thThe 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

SEISMIC ISOLATION RETROFIT OF A MEDICAL COMPLEXBY INTEGRATING TWO LARGE-SCALE BUILDINGS
1

Yoe Masuzawa and Yoshiaki Hisada
12Manager, Risk Management Dept., Engineering and Risk Services Corporation, Japan
Professor, Dept. of Architecture, Faculty of Engineering, Kogakuin University, Japan
Email: masuzawa@ers-co.jp, hisada@cc.kogakuin.ac.jp

2

ABSTRACT :We developed a methodology of the seismic isolation for retrofitting a group of buildings in one unit using
post-tensioned cables, and evaluated their safety and functionary based on the performance based design. We
applied this methodology to two large-scale buildings of Hamamatsu Medical Center, which are the first
hospital retrofitted by the seismic isolation in Japan. The two buildings are both steel-reinforced concrete
frame structures of the nine-story on the ground and the one-story on the underground level. They wereconstructed in 1973 and 1975, based on the old building code before 1981. In the proposed retrofit scheme,
the two buildings were integrated into one structural unit by connecting together at each floor by using
post-tensioned cables through slabs. We confirmed that the integration worked well by comparing
microtremors of the buildings before and after the integration. Seismic isolation devices were set up mainly
in columns of the underground level. We adopted the temporary supporting method using post-tensioned
units to install the devices safely and economically (Masuzawa et al., 2004). In the seismic design phase, wefirst simulated the broadband input earthquake ground motions for a hypothetical M8-class earthquake in thevicinity of the site, using a hybrid method (Hisada, 2000, etc.). Finally, we confirmed the safety andfunctionary of the medical center by evaluating the seismic performance of the buildings, based on the time
history seismic response analysis.
KEYWORDS:

Seismic isolation retrofit, Structural integration of buildings, Medical complex,
Performance-based design, Site-specific strong ground motion prediction

1. INTRODUCTION
Hamamatsu Medical Center, which consists of five buildings for medical treatment functions with more than six
hundred beds, is one of the most important hospitals in the Shizuoka prefecture in Japan. Even though it isexpected that the center has to maintain the building function and the emergency medical operation for a large
earthquake, the two buildings were designed by the old seismic code and required seismic a retrofitting byseismic diagnosis. As an example of the necessary for retrofitting of old hospitals, the Ojiya hospital in the
Ojiya city suffered severe damage during the 2004 Mid Niigata Prefecture earthquake, and could not efficiently
continue emergency medical operations. Even though new three buildings of the hospital suffered little damage,
most medical functions were lost by the damage of the other three old buildings. It was also required a lot oftime to restore and recover the buildings and their functions.
Hamamatsu Medical Center will lose its functions and emergency operations for a major earthquake, because of
the damage of the old two buildings. Therefore, it was necessary to retrofit the two adjoining buildings
effectively. It was also required not to stop the facilities’ functions during retrofitting and constructing. Fromsuch background, we developed a methodology of seismic isolation retrofit by integrating the two buildings into
one unit. In this paper, we first explain the details of the structures of the two buildings, and the retrofitting
method. Second, we explain the methodology for predicting the site-specific strong ground motions for a M8
Tokai earthquake in a subduction zone under the site. Finally, we evaluate the performance and safety of theretrofitted buildings using on time history response analysis to the assumption earthquake.

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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

2. DESCRIPTION OF THE BUILDINGS OF HAMAMATSU MEDICAL CENTER
Figure 1 shows a bird’s-eye view of Hamamatsu Medical Center, and Figure 2 shows the first floor plan andother typical floor plan. The two buildings for the seismic retrofitting are Building No.1 and No.2 in Figure 2.
They are both the steel-reinforced concrete frame buildings of the nine-story on the ground and one-story on theunderground level. Both buildings have penthouses of three floors on their roofs. Building No.1 and No.2 werecompleted in 1973 and 1975, respectively. Table 1 shows the description of the two buildings. The plan of the
Building No.1 is rotated to about 30 degree from the main axis of the hospital buildings; its length is about 72mwith eleven spans, and its width is about 22m with three spans. Building No.2 is almost rectangular with the
50.4m length of eight spans, and with the 22.6m width of three spans.
Table 1 Description of buildings

Figure 1 Bird’s-eye view of Hamamatsu Medical Center

Building No.3

Building nameYear completed
Building area
Total floor area
Address
Number ofstories
Structural type
Eaves height
Structural
systemFoundation type
Bearing stratum

No.1
No.2
1973
1975
2,035m2
1,532m2
2
12,915m
10,008m2
Hamamatsu City, Shizuoka Pref.
nine-story on the ground andone-story on the underground level
Steel-reinforced concrete structure
37.10mMoment-resisting frames withshear wall
Spread foundation
Silty fine sandConnecting building

30m

N

EXP. J

Building No.2
South building

Building No.1

1st floor plan

EXP. J
Building No.3

Building No.2

Building No.1

9th floor plan

Figure 2 First floor plan (left) and typical floor plan (right)

3. METHODOLOGY OF SEISMIC RETROFIT
3.1. Outline of Seismic Retrofit
Figure 3 shows the framing elevation after it retrofits of the two buildings. In the proposed retrofit scheme, first,
the two buildings were connected together at each floor using post-tensioned cables through slabs, and second,
they were isolated mainly on the basement floor using eighty-nine seismic isolation devices. Before theconstruction works, we renewed and moved all the building equipment and facilities from the underground floor
to the rooftop, and also moved medical equipment which would disturbed by the construction works.
Consequently, we could carry out the construction works not to stop the building function and medical services.

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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

3.2. Integration of The Two Buildings and Microtremor Measurements
Figure 4 shows the detail drawing of the connections of the two buildings for the typical floor. We assume theconnections between the existing frames and the newly constructed slabs not to resist against the out-of-plane(bending) of the slabs, because they are connected only with PC cables through the slabs. Accordingly, theconnections not only secure the strength in-plane of the slabs, but also do not give unnecessary stresses on theexisting frames. We secured the safety against the deflection by the PC strand cables with small adhesion, andby reduction of the prestressing force to 80 percent of the allowable load.
We measured the microtremor before and after the integration of the two buildings. Figure 5 shows comparisonsof the predominant directions on the ninth floor during microtremors. The predominant directions of the twobuildings were different before the integration, whereas the directions became almost same after the integrations.
Therefore, we confirmed that the integration worked effectively.
3.3. Seismic Isolation Retrofit
Figure 6 shows the arrangement of eighty-nine devices; seventy-five were in columns of the underground floor,
eight under elevator shafts and six under the entrance base. We used four rectangular natural rubber bearings(diameter 900mm), fifty-one rectangular lead rubber bearings (diameter 900mm), four elastic sliding supports(diameter 300mm), and thirty cross linear bearings (six kinds with a different load limit) for the isolation system.
In order to install the devices safely and economically, we used the temporary supporting method based on
post-tensioned units (Masuzawa et al., 2004), and confirmed their validity through full-scale experiments.
Figure 7 shows the construction process of the temporary supporting system. We used four 3000kN hydrauliclifters on one column, and scheduled less or equal four sets of the temporary supporting systems and the seismic
isolation devices as one unit on the construction process. In order to ensure the earthquake resistant performanceof 0.2 G even under construction, we installed the temporary steel brace and other earthquake resistant elements.

Building No.2

Building No.1

4200

3000

8000

Prestressed concrete slabs

8400

Seismic isolation layer

600

500

600

PC cable

5000

6000

See Figure 4

Figure 3 Framing elevation of No.1 and No.2

Vibration period: 0.59sec
No.3

No.1

No.2

[Before connection]

Figure 4 Plan (up) and section (down) ofconnecting slab
Vibration period: 0.58sec
No.3

No.1

No.2

[After connection]

Figure 5 Predominant directions using microtremor before and after connecting No.1 and No.2

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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

LR
B9
00
LR
LR LR
B9
B9 B9
00
00 00
LR
LR
B9
B9
00
00
LR
LR
LR
B9
B9
B9
00
00
00
LR
LR
LR
B9
B9
B9
00
00
00
C
C
LR
LB
LB
B9
10
10
00
00
00
C
C
LR
LB
L
B7
B9
10
80
00
00
C
LB
25
LR
0 LR
B9
B9
C
00
00
LB
61
LR
LR
LR
C
LB
B
B
B9
90
90
25
00
C
0
0
0
LB
61
LR
C
C
C
LB
LB
LB
B9
25
78
78
00
C
0
0
0
LB
SL
61
30
LR
LR
C
C
C
L
L
LB
0
B
B
B
B
90
90
78
13
78
C
0
0
0
3
0
SL
LB
30
LR
61
LR
LR
LR
C
0
B
LB
B9
B9
B9
90
13
0
00
00
00
SL
C
3
LB
30
L
LR
R
61
0
LR
C
B9
B
LB
SL
LR
90 L
0
B
0
90
13
30
B
0
R
90
C
B9
0
3
0
LB
0 L
00 LR
61
R
LR
C
B9
B9
LB
B9
00 R
00 LR
25
00 LR
B9
B9
0
B9
00 R
00 LR
00 LR
C
B9
B
LB
90 L
B9
00
25 C
0
R
0
B9
0 LR
0 L
B9
B2
00 LR
00 LR
50 C
B9
LB
B9
00
13 C
00 LR
3 L
B
B1
9
00 LR
33 C
LB
B9
13 C
00
3 L
B2
50 C
LB
38
5

RB: Natural rubber bearing (4)LRB: Lead rubber bearing (51)
SL: Elastic sliding support (4)Building No.2

Building No.1

LR
B9
00
LR
B9
00
LR
B9
00
LR
B9
00
LR
B9
00
LR
B9
00
LR
B9
00

R
B9
00

C C
LB LB
38 38
5 5

R
B9
00

CLB: Cross linear bearing (30)

Figure 6 Arrangement of seismic isolation devices

1 retrofit

2 reinforcement

Existing frame before

Structural member

3 installation and tensioning
Steel brackets and cables

Steelbracket8 removal, and completion
Temporary supporting

7 installation and fixation

Seismic isolation device

PC cable

6 Existing column removal

4 and preloading

Hydraulic lifters installation

Hydrauliclifter

5 Existing column cutting
Wire saw installation /Seismic
isolation
deviceWire saw

Figure 7 Construction process by the temporary supporting methodth

The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

4. SIMULATION OF SITE-SPECIFIC STRONG GROUND MOTION
In the seismic design phase, we checked the safety and functionary of the medical center during earthquake bysimulating a hypothetical M8-class Tokai earthquake in proximity to the site, which is located in subduction
zone of the Suruga trough and estimated the 86% occurrence probability in 30 years. In order to makebroadband input earthquake ground motions for the performance based design, we simulated site-specific strong
ground motions using a hybrid method (Hisada, 2000, etc.), which combines theoretical and statistical methods
at low and high frequencies, respectively. Figure 8 shows the hypothetical Tokai earthquake seismic fault model.
The main source parameters and slipping displacements of asperities are shown in Table 2. We made the source
model based on the asperity model of the Central Disaster Management Council of the Cabinet Office,
Government of Japan. We modeled the deep ground structure by a flat-layered structure model from seismic
bedrock (Vs=3000 m/s) to engineering bedrock (Vs=510 m/s) as shown in Table 3. We evaluated the seismic
waves at the building basement (8 m in depth and Vs=220 m/s) using the equivalent-linear earthquake response
analysis based on an one-dimensional stress-strain relationship. We simulated the input seismic motions byconsidering the three different hypocenters as shown in Figure 8. Figure 9 shows pseudo velocity response
spectra of horizontal components. We selected the seismic wave with the largest amplitude level at effective
period of the building after the retrofit (horizontal direction: approx. three seconds, vertical direction: approx.
0.1 seconds). Figure 10 shows the EW components of the acceleration, velocity and displacement, respectively,
for Tokai-3 model, which is the severest case. In addition to the site-specific ground motions, we made thesynthesized several input ground motions, which are required by the current building code. Table 4 shows the
maximum amplitudes of all the earthquake ground motion waveforms used for the time history response
analysis.

°

rake

89.0

(3)

strike

Asp.6
Asp.5

(1)

35°N

35°N

Asp.4
Asp.2

(2)

rake

(3)

Fracture initiation point

Asp.5

(1)

°

strike

Asp.6

Fracture initiation point

Y(E)

208.

Y(E)

208.
89.0

0°

X(N)

0°

X(N)

Asp.4
Asp.3

(2)

SITE

Asp.3

SITE
KiK-net(SZOH28)

1 54

15 2

km

.1k
m

KiK-net(SZOH28)

Asp.2

137°E

138°E

F re
8 5.5 k

m

34°N
eS
urf

ace
km

25 .
2km

dip

ace

8.3

m

Z

15 .0
°

.4k
m
89.2 k

34°N
eS
urf

km

F re

7.3

dip

Asp.1

30

25 .
2km

Z

15 .0
°

30

.4k
m

Asp.1

139°E

137°E

138°E

139°E

Figure 8 Tokai earthquake seismic fault model used for theoretical method (left) and statistical method (right)

Strike208°
Asperity 1
4.8

Table 2 Main source parameter and slipping displacement of each asperity
Dip
LengthWidthUpper depth
Slip15°
154.14 km
89.25 km7.28 km89°
Asperity 2
Asperity 3
Asperity 4
Asperity 5
Asperity 6
6.93
3.35
4.84
2.78
3.9

Rupture vel.
2.7 km/s
back ground
1.78

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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

Table 3 Deep ground structure model
Layer
No.
1
2
3
4
5
6

Depth
m
50～200200～840840～900
900～1000
1000～1900
1900～

Thickness
m
150
640
60
100
900

Density
g/cm3
2.1
2.3
2.5
2.5
2.5
2.6

1000

Vp
m/sec
2020
2280
2870
4140
4600
5300

Vs
m/sec
510
840
1280
1840
2500
3000

Reference origin
KiK-net observation point (SZOH28)
KiK-net observation point (SZOH28)
KiK-net observation point (SZOH28)
KiK-net observation point (SZOH28)
Central Disaster Management Council
Central Disaster Management Council

10
0g
al

Acceleration (gal)1000
tokai-1_NS
tokai-1_EW
tokai-2_NS
tokai-2_EW
tokai-3_NS
tokai-3_EW

Max:624.73gal

0

-1000
100

Pseudo velocity (cm/sec)

100

m
0c
10

Velocity (kine)

Max:92.80kine

0

-100

10

cm
10

Displecement (cm)200

Max:141.58cm

0

-200

1
0.1

1

10

0

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

Period (sec)Time (sec)Figure 9 Velocity response spectra of simulated waves

Figure 10 Simulated waves for Tokai-3_EW

Table 4 Maximum amplitudes of the earthquake ground motion waveforms used for the response analysis(the names of ground motions based on the building code show phase characteristic model)
Acceleration (cm/s2)
Velocity (cm/s)
Displacement (cm)Tokai-3_EW
624.7392.80
141.58Site-specific groundmotions
Tokai-3_UD
215.82
26.57
26.98
Random635.1675.62
22.85
Building code
El centro_NS
657.7776.98
26.76(very rare level)
Taft_EW
717.0975.19
27.16
Hachinohe_NS
630.7598.75
24.37

5. EVALUATION OF SEISMIC PERFORMANCE OF THE BUILDING
5.1. Outline of Performance-Based Seismic Design
In order to secure the seismic safety and functionality of the building after the retrofit, we carried out the time
history seismic response analysis. First, we constructed the three-dimensional frame model as shows in Figure
11. We confirmed that the theoretical vibration modes were nearly equal to those of the microtremor
measurements. Then, using the static load incremental method considering the inelasticity of the structuralth

The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

frame, we analyzed each building and estimated the stress and deflection of component elements, the axial forceof the seismic isolation devices and the ductility of each layer, etc. Next, we made the seismic response analysis
model based on static analytical results, and carried out the time history seismic response analysis. Figure 12
shows the analysis model. We made the parallel multi-lumped mass model by concentrating the masses on eachfloor location. We confirmed that the response results satisfied the seismic performance targets for both theupper structure and the seismic isolation layer
5.2. Seismic Performance Targets
Table 5 shows the earthquake performance targets used in our response analyses. We used I=1.25 as for theimportance factor for the buildings. We set the story drift angle for the main frames of the buildings to be 1/250
radian or less. We assumed that the structural members were less than the shear failure with the yield hinges in someboundary girders. We also assumed that the criterion of the seismic isolation devices was within the safety
deformation level, and the under structure was less than the allowable stresses.
5.3. Results of Evaluation
We evaluated the seismic performance of the retrofitted building based on static and dynamic analyses. Figure
13 shows the time history response analysis results of the upper structure and the seismic isolation layer after
the seismic retrofit. The results confirmed all the target values were satisfied. In addition, we secured thefunctionality of the building and securing medical operations after the earthquake, because the floor response
acceleration roughly becomes 300gal or less.

[Building No.1][Building No.2]
Figure 11 Three-dimensional frame model

Figure 12 seismic response
analysis model

Table 5 Earthquake performance targets for the Site-specific ground motions and building code (very rare level)- within elastic strength level of each layer
Upper structure- below 1/312.5 (=1/250/1.25) of story drift angle
Earthquake performancetargetSeismic isolation layer- within the safety deformation level(below 473.2mm (=591.6/1.25) for the rubber bearings)- within the allowable tensile stress level(below 0.8N/mm2 (=1/1.25) for the rubber bearings)

Foundation structure- within the allowable stress levelth

The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

7

8

6

7

5

6

4

5

3

4

2

3

1

2

0.001

0.002

0.003

9
8

7

7

6

6

5
4

5

3

4
3

2

2

1

X方向
dir.

0.004

0.0

層間変形角
Story drift
angle (rad.)

10.0

20.0

30.0

40.0

50.0

1

Ｙ方向
dir.

60.0

0

絶対変位(cm)
Displacement
(cm)

0.001

0.002

0.003

0.004

dir.
Y方向

0.0

層間変形角
Story drift
angle (rad.)

Ｒ

8

9

7

8

7

8

6

7

6

7

5

6

5

6

5

4

3

4

3

3

2

2

1

第1せん断破壊First shear
発生時/1.25
failure / 1.25

2
1

X方向
dir.
0.000

0.200

0.400

0.600

0.800

1
0.0

層せん断力係数
Story-shearing
force coefficient●：Random○：ElCentro(ELNS)
×：Taft(TAEW)△：Hachinohe(HANS)◇：Tokai-3EW

階

9

9

階

Ｒ

8

4

400.0

600.0

応答加速度(gal)Acceleration
(gal)

800.0

30.0

40.0

50.0

60.0

4
第1せん断破壊First shear
発生時/1.25
failure / 1.25

3
2

Y方向
dir.
0.000

20.0

5

X方向
dir.
200.0

10.0

絶対変位(cm) (cm)Displacement

9

階

階

Ｒ

8

1

X方向
dir.
0.000

1/250/1.25
9

階

8

9

階

Ｒ

階

階

1/250/1.25
9

0.200

0.400

0.600

0.800

1
0.0

層せん断力係数
Story-shearing
force coefficient

Y方向
dir.
200.0

400.0

600.0

800.0

応答解析(gal) (gal)Acceleration●：Random○：ElCentro(ELNS)
×：Taft(TAEW)△：Hachinohe(HANS)◇：Tokai-3EW

Figure 13 Time history response analysis results of the upper structure and the seismic isolation layer(stiffness of seismic isolation layer: hard case)

6. SUMMARY
We developed a methodology of the seismic isolation for retrofitting a group of buildings in one unit using
post-tensioned cables, and applied it to Hamamatsu Medical Center. We showed the details of the methodology
for integrating the two buildings using cables, and for retrofitting those buildings. We also evaluated their safety
and functionary based on the performance based design by simulating realistic strong ground motions from a
Tokai earthquake. Finally, we evaluated the effectiveness of the retrofitting for maintaining the building
function and the emergency medical operation for a large earthquake using the time history response.

ACKNOWLEDGEMENTS
We thanks greatly for the understanding and cooperation of participants of the Hamamatsu City and theHamamatsu Medical Center on the design and construction phase. Also, we would like to express our gratitude
to Mr. Takashi Yamada and Mr. Eiji Yoshikawa, who cooperated in the design and supervision.

REFERENCES
Masuzawa, Y. and Hisada, Y. (2004). Seismic Isolation Retrofit of A Prefectural Government Office Building.
Proceedings of the 13th World Conference on Earthquake Engineering, No.1199.
Hisada, Y. (2000). A Hybrid Method for Predicting Strong Ground Motions at Broad-frequencies Near M8
Earthquakes in Subduction Zones. Proceedings of the 12th World Conference on Earthquake Engineering,763/4/A.