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EERI DISTINGUISHED LECTURE 2001
Seismic Isolation Systems for DevelopingCountriesJames M.
Kelly,a) M.EERI
This paper describes an experimental and theoretical study of
the feasi-bility of using fiber reinforcement to produce
lightweight low-cost elasto-meric isolators for application to
housing, schools and other public buildingsin highly seismic areas
of the developing world. The theoretical analysis cov-ers the
mechanical characteristics of multi-layer elastomeric isolation
bear-ings where the reinforcing elements, normally steel plates,
are replaced by afiber reinforcement. The fiber in the
fiber-reinforced isolator, in contrast tothe steel in the
conventional isolator (which is assumed to be rigid both
inextension and flexure), is assumed to be flexible in extension,
but completelywithout flexure rigidity. This leads to an extension
of the theoretical analysison which the design of steel-reinforced
isolators is which accommodates thestretching of the
fiber-reinforcement. Several examples of isolators in theform of
long strips were tested at the Earthquake Engineering Research
Cen-ter Laboratory. The tested isolators had significantly large
shape factors, largeenough that for conventional isolators the
effects of material compressibilitywould need to be included. The
theoretical analysis is extended to includecompressibility and the
competing influences of reinforcement flexibility
andcompressibility are studied. The theoretical analysis suggests
and the test re-sults confirm that it is possible to produce a
fiber-reinforced strip isolatorthat matches the behavior of a
steel-reinforced isolator. The fiber-reinforcedisolator is
significantly lighter and can be made by a much less
labor-intensive manufacturing process. The advantage of the strip
isolator is that itcan be easily used in buildings with masonry
walls. The intention of this re-search is to provide a low-cost
lightweight isolation system for housing andpublic buildings in
developing countries. [DOI: 10.1193/1.1503339]
INTRODUCTION
The recent earthquakes in India, Turkey, and South America have
again emphasizedthe fact that the major loss of life in earthquakes
happens when the event occurs in de-veloping countries. Even in
relatively moderate earthquakes in areas with poor housing,many
people are killed by the collapse of brittle, heavy, unreinforced
masonry or poorlyconstructed concrete buildings. Modern structural
control technologies such as activecontrol or energy dissipation
devices can do little to alleviate this, but it is possible
that
a) Univ. of California, Pacific Earthquake Engineering Research
Center, 1301 S. 46th St., Richmond, CA 94804
385Earthquake Spectra, Volume 18, No. 3, pages 385406, August
2002; 2002, Earthquake Engineering Research Institute
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386 J. M. KELLYseismic isolation could be adapted to improve the
seismic resistance of poor housing andother buildings such as
schools and hospitals in developing countries.
The theory of seismic isolation (Naeim and Kelly 1999) shows
that the reduction ofseismic loading by an isolation system depends
primarily on the ratio of the isolationperiod to the fixed-base
period. Since the fixed-base period of a masonry block or
brickbuilding may be around 1/10 second, an isolation period of 1
second or longer wouldsignificantly reduce the seismic loads on the
building and would not require a large iso-lation displacement. For
example, the current UBC code for seismic isolation (ICBO1997) has
a formula for minimum isolator displacement which, for a 1.5 second
system,would be around 15 cm (6 inches).
The problem with adapting isolation to developing countries is
that conventional iso-lators are large, expensive, and heavy. An
individual isolator can weigh one ton or more.To extend this
earthquake-resistant strategy to housing and commercial buildings,
thecost and weight of the isolators must be reduced.
The primary weight in an isolator is that of the steel
reinforcing plates used to pro-vide the vertical stiffness of the
rubber-steel composite element. A typical rubber isola-tor has two
large end-plates around 25 mm (1 inch) thick and 20 thin
reinforcing platesaround 3 mm (1/8 inch) thick. The high cost of
producing the isolators reflects the laborinvolved in preparing the
steel plates and laying-up of the rubber sheets and steel platesfor
vulcanization bonding in a mold. The steel plates are cut, sand
blasted, acid cleaned,and then coated with bonding compound. Next,
the compounded rubber sheets with theinterleaved steel plates are
put into a mold and heated under pressure for several hours
tocomplete the manufacturing process. Both the weight and the cost
of isolators could bereduced if the steel reinforcing plates were
eliminated and replaced by fiber reinforce-ment. As fiber materials
are available with an elastic stiffness of the same order as
steel,the reinforcement needed to provide the vertical stiffness
may be obtained by using asimilar volume of very much lighter
material. The cost savings may be possible if theuse of fiber
allows a simpler, less labor-intensive manufacturing process.
If fiber reinforcement were used, it would then be possible to
build isolators in longrectangular strips, with individual
isolators cut to the required size. All isolators are cur-rently
manufactured as either circular or square. Rectangular isolators in
the form oflong strips would have distinct advantages over square
or circular isolators when appliedto buildings where the
lateral-resisting system is walls. When isolation is applied
tobuildings with structural walls, additional wall beams are needed
to carry the wall fromisolator to isolator. A strip isolator would
have a distinct advantage for retrofitting ma-sonry structures and
for isolating residential housing constructed from concrete or
ma-sonry blocks.
The vertical stiffness of a steel-reinforced bearing is
approximated by assuming thateach individual pad in the bearing
deforms in such a way that horizontal planes remainhorizontal and
points on a vertical line lie on a parabola after loading. The
plates areassumed to constrain the displacement at the top and
bottom of the pad. Linear elasticbehavior with incompressibility is
assumed, with the additional assumption that the nor-mal stress
components are approximated by the pressure. This leads to the
well-knownpressure solution that is generally accepted as an
adequate approximate approach for
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calculating the vertical stiffness. The extensional flexibility
of the fiber reinforcementcan be incorporated into this approach,
and the resulting vertical stiffness calculated.
EERI DISTINGUISHED LECTURE 2001: SEISMIC ISOLATION SYSTEMS FOR
DEVELOPING COUNTRIES 387A number of carbon fiber-reinforced rubber
strip isolators were tested on a smallisolator test machine. The
tests show that the concept is viable. The vertical and hori-zontal
stiffnesses of the strip isolator are less than those for the
equivalent steel-reinforced isolator, but still adequate, and they
proved to be easy to cut with a standardsaw, in contrast to
steel-reinforced isolators that are difficult to cut and need
specialsaws. They were light and in use could be put in place
without the use of lifting equip-ment.
Much recent discussion has focused on so-called smart rubber
bearings or intelligentbase isolation systems as the new thrust in
seismic isolation research. While there maybe a role for these
adaptive systems for large expensive buildings in advanced
econo-mies, the development of lightweight, low-cost isolators is
crucial if this method of seis-mic protection is to be applied to a
wide range of buildings, such as housing, schools,and medical
centers in earthquake-prone areas of the world.
VERTICAL STIFFNESS OF FIBER-REINFORCED BEARINGS
The essential characteristic of the elastomeric isolator is the
very large ratio of thevertical stiffness relative to the
horizontal stiffness. This is produced by the reinforcingplates,
which in current industry standard are thin steel plates. These
plates prevent lat-eral bulging of the rubber, but allow the rubber
to shear freely. The vertical stiffness canbe several hundred times
the horizontal stiffness. The steel reinforcement has a
similareffect on the resistance of the isolator to bending moments,
referred to as the bendingstiffness. This important design quantity
makes the isolator stable against large verticalloads.
COMPRESSION OF PAD WITH RIGID REINFORCEMENT
A linear elastic theory is the most common method used to
predict the compressionand the bending stiffness of a thin
elastomeric pad. The first analysis of the compressionstiffness was
done using an energy approach by Rocard (1937); further
developmentswere made by Gent and Lindley (1959) and Gent and
Meinecke (1970). A very detaileddescription of the theory is given
by Kelly (1996).
The analysis is an approximate one based on a number of
assumptions. The kine-matic assumptions are as follows:
(i) points on a vertical line before deformation lie on a
parabola after loading
(ii) horizontal planes remain horizontal
We consider an arbitrarily shaped pad of thickness t and locate
a rectangular Cartesiancoordinate system, (x,y,z), in the middle
surface of the pad, as shown in Figure 1a. Fig-ure 1b shows the
displacements, (u,v,w), in the coordinate directions under
assumptions(i) and (ii):
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388 J. M. KELLYu~x,y,z!5u0~x,y!S12 4z2t2 Dv~x,y,z!5v0~x,y!S12
4z2t2 D
w~x,y,z!5w~z! (1)
This displacement field satisfies the constraint that the top
and bottom surfaces of thepad are bonded to rigid substrates. The
assumption of incompressibility produces a fur-ther constraint on
the three components of strain, xx , yy , zz , in the form
xx1yy1zz50 (2)
and this leads to
~u0,x1v0,y!S12 4z2t2 D1w, z50where the commas imply partial
differentiation with respect to the indicated coordinate.When
integrated through the thickness this gives
u0,x1v0,y53D
2t5
3
2c (3)
where the change of thickness of the pad is D (D.0 in
compression), and c5D/t is thecompression strain.
Figure 1. Coordinate system (a) and constrained rubber pad
(b).
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EERI DISTINGUISHED LECTURE 2001: SEISMIC ISOLATION SYSTEMS FOR
DEVELOPING COUNTRIES 389The other assumptions of the theory are
that the material is incompressible and thatthe stress state is
dominated by the pressure, p, in the sense that the normal stress
com-ponents can be taken as 2p. The vertical shear stress
components are included but thein-plane shear stress is assumed to
be negligible. Linear elastic behavior is assumed.
These assumptions and the equations of stress equilibrium lead
to the pressure so-lution
p, xx1p,yy52p5212GD
t352
12G
t2c (4)
The boundary condition, p50, on the perimeter of the pad
completes the system for thepressure distribution, p(x,y), across
the pad.
The desired result is the effective compression modulus Ec of
the pad. This is ob-tained by computing, p(x,y), in terms of the
compression strain c , integrating, p(x,y),over the area A of the
pad to determine the resultant load P. The effective
compressionmodulus Ec is then given by
Ec5P
Ac(5)
The value of Ec for a single rubber layer is controlled by the
shape factor, S, defined as
S5loaded area
free area
which is a dimensionless measure of the aspect ratio of the
single layer of the elastomer.For example, in an infinite strip of
width 2b, and with a single layer thickness of t, S5b/t, and for a
circular pad of diameter f and thickness t, S5f/(4t), and for a
squarepad of side a and thickness t, S5a/(4t).
The vertical stiffness, Kv , of a rubber bearing made of n pads
is given by
Kv5EcA
tr
where the tr5nt is the total thickness of rubber in the
bearing.
In this paper we are only interested in the theory for and the
testing of a bearing inthe form of a long strip when the effects of
the ends can be neglected and the strip takento be infinite. For an
infinite strip of width 2b (see Figure 2), Equation 3 reduces
to
2p5d2pdx2
5212G
t2c
which, with p50 at x56b, gives
p56G
t2~b22x2!c
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390 J. M. KELLYIn this case the load per unit length of the
strip, P, is given by
P5E2b
bpdx5
8Gb3
t2c (6)
Since the shape factor, S5b/t, and the area per unit length is
A52b,
Ec5P
Ac54GS2 (7)
This result is reasonably accurate for shape factors in the
range 5 to 15, but for largeshape factors the predicted value of
the compression modulus begins to approach thebulk modulus, K, of
the material, which for natural rubber is usually taken as 2000
MPa(300,000 psi). An analysis that includes compressibility is
reviewed in a later section.
COMPRESSION STIFFNESS WITH FLEXIBLE REINFORCEMENT
Developing the solution for the compression of a pad with rigid
reinforcement is al-gebraically simple enough to be treated in two
dimensions and for an arbitrary shape.The problem for the pad with
flexible reinforcement is more complicated, however; forsimplicity,
the derivation will be developed for the long, rectangular strip
isolator. A verydetailed derivation of the solution is given by
Kelly (1999) where the influence of theflexibility of the
reinforcement on the various quantities of interest is studied. In
thisapproach as before, the rubber is assumed incompressible and
the pressure is assumed tobe the dominant stress component. The
kinematic assumption of quadratically variabledisplacement is
supplemented by an additional displacement that is constant through
thethickness and is intended to accommodate the stretching of the
reinforcement. Thus inthis case the displacement pattern given in
Equation 1 is replaced by
u~x,z!5u0~x!S12 4z2t2 D1u1~x! (8)
Figure 2. Infinitely long rectangular pad showing
dimensions.
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EERI DISTINGUISHED LECTURE 2001: SEISMIC ISOLATION SYSTEMS FOR
DEVELOPING COUNTRIES 391w~x,z!5w~z!
The constraint of incompressibility Equation 2 remains, leading
to
u0,x13
2u1,x5
3D
2t(9)
The only equation of stress equilibrium in this case is
txx,x1txz,z50, and the assumptionof elastic behavior means that
txz5Ggxz (10)
which with
gxz528z
t2u0 (11)
from Equation 8, gives
txx,x58Gu0
t2
which with the assumption that txx5tzz52p provides the sole
equation of equilibriumas
p, x528Gu0
t2(12)
The individual fibers are replaced by an equivalent sheet of
reinforcement of thicknesstf . The internal force, F(x), per unit
width of the equivalent reinforcing sheet is relatedto the shear
stresses on the top and bottom of the pad by
dF
dx2txzuz5t/21txzuz52t/250
as shown in Figure 3. The shear stresses on the top and bottom
of the pad are given by
txzuz5t/2528Gu0
2t; txzuz52t/25
8Gu02t
leading to
dF
dx52
8Gu0t
(13)
The extensional strain in the reinforcement f is related to the
stretching force throughthe elastic modulus of the reinforcement Ef
and the thickness tf such that
f5u1, x5F
Eftf(14)
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392 J. M. KELLYwhich when combined with Equation 14, gives
u1,xx528G
Eftftu0
The complete system of equations is
p, x528Gu0
t2(15)
u0, x13
2u1, x5
3D
2t(16)
u1, xx528G
Eftftu0 (17)
The boundary conditions used are the vanishing of the pressure,
p, and the reinforce-ment force, F, at the edges of the strip,
x56b. The results for p and F from Kelly (1999)are
p5Eftf
t S12 cosh ax/bcosh a Dc (18)F~x!5EftfS12 cosh ax/bcosh a Dc
(19)
where
a2512Gb2
Eftft(20)
Figure 3. Force in equivalent sheet of reinforcement.
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EERI DISTINGUISHED LECTURE 2001: SEISMIC ISOLATION SYSTEMS FOR
DEVELOPING COUNTRIES 393The load per unit length of the strip, P,
is given by
P5Eftf
t2E
0
bS12 cosh ax/bcosh a Ddxc52Eftfat b~a2tanh a!cThis result can be
interpreted as an effective compression modulus, Ec , given by
Ec5Eftf
t S12 tanh aa D (21)We note that when a0, i.e., Ef, we have
Ec4GS2 as before. The formula alsoshows that Ec,4GS
2 for all finite values of Ef .
The effect of the elasticity of the reinforcement on Ec can be
illustrated by normal-izing the compression modulus, Ec , by
dividing by 4GS
2, giving from Equation 21
Ec4GS2
53
a2 S12 tanh aa D (22)which is shown in Figure 4 for 0
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394 J. M. KELLYS2 and t/tf are certain to be large. If S is
large and the influence of the stretching of thefiber is
significant it will be necessary to take compressibility of the
elastomer into ac-count.
COMPRESSION STIFFNESS WITH COMPRESSIBILITY OF THE ELASTOMER
When the estimated value of Ec from Equation 7 is comparable to
the bulk modulus,K, of the elastomer, it is necessary to modify the
approach described in the earlier sec-tion to include the influence
of compressibility; the required detailed derivation for thesingle
pad of arbitrary plan-form is given in Kelly (1996). The essential
features of theanalysis for the incompressible pad are retained but
the constraint of incompressibility(Equation 2) is replaced by
xx1yy1zz52pK
(23)
and, as shown in Kelly (1996), the equation for the pressure
becomes
2p212G
t2pK
512G
t2c
and this is solved as before with p50 on the edges of the
pad.For the infinite strip 2b
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EERI DISTINGUISHED LECTURE 2001: SEISMIC ISOLATION SYSTEMS FOR
DEVELOPING COUNTRIES 395Ec4GS2
53
b2b2
32
2b4
15512
24
5
GS2
K
and as b, i.e., S becomes very large
Ec5KF12S K12GD1/2 1
S Gshowing that K is an upper bound to Ec .
FLEXIBLE REINFORCEMENT AND COMPRESSIBILITY
In cases of large shape factors, to estimate Ec including
compressibility in a mannerconsistent with the assumptions of the
previous analysis the equation of incompressibil-ity Equation 2 is
replaced by
xx1zz52pK
(25)
where K is the bulk modulus. Integration through the thickness
leads to the amendedform of Equation 3
2
3u0,x1u1,x1
pK
5c (26)
This is then supplemented by the same equation of stress
equilibrium and by theequation for the forces in the reinforcement,
Equation 14. The system of equations forthe combined effects of
reinforcement flexibility and compressibility is now
p, x528Gu0
t2(27)
u1, xx528Gu0Eftft
(28)
2
3u0, x1u1, x1
pK
5c (29)
Two dimensionless parameters, a and b, as defined in previous
sections, determinethe comparative significance of flexibility in
the reinforcement and compressibility inthe elastomer.
In terms of a and b, Equations 27 and 28 become
~p/K!,x52
3b2u0b
2 (30)
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396 J. M. KELLYu1, xx522
3a2u0 /b
2 (31)
Differentiation of Equation 29 once and substitution of p and u1
from Equations 30and 31 gives
u0, xx2~a21b2!
b2u050
from which we have
u05A cosh lx/b1B sinh lx/b
where
l25a21b2
In turn, using Equations 27 and 28 gives solutions for p and u1
in the forms
u1522
3
a2
l2A cosh lx/b2
2
3
a2
l2B sinh lx/b1C1x1D
and
p/K522
3
b2
lbA sinh lx/b2
2
3
b2
lbB cosh lx/b1C2
The constants of integration are, of course, not independent of
each other but arerelated through the basic equations. Substitution
of the three solutions into Equation 29gives
2
3
l
b~A sinh lx/b1B cosh lx/b!2
2
3
a2
lb~A sinh lx/b1B cosh lx/b!1C1
22
3
b2
l~A sinh lx/b1B cosh lx/b!1C25c
The coefficients of sinh bx/b and cosh bx/b vanish and the
result is C11C25c .
For the particular problem of the compression of the strip it is
useful to consider theobvious symmetries in the solutions. Thus u0
and u1 are anti-symmetric and p is sym-metric on 2b
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EERI DISTINGUISHED LECTURE 2001: SEISMIC ISOLATION SYSTEMS FOR
DEVELOPING COUNTRIES 397The boundary conditions for B and C1 are
that the pressure p at the edges x56b iszero and that the stress in
the reinforcement Efu1,x also vanishes at the edges. Thus
22
3
a2
lbB cos l1C150
22
3
b2
lbB cosh l2C152c
giving
B53
2
l
a21b2b
1
cosh lc
C15a2
a21b2c
and the solution becomes
u053
2b
sinh lx/b
a cosh lc
u15ba2
a21b2 Sxb2 sinh lx/bl cosh l Dcand
pK
5b2
a21b2 S12 cosh lx/bcosh l Dc (32)Integration of p from Equation
32 above and use of Equation 5 gives
Ec5Kb2
a21b2 S12 tanh ll D (33)If the effect of compressibility is
negligible then b0 and l5a and we have
Kb2512Gb2
t2
giving
Ec5Eftf
t S12 tanh aa D (34)which is the same as the result in the
previous section. On the other hand, if the flex-ibility of the
reinforcement is negligible then a0 and lb, giving
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398 J. M. KELLYEc5KS12 tanh bb D (35)If the compression modulus
is normalized by 4GS2, then from Equation 33 we have
Ec4GS2
53
a21b2 S12 tanh~a21b2!1/2
~a21b2!1/2 D (36)which demonstrates how the vertical stiffness
is reduced by both compressibility in theelastomer and flexibility
in the reinforcement.
EXPERIMENTAL RESULTS
Several fiber-reinforced bearings were constructed and tested in
compression andshear. Six specimens manufactured by Dongil Rubber
Belt Co., Ltd. (Pusan, Korea)were shipped in the form of strips
with slightly varying dimensions as given in Table 1.The
width-to-height ratio was very close to 2 and length-to-height
ratio was around 7.5.Each bearing had 99 mm of rubber and was
reinforced by 30 plane sheets of carbonfiber 0.27 mm thick.
The effective vertical stiffness of the bearing was obtained
from a compression testconducted in the following way. The specimen
was monotonically loaded up to the targetvalue of vertical pressure
and then three cycles of vertical loading with small amplitudeabout
this target value were performed. The shear stiffness of a specimen
was obtainedfrom sets of shear cycles with step-wise increasing
amplitude. These shear tests wereconducted for various values of
vertical pressure and for three angles between the testingdirection
and the longitudinal direction of the strip. All tests were
conducted on bearingsthat were not bonded to the test machine.
Table 1. Test specimens
NameLength(mm)
Width(mm)
Height(mm)
Area(mm2) Comments
Presence of rubber cover
East West North South
DRB1 735 183 105 134505 Yes No No NoDRB2 750 190 105 142500 Yes
No Yes NoDRB3 740 190 105 140600 No Yes Yes NoDRB4 365 190 105
69350 cut from 19037553105 No No Yes NoDRB5 390 190 105 74100 cut
from 19037553105 Yes No Yes NoDRB6 377 183 105 68991 cut from
18337553105 No Yes No NoDRB7 377 183 105 68991 cut from 18337553105
No No No NoDRB8 730 185 105 135050 kept as sample
Notes:1) All test specimens were composed from 33 layers of 3-mm
thick rubber and 30 layers of 0.27 mm fiber.2) The in-plane test
machine imposed shear in the east-west direction (representing 0
direction).3) The location angle of the specimen was measured from
the west direction counterclockwise (i.e., at 90 theformer west
side points south).4) The rubber cover of bearing sides reduces the
effective work area of the bearing in the vertical direction.
Thethickness of the rubber cover varies from 5 mm to 9 mm on the
long side of the bearing (north or south) andvaries from 1 mm to 3
mm on the short side of the bearing (east or west).
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EERI DISTINGUISHED LECTURE 2001: SEISMIC ISOLATION SYSTEMS FOR
DEVELOPING COUNTRIES 399These tests were carried out on a test
machine, shown in Figure 5, designed to pro-duce in-plane vertical
and horizontal cyclic loading. The vertical load was appliedthrough
a stiff frame to the specimen by two hydraulic actuators. The
horizontal loadwas applied by a hydraulic actuator. The test
machine had a displacement capacity of6254 mm (10 inches) in the
horizontal direction and a load capacity of 61,140 kN (260kips) in
the vertical direction. The photograph in Figure 6 shows a global
view of a testin progress.
TEST PROGRAM
Specimen DRB1 was tested under vertical load control
monotonically to 1.73 MPa(250 psi) of vertical pressure and three
fully reversed cycles with 60.35 MPa (50 psi)amplitude were
performed, then monotonically unloaded. A similar test at 3.45
MPa(500 psi) vertical pressure with 60.35 MPa (50 psi) amplitude
was performed to studythe vertical stiffness at the greater
vertical load.
The horizontal test was performed under horizontal displacement
control. SpecimenDRB1 was tested in cyclic shear, with three fully
reversed cycles at four peak strain lev-els of 25%, 50%, 75%, and
100% (based on 99 mm rubber thickness) applied at a ver-tical
pressure of 1.73 MPa (250 psi). The vertical pressure was increased
to 3.45 MPa(500 psi) and the shear test was repeated. The peak
value of shear deformation was in-creased by 1.5 and the test
repeated. The shear tests were done for the following se-quence of
the angle between the testing direction and the longitudinal
direction of thestrip: 0, 90, and 45. Specimens DRB2 and DRB3 were
tested under the same testprogram, but with a different sequence of
the angle between the testing direction and thelongitudinal
direction of the strip. For specimen DB2 this sequence was 45, 0,
and 90,and for specimen DRB3 it was 90, 45, and 0.
Figure 5. Testing setup.
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400 J. M. KELLYIn order not to exceed the capacity of the
testing machine, a specimen was cut in twoequal halves, denoted as
DRB4 and DRB5. The half-length specimens DRB4 and DRB5were used to
study behavior at higher levels of vertical load and larger shear
deforma-tion. The value of vertical pre-load was increased to 6.90
MPa (1000 psi) and the sheardeformation was doubled to study the
behavior at a larger shear strains. The angle be-tween the testing
direction in shear and the longitudinal direction of the strip was
0 forspecimen DRB4 and 90 for specimen DRB5.
The possibility of increasing of shear capacity of the bearings
by stacking them (oneon top of the other) was studied by testing
DRB8 and DRB9 in this way. The joint speci-men was vertically
loaded up to 3.45 MPa (500 psi) vertical pressure and then was
testedin shear at 50%, 100%, 150%, and 200% peak shear strains
based on the single bearingthickness.
Figure 6. Test in progress.
Figure 7. Specimen DRB6 at 100% shear deformation (90).
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EERI DISTINGUISHED LECTURE 2001: SEISMIC ISOLATION SYSTEMS FOR
DEVELOPING COUNTRIES 401Finally, specimen DRB9 was cut in two equal
halves, and these were designated asspecimens DRB6 and DRB7. They
differed in that DRB6 had rubber cover at one end,whereas specimen
DRB7 had no side rubber cover. They were tested under the same
testprogram at three levels of vertical pressure: 0.87 MPa (125
psi), 1.73 MPa (250 psi), and3.45 MPa (500 psi). Figure 7 is a
photo of specimen DRB6 under 100% shear deforma-tion testing at 90
to the longitudinal direction. The deformation with the same
magni-tude at 45 to the longitudinal direction is shown in Figure 8
and the specimen under100% shear deformation in the longitudinal
direction is shown in Figure 9.
DISCUSSION OF EXPERIMENTAL RESULTS
Horizontal Test Results
The manufacturer of the test isolators gave the nominal shear
modulus of this naturalrubber compound as 0.690 Mpa (100 psi). The
three full-length uncut specimens had anaverage area of 0.140 m2
and a total rubber thickness of 0.099 m. The horizontal stiff-ness,
KH , of a conventional isolator is given by
Figure 8. Specimen DRB6 at 100% shear deformation (45).
Figure 9. Specimen DRB6 at 100% shear deformation (0).
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402 J. M. KELLYKH5GA/tr
and for these values KH is
KH5970 kN/m
At 100% shear strain and a pressure of 1.73 Mpa (250 psi) the
average horizontalstiffness in the longitudinal loading direction
is 1280 kN/m, in the lateral loading direc-tion is 863 kN/m and at
45 is 1120 kN/m.
The hysteresis loops for the longitudinal loading direction tend
to stiffen when theshear strain is increased from 100% to 150%,
whereas in the lateral direction the loopsturn over so that the
instantaneous tangent stiffness is negative at the larger strains.
How-ever, this effect is reduced at higher pressure levels. The 45
loading does not produceeither stiffening or softening but gives
values intermediate between the 0 and 90 load-ings.
The value of the stiffness at 100% shear strain in the
longitudinal direction is slightlyhigher than would be expected
from the nominal value of the shear modulus but in thetransversal
loading direction the stiffness is lower. At 45 the stiffness is
intermediatebetween the other two. If we assume that the layout of
the strip isolator is orthogonalwith roughly the same number in
each direction, the average between 0 and 90 is closeto the value
at 45 so that the system will have the same period in any direction
of move-ment.
The period, T, can be roughly estimated using the pressure and
the effective shearmodulus. The period is given by
T52pAptrGg
If the average pressure over the system is 3.45 Mpa (500 psi) as
in the tests and themodulus is 0.690 Mpa (100 psi) with 99 mm of
rubber, we have a period of 1.4 seconds.From the code formula this
would produce a displacement of 143 mm (5.64 inches) anda shear
strain of 1.41. Adjusting the values to correspond to the measured
stiffness atg51.5, we find that the period increases to 1.5 seconds
and the displacement to 150 mm(6 inches).
This suggests that if the period of 1.5 seconds is acceptable as
the target value for thedesign of the building, the strip isolators
as tested would be adequate, providing that theaverage pressure can
be at least 3.45 Mpa (500 psi). A longer period can be obtained
byhaving one isolator on top of another. This leads to a period of
2 seconds, a code dis-placement of 200 mm (8 inches) and a g51.0.
It is clear that a wide range of practicalobjectives is possible.
If it is necessary to have an average pressure of less than 3.45MPa
(500 psi) it is possible to use a softer compound. Compounds with
shear moduli, at100% strain, down to 0.40 Mpa (60 psi) are
available.
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EERI DISTINGUISHED LECTURE 2001: SEISMIC ISOLATION SYSTEMS FOR
DEVELOPING COUNTRIES 403Vertical Test Results
The vertical test results are shown in Tables 2, 3, and 4. Since
the dimensions of thebearings are slightly different in each case
it is useful to tabulate the vertical stiffness interms of the
effective compression modulus, Ec , as defined by Equation 5. The
full-length bearings DRB1/2/3 are quite consistent with Ec at
around 414 Mpa. The two setsof half-length bearings have lower
values of Ec at the same vertical pressures of testing.The pair
denoted by DBR4/5 was not tested at 1.73 Mpa (250 psi) but at 3.45
Mpa (500psi) and 6.90 Mpa (1000 psi). The set denoted by DBR6/7 was
tested at 0.87 Mpa (125psi), 1.73 Mpa (250 psi), and 3.45 Mpa (500
psi). At the common test pressure of 3.45Mpa (500 psi) the average
of the two values of Ec of DBR4/5 was the same as that ofDBR6/7, so
that we can interpret the effect of variation of the target
pressure over aneight-fold range. The fact that at the same
pressures the Ec values for the full-lengthbearings are higher than
for the half-length bearings is most likely due to the larger
in-fluence of free ends in the latter case. The theoretical
analysis is developed for the in-finite length strip and for the
full-length bearings the length-to-height ratio of 7.5 islarge
enough that this assumption is valid. At half this value end
effects can be expectedto have some influence. The vertical
stiffness of an elastomeric isolation bearing is al-ways difficult
to measure since the displacements at the vertical loads
corresponding topractical use are extremely small and a great deal
of scatter is to be expected.
Table 2. Vertical test results for 1.73 MPa vertical
pressure
SpecimenNo.
Area(m2)
ImposedLoad (kN)
Average Pressure(MPa)
Average StiffnessKav (kN/M)
Compression modu-lus Ec (MPa)
DRB1 0.135 233.6 1.73 550853.9 404DRB2 0.143 233.6 1.63 602975.0
417DRB3 0.141 233.6 1.66 597053.3 419DRB4 0.069 N/A N/A N/A N/ADRB5
0.074 N/A N/A N/A N/ADRB6 0.069 120.2 1.74 251687.8 361DRB7 0.069
120.2 1.74 278983.5 400
Table 3. Vertical test results for 3.45 MPa vertical
pressure
SpecimenNo.
Area(m2)
ImposedLoad (kN)
Average Pressure(MPa)
Average StiffnessKav (kN/M)
Compression Modu-lus Ec (MPa)
DRB1 0.135 467.3 3.46 791048.8 580DRB2 0.143 467.3 3.27 849319.3
588DRB3 0.141 467.3 3.31 752785.8 529DRB4 0.069 253.7 3.68
349938.19 502DRB5 0.074 253.7 3.43 352040.6 471DRB6 0.069 240.3
3.48 328721.9 472DRB7 0.069 240.3 3.48 351392.3 504
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404 J. M. KELLYIt is clear, however, that there is for all test
specimens a systematic increase in stiff-ness and Ec when the
central value of the pressure around which the load is cycled
in-creases. As the pressure is doubled the average value of Ec for
the three full-length iso-lators increases by 37% to 565 MN/m2. The
half-length bearing tests show an increase inEc over the complete
range of pressure. The increase is not linear in pressure but tends
todecrease with increasing pressure from 55% at the lowest level to
15% at the highest.This is consistent with the type of carbon fiber
used in the bearings. The fiber is woven,two directional, and
epoxied into a thin sheet. As the pressure increases the in-plane
fi-ber sheet tension increases and tends to straighten out the
fiber strands, thus increasingthe effective fiber modulus.
To assess the effect of the various parameters on the vertical
stiffness it is necessaryto estimate the actual shear modulus from
the tests on shear. At a vertical pressure of3.45 Mpa (500 psi) the
average shear stiffness of the first three bearings when tested
inthe longitudinal direction 0 is 1.278 MN/m, which with an average
area of 0.140 m2
and a thickness of rubber of 99 mm implies a shear modulus of
0.904 Mpa (131 psi),which is considerably larger than the nominal
modulus. This use of the longitudinal teststo provide an estimate
of the modulus is warranted by the fact that this case will have
theleast influence from the roll-off due to the unbonded
condition.
A steel-reinforced isolator with this shear modulus, this area
of rubber and this thick-ness would, if compressibility effects
were ignored, have an effective compressionmodulus Ec given by
Equation 5, of 3738 MN/m
2. When compressibility is taken intoaccount the effective
modulus is considerably reduced as given by Equation 35. To
es-timate b, we use K52000 Mpa (290,000 psi), giving b255.3 and
from Equation 35 wehave 1150 MN/m2. The average measured value, 563
MN/m2, can be used to deduce theeffect of the extensibility of the
carbon fiber reinforcement. For this purpose we nowturn to Equation
33 and assume that b255.3. From the results we have Ec /K50.28
andfrom the equation we determine that (a21b2)1/253.7, implying
a258.4. When theknown values of the various parameters are inserted
into the definition of a2 we estimateEf as 14,000 Mpa (2,000,000
psi).
There is no equipment in the laboratory to measure modulus of
the carbon fibersheet directly nor has the fabricator provided a
value. The result is somewhat lower thanothers quoted for
carbon/epoxy sheets and the reason is not clear, but the sheets
appear
Table 4. Vertical test results for extreme values of vertical
pressure
SpecimenNo.
Area(m2)
ImposedLoad (kN)
Average Pressure(MPa)
Average StiffnessKav (kN/M)
Compression Modu-lus Ec (MPa)
DRB1 0.135 N/A N/A N/A N/ADRB2 0.143 N/A N/A N/A N/ADRB3 0.141
N/A N/A N/A N/ADRB4 0.069 507.3 7.35 467372.6 671DRB5 0.074 507.3
6.86 445858.5 596DRB6 0.069 60.1 0.87 175617.3 252DRB7 0.069 60.1
0.87 167190.4 240
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EERI DISTINGUISHED LECTURE 2001: SEISMIC ISOLATION SYSTEMS FOR
DEVELOPING COUNTRIES 405to be a very poor quality fiber with many
spaces and the portion of the thickness that isfiber cannot be
determined from visual analysis. It is certainly possible that the
qualityof the reinforcing could be improved but it is clear that
even this poor quality sheet isadequate for these bearings. An
effective modulus Ec of 563 MN/m
2 at an average pres-sure of 3.45 Mpa (500 psi) implies a
vertical vibrational frequency of 20 Hz, which ismore than is
necessary in any isolation application. The conclusion is that
although thefiber is suspected to be very low quality, and
presumably low-cost, it has sufficient stiff-ness and strength for
application to low-cost isolators.
CONCLUSIONS
The test results show that the concept of the strip isolator
reinforced with carbonfiber is viable. The fact that the isolator
can be made in long, wide sheets and cut to therequired width means
that the cost of the isolator can be reduced to a level that is
ac-ceptable for low-cost public housing. The tests also show that
loading in the direction ofthe strip across a cut surface can be a
source of delamination. In practice this should notbe a problem
since the width of the manufactured sheet will be used as the
length of thestrip isolator and the ends will be finished edges.
Loading in the transverse direction(90), where the edges are cut,
is not so severe, as the rolling of the strip tends to pro-duce
lower forces in this direction. The most vulnerable direction of
loading is at 45,which appears to put a very distorted pattern of
displacement on the isolator and for thisreason it may be advisable
to use either a better cutting method such as water jet, whichwill
leave a smoother finished surface than a steel saw, or to finish
the edge by a cold-bonded surface layer.
The carbon fiber appeared to be very poor quality. The fibers
are laid out in only twodirections, 0 and 90, in a woven sheet with
many spaces. Nevertheless the isolatorsstill functioned acceptably.
It would be possible to make a much better isolator with abetter
quality carbon fiber at little increase in cost.
The unresolved issue from the test program is that of overall
system behavior,namely, can an isolation system made of strip
isolators laid out in an orthogonal gridprotect the masonry wall
superstructure above. Isolators loaded in the longitudinal
di-rection stiffen with increasing displacement and those loaded
across the strip will softenwith displacement above a certain
threshold. The question is can the unbalanced shearbe accommodated
by the wall system. Further research is needed to study this effect
andthe best way to develop a reliable procedure would be to have a
masonry block housemodel on isolators on a large shake table.
It is important to recall that the benefit of isolation is
achieved primarily through theratio of the isolated period of the
building to its fixed-base period. For a constant veloc-ity
spectrum the base shear of the fixed-base building is reduced by
this factor when thesame building is isolated. A masonry wall
structure will have an extremely short fixed-base period, in the
range of 0.10 second. A reduction of a factor of 10 can be
obtainedwith an isolation period of 1 second and this is not
difficult to obtain. In fact, the codeformula for isolation system
displacement that has persisted through all versions of theseismic
isolation codes in the United States since the earliest in 1986
would predict adisplacement of 15 cm. (6 inches) for a 1.5-second
period system, and the isolators
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tested here were tested to displacements of 15 cm. (6 inches)
and more. However, iflarger displacements are needed, the tests
showed that stacking two isolators on top ofeach other was
possible, which would allow even larger displacements.
A shake table test using a full size masonry block house would
help clarify details ofaccess across the isolation interface, the
disadvantages if any of not having the bottomfloor slab-on-grade
isolated, and the extent to which a concrete tie-strip is needed
be-tween the isolators and the block wall. When these remaining
uncertainties are resolvedthis valuable new technology can be
implemented in many highly seismic areas in thedeveloping
world.
ACKNOWLEDGMENTS
406 J. M. KELLYThe sample isolators were made by Dongil Rubber
Belt Co. Ltd. of Pusan, Korea.The test program was conducted at the
Structural Test Facility of the Pacific EarthquakeEngineering
Research Center, University of California, Berkeley. This research
workwas partly supported by the Engineering Research Center for
Met-Shape and Die Manu-facturing of Pusan National University,
Pusan, Korea, which is gratefully acknowledged.
REFERENCES
Gent, A. N., and Lindley, P. B., 1959. The compression of bonded
rubber blocks, Proc. Inst.Mech. Eng. 173 (3), 111117.
Gent, A. N., and Meinecke, E. A., 1970. Compression, bending and
shear of bonded rubber-blocks, Polym. Eng. Sci. 10 (2), 4853.
International Conference of Building Officials (ICBO), 1997.
Earthquake regulations for seis-mic isolated structures, Uniform
Building Code, Appendix Chapter 16, Whittier, CA.
Kelly, J. M., 1996. Earthquake-Resistant Design with Rubber, 2nd
edition, Springer-Verlag,London.
Kelly, J. M., 1999. Analysis of fiber-reinforced elastomeric
isolators, J. Seismic Engrg. 2 (1),1934.
Naeim, F., and Kelly, J. M., 1999. Design of Seismic Isolated
Structures, John Wiley, New York.Rocard, Y., 1937. Note sur le
calcul des proprietes elastique des supports en caoutchouc
adher-
ent, J. Phys. Radium 8, 19.
(Received 13 March 2002; accepted 6 April 2002)