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BASE ISOLATION DESIGN FOR CIVIL COMPONENTS AND CIVIL STRUCTURES

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BASE ISOLATION DESIGN FOR CIVIL COMPONENTS AND CIVIL STRUCTURES Powered By Docstoc
					    PROCEEDINGS, STRUCTURAL ENGINEERS WORLD CONGRESS, SAN FRANCISCO, CALIFORNIA,
                                    JULY (1998)




      BASE ISOLATION DESIGN FOR CIVIL COMPONENTS
                 AND 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 for
isolation 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:
• 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 with
  fundamental 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 Vulnerability 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
                                                                s
tank. A more common type of failure is known as “elephant’ 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
      s
tank’ 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 Isolation 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).
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 safety
requirements, 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 isolated
LNG 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
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 for
elastic response of the tanks up to the SSE level event. It is expected that the tanks will be
operational by 1999.




                                                                            INNER
                                            LIQUEFIED                       STEEL
         OUTER                             NATURAL GAS                      TANK
   PRESTRESSED                                 (LNG)
      CONCRETE
          TANK



       INSULATION
                                                                              CONCRETE
                                                                              UPPER MAT
        CONCRETE
                                                                              SEISMIC
        PEDESTAL
                                                                              ISOLATOR


                            CONCRETE
                            FOUNDATION

                          Figure 1: Schematic of Isolated LNG Tank
 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
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 1300
MWe 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 reactor
assembly 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
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 horizontal
displacement 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.
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 a
costly 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 design
schemes 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.
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 installation
time 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 bearing
unit 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 for
seismic 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.
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 Conference
on 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,
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Fujita, T., Progress of applications, R&D and design guidelines for seismic isolation of civil
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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
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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,
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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.

Tajirian, F. F., and Patel, M. R., Response of seismic isolated facilities, a parametric study of the
ALMR, 12th SMiRT Conference, Stuttgart Germany, August 1993.

Yaghoubian, J., Isolating building contents from earthquake induced floor motions, Earthquake
Spectra, EERI, Vol. 7, No.1 1991.

				
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