Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, JapanOBSERVATION RECORDS OF BASE-ISOLATEDBUILDINGS IN STRONG MOTION AREA DURINGTHE 2011 OFF THE PACIFIC COAST OF TOHOKU
EARTHQUAKE
Goro MIWADA1, Osamu YOSHIDA2, Ritsuko ISHIKAWA3 and Mitsuru NAKAMURA41

Research Engineer, Technical Research Institute, Obayashi Corporation,Tokyo, Japan, miwada.goro@obayashi.co.jp
2D.Sc., Technical Research Institute, Obayashi Corporation,Tokyo, Japan, yoshida.osamu@obayashi.co.jp
3Dr. Eng., Technical Research Institute, Obayashi Corporation,Tokyo, Japan, ishikawa.ritsuko@obayashi.co.jp
4Dr. Eng., Technical Research Institute, Obayashi Corporation,Tokyo, Japan, nakamura.mitsuru@obayashi.co.jp

ABSTRACT: Many buildings in the area of Tohoku and Kanto suffered structural
damage by the 2011 off the Pacific coast of Tohoku earthquake occurred on March 11th.
Although few base-isolated buildings which have suffered structural damage are reported,
there are some reports that nonstructural elements, such as expansion joints, inbase-isolated buildings have suffered damage.
In this paper, strong motion observation records of base-isolated buildings in Miyagi(BLD.A) and Chiba prefectures (BLD.B) are shown. Isolation systems used in both ofthese buildings are high damping rubber bearings. The acceleration reduction effect by
the base-isolation system is confirmed in both of these buildings. The firstfloor/basement acceleration is 200/381 cm/s2 in BLD.A, and is 98/219 cm/s2 in BLD.B.
No structural damage in either building is reported. The maximum relative displacementof the isolated layer is 18 cm in BLD.A and is 5 cm in BLD.B. The cumulated relative
displacement of the isolated layer is 14.6m in BLD.A and is 8.5m in BLD.B, which islarger than one in an ordinary earthquake because of long duration time of the earthquake.
Natural period of the building varied along with the time course on the earthquake due tocharacteristics of high damping rubber bearings, which become softened in proportion tohorizontal displacement.
Based on the analysis of the observation data, it is shown that 1) load-history dependency
of base isolation systems has been observed, 2) high damping rubber bearings haveremained softened after maximum relative displacement of isolation layer and 3) it has
taken about a month until the stiffness of the isolation layer has been back to the initialstiffness.
Key Words: long-period ground motion, base-isolated building, high damping rubber
bearings, load-history dependency

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INTRODUCTION
After the 1995 Hyogoken-Nanbu earthquake, the number of base-isolated buildings has increased. The
total number of constructed base-isolated buildings in Japan has been about 2,900 in 2009 (Tanaka2011). Acceleration reduction effects in base-isolated buildings have been proved by strong motion
records. These records have contributed to spread the base-isolated system. In contrast, there are few
strong motion records in base-isolated buildings during long-period ground motions. The duration timeof the 2011 off the Pacific coast of Tohoku earthquake (i.e. the Tohoku earthquake) occurred on March
11th is much longer than ones of ever observed earthquakes. High damping rubber bearing,
representative one of base-isolated systems, have load-history dependency. There are few reports
about dynamic characteristics of base-isolated buildings using high damping rubber bearings duringlong-period ground motions.
This paper shows strong motion records in two base-isolated buildings using high damping rubber
bearings during the Tohoku earthquake. The load-history dependency of high damping rubber bearings
is confirmed by the variation of the dynamic characteristics obtained through the analysis of therecords.

BASE-ISOLATED BUILDING
BLD.A is a six story base-isolated building, 74.6 m long and 37.0 m wide, in Miyagi prefecture.
BLD.B is an eight story base-isolated building, 30.0m long and 47.6m wide, in Chiba prefecture. Bothof them are office buildings. The superstructure of BLD.A is a moment resisting steel frame structure
and that of BLD.B is moment resisting frame structure consisted of steel encased reinforced concrete
column and steel beam. The elevation plans and accelerometers locations of these buildings are shownin Fig. 1. Isolation systems used in both of these buildings are high damping rubber bearings. The
accelerometers are located at the basement, the floor above the isolated layer and the top floor in bothbuildings. Two accelerometers of NS direction are located at each floor in BLD.A.

RF33.1m

39.8m

5F

B2F
Base74.6m
Base isolation device

UDEW

Base isolation
device

1FUD

Base
30.0m
EW

(b) BLD.A(a) BLD.B

Legend of accelerometers
EW direction
NS direction
UD direction

Fig. 1 Elevation plans and accelerometers locations of the base-isolated buildings

BASE ISOLATION SYSTEM
The base-isolation systems in both buildings consist of only high damping rubber bearings. The
number of bearings is 64 in BLD.A and 35 in BLD.B. The specifications of the base isolation bearings
in BLD.A and BLD.B are shown in Table 1 and Table2. The designed deformation limit of both baseisolation systems is 40 cm. Total rubber layer thickness of base isolation bearings is 150 - 203 mm inBLD.A and 198 – 215 mm in BLD.B.

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Table 1 Specification of the base isolation bearings in BLD.A
Number ofbearings
102020
14Diameter

Rubber layer

Steel plate layer

1,000mm
850mm
800mm
600mm

8.0mm, 25layers
7.0mm, 29layers
6.5mm, 31layers
5.0mm, 30layers

3.1mm, 24layers
2.2mm, 28layers
2.2mm, 30layers
2.2mm, 29layers

1st shape
factor
30.1
30.4
30.8
30.0

2nd shape
factor
5.0
4.2
4.0
4.0

Table 2 Specification of the base isolation bearings in BLD.B
Number ofbearings
51884Diameter

Rubber layer

Steel plate layer

1,180mm
1,000mm
800mm
700mm

9.5mm, 21layers
8.0mm, 25layers
6.5mm, 31layers
6.0mm, 33layers

3.1mm, 20layers
3.1mm, 24layers
2.2mm, 30layers
2.2mm, 32layers

1st shape
factor
31.1
31.3
30.8
29.2

2nd shape
factor
5.9
5.0
4.0
3.5

OBSERVATION RECORDSThe maximum accelerations in two buildings are shown in Table 3 and Table 4. Distribution of themaximum accelerations at each story in EW direction is shown in Fig. 2. Acceleration reduction
effects by the base-isolation systems are confirmed in both of the buildings, as shown in Fig. 2. The
maximum acceleration at the floor above the isolated layer is 41 - 83 % of the basement. Acceleration
records at the basement and the floor above in EW direction are shown in Fig. 3. The first 200 s of theaccelerations and displacements are shown in Fig. 3 and Fig. 4. The duration time of the acceleration
data is 205 s in BLD.A and 713 s in BLD.B during the earthquake. As shown in Fig. 3, the periods ofacceleration wave at the floor above the isolated layer are longer than the one at the basement in bothbuildings. The acceleration reduction effects in almost all the duration time are confirmed throughcomparison of the acceleration amplitudes. Observation data of the first 80 s were missed in BLD.A,
as overwritten by the aftershocks observation data. Therefore, although two principal shocks wereobserved in Miyagi prefecture, only later principal shock was recorded in BLD.A, as shown in Fig.
3(a).
Relative displacements of the isolated layer in EW direction are shown in Fig. 4. The displacements
are calculated via double integration of the acceleration record filtered out the component lower than
0.1 Hz. Maximum relative story displacements of the isolated layer in two buildings are shown in Fig.
5. The maximum relative story displacement in horizontal direction is 19.9 cm in BLD.A, 6.0 cm inBLD.B. These displacements correspond to shear strain 133 % of the thinnest base-isolation bearing in
BLD.A and 30 % in BLD.B.
Cumulative Relative story displacements in horizontal direction of the isolated layers are shown in
Fig.6. The cumulated relative story displacement of the isolated layer is 14.6m in BLD.A and is 8.5min BLD.B. The displacement is much larger than one in ordinary earthquakes because of long duration
time of the earthquake. For example, in the case that maximum acceleration was observed at basementof BLD.B during an earthquake before the Tohoku earthquake, the cumulated relative story
displacement of the isolated layer is 0.67m.

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Table 3 Maximum acceleration in BLD.A (cm/s2)

NS
277
163, 214
240, 231

EW
381
200
209

UD
213
185
424302010

Table 4 Maximum acceleration in BLD.B (cm/s2)

Acceleration
gal 2
cm/s

Acceleration
gal2
cm/s

Base
1F
RF
400.

NS
174
71.7
128

EW
219
97.5
129

BLD.A
BLD.B

Height(m)

Base
B2F
5F

400UD
128
129
295

0500
Maximum Acceleration (gal)(cm/s2)

Fig. 2 Distribution of maximum accelerations
in base-isolated buildings (EW)

BASE-EW

cm/s2
MAX = -380.9 galB2F-EW

MAX = -199.7 cm/s
gal 2-400.
400.

-400.
0.

50.

100.
Time ( sec. )

150.

200.

150.

200.

150.

200.

150.

200.

Acceleration
gal2cm/s

Acceleration
gal2cm/s(a) BLD.A
250.

BASE-EW

cm/s2
MAX = 219.2 gal

1F-EWcm/s2
MAX = -97.5 gal

-250.
250.

-250.
0.

50.

100.
Time ( sec. )Displacementcm(b) BLD.B
Fig. 3 Acceleration records
EW

20.0

MAX = 16.0 cm

0.0-20.00.

50.

100.
Time ( sec. )Displacement
cm(a) BLD.A
EW

5.0

MAX = 4.7 cm

0.0-5.00.

50.

100.
Time ( sec. )(b) BLD.B
Fig. 4 Relative story displacement records of the isolated layer (EW)1020

5Displacement NS(cm)Displacement NS(cm)20100-10-20-20

2.50-2.5-5-5-1001020
Displacement EW(cm)-2.502.5
5
Displacement EW(cm)Fig. 5 Relative story displacements of the isolated layers (Left: BLD.A, Right: BLD.B)

Displacement
cm

1500
750
0050100
Time ( sec. )150200(a) BLD.A
Displacementcm

100050000100200300400
Time ( sec. )500600700(b) BLD.BFig. 6 Cumulative Relative story displacements of the isolated layers

IDENTIFICATION OF DYNAMIC CHARACTERISTICS
Transfer function of 1st floor/basement and roof floor/basement are identified by ERA (Eigensystem
Realization Algorithm, Juang 1985) in BLD.B during the earthquake. Dynamic characteristics areobtained through the identified transfer function. The identified natural period and damping factor are
shown in Table 5. For reference, designed primary natural period in BLD.B is shown in Table 6. In thistable, “small amplitude” corresponds to shear strain ratio smaller than 10% and displacement 10 cm in“Level 1” corresponds to shear strain ratio about 56 %. Identified natural period at 1st mode is slightlarger than designed one in the case of “Small amplitude”, as shown in Table 5 and Table 6. Maximum
shear strain ratio in BLD.B is 30 %. As the observed natural period and the maximum shear strain ratio
are in between the designed value of “Small amplitude” and “Level 1”, the observed dynamic
characteristics approximately agree with the designed one. Comparison of observed and identified
transfer function at 1st floor/basement is shown in Fig. 7. It is considered that damping factors are wellestimated, judging from the identified amplitude and phase being same as observed one.
Table 5 Identified dynamic characteristics in BLD.B

Mode
123

Natural period(s)
1.99
0.63
0.31

NS
Damping factor(%)
15.7
10.2
4.65

1021

Natural period(s)
1.98
0.65
0.27

EW
Damping factor(%)
16.0
12.3
3.54

Table 6 Designed primary natural period in BLD.B

101010EW

Small amplitude

1.81

1.78

Level 1
displacement :10cm
Level 2displacement :20cm

2.50

2.48

2.99

2.97

110

Observation
Identified

Amplitude

Amplitude

10NS0-1

10-2

0.1

1.0
Frequency ( Hz )1010.0

180.0

0-1-2

0.1

90.0

0.0

-90.0
-180.0
0.1

Observation
Identified

1.0
Frequency ( Hz )

10.0

180.0
Observation
Identified

Phase

Phase

90.0

101

Observation
Identified

0.0

-90.0
1.0
Frequency ( Hz )10.0(a) NS Direction-180.00.1

1.0Frequency ( Hz )10.0(b) EW Direction

Fig. 7 Comparison of observed and identified transfer function at 1st floor/basement in BLD.B

DYNAMIC CHARACTERISTICS VARYING ALONG WITH THE TIME COURSEIn BLD.B, the variation of hysteresis characteristics in every 20 s during the earthquake in EW direction
is shown in Fig. 8. Gray line, which connects maximum and minimum loads, shows approximate
stiffness of the isolated layer. The story shear force of the base-isolated layer in Fig. 8 is estimatedfrom the 1st floor acceleration, which is filtered out the component higher than 1 Hz, multiplied by thebuilding mass. As shown from 40 s to 160 s in Fig. 8, the approximate stiffness becomes low alongwith the increase in the relative displacement of the isolated layer. It shows that the base-isolated
system has been softened from the initial condition. After 160 s, although relative displacements of theisolated layer decrease from the maximum displacement, the base-isolated layer stiffness remains
lower than the initial stiffness. It is due to load-history dependency of base isolation systems of highdamping rubber bearings (Torii 1999 and AIJ 2001). Estimated natural period and damping factor
through ARX model (autoregressive model with exogenous input) are shown in Fig. 9. The estimation
was done in every 1 s with the prior 10 s observed data. As shown in Fig. 9, the natural period
increases from the initial period 1.2 s to 2.2 s in around 120 s when the largest relative displacement
was observed. After the largest displacement, although the natural period decreases gradually to 1.4 s
in the end of observation, the natural period remains longer by 0.2 s than the initial period. It showsthat in the end of observation, the base-isolated layer stiffness remains lower than the initial stiffness.
Damping factor also increases to 23 - 30 % from the initial damping factor in around 120 s. After thelargest displacement, damping factor, though varying widely, decreases gradually.

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The relation of the identified natural period and maximum relative displacements of the isolated layers inevery 10 s is shown in Fig. 10. In the displacement larger than 1 cm, the natural period after themaximum relative displacement is about 0.2 s longer. The time course of the natural period in BLD.B isshown in Fig. 11. It is evaluated from the first 10 s of observation data during each earthquake, which hasoccurred from about a month before to a month after the Tohoku earthquake. The time course is smoothedwith a 10-point moving average and is shown in gray line. It has taken about a month until thestiffness of the isolation layers has been back to the initial stiffness.
66Force (kN)

x 10

6x 10

6x 10x 10

2222

20

0

0

00-2-2[40-60s]
05-5
6x 10
Force(kN)

6x 10-56x 10[60-80s]
05-2-56x 10[80-100s]
05-2[100-120s]
05-56x 10-22222200000-2-2[140-160s]
05-5-5[160-180s]
05-2[180-200s]
05-5-2[200-220s]
05-5[120-140s]
05-56x 10-2

[220-240s]
05-5

Relative story displacements of the isolated layer (cm)Damping factor (%)

Fig. 8 Variation of hysteresis characteristics in EW direction along with the time course in BLD.B30NSEW201000100200300400Time (s)500600Fig. 9 Variation of identified period and damping factor along with the time course in BLD.B
1.7

3Time course
210

natural periodsmoothed with a10-point moving
average

1.64
Natural Period (s)Relative story displacementsof theisolated layers
Displacement(cm)(cm)

5

1.5
1.4
1.3
1.2
1.1
11

1.5

2

2.5

2/4

2/11 2/18 2/25

3/4

3/11 3/18 3/25

4/1

Period (s)DateFig. 10 Relation of identified natural period andmaximum relative story displacements of the isolated
layer in EW direction in every10 s in BLD.BFig. 11 Time course of the natural
period in BLD.B evaluated from thethe first 10 s of observation data1023

4/8700CONCLUSIONSIn this paper, the observation records of base-isolated buildings using high damping rubber bearings in
Miyagi and Chiba Prefectures during the Tohoku earthquake have been reported. Dynamics
characteristics obtained from the records in these two base-isolated buildings have been described.
The analyses on the observation records are shown below.
1) The acceleration reduction effects by the base-isolation system are confirmed in both of thebuildings.
2) Load-history dependency of base isolation systems using high damping rubber bearings has beenobserved.
3) High damping rubber bearings have remained softened after maximum relative displacement during
the earthquake.
4) It has taken about a month until the stiffness of the base isolation layer has been back to the initialstiffness.

REFERENCES
Tanaka, Y., Fukuwa, N., Tobita, J., Mori, M. (2011). “Development and analysis of database forbase-isolated buildings in Japan.” Architectural Institute of Japan Journal of Technology andDesign, Vol. 17, No. 35, 79-84. (in Japanese)
Juang J.N., Pappa R.S. (1985). “An eigensystem realization algorithm for modal parameter
identification and model reduction.” Journal of Guidance Control and Dynamics, Vol. 8, 620-627.
Torii, S., Murota, N., Yamane, T., Kitamura, H., Takayama, M., Ohta, M., Yoshizawa, T. (1999). “Astudy of performance test procedure for high damping rubber bearing based on various dependency
evaluation.” Architectural Institute of Japan Journal of Technology and Design, Vol. 8, No. 8,101-106. (in Japanese)
Architectural Institute of Japan (2001). “Recommendation for the design of base isolated buildings”,
third edition, 189-195. (in Japanese)

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