summarized as follows:

• Permit standardization of the design regardless of seismic conditions. This will reduce capital

costs so that future nuclear plants are competitive with those using alternate sources of

energy.

• Enhance the safety and reliability of nuclear power plants to regain public acceptance.

• Facilitate decoupling of the reactor design and development, which is global in nature, from

the balance of plant (BOP) design and licensing which is regional in nature.

To date, six large Pressurized Water Reactor (PWR) units have been isolated in France and South

Africa. The operational four unit Cruas plant in France, where the site safe shutdown earthquake

(SSE) acceleration was 0.2 g, is supported on 1,800 neoprene pads measuring 50x50x6.5 cm,

Postollec (1983). This design was developed for sites with moderate seismicity. For sites with

higher seismicity, sliding plates are placed between the top of the neoprene pads and the upper

mat, Jolivet and Richli (1977). The lower plate is made of a lead-bronze alloy and the upper

plate, which is embedded in the upper mat, is stainless steel. The plate combination provides a


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friction factor of 0.2. When the ground accelerations exceed the friction coefficient, sliding

occurs, thus limiting the shear strains in the pads and the forces in the building to the same level as

that for moderate sites. This design was used by Framatome to isolate a two unit plant in

Koeberg, South Africa, where the site SSE acceleration was 0.3 g. A total of 2000 pads

measuring 70x70x10 cm were used. A similar design was licensed for the Karun River plant in

Iran whose construction was suspended in 1978. Designs were also developed for a single 1300MWe PWR in Laguna Verde Mexico, and a two unit 1400 MWe PWR at Le Carnet in western

France.

In the U.S. the Department of Energy (DOE) sponsored Advanced Liquid Metal Reactor

(ALMR) has adopted seismic isolation to simplify the design, enhance safety margins, and support

the development of a standardized design for the majority of the available U.S. reactor sites. The

nuclear island is being designed for a safe shutdown earthquake (SSE) with a maximum horizontal

and vertical acceleration (PGA) of 0.5g. Detailed seismic analyses of the ALMR have been

performed and the results reported elsewhere by Tajirian and Patel (1993). The ALMR isolated

structural configuration consists of a stiff rectangular steel-concrete box structure, which supports

the reactor vessel, the containment dome, and the reactor vessel auxiliary cooling system stacks.The total isolated weight is about 23,000 tons and is supported on 66 high damping rubber

bearings. The horizontal isolation frequency is 0.7 Hz, and the vertical frequency is greater than

20 Hz.

A qualification program for the ALMR seismic isolation system was established to demonstrate

that all seismic design and safety requirements are met. The essential categories are bearing tests,

both reduced-scale and full-size, bearing environmental tests, and seismic isolation system tests.

To date a large number of bearing tests have been performed and the results reported elsewhere

by Tajirian et al. (1990), Clark et al. (1995).

Another DOE-sponsored project, the Sodium Advanced Fast Reactor (SAFR) houses the reactorassembly module in a standardized shop-fabricated unit housed in a building constructed above

grade with plan dimensions of 38 m by 25 m. The building, which weighs 31,000 tons, is

supported on 100 seismic isolators. What makes the isolation system for SAFR unique is that it

provides vertical as well as horizontal isolation. The design horizontal frequency is 0.5 Hz and

the vertical frequency is 3 Hz. This is achieved by using bearings with thicker rubber layers than

usual which are flexible vertically as well as horizontally. The bearing diameter is 107 cm, the

total height is 41 cm, and consists of three layers of rubber each 10.2 cm thick with a shape factor

of only 2.3. Reduced scale bearings were extensively tested to validate the concept of low shape

factor bearings, Aiken et al. (1989).

In Japan, the Central Research Institute of Electric Power Industry (CRIEPI), under contract with

the Ministry of International Trade and Industry, has carried out a ten-year research and

development program to develop design guidelines for base isolated Fast Breeder Plants. In

1997, the project was completed with the publication of the design guideline document. The

guidelines have been revised to also make them applicable to Light Water Reactors. This means

that a base-isolated nuclear plant can be licensed in Japan. More recently, a research program to


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apply seismic isolation to the ITER (International Thermonuclear Experimental Reactor) fusion

reactor was started, Fujita (1997).

SEISMIC ISOLATION OF INDIVIDUAL COMPONENTS

The benefits of individual component isolation were recognized early on, leading to the isolation

of 230 kV circuit breakers in Southern California, Kircher (1979). This preceded application of

seismic isolation in buildings in the U.S. This was followed by shake table tests at the Earthquake

Engineering Research Center (EERC), which clearly demonstrated the benefits of seismic

isolation of large power plant components as well as light secondary systems, Kelly (1983). More

recent applications of component isolation include the Mark II Detector at the Stanford Linear

Accelerator Center which is part of the Linear Collider, isolated using four lead-rubber bearings,

Buckle and Mayes (1990). The Detector consists of fragile equipment and weighs about 1500

tons. Its dimensions are 7.6m W x 10.5m L x 9.25m H. The seismic criteria consisted of US

Nuclear Regulatory Guide RG 1.60 spectra scaled to 0.6g. The bearings are 74 cm square and 33

cm high and include a lead plug with a diameter of 24 cm. The maximum allowable horizontaldisplacement at 0.6g was 33 cm. Fragile art objects at the J. Paul Getty Museum in Malibu,

California have also been isolated using a sliding isolation device Yaghoubian (1991). The seismic

criterion was a magnitude 6.5 earthquake occurring one mile from the site. Such an event was

estimated to produce a peak acceleration of 0.7g in the horizontal direction and a vertical

acceleration of 0.45g. The isolators consist of off-the-shelf ball transfer units which roll on steel

plates placed on the floor. The centering mechanism consists of a steel bowl and a centering post.

The bowl has a highly polished surface to reduce friction. The post consists of a telescoping mast

whose movement is restrained by a coil spring. Shake table tests were carried out to demonstrate

the efficiency of the isolation system. Several reports are also available of seismic isolation of

electric circuit breakers in Japan and Italy, Bonacina et al. (1994).

Raised floor systems are widely used around the world in computer rooms and clean room

facilities. Most electrical or mechanical equipment are rigidly secured to the floor or are

supported on wheels that are locked from movement. In some cases equipment cabinets are

supported on wheels that are free, thus allowing sliding or rocking to occur. To prevent

overturning during a seismic event, the cabinets are tied to the floor with bungee cords. In Japan,

it is now common practice to isolate raised floor systems. Several systems utilizing a combination

of springs, pneumatic isolators, multi-stage rubber bearings, sliders, and dampers, have been

developed and applied by major construction companies, Fujita (1991). To date, isolated raised

floor systems have not been implemented in the USA. A number of shake table tests at EERC

have been performed on floor isolation systems developed by IBM, Tajirian (1990). In these tests,

the isolation system consisted of elastomeric bearings with Teflon elements sliding on polished

stainless steel plates. Restoring force was provided by a steel-laminated elastomeric bearing.

More recently shake table tests were performed at NCEER, Lambrou and Constantinou, (1994).

The isolation system in these tests consisted of FPS bearings and dampers. Both tests

demonstrated that the isolation system is effective in limiting forces transferred to equipment

supported on the floor.


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REDUCTION OF FOUNDATION LOADS WITH SEISMIC ISOLATION

Seismic isolation is commonly used to reduce seismic loads in buildings and other major

components. However, another use of isolation is to limit the seismic forces exerted by the

isolated structure on the supporting foundation. This is especially useful when new large masses

are introduced in facilities were adequate seismic loads have not previously been considered and acostly upgrade would be required before the increased seismic loads could be accommodated. In

this section, three such examples are given.

Upgraded Titan IV launch vehicles are used to send payloads into space from Vandenberg Air

Force Base, California. Each Titan rocket consists of a liquid-fueled core vehicle, and two strap-

on solid rocket motor units (SRMU). Each SRMU is divided into three segments: Aft, Center,

and Forward. The weights of each unit are 167, 144, and 67, ton (metric) respectively. Before

launch, the SRMU segments are stored and checked in an existing facility near the launch site.

This facility required modifications to accommodate the increased loads caused by the SRMUs

while they are in storage. There was concern that personnel could be endangered due to

movement or failure of SRMU segments during an earthquake. Several conventional designschemes to restrain the SRMU segments were evaluated, including rigidly attaching the bases of

the segments to their foundations and using existing steel platform stands to provide lateral

support for the SRMUs. It was concluded that the Forward segments, which are lighter than the

other segments could be supported at the base without requiring major additional modifications to

the supports. However, for the Aft and Center segments the implementation of rigid foundations

would have required the excavation of the existing foundation and installation of new foundations

underneath the segments’ support stands. These modifications would have required an extended

construction period and thus would not have been compatible with program objectives. Seismic

isolation was therefore selected for the Aft and Center Segments. This minimized the requirement

for major modifications to the existing foundations and structures.

A site specific response spectrum with a peak ground acceleration of 0.6 g was used as input.

Each segment was supported on a steel frame supported on four high damping rubber bearings.

A medium stiffness compound with a shear modulus of 0.55 MPa at 100 percent shear strain was

used. The bearings had a diameter of 38 cm, and a total height of 25.4 cm. The rubber stack

consisted of 23 layers with a thickness of 6.4 mm. The isolation frequencies were 0.52 Hz for the

Aft segment and 0.56 Hz for the Center segment at the design displacement. The total maximum

displacements for the Aft and Center segments were 22.4 cm and 20 cm respectively.

In the second example, a situation arose in which it was necessary to provide a back-up

emergency system in case of a major malfunction of the main turbine generator exciter in the

Diablo Canyon Power Plant in California. The exciter is not a safety-related piece of equipment,

but is needed for the electric power generation. One solution that was considered was the use of

two existing mobile exciters which would be placed in the Turbine building to replace the

malfunctioning exciter until it could be repaired and put back in service. The mobile exciters

consist of transformers and switch gear mounted on a truck trailer, each weighing about 32 tons.


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Pacific Gas and Electric needed to demonstrate that the mobile exciters would not fail during the

design-basis earthquake in such a manner as to compromise other nearby safety-related structures.

Calculations showed that the exciters would exert large seismic reaction forces; as a result: (1)

strengthening of the truck trailers and equipment anchorages would be required, (2) an

excessively large mounting skid would be required, and (3) the number of foundation bolts would

be excessive. These modifications were undesirable because it would lengthen the installationtime of the mobile exciters, increasing the duration of a forced outage. An alternate approach

using seismic isolation was developed in which four high-damping rubber bearings were used to

support each trailer. The design-basis earthquake consisted of horizontal and vertical floor

response spectra with a maximum horizontal acceleration of 0.75 g. The peak spectral

acceleration between 2 and 9 Hz was 1.8 g, and a peak velocity of 127 cm/sec between 0.5 and 2

Hz. Three preliminary isolator bearing designs were developed giving isolation frequencies of 0.5

Hz, 0.75 Hz, and 1.0 Hz. Because of the relatively low weight of the trailer, the 0.5 and 0.75 Hz

conventional bearing designs using the available high damping rubber compound at the time

(shear modulus of about 0.76 MPa at 100 percent shear strain) would have required a very tall

and slender bearing which would not have had the required stability during large horizontal

displacements. Consequently, a stacked multi-stage bearing design was developed. Each bearingunit was composed of a stack of small bearings bolted to the corner of stabilizing steel plates.

Alternatively, by selecting the 1.0 Hz system, the displacements were smaller and allowed the use

of a conventional bearing design, while reducing forces sufficiently to eliminate the need for

strengthening the supporting floor. Four high damping rubber bearings with an outside diameter

of 42 cm and a total rubber height of 30.5 cm were used. The maximum design horizontal

displacement was 30.5 cm.

In the third example, a partially completed offshore structure, situated in the Caspian Sea, was

retrofitted with seismic isolation, Medeot and Infanti (1997). The original structure, located in

120 m of water, consisted of two, back-to-back steel jackets, each supported on ten piles.

Construction of the original jacket was completed in 1993. The structure was not designed forseismic loads. A detailed seismic analysis of the structure as built demonstrated that the piles

could not sustain the level of loads associated with the MCE event (1000 year return period).

Two alternatives were considered for strengthening the structure. The first alternative considered

strengthening the foundation by installing additional skirt piles. This option was eliminated as too

costly and schedule-intensive. The second option was to reduce the seismic load on the

foundation by isolating the top deck structure from the jacket. The isolation system comprised of

spherical PTFE sliding bearing equipped with steel hysteretic dampers. The isolation system was

equipped with sacrificial restraints, which were designed to break during an earthquake. This

prevents the movement of the energy dissipators during wind and wave loading.

References

Aiken, I. D., Kelly, J. M., and Tajirian, F. F., Mechanics of low shape factor elastomeric seismic

isolation bearings, Report No. UCB/EERC-89/13, University of California, Berkeley,1989.


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Bomhard, H., and Stempniewski, L., LNG Tanks for seismically highly affected sites, Intl. Post-

SMiRT Conference Seminar on Isolation, Energy Dissipation and Control of Vibrations of

Structures, Capri, Italy, 1993.

Bonacina, G., et al., Seismic base isolation of gas insulated electrical substations: design,

experimental and numerical activities, evaluation of the applicability, 10th European Conferenceon Earthquake Engineering , Vienna, Austria, 1994.

Buckle, I. G., and Mayes R. L., Seismic isolation: history, application, and performance - A world

view, Earthquake Spectra, EERI, May 1990.

Clark, P. W., Aiken, I. D., Kelly, J. M., Tajirian, F. F., and Gluekler, E. L., Tests of reduced-scale

seismic isolation bearings for the Advanced Liquid Metal Reactor Program (ALMR) program,

ASME/JSME Pressure Vessel and Piping Conference, Honolulu, Hawaii, July 1995.

Constantinou, M., Personal Communication, November 1997.

Fujita, T. ed., Seismic isolation and response control for nuclear and non-nuclear structures,

Special Issue for the Exhibition of the 11th International Conference on Structural Mechanics in

Reactor Technology, Tokyo, Japan, August 18-23, 1991.

Fujita, T., Progress of applications, R&D and design guidelines for seismic isolation of civil

buildings and industrial facilities in Japan, Int. Post-SMiRT Conference Seminar on Seismic

Isolation, Passive Energy Dissipation and Active Control of Seismic Vibrations of Structures,

Taormina, Italy, August 25-27, 1997.

Jolivet, J., and Richli, M. H., Aseismic foundation system for nuclear power stations, SMiRT-4,

Paper K.9/2, San Francisco, 1977.

Kelly, J. M., The use of base isolation and energy-absorbing restrainers for the seismic protection

of a large power plant component, EPRI NP-2918, Research Project 810-8, March (1983).

Kircher, C. A., et al., Performance of a 230 KV ATB 7 power circuit breaker mounted on

GAPEC seismic isolators, JABEEC No. 40, Dept. of Civil Engineering, Stanford University,

1979.

Koh, H. M., Progress of applications, new projects, R&D and development of design rules for

base isolation of civil buildings, bridges and nuclear and non-nuclear plants in Korea, Int. Post-

SMiRT Conference Seminar on Seismic Isolation, Passive Energy Dissipation and Active

Control of Seismic Vibrations of Structures, Taormina, Italy, August 25-27, 1997.

Lambrou, V. and Constantinou, M. C., Study of seismic isolation systems for computer floors,

Technical Report NCEER-94-0020, July, 1994.


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Malhotra, P. K., Method for seismic base isolation of liquid-storage tanks, Journal of Structural

Engineering , ASCE, Vol. 123, No. 1, January, 1997.

Medeot, R., and Infanti, S., Seismic retrofit of Chirag 1 offshore platform, Int. Post-SMiRT

Conference Seminar on Seismic Isolation, Passive Energy Dissipation and Active Control of

Seismic Vibrations of Structures, Taormina, Italy, August 25-27, 1997.

Postollec, J-C, Les foundations antisismiques de la Centrale Nucleare de Cruas-Meysse, notes du

service etude geni civil d'EDF-REAM, 1983.

Tajirian, F. F., Seismic isolation study final report, Technical Report, Bechtel Corporation, 1990.

Tajirian, F. F., Kelly, J. M., and Aiken, I. D., Seismic isolation for advanced nuclear power

stations, Earthquake Spectra, 1990, Vol. 6, no. 2, May.

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ALMR, 12th SMiRT Conference, Stuttgart Germany, August 1993.

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Spectra, EERI, Vol. 7, No.1 1991.

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BASE ISOLATION DESIGN FOR CIVIL COMPONENTS.pdf