seismic isolation retrofit of a medical complex by integrating two large-scale buildings

Masuzawa, Y. and Hisada, Y.

Paper:

Seismic Isolation Retrofit of a Medical Complex by IntegratingTwo Large-Scale BuildingsYoe Masuzawa and Yoshiaki Hisada

Risk Management Department, Engineering and Risk Services Corporation, JapanAkasaka Kikyo Bldg., 3-11-15 Akasaka, Minato-ku, Tokyo 107-0052, Japan

E-mail: [email protected] of Architecture, Faculty of Engineering, Kogakuin University, Japan

1-24-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo, 163-8677, JapanE-mail: [email protected]

[Received March 8, 2009; accepted May 25, 2009]

We developed a methodology of seismic isolationretrofit integrating adjacent buildings using pre-stressed concrete slabs, and applied it to two large-scale buildings in Hamamatsu City in Shizuoka Pre-fecture in Japan. It is the first seismic isolation retrofitof hospital in Japan. The two steel-reinforced concretebuildings were nine stories high with one basement,and had been constructed in 1973 and 1975 basedon an old structural design code. The two buildingswere integrated into one building by connecting indi-vidual floors using post-tensioned prestressing cablesthrough slabs. A comparison of microtremors beforeand after the integration confirmed that the integra-tion worked well. Seismic isolation devices were setup mainly in basement columns using temporary sup-port involving steel brackets and prestressing cables toinstall devices safely and economically (Masuzawa etal., 2004 [1]). In the seismic design phase, broadband-generated earthquake ground motions for a hypothet-ical Magnitude 8 earthquake near the site were simu-lated using a hybrid method (Hisada, 2000 [2], etc.).Safety and functionality were verified by evaluatingstructural seismic performance based on time-historyseismic response analysis.

Keywords: seismic isolation retrofit, structural integra-tion of buildings, medical complex, performance-baseddesign, site-specific strong ground motion prediction

1. IntroductionSeismic risk mitigation of the hospitals which become

medical treatment bases at the time of disasters is very im-portant in high seismicity countries such as Japan. Thosehospitals need to ensure not only the safety of buildingsbut also the operability of medical treatment even dur-ing and after large earthquakes. The seismic isolation isone of the most effective and practicable countermeasuresagainst earthquakes because it drastically reduces seis-mic response due to devastating ground shaking. Sinceextensive damage of hospitals was experienced by theGreat Hanshin-Awaji Earthquake in 1995, seismic isola-tion structures have been adopted for a lot of new hospital

buildings in Japan. On the other hand, old and new build-ings often exist together adjacently in large-scale hospi-tals, and earthquake damages of the old buildings maysignificantly reduce the entire functions of medical treat-ment. The need to retrofit aging hospitals was made alltoo clear when three old structures of a six-building facil-ity in Ojiya City were so severely damaged in the 2004Mid-Niigata Prefecture earthquake that emergency medi-cal operations could not be maintained [3]. Typical struc-tural damage involved building joints, nonstructural com-ponents, furniture, and equipment, as shown in Fig. 1.It also took much time to restore the buildings and theirfunction.

A similar situation will probably happen in the hospi-tal of Hamamatsu City that is located in a high seismic-ity area in Japan. The hospital will lose its functions andemergency operations for a large earthquake, because ofdamage of the two old buildings. Therefore, it was nec-essary to retrofit those two buildings effectively. Givenits central location and importance as a medical treat-ment facility, the hospital would have to continue its func-tions and emergency service during retrofitting and recon-struction. The sections that follow provide a backgroundof the retrofitting methodology, building integration, andthe evaluation of microtremor measurement. Site-specificstrong ground motions in a hypothetical Magnitude 8earthquake in a subduction zone under the site are thensimulated, and the performance and safety of retrofittedbuilding evaluated using time-history response analysis ofsimulated earthquake motion.

2. Hamamatsu Medical CenterHamamatsu Medical Center, a five-building treatment

complex having over 600 beds, is one of Shizuoka Pre-fectures most important medical facilities, and one whosebuilding function and emergency medical services wouldbe needed in a large earthquake. The two buildings, de-signed under the old seismic design code, were found inseismic diagnosis to be inadequate under the current code.

Figure 2 shows a birds-eye view of Hamamatsu Med-ical Center and Fig. 3 the first floor and typical floor plansof existing hospital. Each building was structurally in-

208 Journal of Disaster ResearchVol.4 No.3, 2009

Seismic Isolation Retrofit of a Medical Complex by IntegratingTwo Large-Scale Buildings

(a) Structural damage (b) Building joint damage [3]

(d) Furniture damage [3] (e) Overhead tank movement [3] (f) Piping joint damage

(c) Nonstructural component damage [3]

Fig. 1. Typical damage of a hospital in the 2004 Mid-Niigata Prefecture earthquake (Photos were taken by Yoe Masuzawa).

South building

Connecting building

Building No.1

Building No.2

Building No.3

Fig. 2. Birds-eye view of Hamamatsu Medical Center (thephoto referred to the medical center brochure).

dependent, but arranged adjacently, and connected withthe expansion joint mutually. Building No.1, built in1973, and Building No.2, built in 1975, and now to beretrofitted, have steel-reinforced concrete frames, ninestories and one basement, and three-story penthouses ontheir roofs, as detailed in Table 1. Building No.1 is aplan rotated 60 degrees at the center of building. BuildingNo.2 is nearly rectangular. In those buildings, diagnosisand treatment sections are arranged in low layer floors,and medical wards are located in upper floors.

3. Seismic Retrofitting Methodology

3.1. Overview

Seismic retrofitting, started in autumn 2006, was com-pleted as scheduled in autumn 2009. Fig. 4 shows fram-ing elevation of the two buildings after retrofitting. Un-der proposed retrofitting, prestressed concrete slabs would

Building No.1Building No.3 Building No.2

9th floor plan

30m

EXP. J

Building No.1

Connecting buildingBuilding No.3

South building

Building No.2

N

1st floor plan

EXP. J

Fig. 3. Floor plans of Hamamatsu Medical Center.

Table 1. Building description.

Building No. 1 2 Year completed 1973 1975

Building area (m2) 2,035 1,532 Floor area (m2) 12,915 10,008

Site Hamamatsu City, Shizuoka Prefecture Stories 9 plus 1 basement

Building material Steel-reinforced concrete structure Eave height (m) 37.10

Structure Moment-resisting frames and shear wallsFoundation Spread Site stratum Fine silt sand

Journal of Disaster ResearchVol.4 No.3, 2009 209


Connection by prestressed concrete slab (Fig. 5.)

Seismic isolation device

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

50400 5000 300006000 6000 6000 6000 6000 6000 6000 8400 6000 6000 6000 6000 6000

16 17 18 19 20 21 22 23 24

42000

GL

3000

5260

023

0050

0042

5043

0044

0039

5039

0039

0038

5038

5038

0030

0026

00

B1FL

1FL

2FL

3FL

4FL

5FL

6FL

7FL

8FL

9FL

RFL

PH2FL

PH3FL

PHRFL

B.PL

5150

6000 6000 6000 6000 6000 6000 6000

Building No.165

025

0Building No.2Building No.3 Building No.2

Fig. 4. Framing elevation of Building No.1 and No.2 (unit: mm).

be installed and connected between the two buildings oneach floor. Buildings would then be isolated mainly un-derground using 89 seismic isolation devices. Before re-construction, all building equipment and facilities wererenewed and moved from the basement to the roof andmedical equipment potentially disturbed by reconstruc-tion work moved, enabling reconstruction to be conductedwhile building functions and medical services continued.

3.2. Building Integration and Microtremor Mea-surement

3.2.1. Integration OverviewFigure 5 shows the connection section and plan for

a typical floor. To ensure joint strength and ductility,slabs were connected to buildings using post-tensionedprestressing cables penetrating both slabs and adjacentgirders and anchored to existing building frames. An-chorage zones were fastened to frames by anchor dow-els to transmit stress between slabs and frames. Connec-tions were assumed to not resist slab out-of-plane (bend-ing) because they were connected only with prestressingcables through the centers of slabs. Connections thus bothensured slab in-plane strength and avoided placing un-due stress on existing frames. Safety against cable strandelongation was ensured with minimum adhesion and byreducing prestressing force to 80% of the allowable load.Seismic performance is evaluated in detail Section 5.1.

3.2.2. Microtremor Measurement and Building Vi-bration Features

To investigate building vibration features before and af-ter integration, microtremors were measured on Novem-ber 21, 2004, before connection, and on June 16, 2007,

Building No.1Building No.2

4200 3000

600

600

500

8000

Prestressing cable

Plan

Section

Rebar dowels

5000

Anchorage zone

Fig. 5. Detail of buildings connection (unit: mm).

after connection [4, 5]. The integrated building was mea-sured when connection was completed on floors 7-9 andthe roof before seismic isolation construction was started.Building microtremor records, natural periods, particleorbits, and vibration-mode shapes were obtained, and theeffects of connection confirmed. Fig. 6 shows where mi-crotremor sensors were deployed. Recording used servovelocity sensors, a 16-bit analog-to-digital converter, anda notebook PC. Longitudinal (X) and transverse (Y) com-ponents of microtremors in velocity were recorded at eachlocation. The sampling rate was 100 Hz and the recordingof each record was 180 seconds long. Three sets of sam-plings were recorded for each pattern. The following fivepatterns were observed in simultaneous measurement byup to eight sensors:

Pattern 1: 1C, 1E, and 1W on the ninth floor by recording



Building No.1Building No.2Building No.3 37

400

5000

8550

1225

011

600

B1F1F

3F

6F

9F

[2W] [2C] [2E][1W] [1C]

[1E]

[12C]

[3C]

Fig. 6. Locations of microtremor sensors (unit: mm).

Building No.1Building No.2 Building No.3

X

Y

CH-1

CH-2

CH-3

CH-4

CH-5

CH-6

CH-7

CH-8

Connecting location

Fig. 7. Sensors layout in observation pattern 3 at ninth floor.

in two directions simultaneously.

Pattern 2: 2C, 2E, 2W, and 1C on the ninth floor byrecording in two directions simultaneously. 1C was setonly for the integrated building.

Pattern 3: 1C, 2C, 3C, and 12C on the ninth floor byrecording in two directions simultaneously. 12C was setonly for the integrated building.

Pattern 4: 1C on the ninth, sixth, third, first, and basementfloors by recording alternately in two directions. The sen-sor in the basement was set only for the existing building.

Pattern 5: 2C on the ninth, sixth, third, first, and basementfloors by recording alternately in two directions.

Figure 7 shows the sensor layout for Pattern 3. Mi-crotremor sensors were installed on the ninth floor nearthe center of gravity of each building and in the connect-ing location. Fig. 8 shows Fourier amplitude spectra formicrotremors obtained in the same observation pattern.Histories 20.48 seconds long were selected from records,followed by zeroes 20.48 seconds long, and put throughFourier transformation to obtain spectra smoothed with a0.2 Hz Parzen window. Before the buildings were con-nected, the predominant period in the transverse (Y) di-rection of Building Nos.1 and 2 were equaled 0.59 sec-onds, but a variation in the peak period was also confirmedin spectra. Note that after connection, Building Nos.1and 2 and the connecting location show concordance inpredominant periods in each direction. To determine thepredominant direction in each peak period, the horizon-tal particle orbit was obtained with each velocity recordintegrated to displacement with a band-pass filter whose

[After connection]

[Before connection]

0.2 0.3 0.4 0.5 0.6 0.7 0.80

1

2

3 CH-1 Y CH-2 X CH-3 Y CH-4 X CH-5 Y CH-6 X CH-7 Y CH-8 X

0.2 0.3 0.4 0.5 0.6 0.7 0.80

1

2

3 CH-1 Y CH-2 X CH-3 Y CH-4 X CH-5 Y CH-6 X

Four

ier s

pect

rum

(mki

ne*s

ec)

Period (sec) Fig. 8. Fourier amplitude spectra for microtremors obtainedat ninth floor (Pattern 3).

Vibration period: 0.59 sec

Vibration period: 0.58 sec [After connection]

[Before connection] Building No.1Building No.2 Building No.3

Fig. 9. Predominant directions using microtremor beforeand after connecting No.1 and No.2.

typical period is the predominant period of each mode.From displacement time histories in the longitudinal andtransverse directions, 1-second sections with high ampli-tude were selected and horizontal particle orbits shown.Vibration-mode shape for each natural period was ob-tained the same as for horizontal particle orbits. Fig. 9shows horizontal particle orbits obtained on the ninth floorin Pattern 3 with band-pass filters at each of 0.59 sec-onds before buildings were connected and 0.58 secondsafter connection when the peak periods of Building Nos.1and 2. The predominant directions of the two buildingsdiffered before connection, but corresponded after con-nection, clearly showing that integration was effective.Note also that Building No.3, which was not connectedto the integrated buildings, vibrated independently. Natu-ral periods and the vibration modes for individual and in-tegrated buildings are shown based on microtremor mea-



Building No.2

Building No.1

+

,

4000

2260

061

0086

0079

0025

0061

00

50400 5000 300006000 6000 6000 6000 6000 6000 6000 8400 6000 6000 6000 6000 6000

4200

0

6000

6000

4500

7000

8000

70002500

16797

6088

5450

&

:

;

:

;

4000

300030003000300030003000300030003000300030002000

3110

3600

1700

766

1670x465CLB250CLB133CLB061

1570x465

51900x900LRB900S

CLB780CLB1000 2480x1270

53

1970x740

1500x345SL300S 300x300 4RB900S 900x900 4

CLB385 1770x555 3

Symbol NumberType Size (mm)Lead rubber bearing

Cross linear bearing

Elastic sliding supportNatural rubber bearing

Seismic isolation device

6000

6000

6000

6000

6000

22000

5000

6088

4620

Building No.3

Connecting building

NLmax (kN)

Note: "NLmax" shows calculated maximum reaction force under normal load

2,3801,515712

6,036

8,79812,054

6445,311

3,743

Fig. 10. Arrangement of seismic isolation devices (unit: mm).

900

100

225

225

1350

LRB900S RB900S

419.

140

4049

9.1

900

100

225 2251350

368

4040

448

465

1670

4191650

615.5 615.5

132.

625

28

185.

655

015

50

6251500250 625

300

CLB250 SL300S

Lead plugStainless steel plate with special lubrication filmDowel

Dowel

Rubber shim

Linear Motion railLinear Motion block

Laminated-rubber

Linear Motion rail

Linear Motion block

Laminated-rubberPTFE sliding surfaceRubber layers: 34 x 5.8 mm

Steel plates: 33 x 4.3 mm

Rubber layers: 12 x 3.0 mmSteel plates: 11 x 2.2 mm

Gross weight: 9.4 kNFriction coefficient: under 0.01

Gross weight: 13.8 kNFriction coefficient: 0.01Shear modulus of rubber: 0.588 MPaGross weight: 26.9 kN (LRB) / 26.6 kN (RB)

Shear modulus of rubber: 0.392 MPa

Fig. 11. Constitution of seismic isolation devices (unit: mm).

surement in conjunction with eigenvalue analysis resultsin Section 5.1.

3.3. Seismic Isolation RetrofitFigure 10 shows the arrangement of 89 seismic iso-

lation devices 75 in basement columns, 8 under eleva-

tor shafts, and 6 under the entrance base. We used 51lead rubber bearings (LRB) 900 mm on a side, 4 naturalrubber bearings (RB) 900 mm on a side, 4 elastic slid-ing supports (SL) 300 mm on a side, and 30 cross-linearbearings (CLB) of 6 different types with different loadlimits for isolation. Fig. 11 shows four types of seismic



300 10050

Existing column

400PC cable (SEEE F200)

580

140019

50

175

250

1350

1000

Steel bracket(grade: SS400 (JIS G 3101))

220

580

435 830 435

270

1100

1950

Non-shrink mortar(40 mm thickness)

400

175

1350

1000

300 3001000

1600

300 3001000

1600

90

100

400

600

400 150

235

400 150 400 100

235

1700

6565

Bond surface of steel bracket

Shear cotter bar(bar size: D13 (13 mm rebar))(grade: SD295A (JIS G 3112))

1600

300

Reinforcement column(specified compressive strengths: 36 MPa)

Hydraulic jack(capacity: 3000 kN)

Fig. 12. Temporary supporting system (six-cable type)(unit: mm).

isolation device in the seismic isolation layer. Bearingratio to the building weight (sustained loading) of LRB,RB, CLB, and SL was 64%, 5%, 30%, and 1% respec-tively. Square rubber bearings were used to make rein-forcement columns as small as possible. The cross-linearbearing combines orthogonal linear motion (LM) guidesconsisting of LM rails and blocks up and down and vary-ing in size with the load capacity. Elastic sliding supportsconsist of laminated rubber with polytetrafluoroethylene(PTFE) friction surfaces and stainless steel plates withspecial lubrication film.

Steel brackets and prestressing cables were used in tem-porary support to ensure that isolation devices were in-stalled safely and economically (Masuzawa et al., 2004[1]). Fig. 12 shows temporary support used to insert aseismic isolation device in a column. The number of pre-stressing cables used depended on the maximum reactioncalculated for each column. The feasibility of temporarysupport was confirmed through full-scale experiments [6],with an example of experimental results shown in Fig. 13.Note the load-displacement relationship of the six-cablespecimen, which was measured at prestressed joints be-tween steel brackets and reinforcement columns. Full-

0

5

10

15

20

0 1 2 3 4 5Vertical displacement (mm)

Ver

tical

load

P (M

N).

0

5

10

Shea

r stre

ngth

(M

Pa)/*=1.0

/*=0.5

calculated reaction force of column

*: confinement stress

Pmax=17.677 MN

Fig. 13. Load-displacement relation of the prestressed joint(six-cable type).

scale tests showed that vertical load support was suffi-cient. Fig. 14 shows construction of temporary supportfrom phases 1 to 8. In phases 1 to 2, structural members ofthe existing frame underground are reinforced except forintermediate parts of the column. In phase 3, steel brack-ets and prestressing cables are installed and prestress in-stalled in cables. In phase 4, hydraulic jacks are installed,preloading force acts on brackets, and the axial force ofthe column is released. In phases 5 to 6, a diamond wiresawing machine is installed on the column and the ex-isting column cut off and removed. In phase 7, seismicisolation devices are installed and upper and lower jointsfixed using high-flow concrete or nonshrink mortar. Inphase 8, all temporary support components are removed,completing the job. A maximum of four temporary sup-port sets were used together and rotated in the construc-tion flow. To ensure earthquake resistance of 0.2 G evenin the middle of construction in the basement, temporarysteel braces and other earthquake-resistant elements wereinstalled. In basement usable as floor area, seismic isola-tion retrofitting was implemented, and then fireproof pan-els attached to columns to enclose seismic isolation de-vices.

4. Site-Specific Strong Ground Motion Simula-tion

A hypothetical Magnitude 8 earthquake near the sitewas simulated in the seismic design phase. A fault modelwas located in a subduction zone of the Suruga Troughwhere a very high possibility exists of earthquake oc-currence in the near future. To create broadband inputearthquake ground motion for performance-based design,site-specific strong ground motion was simulated usinga hybrid combination (Hisada, 2000 [2], etc.) of the-oretical methods at low frequency and statistical meth-ods at high frequency. Fig. 15 shows the hypotheticalseismic fault earthquake model, with main source param-eters and asperity slipping displacements shown in Ta-ble 2. The source model was defined based on the asper-



Existing frame before retrofit1 Structural member reinforcement2

PC cableSteel bracket

Steel brackets and cables installation and tensioning3

Hydraulic jack

Hydraulic jacks installationand preloading4

Wire saw

Wire saw installation / Existing column cutting5Existing column removal6

Seismic isolation

device

Seismic isolation deviceinstallation and fixation7Temporary supportingremoval, and completion8

Fig. 14. Construction process by the temporary supporting method.

SITEKiK-net(SZOH28)

30.4

km

15.0

X(N)

Y(E)

strike

q

dip

rakeq

Free Surface

Z 25.2

km

(3)

Fracture initiation point

(1)

(2)

34qN

35qN

137qE 139qE138qE

7.3k

m89.2km

154.

1km

Asp.1

Asp.5

Asp.3

Asp.6

Asp.2

Asp.4

SITEKiK-net(SZOH28)

30.4

km

15.0

X(N)

Y(E)

strike

q

dip

rakeq

Free Surface

Z 25.2

km

(3)

Fracture initiation point

(1)

(2)

34qN

35qN

137qE 139qE138qE

8.3k

m85.5km

152k

m

Asp.1

Asp.4

Asp.3Asp.2

Asp.6

Asp.5

Fig. 15. Tokai earthquake seismic fault model used for theoretical method (left) and statistical method (right).

ity model of the Central Disaster Management Councilof the Cabinet Office, Government of Japan [7]. Table 3shows the deep ground structure using a flat-layered struc-ture model from seismic bedrock (Vs=3000 m/s) to en-gineering bedrock (Vs=510 m/s). Parameters of individ-

ual layers reference KiK-net observation point data [8],etc. Seismic waves at the building basement 8 m deepand Vs=220 m/s were evaluated using equivalent-linearearthquake response analysis based on a one-dimensionalstress-strain relationship. Input earthquake motions were



Table 2. Main source parameters and asperity slipping dis-placement.

Parameter Displacement Strike 208 deg Asperity 1 4.80 m Dip 15 deg Asperity 2 6.93 m

Length 154.14 km Asperity 3 3.35 m Width 89.25 km Asperity 4 4.84 m

Upper depth 7.28 km Asperity 5 2.78 m Slip 89 deg Asperity 6 3.90 m

Rupture velocity 2.7 km/s Background 1.78 m

Table 3. Deep ground structure model.

LayerNo.

Depth (m)

Thickness (m)

Density (g/cm3)

Vp (m/s)

Vs (m/s)

1* 50-200 150 2.1 2,020 5102* 200-840 640 2.3 2,280 8403* 840-900 60 2.5 2,870 1,2804* 900-1,000 100 2.5 4,140 1,8405** 1,000-1,900 900 2.5 4,600 2,5006** 1,900- 2.6 5,300 3,000

Reference: *KiK-net observation point (SZOH28) [8]**Central Disaster Management Council [7]

Table 4. Maximum ground motion waveform amplitudes inbuilding response analysis.

Ground motion Acc

(cm/s2) Vel

(cm/s)Dis

(cm) Tokai-3_EW 624.73 92.80 141.58Site-specific

ground motion Tokai-3_UD 215.82 26.57 26.98Random* 635.16 75.62 22.85

El Centro_NS* 657.77 76.98 26.76Taft_EW* 717.09 75.19 27.16

Building code (very rare level)

Hachinohe_NS* 630.75 98.75 24.37Note: *Ground motion names indicate phase characteristic

models.

simulated by considering three different hypocenters, asshown in Fig. 15. Hypocenter model 3 is the worst-casescenario for the site because of the forward directivityeffects of the fault rupture. Fig. 16 shows pseudo ve-locity response spectra of horizontal components in allhypocenter models. The Tokai-3 model was selected asthe severest case at the effective period after seismic iso-lation retrofitting (horizontal, roughly 3 seconds). Fig. 17shows EW components of acceleration, velocity, and dis-placement for the Tokai-3 model. Several synthesized in-put ground motions required by the current building codewere applied in addition to site-specific ground motion.Table 4 shows maximum waveform amplitudes for site-specific ground motion and ground motion based on thebuilding code at a very rare level used for the time-historyresponse analysis.

10cm

100cm

100g

al

1

10

100

1000

0.1 1 10Period (sec)

Pseu

do v

eloc

ity (c

m/s

ec)

tokai-1_NStokai-1_EWtokai-2_NStokai-2_EWtokai-3_NStokai-3_EW

Fig. 16. Velocity response spectra of simulated waves.

-1000

0

1000

Acce

lera

tion

(gal

) Max:624.73gal

-200

0

200

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180Time (sec)

Disp

lece

men

t (cm

) Max:141.58cm

-100

0

100

Velo

city

(cm

/sec

) Max:92.80cm/sec

Fig. 17. Simulated waves for Tokai-3 EW.



Building No.1

Building No.2

Fig. 18. Three-dimensional frame model.

Table 5. Building model weight.

Table 6. Natural period by eigenvalue analysis and mi-crotremor measurement.

Eigenvalue analysis Microtremors Building Condition

Modeorder Period

(s) Vibration

mode Period

(s) Vibration

mode 1st 0.61 Transverse 0.57 Transverse2nd 0.50 Longitudinal 0.48 Longitudinal

No.1 in existing structure 3rd 0.41 Rotation 0.31 Rotation

1st 0.55 Transverse 0.47 Transverse2nd 0.53 Longitudinal 0.40 Longitudinal

No.2 in existing structure 3rd 0.44 Rotation 0.24 Rotation

1st 0.63 Transverse 0.58 Transverse2nd 0.57 Longitudinal 0.50 Longitudinal

No.1+No.2before

isolation 3rd 0.51 Rotation 0.47 Rotation

5. Seismic Performance Evaluation

5.1. Performance-Based Seismic Design Overview5.1.1. Three-Dimensional Frame Model

To ensure the seismic safety of the building afterretrofitting, structural and vibration features of buildingsand the seismic isolation layer were evaluated. Fig. 18shows a three-dimensional frame model for individualbuildings for static and eigenvalue analysis. Individualfloor weights in building models are shown in Table 5.Static analysis was conducted using a load incremen-tal method taking structural frame inelasticity into ac-

-60000

-40000

-20000

0

20000

40000

60000

-500 -400 -300 -200 -100 0 100 200 300 400 500Displacement D (mm)

Shea

ring

forc

e Q

(kN

).

StandardHard caseSoft case

Qy

Dy

Kb1

Kb2

Fig. 19. Design shearing force-displacement relationship inseismic isolation layer.

Table 7. Bilinear loop parameters in seismic isolation layer.

count. Analysis evaluated results for component elementstress and deflection, seismic isolation device axial force,layer ductility, etc., as structural building features. Nat-ural periods and vibration modes for existing and inte-grated buildings calculated by eigenvalue analysis basedon three-dimensional frame models are calculated in Ta-ble 6, which also shows results of microtremor measure-ment in Section 3.2. It was confirmed that theoretical vi-bration modes of both existing and integrated buildingscorrespond roughly to those in microtremormeasurement.

5.1.2. Characteristics of Structural ModelRestoring force characteristics of the seismic isolation

layer were made as a bilinear model that integrated shear-ing force-displacement relationships of all the seismic iso-lation devices. In the shearing force-displacement rela-tionships of each kind of device, the natural rubber bear-ing was assumed to be linear, the read rubber bearingand the elastic sliding support were assumed to be bilin-ear. The shearing force of the cross-linear bearing wasignored because the coefficient of friction is very small atbelow 0.001. Fig. 19 shows the restoring force character-istics in the seismic isolation layer at large earthquakes.Due to a dependence on shear strain in restoring forcecharacteristics of lead rubber bearings, a shearing force-displacement relation in horizontal displacement of 430mm, i.e., shear strain of 218% in rubber bearings corre-sponding to the maximum response in the seismic isola-tion layer was used for the analyses. Parameters of thebilinear restoring force characteristics in the seismic iso-lation layer are tabulated in Table 7. Variations in thestiffness and yield strength of the bilinear model origi-nating in product error margin, secular change, temper-



ature change, and device-dependent factors as variationsin seismic isolation layer performance were considered.In addition to standard conditions, hard case stiffness in-creased 21% soft case stiffness 14% in seismic isolationlayer stiffness were taken into account in the design as asummation of the amount of variation in individual deviceperformance.

In analysis after seismic isolation retrofitting, seismicisolation layers of each building were designed for thesame period features. In horizontal displacement of 430mm, the equivalent period of the seismic isolation layer inthe standard condition was 3.4 seconds and that of the tan-gent period 4.0 seconds. From building model eigenvalueanalysis results after integration and seismic isolation,when secant stiffness at the same deformation of seismicisolation devices was used in the three-dimensional framemodel, the natural period in the longitudinal (X) directionwas 3.15 seconds and that in the transverse (Y) direction3.16 seconds.

5.1.3. Seismic Response Analysis ModelA seismic response analysis model was made based

on static analysis results and time-history seismic re-sponse analysis was conducted. Fig. 20 shows the analy-sis model. A parallel multi-lumped mass model was madeby concentratingmasses on individual floors. Shear force-displacement relation of the superstructure was modeledby the tri-linear equivalent shear model that substitutedrelationships between story-shear force and relative storydisplacement of each story based on the static analysis.A damping factor of the superstructure was assumed tobe 3% in proportion to an initial stiffness. For the seis-mic isolation layer, only a hysteresis damping based onthe restoring force characteristics was considered. Eachmulti-lumped mass model in seismic response analysisassumed at the center of gravity in each building. Par-allel lumped masses were connected basically by a rigidspring in the horizontal direction. One-third of the elasticstiffness of the slab in axial and shear force was also ana-lyzed as a case of insufficient stiffness of the connection.To prevent torsion in the seismic isolation layer, the ec-centricity ratio in horizontal maximum response displace-ment of the seismic isolation layer was minimized to lessthan 1% and permissible deformation of seismic isolationdevices included a margin of 10% for maximum responsedisplacement. Minimum clearance in building circumfer-ence was ensured at 600 mm.

5.1.4. Structural Integration of BuildingThe following considerations were taken for new estab-

lishment slabs using prestressing cables:

a) For in-plane force, stress acting on a new slab wascalculated in both translational modes of the twobuildings based on seismic response analysis forthe parallel multi-lumped mass model and the othermode that flexes via the joint between the two build-ings based on static and modal analysis of a three-

Seismic isolation layer

Building No.1 Building No.2

Connecting element

RF

9F

8F

7F

6F

5F

4F

3F

2F

1F

Fig. 20. Seismic response analysis model.

Table 8. Seismic performance targets for site-specificground motion and building code (very rare level).

Upper structure

a) within elastic strength of individual layer b) below 1/312.5 (=1/250/1.25) of story drift angle

Seismic isolation

layer

a) within safety deformation -- below 473.2 mm (=591.6/1.25) for rubber bearings

b) within allowable tensile stress -- below 0.8 N/mm2 (=1/1.25) for rubber bearings

Foundation a) within allowable stress

dimensional model, and it was confirmed that this isbelow allowable slab stress.

b) For out-of-plane force, slab bending resistance is notexpected in design, shown above, but it was con-firmed that the amount of allowable strain in pre-stressing cable set based on yield strain of the cablehad sufficient margin for strain increments originat-ing in bending deformation caused at slab edges bythe relative displacement of the two buildings. Work-ing bending moment was also calculated for connec-tion when the prestressing cable was extended to themaximum, and it was also confirmed that margin wassufficient for bending strength when axial force de-termined in seismic response analysis acted on theconnection.

5.2. Seismic Performance TargetsTable 8 shows seismic performance targets for the up-

per structure, seismic isolation layer, and foundation usedin response analysis. To maintain the upper structure tobe elastic range at each building layer, story drift angle ofmain building frames was configured at 1/250 radian orless. A seismic performance of seismic isolation deviceswas desired to be less than safety deformation and allow-able tensile stress, and foundation structure was desired to



[Longitudinal (X) direction] [Transverse (Y) direction]

0.000 0.001 0.002 0.003 0.004Story-drift angle (rad.)

Stor

y

9

8

7

6

5

4

3

2

1

1/312.5

0 20000 40000 60000 80000Story-shearing force (kN)

Stor

y

Force of firstshear failure/1.25

9

8

7

6

5

4

3

2

1

0 10 20 30 40 50 60Displacement (cm)

Floo

r

9

8

7

6

5

4

3

2

1

R

47.32 cm

0 200 400 600 800Acceleration (gal)

Floo

r

9

8

7

6

5

4

3

2

1

R

Tokai-3_EW Random El centro_NS Taft_EW Hachinohe_NS

0.000 0.001 0.002 0.003 0.004Story-drift angle (rad.)

Stor

y

9

8

7

6

5

4

3

2

1

1/312.5

0 200 400 600 800Acceleration (gal)

Floo

r

9

8

7

6

5

4

3

2

1

R

0 10 20 30 40 50 60Displacement (cm)

Floo

r

9

8

7

6

5

4

3

2

1

R

47.32 cm

0 20000 40000 60000 80000Story-shearing force (kN)

Stor

y

Force of firstshear failure/1.25

9

8

7

6

5

4

3

2

1

Tokai-3_EW Random El centro_NS Taft_EW Hachinohe_NS

Fig. 21. Time history response analysis results of the upper structure and the seismic isolation layer (stiffness of seismic isolationlayer: hard case).

be less than allowable stresses. In all target values, an im-portance factor (I=1.25) was configured as a safety marginfor seismic performance.

5.3. Evaluation Results

Seismic retrofitted building performance was evaluatedbased on static and dynamic analysis. Fig. 21 showsseismic response analysis results for the upper structureand seismic isolation layer after retrofitting for hard casestiffness variations as the severest case. Figures at leftshow results in the longitudinal direction and those atright results in the transverse direction. Each directionshows maximum story drift angle, displacement, story-shearing force, and floor response acceleration. The re-sult of the response analysis, which was conducted underthe worst case scenario of site-specific ground motions(Tokai-3 model), revealed that the maximum base shear-to-weight ratio of upper structure and the maximum dis-placement of seismic isolation layer are 0.188 and 419.3mm, respectively. Response results were confirmed to sat-isfy seismic performance targets for both upper structureand seismic isolation layers, and that floor response ac-celeration was roughly 300 gal or less. From these anal-ysis results, retrofitting for maintaining building functionand emergencymedical activities in large earthquakeswasthus evaluated as effective.

6. Conclusions

We have developed a methodology of seismic isola-tion retrofit by integrating a couple of adjacent buildings,and actually applied it to the two large-scale buildings atthe Hamamatsu Medical Center. This is the first hospi-tal retrofitting using seismic isolation in Japan. We havedetailed the seismic retrofit scheme integrating the twobuildings using prestressed concrete slabs. From the mi-crotremor measurements and evaluated building vibrationbefore and after integration, confirming that integrationwas successful. During retrofitting, we used temporarysupport with steel brackets and prestressing cables to in-stall seismic isolation equipment safely and economically.In the seismic design phase, we simulated broadband in-put earthquake ground motion for a hypothetical Magni-tude 8 earthquake near the site, and confirmed structuralsafety and functionality by evaluating seismic buildingperformance based on time-history seismic response anal-ysis.

AcknowledgementsWe thank the staffs of Hamamatsu City and Hamamatsu Medi-cal Center for their generous understanding and cooperation dur-ing design and construction phases. Overall building renovationwas designed by Yokogawa Architects and Engineers, Inc. We



thank Messrs. Takashi Yamada and Eiji Yoshikawa for their en-couraging support in project design and supervision. We thankDr. Takumi Toshinawa of Meisei University for microtremor mea-surement.

References:[1] Y. Masuzawa and Y. Hisada, Seismic Isolation Retrofit of a Prefec-

tural Government Office Building, Proc. of the 13th World Confer-ence on Earthquake Engineering, CD-ROM, 2004.

[2] Y. Hisada, A Hybrid Method for Predicting Strong Ground Mo-tions at Broad-frequencies Near M8 Earthquakes in SubductionZones, Proc. of the 12th World Conference on Earthquake Engi-neering, CD-ROM, 2000.

[3] Report on the Damage Investigation of the October 23, 2004 MidNiigata Prefecture Earthquake, Architectural Institute of Japan,2006.8. (in Japanese).

[4] T. Toshinawa and Y. Masuzawa, Vibration Characteristics of 9-Story SRC Buildings Connected with Expansion Joints, Sum-maries of Technical Papers of Annual Meeting, Architectural In-stitute of Japan, B-2, pp. 73-74, 2005.9. (in Japanese).

[5] Y. Masuzawa and T. Toshinawa, Vibration Characteristics of 9-Story SRC Buildings Connected with Expansion Joints, Part 2:Vibration Characteristics After Integrating Two Buildings, Sum-maries of Technical Papers of Annual Meeting, Architectural Insti-tute of Japan, B-2, pp. 157-158, 2008.9. (in Japanese).

[6] Y. Masuzawa and Y. Hisada, Development a Temporary Support-ing Method for Seismic Isolation Retrofit and Evaluation of Verti-cal Load Support Capacity Based on Full Scale Tests, Journal ofStructural and Construction Engineering, Architectural Institute ofJapan, Vol.74, No.638, pp. 701-710, 2009.4. (in Japanese).

[7] The 7th material of Special Investigation Committee for a Tokaiearthquake, Central Disaster Management Council secretariat,Cabinet Office, Government of Japan, 2001.8. (in Japanese).

[8] Digital Strong-Motion Seismograph Network (KiK-net), NationalResearch Institute for Earth Science and Disaster Prevention.http://www.kik.bosai.go.jp/

Name:Yoe Masuzawa

Affiliation:Assistant Business Promotion Manager, RiskManagement Department, Engineering & RiskServices Corporation

Address:Akasaka Kikyo Bldg., 3-11-15 Akasaka, Minato-ku, Tokyo 107-0052,JapanBrief Career:1995 Taisei Corporation1997 Yokogawa Architects & Engineers, Inc.2006- Engineering & Risk Services Corporation2009- Visiting Fellow of Kogakuin UniversitySelected Publications: Y. Masuzawa and Y. Hisada, Seismic Isolation Retrofit of a PrefecturalGovernment Office Building, Proc. of the 13th World Conference onEarthquake Engineering, CD-ROM, 2004. Y. Masuzawa and Y. Hisada, Development a Temporary SupportingMethod for Seismic Isolation Retrofit and Evaluation of Vertical LoadSupport Capacity Based on Full Scale Tests, Journal of Structural andConstruction Engineering, Architectural Institute of Japan, Vol.74, No.638,pp. 701-710, 2009.4. (in Japanese).Academic Societies & Scientific Organizations: Architectural Institute of Japan (AIJ) Japan Association for Earthquake Engineering (JAEE)

Name:Yoshiaki Hisada

Affiliation:Dr. of Eng., Professor, Department of Architec-ture, Faculty of Engineering, Kogakuin Univer-sity

Address:1-24-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo 163-8677, JapanBrief Career:1989 Research Associate, Waseda University1993 Research Associate, University of Southern California1995 Lecturer, Kogakuin University1999 Associate Professor, Kogakuin University2004- Professor, Kogakuin UniversitySelected Publications: Y. Hisada, Broadband Strong Motion Simulation in Layered Half-SpaceUsing Stochastic Greens Function Technique, Journal of Seismology,Vol.12, No.2, pp. 265-279, 2004. Y. Hisada, M. Murakami, and S. Zama, Quick Collection of EarthquakeDamage Information and Effective Emergency Response by CollaborationBetween Local Government and Residents, Proc. of the 14th WorldConference on Earthquake Engineering, DVD, 2008.Academic Societies & Scientific Organizations: Architectural Institute of Japan (AIJ) Seismological Society of Japan (SSJ) Seismological Society of America (SSA) Japan Association for Earthquake Engineering (JAEE)


seismic isolation retrofit of a medical complex by integrating two large-scale buildings

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