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29.2 Seismic Base Isolation

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29.2.1 Historical Development of Base Isolation

Seismic base isolation is not a very new idea. More

than a century ago, John Milne, a professor of

engineering in Japan, built a small house of wood

and placed it on ball bearings to demonstrate that

a structure could be isolated from earthquake

shaking (Housner et al., 1997). In 1891, after the

Narobi earthquake ðM ¼ 8:0Þ; Kawai, a Japanese

person, proposed a base-isolated structure with

timber logs placed in several layers in the

longitudinal and transverse direction (Izumi,

1988; see Figure 29.4). In 1906, Jacob Bechtold

of Germany applied for a U.S. patent in which he

proposed to place building on rigid plate,

supported on spherical bodies of hard material

(Buckle and Mayes, 1990). In 1909, a medical

doctor from England, Calentarients, applied

for patents for his invention comprising

isolation system for earthquake-proof building

(see Figure 29.5). He proposed separating a

building from its foundation with a layer of sand

or talc (Kelly, 1986). The Imperial Hotel in Tokyo,

constructed in 1921, was intended to float on an underlying layer of mud. The building was founded on

an 8-ft thick layer of firm soil under which exists a 60- to 70-ft thick layer of soft mud. The building was

Earthquake Performance Level

Fully

Operational

Life safe

Near collapse

Frequent

(T = 43 years)

Occasional

(T = 72 years)

Rare

(T = 475 years)

Very rare

(T = 970 years)

Unacceptable Performance

Earthquake (for New Construction)

Design

Level

FIGURE 29.3 Public demand for seismic performance of infrastructure.

FIGURE 29.4 Base isolation by timber logs. (Source:

JSSI, Introduction of Base Isolated Structures, Japan

Society of Seismic Isolation, Ohmsa, Tokyo, 1995. With

permission.)

29-4 Vibration and Shock Handbook

highly decorative with many appendages. The soft mud acted as isolation system and the building

survived the devastating 1923 Tokyo earthquake (Kelly, 1986; Buckle and Mayes, 1990).

Attempts were made in the 1930s to protect the upper floors of multistory buildings by designing

very flexible first-story columns. In a later modification of the flexible first-story columns approach, it

was proposed that the first-story columns should be designed to yield during an earthquake to produce

an energy-absorbing action. However, to produce enough damping, several inches of displacement is

required, and a yielded column has greatly reduced buckling load, proving the concept to be

impractical. It was then proposed that, if the soft first story is constructed underground, then energy

dissipaters can be installed at the top of this story (which approximates ground level) that prevent the

superstructure from moving too far, and dissipate the energy of ground motion before it enters the

building. The superstructure, from the first floor up, can be an economically braced, nonductile

concrete frame requiring no internal shear walls (Arnold, 2001). To overcome the inherent dangers of

soft supports at the base, many types of roller-bearing systems have been proposed. The rollers and the

spherical bearings are very low in damping and have no inherent resistance to lateral load, and

therefore some other mechanism that provides wind restraint and energy-absorbing capacity is

needed. A long duration between two successive earthquakes can result in the cold welding of

bearings and plates, thus causing the system to become rigid after a time. Therefore, the application of

rolling supports was restricted to the isolation of special components of low or moderate weight

(Caspe, 1984).

Parallel to the development of the soft first-story approach, the flexibility of natural rubber was also seen

as another solution for increasing the flexibility of the system. In 1968, large blocks of hard rubber, 54 in

number, were used to isolate the three-story Heinrich Pestalozzi School in Skopje, Republic of Macedonia.

The building is constructed of reinforced concrete

shear walls. This is the first building for which

rubber bearings were used as base isolation against

strong earthquakes. These rubber blocks are unreinforced

and bulge sideways under the weight of

this concrete structure (see Figure 29.6). Owing to

having the same stiffness of the isolation system in

all the directions, the building bounces and rocks

backwards and forwards (Jurukovski and Rakicevic,

1995). These types of bearings are unsuitable

for the earthquake protection of structures. The

subsequent development of laminated rubber

bearings has made base isolation a practical reality

(Figure 29.7). Later, a large number of isolation

devices were developed, and now base isolation has

reached the stage of gaining acceptance and

replacing the conventional construction, at least

for important structures.

FIGURE 29.5 Calentarients’ idea of seismic base isolation. (Source: JSSI, Introduction of Base Isolated Structures,

Japan Society of Seismic Isolation, Ohmsa, Tokyo, 1995. With permission.)

FIGURE 29.6 Unreinforced rubber blocks. (Source:

Ohashi, U.G. Earthquakes and Base Isolation, Pub.

Asakura, Tokyo, 1995. With permission.)

FIGURE 29.7 Rubber bearings with steel shims.

Seismic Base Isolation and Vibration Control 29-5

29.2.2 Basic Principle

Seismic base isolation is basically a lengthening of the fundamental time period of the structure with

the help of a specially designed system that is placed between its superstructure and substructure

(see Figure 29.8). Besides other advantages, the concept gained widespread acceptance due to the fact that

most of the earthquake motions around the world have dominating frequencies in the range of 1.0 to

10 Hz, and the majority of conventionally designed structures also has their fundamental frequency of

vibration lying in this range. Owing to this unwanted matching of the frequencies, these structures are

subjected to high forces during earthquakes. The application of seismic base isolation shifts the

fundamental time period away from the dominating frequencies of earthquake motions and thus detunes

the frequencies. In other words, base isolation consists in filtering out high-frequency waves from the

ground motion, thereby preventing the transmission of high energy in the structure. The effect of base

isolation on reduction in force is shown schematically in Figure 29.9. Under favorable conditions, seismic

base isolation can reduce drift to 0.2 to 0.5 of that which would occur if the building were fixed base

(Figure 29.10). Reduction in acceleration has more influence on the force – deflection characteristics of

the isolation system and may not be as significant as the reduction of drift (FEMA 356, 2000). However,

the additional flexibility required for this period shift give rise to excessive relative displacement at the

isolation level. Additional damping is introduced in the isolation system to limit this displacement

response to within feasible limits. Still, it is necessary to provide an adequate seismic gap that can

accommodate displacements at the isolation level. Most of the isolators have inherent damping, although

sometimes supplemental energy dissipation devices may also be required at the isolation level. Various

types of energy dissipation devices like metallic dampers and hydraulic dampers have been developed and

can be used for this purpose (Skinner et al., 1993). The isolation damping also suppresses the resonance

FIGURE 29.8 Application of seismic isolation for different structures.

Natural period (seconds)

4.0

0.0 1.0 2.0 3.0 4.0 5.0

1.0

2.0

3.0

0.0

Conventionally

designed structure

Seismically

isolated structure

Normalized acceleration

response

V =15%

V =2%

FIGURE 29.9 Conceptual diagram for seismic isolation.

29-6 Vibration and Shock Handbook

resulting due to higher period contents of the

earthquake motion. Although damping is useful in

reducing the required seismic gap, excess damping

may result in an increase in acceleration that may

affect the performance of nonstructural elements

and contents. Thus, an isolation system should

essentially be able to (1) support a structure,

(2) provide horizontal flexibility, and (3) dissipate

energy. These three functions can be incorporated

in a single device or can be provided by means of

different components. In addition, it may be

necessary to provide buffers, which can limit the

isolator displacements during extreme

earthquakes.

29.2.3 Issues in Seismic Base Isolation

A number of issues for seismic isolation design have been identified based on experiences of their

behavior. Some of the issues that should be considered before choosing the base-isolation approach for a

project are touched in the following sections.

29.2.3.1 Performance Criteria

The performance criteria for the structure needs to be established in order to evaluate alternative seismic

resisting systems; for example, it must be established whether the structure is required to be functional

during and after major earthquakes, or if it is to be preserved for its historical importance. Whether

seismic base isolation is a suitable design strategy for a particular project will depend primarily on the

performance required. To achieve the fully operational or operational performance level, one can consider

seismic base isolation as a possible design strategy, but if life safe is the required structural performance

level, it may not be practical to choose seismic base isolation.

29.2.3.2 Type of Structure

Significant benefits obtained from isolation exist in structures for which the fundamental period of

vibration without base isolation is short, that is, less than 1 sec. Certain structures may not be suitable for

base isolation because of their shape; for example, this is true for slender high-rise buildings that have a

natural period long enough to attract low earthquake forces without isolation. Therefore, seismic

isolation is mostly used for low-rise buildings. Historical buildings, which generally are stiff masonry

structures, can be appropriate structures for seismic base isolation. Bridges are the structures for which

application of seismic isolation is very convenient. The provision of bearings at the tops of piers adds

flexibility to stiffer piers and in turn avoids yielding of piers. It is easy to examine these bearings after a

seismic event and replace them if needed.

29.2.3.3 Site Characteristics

In base-isolation design, the basic objective is to filter out the high frequencies of the ground

motion by lengthening the time period of vibration to approximately 2 sec. Thus, conventional

base isolation is not suitable for structures on soft soils where the ground motions are

dominated by low frequencies. Therefore, a detailed investigation of the site must be carried out

before possible isolation can be considered. Another important aspect is near-fault ground motions.

Waves from such motions usually have long-period velocity pulses, which impart lot of momentum

to the structure. This is particularly damaging to base-isolated structures because it may cause

large horizontal base displacements. The displacement can result in instability of the structure, or it

FIGURE 29.10 Behavior of (a) fixed-base and (b) baseisolated

building.

Seismic Base Isolation and Vibration Control 29-7

can result in impact with moat walls, which can affect the sensitive equipment housed in the

building. An isolator with bilinear force – displacement behavior and a large ratio of yield-force to

supported weight can substantially reduce the displacement. The provision of high damping in an

isolation system can also work. However, the degree of isolation during relatively frequent

earthquakes without near-fault pulses is much reduced due to these provisions (Skinner and

McVerry, 1996).

29.2.3.4 Retrofit Issues

In selecting a suitable retrofit system and properties of seismic isolation system, consideration should

be given to the characteristics of the existing building, such as foundation capacity and strength of the

superstructure. For retrofit of buildings, successful implementation of a seismic isolation system

requires that the sequences of temporary bracing, shoring, cutting of existing columns and walls,

and installation of isolators be well planned. Base isolation for the retrofit of bridges is simpler as they

usually have thermal bearings, which can easily be replaced by seismic isolation bearings. The

retrofitting of monuments or buildings of historical importance requires special efforts to cope with

the need of minimum alterations. Provisions must be made to protect them from any seismic event

during the retrofitting. Also it must be identified whether workable spaces and access to the work area

is available.

29.2.3.5 Design of Building Services

Depending on the base-isolation system, base-isolated structures under earthquake motions can exhibit

significant base slab displacements due to the low horizontal stiffness of the isolation elements. This may

create problems on the supply lines transitioning between the parts of structure below and above

isolation level. Therefore, special attention is to be given to installations of building services such as water

supply, sewerage, gas, air-conditioning, and so on in order to prevent any damage to these supply lines,

which might cause secondary effects. In the case of isolation of two or more structural units founded on a

common foundation and connected by expansion joints, special care is needed regarding the proportions

of the expansion joint in order to prevent the pounding of buildings during earthquakes.

29.2.3.6 Expected Life of Isolator

The isolation system should remain operational for the expected lifetime of the isolated structure. It

should not require frequent maintenance during this period. Although the functioning of an isolator may

be required few times during the lifetime of structure, it must perform well at such times. If life of the

isolator is less than the life of structure, then it may be necessary to provide a mechanism for the

inspection and replacement of the isolation system. Another related aspect is the protection of isolation

elements against fire, and measures should to be taken for this. Furthermore, as an isolation system is

provided mostly at the base, its resistance to chemical and biological reactions is also important

(Jurukovski and Rakicevic, 1995).

29.2.4 Seismic Isolation Devices

The successful seismic isolation of a particular structure is strongly dependent on the appropriate choice

of the isolation system. In addition to providing adequate horizontal flexibility and appropriate

damping, the isolation system should essentially have the capability of self-centering after deformation,

high vertical stiffness to avoid rocking, and enough initial stiffness to avoid frequent vibration from wind

and minor seismic events. Different types of isolators have been developed and proposed to achieve these

properties, and some of them are discussed below.

29-8 Vibration and Shock Handbook

29.2.4.1 Laminated Rubber Bearings

Rubber bearings offer the simplest method of

seismic base isolation and are relatively easy to

manufacture. The bearings are made by vulcanization

bonding of sheets of rubber to thin steel

reinforcing plates. Initially, the main function of

the laminated rubber bearings was to provide

flexibility for thermal displacements in bridges.

Later, similar bearings found application in the

isolation of buildings from vibration due to

underground railways, and these bearings have

performed well over a substantial period of time

(Kelly, 1990). The bearings are very stiff in the

vertical direction and flexible in the horizontal

direction. High vertical stiffness of these bearings is

achieved through the laminated construction of

the bearing using steel plates. The cross section of a

typical rubber bearing is shown in Figure 29.11.

The most common elastomers used in elastomeric

bearings are natural rubber, neoprene rubber,

butyl rubber, and nitrile rubber. The mechanical

(tear strength, high strain fatigue resistance, creep

resistance) and low-temperature properties of

natural rubber are superior to those of most

synthetic elastomers used for seismic isolation

bearings. Therefore, natural rubber is the most

frequently recommended material for use in

elastomeric bearings, followed by neoprene. Butyl

rubbers are suitable for low-temperature applications

and nitrile rubber has limited application

in offshore oil structures (Taylor et al., 1992).

The damping ratio (i.e., the fraction of critical

damping) achieved from natural rubber is low, in

the order of 0.02 to 0.04, and therefore it is unusual

to use it without some other element that is able to provide increased damping. In order to achieve better

performance in a single unit, rubber used in the bearing is a compound formed with some filler agents.

This compounding results in desired properties, such as (1) high damping and (2) high horizontal

stiffness at low values of shear strain. The damping ratio (i.e., the fraction of critical damping) achieved is

in the order of 0.10 to 0.20. These high-damping rubber bearings, originally developed in England, found

several applications in Japan and United States. A number of fillers are employed, such as metal oxides,

clay, and cellulose, but the filler that is most commonly used in seismic isolation bearings is carbon black

(Taylor et al., 1992). Force – displacement behavior of these bearings depends upon the type of

compounding. Figure 29.12 shows the results of cyclic loading test conducted on a four-layer highdamping

rubber bearing specimen (Tanzo et al., 1992). In the experiment, the tests were carried out up to

200% (96 mm) shear strain. Vertical load for the tests was kept as 40 tonf (64 kgf/cm2).

The application of steel shims in laminated rubber bearings provides necessary vertical stiffness, but at

the same time makes these isolators heavy and expensive. Recently, Kelly (2001) proposed a

seismic isolation system for developing countries, in which steel plates are replaced by fiber mesh.

The fiber-reinforced isolator is expected to be significantly lighter and could lead to a much less laborintensive

manufacturing process.

FIGURE 29.11 Laminated rubber bearing.

−100 0 100

−20

Displacement (mm)

Load (ton)

FIGURE 29.12 Load – displacement loops for highdamping

rubber bearing. (Source: Tanzo, W. et al. Res.

Rpt. 92-ST-01, Kyoto Univ., 1992. With permission.)

Seismic Base Isolation and Vibration Control 29-9

29.2.4.2 Lead Rubber Bearing

This isolation system consists of a cylinder of lead

enclosed in a rubber bearing. The system is also

known as the NZ bearing system, and its

components are shown in Figure 29.13. The lead

plug produces a substantial increase in damping,

from about 3% of critical damping in the natural

rubber to 10 to 15%, and also increases the

resistance to frequently occurring loads such as

minor earthquakes or wind. The reason for

choosing lead is that it is a crystalline material,

and under normal conditions it changes its crystal

structure under deformation but almost instantly

regains its original crystal structure when the

deformation ceases. Thus, repeated yielding due to

cyclic loading does not cause fatigue. Lead yields in

shear at the relatively low stress of about 8 to 10 N/

mm2, and therefore produces stable hysteretic

behavior and dissipates significant energy in

strong ground motion. Load – displacement behavior

of a lead rubber bearing is shown in Figure

29.14 (JSSI, 1995). The hysteretic behavior of lead

rubber bearings can be treated as bilinear, with

initial stiffness in the range of 9 to 16 times the

postyield stiffness. These bearings provide an

economical and effective solution, incorporating

period shifting, increased damping, and high

stiffness at low strains, and providing vertical

support in a single device (Skinner et al., 1993).

It has found several applications in new constructions

as well as for retrofitting of buildings and

bridges in different parts of the world.

29.2.4.3 Friction-Based Systems

In this class of isolators, the superstructure is

allowed to slide during major seismic events.

The structure slides whenever the lateral force

exceeds the friction force at the sliding interface. The horizontal friction force at the sliding surface

offers resistance to motion and dissipates energy. Pure sliding systems have no inherent natural

period and therefore are insensitive to variations in the frequency content of ground excitation. The

acceleration at the base of the structure is limited to the coefficient of friction at the sliding

interface. Thus, by keeping this coefficient of friction low, the acceleration felt by the structure can

be reduced. Systems based on a Teflon (PTFE) and stainless steel interface have the potential to

provide high levels of protection against floor acceleration because of the low friction of the

materials. However, the friction coefficient cannot be reduced arbitrarily, as the sliding displacement

may exceed the acceptable value. Another feature of this isolation system is that the frictional force

is proportional to the vertical load coming on the bearing, and therefore the center of mass and the

center of resistance of the sliding support coincide. As a result, the torsional effects produced by the

asymmetry of building are diminished.

FIGURE 29.13 Lead rubber bearing. (Source: Skinner,

R.I. et al. An Introduction to Seismic Isolation, Wiley,

Chichester, UK, 1993. With permission.)

−300 0 300

−15

Shear strain (%)

Stress (kg/cm2)

FIGURE 29.14 Behavior of lead rubber bearing under

cyclic loading. (Source: JSSI, Introduction of Base Isolated

Structures, Japan Society of Seismic Isolation, Ohmsa,

Tokyo, 1995. With permission.)

29-10 Vibration and Shock Handbook

The main drawback of the system is the

absence of restoring force due to which a large

residual displacement from the original position

of the structure may be left after a major

earthquake event. There are other practical

problems that can affect the efficiency of sliding

isolation, such as cold welding, freezing, and

deterioration of the sliding surfaces. Changes

may occur in the friction coefficient due to

ageing, environmental attack, temperature variation,

or wear during use (Skinner et al., 1993).

As the sliding depends upon the coefficient of

friction, the system may require regular inspection to maintain its coefficient of friction. The

unsatisfactory predictability of friction coefficient and the absence of any centering force suggest that

Teflon bearings should be used as seismic isolators only in combination with some other centering

devices like steel dampers or rubber bearings (Priestley et al., 1996). Based on this concept another

system called the resilient friction bearing (R-FBI) system was developed (Mostaghel and

Khodaverdian, 1987). The system consists of several concentric layers of Teflon-coated friction plates

that are in friction contact with each other and a central core of rubber. The rubber provides the

resilient force while the friction forces dissipate the energy.

An effective mechanism to provide recentering force by gravity has been utilized in a friction

pendulum system (FPS). In this system, the sliding surface takes a concave spherical shape so that

the sliding and recentering mechanisms are integrated in one unit. As the name indicates, there are

two mechanisms that are employed to achieve isolation, namely, sliding friction and pendulum

motion. The internal components consist of a stainless steel concave surface upon which slides a

stainless steel articulated slider surfaced with a high-load capacity and a low friction bearing

material composite (see Figure 29.15). The radius of curvature determines the sliding or isolation

period of the system. The period of the structure supported on the FPS is independent of the

structure mass and therefore the period does not change if the structure weight changes or is

different than assumed. This results in better control over the response of the systems, like liquid

storage tanks in which weight varies in time because of variable liquid storage level. However, since

the restoring force is linearly proportional to the sliding displacement, in case of high-intensity

earthquakes or a low coefficient of friction, the additional sliding introduces additional energy in the

structure. To overcome this problem, Pranesh and Sinha (2002) proposed the variable frequency

pendulum isolator, in which the geometry of the concave surface is designed such that its frequency

decreases with the increase in sliding displacement and asymptotically approaches zero at very large

displacement.

29.2.4.4 Other Systems

A base-isolation system, which has found many applications in Japan, comprises of low-damping rubber

bearings with yielding metal devices. The yielding of metals provides the necessary dissipation of energy.

The commonly used dampers are shown in Figure 29.16 (Aoyagi et al., 1988; Takayama et al., 1988;

Yasaka et al., 1988).

To standardize the designs of the sensitive structures of nuclear power plants for regions of

different seismicity, an isolation system called the EDF (Electricite de France) system was developed.

This is a laminated rubber bearing with friction plate at the top. An attractive feature of the EDF

isolator is that, for the lower amplitude ground excitations, the lateral flexibility of the rubber provides

the base isolation. At a high level of excitation, however, sliding will occur assuring maximum

acceleration transmissibility of mg, where m is the coefficient of friction between materials of sliding

surface (Constantinou, 1994).

FIGURE 29.15 Schematic diagram of friction pendulum

system.

Seismic Base Isolation and Vibration Control 29-11

Taisei Corporation, Japan, developed the TASS (TAISEI Shake Suppression) system (Figure 29.17),

which is composed of PTFE-elastomeric bearings and neoprene springs (Kawamura et al., 1988).

PTFE-elastomeric bearings support the vertical load of the superstructure and reduce the horizontal

seismic forces by sliding against severe earthquake motion. Horizontal springs provide weak lateral

stiffness and restrain displacement. The TASS system never resonates to any type of excitation, stably

supports superstructure, and limits the horizontal force transmitted to the superstructure to that equal

to friction force.

In situations where circumstances may result in substantial tension forces on the bearings, it has

generally been accepted that elastomeric, sliding, or roller isolators alone are not suitable. For this, a

system that controls uplift was developed. The system consists of rubber bearings with a central hole to

accommodate a tension device (Kelly and Chalhoub, 1990). Logiadis et al. (1996) proposed a prestressed

bearing isolation system in which vertical prestressed tendons are located in pairs, symmetrical on both

sides of each bearing to avoid tension in the rubber bearing.

FIGURE 29.17 Elements of a TASS system. (Source: Kawamura, S. et al. 1998. Proc. IX World Conf. on Earthquake

Engineering, pp. 735 – 740. Tokyo, Japan. With permission.)

FIGURE 29.16 (a) Elasto-plastic steel damper; (b) lead damper; (c) steel rod damper.

29-12 Vibration and Shock Handbook

Tarics (1996) developed a composite isolator to

provide equal protection in minor, moderate, or

major earthquakes (see Figure 29.18). The

composite seismic isolator has two distinctly

different stiffnesses, which are activated by the

displacement demand. During minor or moderate

earthquakes, the upper isolator provides

flexibility. In the event of major earthquake, the

higher stiffness of the lower isolator prevents

excessive displacement. This isolator seems to be

a promising alternative for near-fault ground

motions. With similar objectives, Shimoda et al.

(1992) proposed lead rubber bearings with a

stepped lead plug. Several other systems, for

example, the sleeved pile system, GERB, alexisismon,

and so on, were developed by different researchers (Kelly, 1986; Buckle and Mayes, 1990) and the

search for new isolation systems, which can give better performance under different conditions, is still

continuing.

29.2.5 Design of Isolation Devices

As discussed above, several types of isolators have been proposed so far. Only a few bearings, namely,

laminated rubber bearings, lead rubber bearing, FPS bearings, and Teflon – steel sliding bearings with a

restoring device, have been developed to the stage of practical application. These isolators are in general

observed to be effective in reducing seismic response; however, their effectiveness depends on the

situation and design constraints. For example, rubber bearings are most suitable for high-frequency

motions, while friction-type isolators may also be applied at the site where ground motion can have lowfrequency

content. Buildings with sophisticated electronic equipment and loose contents should

preferably be isolated with rubber bearings to avoid transmission of high frequencies to the

superstructure. In the case where, for some reason, it is not possible to provide large seismic gaps

or for the conditions where light weight structure is to be isolated, the FPS-type isolators are more

suitable. The main requirements for the design of a base-isolation system are (1) the ability to

sustain gravity loads, (2) low horizontal stiffness that can lengthen the fundamental time period to the

desired value, (3) large vertical stiffness to minimize amplification in vertical direction and complications

due to rocking, (4) energy dissipation capacity to keep displacements at the isolation level

within acceptable limits, and (5) sufficient initial stiffness to avoid unwanted vibrations due to wind

loads and frequent minor seismic events. The design procedure of most widely used isolation systems,

namely, high-damping rubber bearings, lead rubber bearings, and the FPS, is discussed in the

following sections.

29.2.5.1 High-Damping Rubber Bearing

The design of high-damping laminated rubber bearings involves determining the plan size, the number

of rubber layers, the thickness of each rubber layer, and steel shim. The steps involved in the design of

high-damping rubber bearings are as follows:

1. Specify the design vertical load on the bearing ðW Þ; the design period for the isolated structure

ðTIÞ; and site conditions.

The load, W ; is computed for the dead load and live load combination. The design period, TI;

depends on site conditions. Its minimum value is usually taken as three times the fundamental

time period of the structure when it is fixed base.

FIGURE 29.18 Composite isolator and its force –

displacement diagram. (Source: Tarics, A.G. Proc. XI

World Conf. on Earthquake Engineering, 1996. With

permission.)

Seismic Base Isolation and Vibration Control 29-13

2. Find corresponding horizontal stiffness of the bearing, Kh; using

Kh ¼

g ð2p=TIÞ2 ð29:1Þ

3. Assign the values to quantities such as the maximum permissible shear strain ðgÞ; Young’s

modulus ðEÞ; and the shear modulus ðGÞ for the rubber compound.

Generally, g is considered to be in the range of 100 to 150%. G can be taken from design

specifications provided by the manufacturer. The value of G for this range of shear strains varies

from 0.69 to 0.86 MPa, depending on the rubber composition (Kelly, 1997).

4. Specify the effective equivalent damping in the isolation system and allowable pressure on the

bearing ðpÞ for vertical loads.

The allowable pressure ðpÞ for bearings with metal reinforcement can be taken as about

6.9 N/mm2 (Kelly, 1997). Damping can be taken as 15 to 20%, subject to verification.

5. Using a response spectrum or code formulae, find the maximum horizontal displacement, that

is, the design displacement ðDÞ: This depends on the spectral coefficient (i.e., the site condition),

design period, and damping.

6. The plan area of the rubber ðAÞ in bearing is primarily controlled by the design vertical load ðW Þ

and allowable pressure ðpÞ; and is determined by the expression

A ¼ W =p ð29:2Þ

7. The total thickness of rubber, tr; in the bearing is determined from the following equation:

tr ¼ GA=Kh ð29:3Þ

Total thickness, tr; should not be less than D=g:

8. Evaluate the shape factor ðSÞ for the desired value of ratio of vertical stiffness ðKv Þ and horizontal

stiffness ðKhÞ using following expressions:

6S2 ¼ Kv =Kh for circular bearings ð29:4aÞ

6:73S2 ¼ Kv =Kh for square bearings ð29:4bÞ

The minimum recommended value for the stiffness ratio is 400. These expressions are fairly

accurate for shape factors of ten or less. However, for higher values of S; the effect of

compressibility should also be taken into account (Kelly, 1997).

9. Using a basic definition of shape factor as the ratio of the cross-sectional area of the bearing to

the force-free area of a single layer of rubber, the thickness of each rubber layer can now be

determined by

t ¼ f=4S for circular bearings ð29:5aÞ

t ¼ b=4S for square bearings ð29:5bÞ

where f is the diameter for circular bearing and b is the side for square bearing. Knowing the

values of tr and t; the number of rubber layers ðnÞ can be computed.

10. The thickness of steel shims provided for laminated construction should be in the range of one

tenth to one eighth of an inch (Kelly, 1997).

The material properties for rubber compounds used for isolation devices can be related to the

rubber hardness. Properties for the range of rubber hardness normally used in bearing are listed

in Table 29.1 (Bridgestone, 1990).

The designed bearing should be checked for the following criteria also.

29.2.5.1.1 Stability against Buckling

Buckling load may become critical in situations where bearings with a high rubber thickness relative to

the plan dimension are to be provided. For example, in the case of base isolation for low vertical loads,

29-14 Vibration and Shock Handbook

rubber bearings are generally slender. Average compressive stress in the bearing ðW =AÞ should not exceed

the critical stress ðpcrÞ given by Kelly (1997):

pcr ¼

ffiffi

2 p pGS

r ð29:6Þ

where r is radius of gyration (b=2

ffiffi

3 p for a square bearing and f/4 for a circular bearing).

Maximum shear strain is described as follows. Under vertical loading only, as per AASHTO

recommendations for service loads (i.e., for the dead and live load), a factor of safety of three should be

adopted for maximum shear strain values. Thus, maximum shear strain ðgsÞ should satisfy the following

condition:

gs i:e: 6S

EcA

where 1b is the minimum tensile strain at which rubber breaks and Ec is compression modulus for

bearing and is given by Ec ¼ Eð1 þ kS2Þ: E and k, for a given value of rubber hardness, can be taken from

Table 29.1.

Under earthquake loading, the following applies. In the case of ultimate load that includes effect of

earthquake loads, AASHTO recommends a factor of safety of 1.33. Thus, maximum shear strain (gu) due

to combined effect of compression (gc), torsion (gt), and lateral load (geq) should not exceed 1b=1:33;

that is, gc þ geq þ gt # 1b=1:33: geq is given by D=tr; and gt can be evaluated by

gt ¼

2ttr

12De

ðB2 þ L2Þ ð29:7Þ

Here, B and L are dimensions of a structure with rectangular plan and e is the eccentricity.

gcð¼ 6SW 0=EcArÞ is computed for vertical load (W 0), which also includes the effect of earthquake load.

In this case, the effective area (Ar) is reduced due to lateral displacement (D) and it is the overlapping area

of top and bottom in displaced condition. Ar can be determined using the following expressions:

Ar ¼ A 1 2

􀀏 􀀐

for square bearings ð29:8aÞ

Ar ¼

f2 sin21

ffiffiffiffiffiffiffiffiffiffiffi

f2 2 D2

2 D

ffiffiffiffiffiffiffiffiffiffiffi

f2 2 D2

( 􀁻 q !)

for circular bearings ð29:8bÞ

29.2.5.1.2 Stability against Roll Out

Maximum horizontal displacement should not exceed the roll-out displacement (d) given by

d ¼

WEb

WE þ Khh ð29:9Þ

where vertical load (WE) also includes the effect of earthquake load, b is the side for square and the

diameter for the circular bearing, and h is the overall height of the bearing (Figure 29.19).

TABLE 29.1 Relationship between Rubber Hardness and Other Properties

Rubber Hardness

(IRHD ^2)

Young’s Modulus, E

(MPa)

Shear Modulus, G

(MPa)

Material Constant, k Minimum Elongation at Break

(%)

35 1.18 0.37 0.89 650

40 1.50 0.45 0.85 600

45 1.80 0.54 0.80 600

50 2.20 0.64 0.73 500

55 3.25 0.81 0.64 500

60 4.45 1.06 0.57 400

Seismic Base Isolation and Vibration Control 29-15

The check should be made for bearings with

bolted connections also, so as to avoid the

development of tensile stresses.

29.2.5.2 Lead Rubber Bearings

The design of lead rubber bearings can be split into

two parts, namely, (a) the design of the lead

plug and (b) the design of the laminated rubber

bearing.

29.2.5.2.1 Design of Lead Plug

In fact, the lead rubber bearing can be designed

effectively by assuming its force – displacement

behavior comprises two distinct stiffnesses,

namely, the preyielding and postyielding stiffnesses,

as shown in Figure 29.20. The characteristic

strength, Qd ; for the desired energy dissipation

(WD) or the known effective damping (jeff) can be

computed using

Qd ¼ WD=4ðD 2 dy Þ

¼ 2pKeff D2jeff =4ðD 2 dyÞ ð29:10Þ

Qd and yield displacement (dy) are the unknowns.

So for a first trial, considering dy is very small as

compared with design displacement (D), we have,

as a first approximation

Qd ¼ WD=4D ð29:11Þ

Using this approximate value for Qd ; the postyield

stiffness (Kp) can be evaluated by

Keff D ¼ Qd þ KpD ð29:12Þ

This may lead to modified value of yield displacement, dy ; given by

dy ¼ Qd =ðKe 2 KpÞ ð29:13Þ

where, depending upon the physical parameters of the laminated rubber bearing, Ke can be assumed to

be about ten times the value of Kp:

This value of dy can be used in the next trial to compute revised values of Qd ; Kp; and dy : Trials are to

be repeated until dy converges.

Now, the yield force can be evaluated by

Fy ¼ Kedy ð29:14Þ

Thus, the area of lead plug (Ap) can be determined using Ap ¼ Fy =f y ; in which f y is the yield strength

of the lead in shear and has a value of 10.5 MPa (Skinner et al., 1993). In design, owing

to considerations for the confinement of the plug, a reduced value of 7 to 8 MPa is generally

considered.

dy Displacement

Keff

FIGURE 29.20 Design parameters for lead rubber

bearing.

FIGURE 29.19 Stability against roll out. (Source: Kelly,

J.M. Earthquake Resistant Design with Rubber, Springer,

London, 1997. With permission.)

29-16 Vibration and Shock Handbook

29.2.5.2.2 Design of Laminated Rubber Bearing

Postyield stiffness of the bearing (Kp) is typically higher than the corresponding stiffness of the bearing

without the lead plug (Kh):

Kp ¼ f LKh

where f L is a factor that takes into account the effect of lead plug and is always larger than unity. Typically,

f L is 1.15 (FEMA 356, 2000).

Another expression for computing Kp is given by (Yang et al., 2003)

Kp ¼ Kh 1 þ 12

􀀒 􀀓

ð29:15Þ

where A0 is the area of the bearing based on allowable normal stress under the vertical load.

The final value of Kp obtained during design of the lead plug is used in these expressions to find Kh:

This contribution (Kh) towards combined stiffness (Kp) controls the design of the laminated rubber

bearing. The steps from Step 3 onwards, described earlier in the design of high-damping rubber bearings,

can be followed to design this laminated rubber bearing.

One additional check for the lead plug should be applied:

1:25 #

# 5:0

where hp is effective height of the lead plug and fp is its diameter (Yang et al., 2003).

29.2.5.3 Design of Friction Pendulum System

The basic design variables in FPS are (1) the radius of curvature (R), (2) the material friction coefficient

ðmÞ; and (3) the plan dimension. The desired isolation period governs the radius of curvature, whereas

the plan dimension is controlled by the design displacement. R for a known value of the design period of

the isolated structure (TI) can be computed using

R ¼ gðTI=2pÞ2 ð29:16Þ

where g is acceleration due to gravity.

The coefficient of friction ðmÞ; along with the displacement, controls the energy dissipation. The

effective damping (b) for this system is given by expression

b ¼

m þ D=R

􀀏 􀀐

ð29:17Þ

The desired damping, depending upon the properties of the system, may vary from 10% to 20% of

critical damping. As the design displacement (D) depends upon damping (b), it becomes a trial-anderror

process and requires few iterations to get the values of b, m, and D for desired response.

The effective stiffness of the isolation system at design displacement can be evaluated by

Keff ¼

R þ

D ð29:18Þ

29.2.6 Verification of Properties of Isolation Systems

As they are the most critical part of the base-isolated structure, the quality of isolation bearings is of

paramount importance. Looking into uncertainties in the manufacture, the deformation characteristics

and damping values of the isolation system must be verified by tests. Therefore, the selected units of the

isolation system have to undergo testing for examining quality control and determining their actual

load – deformation behavior. Force – deformation characteristics of the isolation system obtained from

tests are used for analyzing the base-isolated structures. These tests also serve as a means to verify the

Seismic Base Isolation and Vibration Control 29-17

ability of the isolation bearings to withstand long-term vertical loading and numerous cycles of shear

displacement during design basis and maximum capable earthquake. The primary function of an

isolation system is called on during major earthquakes only and, for most of its design life, it is subjected

to nonseismic loads. Therefore its behavior for such loads should also be verified by tests.

29.2.6.1 Tests for Isolation Systems

System characterization tests, prototype tests, and quality control tests are the three levels of tests that are

recommended for an isolation system. The first tests provide the fundamental characteristics of the

isolation system; the second allow one to know the design parameters of the isolation system fabricated

for the project; and the third verify the quality control and consistency achieved in manufacturing. These

tests are briefly discussed in the following sections (Taylor et al., 1995).

29.2.6.1.1 System Characterization Tests

These tests are performed to evaluate the basic characteristics of the isolation systems, especially its

dependence on temperature, frequency, bilateral loading, wear and fatigue, and so on. The tests are also

intended to provide the ultimate and reserve capacity of a device for various loading conditions. These

tests are conducted only once for a system of a given design, material, and construction. However, they

should be repeated if there are major changes. These tests are formally not required by codes; however, in

practice, they are conducted for a new isolation system and the test results provide some idea about the

suitability of the isolation system for a particular project.

29.2.6.1.2 Prototype Tests

These tests are conducted to verify the design properties of an isolation system prior to its construction.

Effective stiffness, energy dissipation capacity, stability of hysteretic behavior (in a check for system

degradation), and stability at maximum seismic displacement are the properties generally verified

through these tests. The stability of an isolation system is verified for maximum and minimum vertical

load conditions. Tests are also carried out for nonseismic loads. In the case of isolation systems for

buildings, the major nonseismic force is wind load, and for bridges it may be thermal displacements and

braking or centrifugal forces. If the isolation system is frequency-dependent, then prototype tests should

be performed dynamically for a range of frequency that represents full-scale prototype loading rates.

Similarly, if the behavior of the isolation system is significantly different for unilateral and bilateral

loadings, then the isolation system should be tested for different combinations of displacements in two

orthogonal directions. Specimens in these tests are subjected to extreme conditions and therefore are not

to be used for construction.

29.2.6.1.3 Quality Control Tests

These tests are carried out to verify the quality and consistency of the manufacturing process, and to

measure the as-built properties of the isolation system prior to installation. The quality control tests are

divided into production tests and completed unit tests. Production tests are carried out on materials or

components that are used in the making of the isolation unit. In the case of elastomeric bearings, the

elastomer is tested for hardness, tensile strength and elongation at break, bond strength, compression set,

low-temperature properties, high-temperature aging, and ozone resistance. Elements used for a sliding

system are supposed to be tested for surface roughness, trueness of surface, interface material properties,

bearing pad attachment, and sliding interface attachment. All completed units are tested for (1) sustained

compression, and (2) combined compression and shear. The purpose of the sustained compression test is

to verify (1) the quality of the bond between the elastomer and the steel for laminated elastomeric

bearings and (2) the bond between the bearing liner and the metal backing plate for sliding bearings.

A combined compression and shear test is conducted to verify that actual values of effective stiffness and

damping are in close agreement with design values. After quality control testing, laminated elastomeric

bearings are visually inspected for rubber-to-steel bond, surface cracks, laminate placement, and

permanent deformation, while sliding bearings should be inspected for bearing liner-to-metal bond,

29-18 Vibration and Shock Handbook

scoring of the stainless steel plate, leakage, and permanent deformation. In the case that the bearings do

not meet the requirements, they should be rejected.

29.2.6.2 Uniform Building Code Recommendations

Uniform Building Code (UBC, 1997) recommends

that the deformation characteristics and damping

values of the isolation system used in the design

and analysis of seismic-isolated structures is based

on the sequence of cyclic loading tests of a selected

sample of the components (Figure 29.21). First,

the specimen is tested under a cyclic load

corresponding to the design wind load. Subsequently,

it is tested, for a prescribed number of

cycles, with cyclic displacements varying from one

fifth of the design displacement to the total maximum design displacement. Vertical load (dead load plus

50% of the live load) for all the above tests is prescribed as the average load on all isolators of common

type and size. If an isolator unit is also a vertical load-carrying element, the cyclic load tests should be

performed for maximum and minimum vertical loads, which includes the effects of an earthquake

overturning evaluated corresponding to the test displacement.

29.2.6.2.1 Units Dependent on Loading Rates

If there is greater than 10% difference in effective stiffness values at the design displacement (i) obtained

by performing test on an isolator unit at a frequency equal to the inverse of the effective period TI of the

isolated structure, and (ii) obtained by performing test on the same unit at any frequency in the range of

0.1 to 2.0 times the inverse of TI, then the force – deflection properties of the isolator unit are considered

to be dependent on rate of loading. In such cases, the tests are prescribed to be performed at a frequency

equal to the inverse of TI.

29.2.6.2.2 Units Dependent on Bilateral Load

If the bilateral and unilateral force – deflection properties have greater than a ^10% difference in effective

stiffness at the design displacement, the force – deflection properties of an isolator unit are be considered

to be dependent on bilateral load. In such cases, the tests specified above shall be augmented to include

bilateral load increments of the total design displacement 0.25 and 1.0, 0.50 and 1.0, 0.75 and 1.0, and

1.0 and 1.0.

29.2.6.2.3 System Adequacy

The performance of the test specimens is assessed as adequate if the following conditions are

satisfied:

1. The load – displacement curves for all tests have a positive incremental load-carrying capacity.

2. The difference between the average values of effective stiffness of the two test specimens of a

common type and size does not exceed the prescribed value of 10%.

3. The effective stiffness of each test specimen for each cycle of test is within ^10% of the average

value of effective stiffness for the specimen.

4. There is no greater than a 20% change in the initial effective stiffness of each test specimen over the

prescribed number of cycles.

5. There is no greater than a 20% decrease in the initial effective damping of each test specimen over

the prescribed number of cycles.

6. All specimens of vertical load-carrying elements remain stable at the total maximum displacement

for the prescribed vertical load.

FIGURE 29.21 Schematic diagram for cyclic loading

test of bearing.

Seismic Base Isolation and Vibration Control 29-19

29.2.7 Analysis of Base-Isolated Structures

Depending upon the code requirements, one can choose the static or dynamic analysis procedure for a

particular project. Regarding the static analysis method, codes generally have some bindings, whereas

time-history analysis can be used for all types of situations. In UBC (1997), minimum values of some

design parameters to be used in dynamic analysis are defined in terms of percentage of values obtained by

static analysis and thus most of the steps of static analysis are necessary for a designer, even if the dynamic

analysis procedure is followed. These analysis methods require the modeling of the base-isolation system

and the superstructure. In most of the cases, the linear model for the superstructure of a base-isolated

building is enough. However, the isolation systems generally require nonlinear model.

29.2.7.1 Modeling of a Superstructure

The superstructure of a base-isolated building can

be modeled as rigid blocks, a two-dimensional and

a three-dimensional linear model, or nonlinear

models. The rigid block assumption is the simplest

one, which is used in the design of isolation

system, for preliminary analysis and feasibility

studies. Moreover, it can also be used as a tool to

crosscheck the results obtained from other rigorous

analysis methods. Assuming the superstructure

as a single-degree-of-freedom (single-DoF)

model and the isolation system as linear spring

plus linear viscous damper, Kelly (1990) developed

linear theory for a base-isolated structure (Figure

29.22). In general, an isolation system aims at

providing significant protection not only to the

building but also to nonstructural components and contents. Therefore, a three-dimensional linear

model for the superstructure is generally sufficient for analyzing a base-isolated structure. However, for

the situations when the response to extreme earthquakes must be investigated, nonlinear models for

superstructure should be used.

In case of the analysis of seismically isolated bridges, it may be important to take into account the pier

masses and their own modes of vibration. The pier can be modeled using a linear beam element with

mass, and the deck can be modeled with linear beams with mass. However, for very squat piers, the shear

flexibility should also be considered. In the case of a regular structure, the coupling effect of the deck can

be neglected, and a bend can be considered independent of the others. However, if a stiff deck is

supported on piers of different flexibility, the structure should be modeled as a linear multi-DoF system.

To ascertain the response for extreme loading conditions, the piers should be modeled with nonlinear

models; however, the deck can still be simulated with linear elements. In such cases, proper account

should be made for change in the effective stiffness of piers due to cracks (Priestley et al., 1996). In the

case of equipment, the modeling depends upon the level of protection; however, the rigid block

assumption for equipment generally suffices. Liquid storage containers should be modeled with proper

consideration to the sloshing affect of liquid during base motion.

29.2.7.2 Modeling of Isolation System

The isolation system can be modeled as a linear or nonlinear element. The linear model of an isolation

system consists of effective stiffness and effective damping at the design displacement. These values are

derived from force – displacement test data for the isolation system. However, except in preliminary

designs, the isolation system is generally modeled as nonlinear. Commonly used isolation systems,

namely, high-damping rubber bearings, lead rubber bearings, friction bearings, and FPSs, can be

modeled by bilinear model (Figure 29.23). However, for more precise results, the isolation elements

should be modeled by their actual force – displacement behavior, for example, in the modeling of

FIGURE 29.22 A two-DoF model of base-isolated

building. (Source: Kelly, J.M. Earthquake Spectra, 6(2),

223 – 244, 1990. With permission.)

29-20 Vibration and Shock Handbook

high-damping rubber bearings, which in extreme

conditions follow a trilinear model with stiffening

at larger strains. The model used for analysis

should be verified with force – displacement

characteristics obtained by prescribed tests, discussed

in Section 29.2.6. Moreover, the isolation

system should be modeled with considerations for

the following requirements (UBC, 1997):

1. Account for the spatial distribution of

isolator units.

2. Calculate translation, in both horizontal

directions, and the torsion of the structure above the isolation interface, considering the most

disadvantageous location of mass eccentricity.

3. Assess overturning or uplift forces on individual isolator units.

4. Account for the effects of vertical load, bilateral load, and/or the rate of loading if the force –

deflection properties of the isolation system are dependent on one or more of these attributes.

29.2.7.3 Static Analysis Procedure

This procedure may be sufficient for analyzing a base-isolated structure that satisfies certain code

requirements related to the height of a building, soil conditions, geometry of a structure, effective period

of base-isolated building, force – deflection, and restoring-force characteristics of the isolation system,

and so on. The procedure involves the use of simple expressions for computing design displacements (D)

and lateral forces. Using the response spectrum or code formula, the design displacement is computed for

a target period of the isolation system, its effective damping, and site conditions. Since time period and

effective damping itself depends on design displacement, it becomes an iterative procedure. Appropriate

values of damping and the target time period can be assumed for the first iteration. Force – displacement

data for the isolation system, obtained from tests, is used to determine effective stiffness ðkeff Þ and

damping at design displacement.

The design lateral seismic force for the isolation system and structural elements at or below the

isolation system is simply evaluated as

Vb ¼ keff D ð29:19Þ

Total lateral force for the structure above the isolation system is given by

Vs ¼

keff D

RI ð29:20Þ

The factor RI is based on the type of lateral force resisting system used for the structure above the

isolation level.

The value of Vs is not to be taken as less than the following (UBC, 1997):

1. The lateral seismic force required for a fixed-base structure of the same weight, W, and a period

equal to the isolated period

2. The base shear corresponding to the design wind load

3. One and a half times the lateral seismic force required to fully activate the isolation system

The total lateral force shall be distributed over the height of the structure in accordance with the formula

Fx ¼

Wx hx

i¼1

WIhI

Vs ð29:21Þ

where Wx is weight at level x and hx is the distance of level x from the base. Fx ; the lateral force at level x, is

applied over the area of the building at that level in accordance with the mass distribution. The maximum

F F

(a) (b)

msW msW

FIGURE 29.23 Analytical models for isolators: (a) pure

friction system; (b) friction pendulum system.

Seismic Base Isolation and Vibration Control 29-21

displacement of the isolation system is calculated in the most critical direction and for maximum capable

earthquake. The total design and maximum displacement of the elements of the isolation system should

include the additional displacement due to actual and accidental torsion, calculated by taking into account

the spatial distribution of the lateral stiffness of the isolation system and the most disadvantageous

location of mass eccentricity.

29.2.7.4 Dynamic Analysis Procedure

The method involves response spectrum or time-history analysis, and is essential when, owing to code

requirements, it is not possible to use the static analysis procedure alone. However, for the situations

when (1) the structure above isolation system may reach the nonlinear range for the design earthquake

motion, (2) the soil is soft or site conditions require site specific evaluation, or (3) the isolation system

does not meet certain prescribed code criteria, even the use of the response spectrum method is not

sufficient and the time-history analysis procedure becomes essential.

29.2.7.4.1 Response Spectrum Method

This method of analysis is very convenient for the laminated rubber bearing-type isolation system. A

design spectrum is constructed for the design basis and the maximum capable earthquake. Total design

displacement of the isolation system and the lateral forces and displacements of the isolated structure are

computed for the design basis earthquake. Maximum displacement of the isolation system should be

obtained for the maximum capable earthquake. In this method of analysis, damping for fundamental

mode in the direction of interest is generally restricted to the effective damping of the isolation system or

30% of the critical damping, whichever is less. Damping for higher modes is consistent with those

appropriate for the response spectrum analysis of similar fixed-base structure. To take into account the

effect of bidirectional excitation, the total maximum displacement includes simultaneous excitation of

the model by 100% of the most critical direction of the ground, and not less than 30% of the ground in

the orthogonal direction (UBC, 1997). The maximum displacement of the isolation system is calculated

as the vector sum of the two orthogonal displacements. The total lateral force shall be distributed over the

height of the structure in accordance with Equation 29.21.

29.2.7.4.2 Time-History Analysis

This method of analysis can be used for base-isolated structures under all types of situation. The method

is necessary for conditions where (1) isolation systems have a sliding system, (2) the structure is founded

on very soft soil, (3) the isolation system allows low displacement, or (4) the force – displacement

behavior of the isolation system depends on (a) the rate of loading, (b) the vertical load, or (c) the

bilateral load. Appropriate horizontal ground motion time histories are selected, and their magnitudes,

fault distances, and source mechanism should be consistent with those that control the design basis (or

the maximum capable) earthquake. Where appropriate recorded ground motion time-history pairs are

not available, appropriate simulated ground motion time-history pairs may be used. Each pair of time

histories is applied simultaneously to the model, considering the most disadvantageous location of mass

eccentricity. The maximum displacement of the isolation system is calculated from the vectorial sum of

the two orthogonal displacements at each time step.

29.2.7.5 Software for Analysis

The need for an isolation system that is stiff under low levels of frequently occurring loads but flexible

under higher levels necessarily leads to a nonlinear system. The force – displacement behavior of most of

the isolators can be modeled as bilinear. However, in the case of high-damping rubber bearings, in

extreme conditions, a trilinear model with stiffening at large shear strains better represents the behavior.

These nonlinear force – displacement relationships of the bearings need the use of specialized computer

programs to analyze base-isolated structures. The program should be capable of analyzing base-isolated

29-22 Vibration and Shock Handbook

buildings with isolator models made of combination of discrete nonlinear elements and superstructure

models that are fully elastic or that permit some localized nonlinear behavior.

Several programs are available for this purpose. The computer code NPAD has plasticity-based

nonlinear elements that can be used to model certain types of elastomeric bearings. The program simulates

the superstructure using a three-DoF per floor model and can analyze base-isolated structures with linear

superstructure (Tospelas et al., 1994). The justification behind the approach of considering superstructure

as linear is that seismic base isolation is generally provided to reduce the force transmitted to the

superstructure to the point where it can be assumed to remain elastic. The computer program SADSAP has

the capability to analyze three-dimensional structures with localized nonlinearities. The nonlinear

elements available in the software can be combined to simulate the behavior of various types of isolation

systems. The program may also be useful in analyzing base-isolated buildings with nonlinear bracing

elements provided for supplemental damping (Kelly, 1997). Some general-purpose programs such as the

DRAIN series and ANSR have the capability of nonlinear analysis of two- and three-dimensional

structures and have provisions that can be used to simulate seismic isolators, which exhibit bilinear

behavior (Tospelas et al., 1994). Some commonly used finite element analysis software such as ABAQUS,

ANSYS, and COSMOS also have the capability for analyzing complex superstructures on nonlinear

isolators (Kelly, 1997). However, sliding bearings cannot be modeled precisely with these programs.

SAP2000 (2002) can be used to analyze these structures and also has the facility to model friction

bearings. The 3D-BASIS group of programs is a group of special purpose programs developed for

nonlinear dynamic analysis of three-dimensional base-isolated structures, and has been used for the

analysis of many base-isolated structures (Constantinou, 1994). The programs assume that the

superstructure is elastic and the isolation system is nonlinear. Models used for modeling the isolation

elements can also capture precisely frictional behavior. It is not possible to account for stiffening of

laminated rubber bearings at higher strain levels using the programs discussed above. The program LPM

(lumped parameter model), originally developed for nonlinear three-dimensional analysis of masonry

structures, has the capability to model this stiffening and at the same time can also incorporate a

nonlinear element in the superstructure (Kelly, 1997).

29.2.8 Experimental Methods for Isolated Structures

Although analytical simulation has progressed tremendously through the use of powerful computers

utilizing accumulated theoretical and experimental data on structures, it is not yet possible to analyze

and predict the inelastic response of base-isolated structures with total confidence. The responses of

base-isolated structures are very much dependent on the characteristics of the input earthquake

motion, isolation system, and the supported structures. Experimental tests have been used to

verify and calibrate mathematical models, as well as to provide data for the design of structures.

However, extensive tests are needed to study the complex behavior of base-isolated structures under

realistic conditions that are likely to occur during earthquakes. Different test methods such as

quasistatic testing, shaking table testing, online hybrid testing, and substructured online testing have

been proposed. A typical problem that appears when performing quasistatic tests on an isolated

structure is the alteration of the restoring forces due to strain-rate effect existing for this material. This

problem does not exist for shaking table tests performed at the real-size and real-time scale. Shaking

table tests of base-isolated specimens have been used successfully for investigating the behavior.

However, it requires a large-capacity table for testing a full-scale specimen. The involvement of high

cost for large-size shaking tables and the limited capacity of available shaking tables necessitate the

structure to be drastically scaled down. This scaling of a structure presents some limitations for the

extrapolation of the results to the real scale.

As a promising option, the online hybrid test method seems to be well suited for obtaining the

seismic response of isolated structures thanks to available quasistatic loading equipment and highspeed

computers. However, this technique involves the fabrication of a complete structural prototype

Seismic Base Isolation and Vibration Control 29-23

for proper modeling. Thus, test models must be simplified, for example, into a single-DoF model,

and/or specimens have to be reduced in scale.

By incorporating substructure concepts into the online hybrid test technique, a substructured online

hybrid test method can be used, in which a complete system is considered to be divided into analytical

substructures and experimental substructures (Tanzo et al., 1992). In the case of seismically isolated

structures, inelastic deformations are designed to occur only in isolation system. The isolation system,

which is generally difficult to model mathematically, is taken as experimental substructures (Figure

29.24). The remaining part of the structure is taken as analytical substructures in which presently

available analytical models are used to describe their restoring-force characteristics. For the experimental

substructure, restoring-force information is directly measured from a specimen loaded according to its

current deformation state. The substructured online hybrid test procedure for the earthquake response of

base-isolated structure is explained in Figure 29.25.

29.2.9 Implementation of Seismic Isolation

29.2.9.1 New Construction

29.2.9.1.1 Buildings

In the early stages of the development of seismic isolation, the main target of the earthquake-resistant

design of the structure was the prevention of collapse. However, later, other additional considerations,

such as the value of structure or its content, have exerted their importance on the seismic design of

structures. For example, facilities of postearthquake importance such as fire stations, police stations,

hospitals, communication centers, and so on, have a critical role and are required to remain operational

immediately after a major seismic event. Similar reasoning applies to museums housing unique artifacts

or buildings that are architecturally important. The required low level of structural and nonstructural

damage may be achieved by using an isolation system, which limits structural deformations and ductility

demands to low values. The isolation system can be provided at different levels. However, while deciding

the isolation level, considerations should be given for (1) the seismic gap, (2) the continuity of supply

lines, stairways, and elevators, and (3) details of cladding below the isolation level. In addition, provisions

for (1) access to bearings for inspection and replacement, (2) backup system for vertical loads, and

(3) full diaphragm for the uniform distribution of lateral load to individual bearings also need to be

considered. Figure 29.26 shows typical positions for bearings in buildings (Mayes and Naeim, 2001).

The first building constructed using modern isolation bearings was a government facility, the William

Clayton Building in Wellington, New Zealand, which was completed in 1981. After that, a number of

seismically isolated buildings have been constructed there, and in most of them the isolation system used

is lead rubber bearings. The most widespread use of seismic isolation systems is in Japan. The first

seismically isolated structure to be completed in Japan was the Yachiyodai Residential Dwelling,

a two-story building completed in 1982. Better performance of seismically isolated buildings during the

FIGURE 29.24 Substructured hybrid testing for seismically isolated structures. (Source: Tanzo, W. et al. Res. Rpt.

92-ST-01, Kyoto Univ., 1992. With permission.)

29-24 Vibration and Shock Handbook

FIGURE 29.25 Procedure for substructured online test method for a base-isolated structure. (Source: Tanzo, W.

et al. Res. Rpt. 92-ST-01, Kyoto Univ., 1992. With permission.)

Seismic Base Isolation and Vibration Control 29-25

Kobe earthquake of 1995 triggered the rapid increase in number of seismic isolation projects. Before the

Kobe earthquake, the number of such buildings in Japan was 82; this suddenly increased to about 650

within 4 years of the earthquake. Besides rubber bearings, there have been some applications of frictional

sliding systems also for seismic isolation of buildings (Nishitani, 2000). The first newly constructed baseisolated

building in the United States was the Foothill Communities Law and Justice Center in California,

which was completed in 1986. Until 1990 there were four isolated buildings. After the good performance

of base-isolated buildings during the Northridge earthquake, the number increased and, at the end of

1998, there were approximately 40 isolated buildings completed or in construction in the United States

(Clark et al., 2000). Some buildings in other countries, including Italy, France, Canada, Mexico, China,

England, Russia, Armenia, Indonesia, Iran, Chile, and India, now use these systems. Until now, the

emphasis on base-isolation applications has mostly been for structures with sensitive and expensive

contents or high-risk structures like nuclear power plants and computer centers. However, there is an

increasing interest in applying this technology to public housing and schools in developing countries

(Fuller and Muhr, 1995). Most of the base-isolation projects have made use of laminated rubber bearings

and lead rubber bearings. In Japan, laminated rubber bearings are often used with energy-dissipating

devices, both with and without provision for recentering.

29.2.9.1.2 Bridges

In most cases, bridges are strategic structures and require a higher degree of protection to ensure their

functionality after a seismic event. For many simple bridges, it has been found that seismic isolation of

the superstructure gives improved seismic resistance, often at a reduced cost, while it is also effective for

thermal expansion of the superstructure (Skinner et al., 1993). The aim of seismically isolating bridge

superstructures is usually to protect the piers and their foundations, and sometimes to protect the

abutments also. With this approach, bearings are placed between the superstructure and the top of the

substructure (the piers and abutments). Under normal conditions, they behave like regular thermal

bearings. However, in the event of a strong earthquake, they add flexibility to the structure by elongating

its period and dissipating energy. In addition, most of the mass of a bridge is concentrated at the deck

level, which is inherently strong and can be assumed to be rigid. This permits the superstructure

to oscillate at a lower frequency than its piers, which results in the reduction or elimination of

deformation of the substructure components beyond their elastic range, particularly at locations that are

difficult to inspect or repair (e.g., the piles). Superstructure isolation systems are designed, as far as is

practical, to provide moderate flexibility, high damping, torsional balance, and an appropriate

distribution of seismic loads between the superstructure supports.

Isolated bridges in the United States are generally designed for the effects of a design-basis earthquake;

however, the components of the isolation system are provided with increased displacement capacity and are

tested for the effects of maximum capable earthquake. This design philosophy is based on observations

of bridge failures in past earthquakes, which were primarily due to excessive bearing displacement and

the loss of bearing support rather than the collapse of substructure. Providing for strong restoring force in

this design philosophy prevents the accumulation of significant permanent displacements and allows for

a relatively reliable estimation of peak bearing displacement and substructure force.

FIGURE 29.26 Typical positions of isolation level for buildings. (Source: Mayes, R.L. and Naeim, F. 2001. The

Seismic Design Handbook, 2nd ed., Kluwer, Boston. With permission.)

29-26 Vibration and Shock Handbook

Isolation systems adopted by Italian engineers restrict the transmission of force to elements of the

substructure to a predetermined level, which is independent of seismic action. Thus, the Italian engineers

chose to limit the force transmitted to the substructure to a desired level, at the expense, however, of large

variation in peak bearing displacements and the development of permanent displacements. In Japan,

menshin (reduction of response) design is followed for the design of bridges. Although menshin design is

closely related to the seismic isolation design, the natural period of a bridge is not forcibly elongated in

this method, because there are various restrictions in increasing the natural period. Instead of elongating

the period, the emphasis in the menshin design is on increasing the energy-dissipating capability and

distribution of lateral forces to as many substructures as possible in order to decrease the lateral forces for

the design of substructures (Iemura, 1994a, 1994b). It is common to restrain the transverse displacement

of bridge isolators, and therefore many bridges in Japan are isolated in longitudinal direction only.

The application of seismic isolation for bridges started in the early 1970s. Bridges isolated in

New Zealand before 1978 used metallic dampers for seismic isolation. Later, lead rubber bearings

replaced the metallic dampers for their seismic isolation. Italian engineers also started the application of

modern technology of seismic isolation for bridges in the mid-1970s and now have a long list of isolated

bridges. The first seismically isolated bridge in Japan was the Hokuso line viaduct in Chiba Prefecture.

The viaduct was completed in 1990 and used lead rubber bearings for its isolation system. The United

States also obtained its first seismically isolated new bridge in 1990, when two bridges, namely, Sexton

Creek Bridge in Illinois and Toll Plaza Road Bridge in Pennsylvania, were completed. Lead rubber

bearings were used as the isolation system for both the bridges (Skinner et al., 1993).

29.2.9.2 Seismic Retrofit

Postearthquake analysis of recent earthquakes around the world shows that the damage was especially

concentrated in the buildings and bridge structures that were not designed for earthquake loads or were

damage was due to nonstructural elements and the contents housed in the buildings (Kelly, 1997). In

order to prevent the recurrence of damage in such seismically deficient structures, it is vital to understand

their vulnerability and retrofit them for expected future seismic events. In the case of important

structures, retrofit techniques are required to provide a solution that can also lead to continued

functioning during and after major earthquakes, protection of nonstructural components and contents,

and, sometimes, to preserve their historic character.

29.2.9.2.1 Buildings

Conventional retrofitting has been the most common way of enhancing the performance of existing,

seismically deficient structures. However, to maintain the usability of important structures after a big

earthquake and to protect the valuable contents housed in the buildings, conventional methods can help

a little. Simple strengthening or stiffening of a structure through reducing inelastic response displacement,

however, may increase the response acceleration and the forces. Some of the other demerits of this

approach are (i) the large amount of labor required to increase the strength and ductility of critical

sections distributed throughout the complex structural system and (ii) damage may occur due to

inelastic behavior of structural elements during a major seismic event (Iemura and Adachi, 2003). Besides

this, conventional methods of retrofitting often require restricting the use of the building during the

retrofit. Some buildings, housing important activities, cannot afford to have their routine usage stopped

and therefore require retrofitting through an earthquake-resistance system that enables continued

building use even during the retrofit operation.

Seismic isolation has proved to be very effective not only in enhancing the safety of existing seismically

deficient buildings, but also to preserve the function and protect the contents. Unlike the conventional

approach, this retrofit method aims to reduce the earthquake force to a level lower than the strength of

the existing structure (Figure 29.27). Seismic isolation is the method that can mitigate both interstory

drift and high-floor accelerations simultaneously, and it has been used in retrofitting of important

buildings, buildings housing valuable contents, and structures of postearthquake importance.

Seismic Base Isolation and Vibration Control 29-27

Moreover, as the modification or demolition of building features is minimized, the method has found

application in many monuments of historical importance, protecting them from future earthquakes

(Buckle, 1995). Recently, the old masonry Chapel of Rikkyo (St Paul’s) University, Tokyo, constructed in

1920, was retrofitted with seismic isolation to preserve this historical asset to the university (Seki et al.,

2000). Retrofitting using seismic isolation of the 72-year-old Clock Tower, a symbol of Kyoto University,

Japan, is presently underway (Ogura et al., 2003).

The seismic isolation method is found to be more appropriate for the retrofitting of seismically

vulnerable buildings that are required to function even during retrofit operation. To meet such a demand,

the retrofitting method should affect minimum alterations to the existing structures and should produce

a low level of noise and vibration. The technique has successfully been applied to a number of such

buildings. In the retrofitting of Rankine Brown Building of Victoria University, Wellington, New Zealand,

which houses the main library and administration of the university, one of the main requirements was

the building’s uninterrupted usage during the retrofit operation (Robinson, 2003). Meeting a similar

challenge, the government office of Toshima Ward, in Tokyo, has been retrofitted with rubber and sliding

isolators below the existing foundation and was used as usual by the officials and citizens even during the

retrofit works. Similarly, a building of Nihon University in Tokyo, Japan, in which seismic isolation was

applied at the top of the columns of basement floor, was available for research and education during the

retrofit operation (Kawamura et al., 2000).

Although seismic isolation reduces earthquake forces, it does not eliminate them. Consequently, the

strength and ductility of an existing structure must at least be sufficient to resist the reduced earthquake

forces that remain even after isolation. Further, the requirement of space for the seismic gap and

provisions for services, elevators, and so on to follow large displacements at isolation level are some issues

that need prior attention. Although not applicable to all structures and all site conditions, seismic

isolation still has great potential to reduce the risk in existing buildings and bridges that are not capable of

resisting expected future earthquakes. This method, in the beginning, was used generally for preservation

of cultural assets, but now has many applications in the retrofitting of other buildings also.

29.2.9.2.2 Bridges

A large number of old bridges were constructed with little or no consideration for seismic requirements.

Some of them have been retrofitted, mostly by conventional methods. However, the remaining bridges

are waiting for the enhancement of their strength to meet the latest code requirements. In existing

bridges, bearings have generally been provided to accommodate longitudinal thermal movements

between the superstructure and the supports. Therefore, little structural modification is required to use

seismic isolation as a retrofit method. A common structural configuration, a continuous deck supported

Note: Region above the required strength curve indicates safe zone.

(a)

Before retrofit

After retrofit

(b)

Fundamental Period

Acceleration

(Strength)

Required strength

Lack of strength

Safety margin

Lack of strength

Seismic

isolation

Before retrofit

Retrofit by seismic isolation

Required strength

Fundamental Period

Safety

margin

FIGURE 29.27 Concept of retrofitting methods: (a) conventional; (b) seismic isolation. (Source: Sugano, S., Proc.

XII World Conf. on Earthquake Engineering, 2000. With permission.)

29-28 Vibration and Shock Handbook

on bearings at the tops of piers, attracts the option of seismic upgrading of existing bridge structures by

seismic isolation and is often practical and relatively inexpensive. Seismic isolation has been used for the

retrofitting of number of bridges in New Zealand. During the early period of its application in the United

States, seismic isolation was mainly used for the retrofitting of bridges and the isolation system used was

mainly lead rubber bearings (Skinner et al., 1993).

However, to replace an existing bearing with an isolation bearing, enough space is required between

the deck and pier top. All the jacks used for replacing the bearings should be driven by the same hydraulic

pump and raised at the same time to prevent stressing any cross members between girders. The dampers

can be connected to the substructure using heavy-duty mechanical or chemical anchorages, with the

other end bolted to the girders. Since an earthquake can shift a bridge in any direction, universal joints

are recommended on each end of the damping device to prevent the shaft from being bent during

movements other than axial ones. The damper should be set to the bridge in its midstroke position or a

position that takes full advantage of the free movements (Hipley, 1997).

In situations where rigid connection of footing and piers has made the bearing capacity of existing piles

inadequate for the new seismic requirements, seismic isolation bearings can be used at the base of

columns. Sometimes, it may be costly to retrofit a seismically deficient pier and foundation by

conventional methods due to submerged conditions that may require construction of cofferdams. In such

cases, seismic isolation can reduce the demand on the seismically deficient piers by redistributing the

force and transferring it to the abutments, which are significantly less expensive to retrofit. Seismic

retrofits are intended to bring bridges up to reasonable earthquake standards; however, in some old

bridges, the retrofit method is also required to have the least effect possible on the historic appearance

and seismic isolation is the appropriate alternative for this requirement.

29.2.9.3 Other Implementations

Apart from in buildings and bridges, seismic isolation has found application in other structures. It

has been used for a number of reactor units in France for which the site safe shutdown earthquake

acceleration was 0.2g. An isolation system of neoprene pads with topping of lead – bronze alloy limits the

acceleration to a value of 0.2g. A similar system was used for reactor units in Koeberg, South Africa, where

the site safe shutdown earthquake acceleration was 0.3g (Tajiran, 1998). Seismic isolation has a lot of

potential for the protection of sensitive equipment that can malfunction when the acceleration reaches

beyond its allowable limit. Communication centers, hospitals, computer centers, and other facilities

housing such sensitive equipment can use seismic isolation to remain functional even after a major

seismic event. Proposed systems for the seismic isolation of sensitive equipment are to (1) base isolate

the entire building or a portion of the building, (2) isolate essential flooring systems only (raised floors),

and (3) isolate individual pieces of equipment by placing them on separate isolation bearing systems

(see Figure 29.28). In Japan, there are many buildings that are using the raised floor system to protect

computers and other sensitive equipment. Seismic isolation is found to be very useful for semiconductor

facilities, in which cost of the delicate items is as high as 75% of the total investment (Amick et al., 1998).

Liquid storage tanks are important components of chemical factories and are also used in increasing

FIGURE 29.28 Seismic isolation for protection of equipment. Unreinforced rubber blocks. (Source: Ohashi, U.G.

Earthquakes and Base Isolation, Pub. Asakura, Tokyo, 1995. With permission.)

Seismic Base Isolation and Vibration Control 29-29

number to store liquefied natural gas. The FPS was

applied for seismic isolation of a large-size tank in

Greece, and steel – rubber bearings were used for

another tank in Korea, storing liquefied natural gas

(Tajiran, 1998).

Seismic base isolation can also be used for the

seismic protection of antique pieces. Iemura and

coworkers designed a special seismic isolation

system to protect a wooden carved statue of

Moroku Bosatsu (Maitreya). It is an antique

cultural artifact, considered to be the first wooden

carved statue in Japan. The isolation system

provides protection in all three directions

(Figure 29.29). The details of the isolation system

are provided in Table 29.2.

The overall size of the isolation system is shown in Figure 29.30. The shaking table test was also carried

out to verify the effectiveness of the isolation system. Results of the experiment are presented in Table 29.3

and clearly show that the isolation system is very effective in reducing the accelerations.

29.2.10 Performance during Past Earthquakes

Analytical models and experimental tools have been developed by researchers to simulate the behavior of

a structure under earthquake motion. However, none of these methods can validate the behavior of a

structure as does a real earthquake. In fact, many of the concepts of earthquake-resistant design are

FIGURE 29.29 Three-dimensional base-isolation system for Moroku Bosatsu statue.

FIGURE 29.30 Overall size of the three-dimensional

base-isolation system.

TABLE 29.2 Details of Isolation System and Dynamic Characteristics of

the Structure

Horizontal Direction Vertical Direction

Elastic support Coil spring Coil spring

Damping mechanism Friction damper Air damper

Natural frequency (Hz) 0.25 – 0.35 2.0 – 2.5

29-30 Vibration and Shock Handbook

the outcome of the analysis of observations made or recorded during past earthquakes. In the case of base

isolation, as it is a relatively newer approach, it can help to assess the modeling techniques, existing

analysis and design procedures, and methods followed in construction. Such analyses may help in

evaluating the effectiveness of base isolation and also exposing weak aspects of designs. Strong motionrecording

instrument networks have been installed on some of the base-isolated structures in different

parts of the world in order to record the response of those structures during earthquakes. During the

Northridge, Kobe, and Turkey earthquakes, a few base-isolated structures had their isolation system

reaching nonlinear range, and the recorded performance of these structures can give a fair idea of the

behavior of base-isolated structures during strong earthquake motions.

29.2.10.1 Performance of Buildings

The Northridge earthquake of January 1994 saw thousands of conventionally designed buildings and

their contents suffer extensive damage and be closed for several days. However, two base-isolated

buildings, namely, University of Southern California Hospital building and Fire Command and

Control Facility, performed well during the earthquake. During the event, the seven-story baseisolated

hospital building experienced a peak ground acceleration of 0.37g. The recorded peak

acceleration at the roof was 0.21g. Analytical study showed that presence of the isolation system

reduced the peak story shear at the base to about one third of that if the building had its base fixed

(Nagarajaiah et al., 1996). “Workers in a large supply room within the hospital reported no

disruption of supplies, or material falling off shelves, as a result of the earthquake. In contrast a

pharmacy at the ground level in an adjacent medical building with fixed-base reported substantial

disruption of the supplies” (EERI, 1996). The two-story base-isolated Fire Command and Control

Facility could not perform as per expectation due to minor negligence in detailing the seismic gap.

The building is base-isolated by high-damping rubber bearings and experienced peak ground

acceleration of 0.19g. The recorded response shows the presence of sharp acceleration spikes that were

due to pounding. However, the analytical study by Nagarajaiah et al. (2001) shows that, even with

pounding, maximum base story shear of the base-isolated building was half that of the fixed-base case

and without pounding it would have been one fourth.

In January 1995, the unexpected earthquake of magnitude 7.2 in Kobe shocked the engineering

community. The 7 sec of strong shaking caused a heavy loss of life (over 5000 people died) and a

catastrophic loss to infrastructure. The strong earthquake tested two existing base-isolated buildings

in the region, namely, the six-story building of West Japan Postal Savings Computer Center

(WJPSCC) and the three-story building of Technical Research Institute of Matsumura Gumi

Corporation (TRIMGS). The six-story WJPSCC building that is base-isolated with elastomeric

isolators, lead rubber bearings, and steel coil dampers experienced peak ground acceleration of

0.306g. A nearby six-story fixed-base building experienced peak ground acceleration of 0.27g.

However, during the event, the maximum acceleration recorded at the roof of the fixed-base

building was about nine times of that recorded at the roof of the WJPSCC building. Similarly, a

TABLE 29.3 Results of Shaking Table Tests of a Three-Dimensional Base Isolation

System

Direction Level

Acceleration (gal) Relative Displacement

of Base Isolation (cm)

Shaking Table Above Base-Isolation System

NS 795 166 15.3

EW 505 136 10.0

Vertical 314 250 1.1

Seismic Base Isolation and Vibration Control 29-31

comparison of the roof acceleration of the three-story base-isolated TRIMGS building with that of a

nearby three-story fixed-base building reveals that, due to base isolation, the peak accelerations at the

roof were reduced by a factor of approximately 4.9 in the north – south direction and 2.5 in the

east – west direction. It was later realized that the performance of this base-isolated building

might be adversely affected due to freezing cold, when the rubber has a tendency to become stiffer

(Izawa et al., 1996).

29.2.10.2 Performance of Bridges

In the year 1999, Turkey faced two major earthquakes, the first one on 17th August and the second

on 12th November. At the time of the events, there were two base-isolated structures in the region,

namely, Bolu Viaduct and Bolu Bridge. The structures were designed for a peak ground acceleration

of 0.4g with isolation system comprising of a friction system and crescent moon-type hysteretic

elements. During the earthquake of August 17, 1999, when the estimated ground acceleration at the

Bolu viaduct site was 0.39g in the longitudinal direction and 0.31g in the transverse direction, the

viaduct survived without any damage. The residual displacement was of the order of 1 mm only.

However, the second event produced near-fault, or rather on-fault, pulse-type motion with a peak

ground acceleration at the viaduct site in the order of 1.0g. The unexpected level of ground

acceleration caused heavy damage to the viaduct. It was reported that the sliding bearings fell down

from most of the piers before any cyclic movement. This performance again raised the issue of

feasibility of base isolation for such near-fault pulse-type motions. During the same event, Bolu

Bridge experienced the estimated peak ground motion of 0.65g, much higher than the design value.

However, the only significant damage suffered by the bridge was the failure of some elements of the

expansion joints because they were designed for a movement of 100 mm only (corresponding to the

earthquake with a return period of 50 years). After the earthquake, the bridge showed a residual

displacement in longitudinal direction of 45 mm and in transversal direction varying from 60 to

100 mm (Marioni).

The Eel River Bridge (U.S.), after being retrofitted with lead laminated rubber bearings, was hit by

the magnitude 7.0 Cape Mendocino (Petrolia) earthquake in the year 1992. The peak ground

acceleration recorded at a nearby recording station was 0.55g. The bridge suffered structural damage

limited to the spalling of concrete at joints. Another base-isolated bridge, Sierra Point overpass (U.S.),

was subjected to the 1989 Loma Prieta earthquake when peak ground acceleration at the base of

bridge columns was 0.090g. The bridge, first constructed in 1956, was later retrofitted with lead

rubber bearings in 1985. However, the abutments were not modified for the larger clearance required

in isolation. This design weakness was exposed during the Loma Prieta earthquake and caused

significant amplification in the steel superstructure of the bridge. However, no damage was sustained

(Lee et al., 2001). Rangitaiki River Bridge at Te Teko, New Zealand, with lead rubber bearings on each

pier and plain rubber bearings on abutments, suffered minor damage and a small permanent

displacement during the 1987 Bay of Plenty earthquake. The peak acceleration, recorded at about

11 km from the bridge site was 0.33g. It was observed that owing to the inadequate fastening

details for the isolators, one of the two bearings on abutment was dislocated (McKay et al., 1990).

Bai-Ho bridge (Taiwan), a three-span, continuous nonprismatic prestressed concrete girder bridge,

was subjected to the magnitude 6.0 Gia-I earthquake in year 1999. The bridge is seismically isolated

in its longitudinal direction with lead rubber bearings; however, shear keys and specially designed

steel rods are installed on both abutments to restrict the transverse movement of the

superstructure. The peak acceleration recorded on the deck in longitudinal direction was slightly

higher than that in the foundation while, owing to restraints, the peak acceleration in the

transverse direction was 2.5 times than that of foundation (Lee et al., 2001). During the Kobe

earthquake, the base-isolated Matsunohama Bridge had good recorded performance; however,

the ground acceleration was not profound enough to test the effectiveness of the isolation system

(Izawa et al., 1996).

29-32 Vibration and Shock Handbook

In summary, analysis of the records obtained from a variety of base-isolated structures during real

earthquakes presents some very interesting facts. In general, it was observed that, during strong

earthquakes, numerous conventionally designed buildings suffered damage and remained closed for

several days; however, the base-isolated buildings in the same region not only survived but continued

to operate. On the other hand, in some cases, strong earthquakes exposed the lack of attention

to minor construction details and the effectiveness of base isolation for near-fault earthquakes.

Better performance of base-isolated structures during the Northridge and Kobe earthquakes have now

persuaded the structural engineers and owners and triggered a rapid increase in the number of baseisolated

buildings. In Japan alone, the number of buildings of this kind constructed during 1995 was

almost the same as that of those constructed during the period from 1985 to 1994.

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