Paper:

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

Yoe Masuzawa* and Yoshiaki Hisada**

*Risk Management Department, Engineering and Risk Services Corporation, Japan

Akasaka Kikyo Bldg., 3-11-15 Akasaka, Minato-ku, Tokyo 107-0052, Japan

E-mail: masuzawa@ers-co.jp

**Department of Architecture, Faculty of Engineering, Kogakuin University, Japan

1-24-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo, 163-8677, Japan

E-mail: hisada@cc.kogakuin.ac.jp

We developed a methodology of seismic isolation retrofit integrating adjacent buildings using prestressed concrete slabs, and applied it to two large-scale buildings in Hamamatsu City in Shizuoka Prefecture in Japan. It is the first seismic isolation retrofit of hospital in Japan. The two steel-reinforced concrete buildings were nine stories high with one basement, and had been constructed in 1973 and 1975 based on an old structural design code. The two buildings were integrated into one building by connecting individual floors using post-tensioned prestressing cables through slabs. A comparison of microtremors before and after the integration confirmed that the integration worked well. Seismic isolation devices were set up mainly in basement columns using temporary support involving steel brackets and prestressing cables to install devices safely and economically (Masuzawa et al., 2004 [1]). In the seismic design phase, broadband-generated earthquake ground motions for a hypothetical Magnitude 8 earthquake near the site were simulated using a hybrid method (Hisada, 2000 [2], etc.). Safety and functionality were verified by evaluating structural seismic performance based on 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

Seismic risk mitigation of the hospitals which become medical treatment bases at the time of disasters is very important in high seismicity countries such as Japan. Those hospitals need to ensure not only the safety of buildings but also the operability of medical treatment even during and after large earthquakes. The seismic isolation is one of the most effective and practicable countermeasures against

earthquakes because it drastically reduces seismic response due to devastating ground shaking. Since extensive damage of hospitals was experienced by the Great Hanshin-Awaji Earthquake in 1995, seismic isolation structures have been adopted for a lot of new hospital buildings in Japan. On the other hand, old and new buildings often exist together adjacently in large-scale hospitals, and earthquake damages of the old buildings may significantly reduce the entire functions of medical treatment. The need to retrofit aging hospitals was made all too clear when three old structures of a six-building facility in Ojiya City were so severely damaged in the 2004 Mid-Niigata Prefecture earthquake that emergency medical operations could not be maintained [3]. Typical structural damage involved building joints, nonstructural components, furniture, and equipment, as shown in Fig. 1. It also took much time to restore the buildings and their function.

A similar situation will probably happen in the hospital of Hamamatsu City that is located in a high seismicity area in Japan. The hospital will lose its functions and emergency operations for a large earthquake, because of damage of the two old buildings. Therefore, it was necessary to retrofit those two buildings effectively. Given its central location and importance as a medical treatment facility, the hospital would have to continue its functions and emergency service during retrofitting and reconstruction. The sections that follow provide a background of the retrofitting methodology, building integration, and the evaluation of microtremor measurement. Site-specific strong ground motions in a hypothetical Magnitude 8 earthquake in a subduction zone under the site are then simulated, and the performance and safety of retrofitted building evaluated using time-history response analysis of simulated earthquake motion.

2. Hamamatsu Medical Center

Hamamatsu Medical Center, a five-building treatment complex having over 600 beds, is one of Shizuoka

Prefecture’s most important medical facilities, and one whose building function and emergency medical services would be needed in a large earthquake. The two buildings, designed under the old seismic design code, were found in seismic diagnosis to be inadequate under the current code.

Fig. 2 shows a bird’s-eye view of Hamamatsu Medical Center and Fig. 3 the first floor and typical floor plans of existing hospital. Each building was structurally independent, but arranged adjacently, and connected with the expansion joint mutually. Building No. 1, built in 1973, and Building No. 2, built in 1975, and now to be retrofitted, have steel-reinforced concrete frames, nine stories and one basement, and three-story penthouses on their roofs, as detailed in Table 1. Building No.1 is a plan rotated 60 degrees at the center of building. Building No. 2 is nearly rectangular. In those buildings, diagnosis and treatment sections are arranged in low layer floors, and medical wards are located in upper floors.

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

(a) Structural damage (b) Buildings joint damage

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

(c) Nonstructural component damage

Fig. 1. Typical damage of a hospital in the 2004 Mid-Niigata Prefecture earthquake

Fig. 3. Floor plans of Hamamatsu Medical Center

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. 2. Bird’s-eye view of Hamamatsu Medical Center (The photo referred to the medical center brochure)

South building

Connecting building

Building No.1

Building No.2

Building No.3

3. Seismic Retrofitting Methodology

3.1. Overview

Seismic retrofitting, started in autumn 2006, was completed as scheduled in autumn 2009. Fig. 4 shows framing elevation of the two buildings after retrofitting. Under proposed retrofitting, prestressed concrete slabs would be installed and connected between the two buildings on each floor. Buildings would then be isolated mainly underground using 89 seismic isolation devices. Before reconstruction, all building equipment and facilities were renewed and moved from the basement to the roof and medical equipment potentially disturbed by reconstruction work moved, enabling reconstruction to be conducted while building functions and medical services continued.

3.2. Building Integration and Microtremor

Measurement

3.2.1. Integration overview Figure 5 shows the connection section and plan for a

typical floor. To ensure joint strength and ductility, slabs were connected to buildings using post-tensioned prestressing cables penetrating both slabs and adjacent girders and anchored to existing building frames. Anchorage zones were fastened to frames by anchor dowels to transmit stress between slabs and frames. Connections were assumed to not resist slab out-of-plane (bending) because they were connected only with prestressing cables through the centers of slabs. Connections thus both ensured slab in-plane strength and avoided placing undue stress on existing frames. Safety against cable strand elongation was ensured with minimum adhesion and by reducing prestressing force to 80% of the allowable load. Seismic performance is evaluated in detail Section 5.1.

3.2.2. Microtremor Measurement and Building Vibration

Features To investigate building vibration features before and after

integration, microtremors were measured on November 21, 2004, before connection, and on June 16, 2007, after connection [4, 5]. The integrated building was measured when connection was completed on floors 7-9 and the roof before seismic isolation was completed. Building microtremor records, natural periods, particle orbits, and vibration-mode shapes were obtained, and the effects of connection confirmed. Fig. 6 shows where microtremor sensors were deployed. Recording used servo velocity sensors, a 16-bit analog-to-digital converter, and a notebook PC. Longitudinal (X) and transverse (Y) components of microtremors in velocity were recorded at each location. The sampling rate was 100 Hz and the recording of each record

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

Building No.1Building No.2

4200 3000

600

600

500

8000

Prestressing cable

Plan

Section

Rebar dowels

5000

Anchorage zone

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)

Connection by prestressed concrete slab (Fig. 5.)

Seismic isolation device

was 180 seconds long. Three sets of samplings were recorded for each pattern. The following five patterns were observed in simultaneous measurement by up to eight sensors:

Pattern 1: 1C, 1E, and 1W on the ninth floor by recording in two directions simultaneously.

Pattern 2: 2C, 2E, 2W, and 1C on the ninth floor by recording in two directions simultaneously. 1C was set only for the integrated building.

Pattern 3: 1C, 2C, 3C, and 12C on the ninth floor by recording in two directions simultaneously. 12C was set only for the integrated building.

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

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

Fig. 7 shows the sensor layout for Pattern 3. Microtremor sensors were installed on the ninth floor near the center of gravity of each building and in the connecting location. Fig. 8 shows Fourier amplitude spectra for microtremors obtained in the same observation pattern. Histories 20.48 seconds long were selected from records, followed by zeroes 20.48 seconds long, and put through Fourier transformation to obtain spectra smoothed with a 0.2 Hz Parzen window. Before the buildings were connected, the predominant period in the transverse (Y) direction of Building Nos. 1 and 2 were equaled 0.59 seconds, but a variation in the peak period was also confirmed in spectra. Note that after connection, Building Nos. 1 and 2 and the connecting location show concordance in predominant periods in each direction. To determine the predominant direction in each

peak period, the horizontal particle orbit was obtained with each velocity record integrated to displacement with a band-pass filter whose typical period is the predominant period of each mode. From displacement time histories in the longitudinal and transverse directions, 1-second sections with high amplitude were selected and horizontal particle orbits shown. Vibration-mode shape for each natural period was obtained the same as for horizontal particle orbits. Fig. 9 shows horizontal particle orbits obtained on the ninth floor in Pattern 3 with band-pass filters at each of 0.59 seconds before buildings were connected and 0.58 seconds after connection when the peak periods of Building Nos. 1 and 2. The predominant directions of the two buildings differed before connection, but corresponded after connection, clearly showing that integration was effective. Note also that Building No. 3, which was not connected to the integrated buildings, vibrated independently. Natural periods and the vibration modes for individual and integrated buildings are shown based on microtremor measurement in conjunction with eigenvalue analysis results in Section 5.1.

Fig. 8. Fourier amplitude spectra for microtremors obtained at ninth floor (Pattern 3)

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

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

3740

050

0085

5012

250

1160

0

B1F1F

3F

6F

9F

[3C] [2W] [2C] [2E] [1W] [1C]

[1E]

[12C]

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

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

Vibration period: 0.59 sec

Vibration period: 0.58 sec [After connection]

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

3.3. Seismic Isolation Retrofit

Figure 10 shows the arrangement of 89 seismic isolation devices -- 75 in basement columns, 8 under elevator shafts, and 6 under the entrance base. We used 51 lead rubber bearings (LRB) 900 mm on a side, 4 natural rubber bearings

(RB) 900 mm on a side, 4 elastic sliding supports (SL) 300 mm on a side, and 30 cross-linear bearings (CLB) of 6 different types with different load limits for isolation. Fig. 11 shows four types of seismic isolation device in the seismic isolation layer. Bearing ratio to the building weight (sustained loading) of LRB, RB, CLB, and SL was 64%, 5%, 30%, and 1% respectively. Square rubber bearings were

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

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

17

18

19

20

21

22

23

24

16

Ｇ

Ｅ

Ｂ

Ａ

12

16

20

21

Ｈ

Ｇ

Ｅ

Ｃ

Building No.2

Building No.1

I

J

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

D

X

Y

X

Y

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

60884620

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. 11. Constitution 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

used to make reinforcement columns as small as possible. The cross-linear bearing combines orthogonal linear motion (LM) guides consisting of LM rails and blocks up and down and varying in size with the load capacity. Elastic sliding supports consist of laminated rubber with polytetrafluoroethylene (PTFE) friction surfaces and stainless steel plates with special lubrication film.

Steel brackets and prestressing cables were used in temporary support to ensure that isolation devices were installed safely and economically (Masuzawa et al., 2004 [1]). Fig. 12 shows temporary support used to insert a seismic isolation device in a column. The number of prestressing cables used depended on the maximum reaction calculated for each column. The feasibility of temporary support was confirmed through full-scale experiments [6], with an example of experimental results shown in Fig. 13.

0

5

10

15

20

0 1 2 3 4 5Vertical displacement δ (mm)

Ver

tical

load

P (M

N).

0

5

10

Shea

r stre

ngth

τ (M

Pa)τ/σ*=1.0

τ/σ*=0.5

calculated reaction force of column

*σ: confinement stress

Pmax=17.677 MN

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

Fig. 14. Construction process by the temporary supporting method

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 supporting

removal, and completion8

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

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)

Note the load-displacement relationship of the six-cable specimen, which was measured at prestressed joints between steel brackets and reinforcement columns. Full-scale tests showed that vertical load support was sufficient. Fig. 14 shows construction of temporary support from phases 1 to 8. In phases 1 to 2, structural members of the existing frame underground are reinforced except for intermediate parts of the column. In phase 3, steel brackets and prestressing cables are installed and prestress installed in cables. In phase 4, hydraulic jacks are installed, preloading force acts on brackets, and the axial force of the column is released. In phases 5 to 6, a diamond wire sawing machine is installed on the column and the existing column cut off and removed. In phase 7, seismic isolation devices are installed and upper and lower joints fixed using high-flow concrete or nonshrink mortar. In phase 8, all temporary support components are removed, completing the job. A maximum of four temporary support sets were used together and rotated in the construction flow. To ensure earthquake resistance of 0.2 G even in the middle of construction in the basement, temporary steel braces and other earthquake-resistant elements were installed. In basement usable as floor area, seismic isolation retrofitting was implemented, and then fireproof panels attached to columns to enclose seismic isolation devices.

4. Site-Specific Strong Ground Motion Simulation

A hypothetical Magnitude 8 earthquake near the site was

simulated in the seismic design phase. A fault model was located in a subduction zone of the Suruga Trough where a very high possibility exists of earthquake occurrence in the near future. To create broadband input earthquake ground motion for performance-based design, site-specific strong ground motion was simulated using a hybrid combination (Hisada, 2000 [2], etc.) of theoretical methods at low

Table 2. Main source parameters and asperity slipping displacement

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) **Central Disaster Management Council

SITEKiK-net(SZOH28)

30.4

km

15.0

°

X(N)

Y(E)

strike

208.0°

dip

rake89.0°

Free Surface

Z 25.2

km

(3)

Fracture initiation point

(1)

(2)

34°N

35°N

137°E 139°E138°E

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

208.0°

dip

rake89.0°

Free Surface

Z 25.2

km

(3)

Fracture initiation point

(1)

(2)

34°N

35°N

137°E 139°E138°E

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)

frequency and statistical methods at high frequency. Fig. 15 shows the hypothetical seismic fault earthquake model, with main source parameters and asperity slipping displacements shown in Table 2. The source model was defined based on the asperity model of the Central Disaster Management Council of the Cabinet Office, Government of Japan [7]. Table 3 shows the deep ground structure using a flat-layered structure model from seismic bedrock (Vs=3000 m/s) to engineering bedrock (Vs=510 m/s). Parameters of individual layers reference KiK-net observation point data [8], etc. Seismic waves at the building basement 8 m deep and Vs=220 m/s were evaluated using equivalent-linear earthquake response analysis based on a one-dimensional stress-strain relationship. Input earthquake motions were simulated by considering three different hypocenters, as shown in Fig. 15. Hypocenter model 3 is the worst-case scenario for the site because of the forward directivity effects of the fault rupture. Fig. 16 shows pseudo velocity response spectra of horizontal components in all hypocenter models. The Tokai-3 model was selected as the severest case at the effective period after seismic isolation retrofitting (horizontal, roughly 3 seconds). Fig. 17 shows EW components of acceleration, velocity, and displacement for the Tokai-3 model. Several synthesized input ground motions required by the current building code were applied in addition to site-specific ground motion. Table 4 shows maximum waveform amplitudes for site-specific ground motion and ground motion based on the building code at a very rare level used for the time-history response analysis.

5. Seismic Performance Evaluation

5.1. Performance-Based Seismic Design Overview

5.1.1. Three-Dimensional Frame Model To ensure the seismic safety of the building after

retrofitting, structural and vibration features of buildings and the seismic isolation layer were evaluated. Fig. 18 shows a three-dimensional frame model for individual buildings for static and eigenvalue analysis. Individual floor weights in building models are shown in Table 5. Static analysis was conducted using a load incremental method taking structural frame inelasticity into account. Analysis evaluated results for component element stress and deflection, seismic isolation device axial force, layer ductility, etc., as structural building features. Natural periods and vibration modes for existing and integrated buildings calculated by eigenvalue analysis based on three-dimensional frame models are

Table 4. Maximum ground motion waveform amplitudes in building 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.

Fig. 16. Velocity response spectra of simulated waves

100g

al

100cm

10cm

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. 17. Simulated waves for Tokai-3_EW

-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

calculated in Table 6, which also shows results of microtremor measurement in Section 3.2. It was confirmed that theoretical vibration modes of both existing and integrated buildings correspond roughly to those in microtremor measurement.

5.1.2. Characteristics of Structural Model

Restoring force characteristics of the seismic isolation layer were made as a bilinear model that integrated shearing force-displacement relationships of all the seismic isolation devices. In the shearing force-displacement relationships of each kind of device, the natural rubber bearing was assumed to be linear, the read rubber bearing and the elastic sliding support were assumed to be bilinear. The shearing force of the cross-linear bearing was ignored because the coefficient of friction is very small at below 0.001. Fig. 19 shows the restoring force characteristics in the seismic isolation layer at large earthquakes. Due to a dependence on shear strain in restoring force characteristics of lead rubber bearings, a shearing force-displacement relation in horizontal displacement of 430 mm, i.e., shear strain of 218 % in rubber bearings corresponding to the maximum response in the seismic isolation layer was used for the analyses. Parameters of the bilinear restoring force characteristics in the seismic isolation layer are tabulated in Table 7. Variations in the stiffness and yield strength of the bilinear

model originating in product error margin, secular change, temperature change, and device-dependent factors as variations in seismic isolation layer performance were considered. In addition to standard conditions, hard case stiffness increased 21% soft case stiffness 14% in seismic isolation layer stiffness were taken into account in the design as a summation of the amount of variation in individual device performance. In analysis after seismic isolation retrofitting, seismic isolation layers of each building were designed for the same period features. In horizontal displacement of 430 mm, the equivalent period of the seismic isolation layer in the standard condition was 3.4 seconds and that of the tangent period 4.0 seconds. From building model eigenvalue analysis results after integration and seismic isolation, when secant stiffness at the same deformation of seismic isolation devices was used in the three-dimensional frame model, the natural period in the longitudinal (X) direction was 3.15 seconds and that in the transverse (Y) direction 3.16 seconds.

Table 5. Building model weight Building No.1 Building No.2 Total

Floor Hi(m) Wi

(kN)∑Wi(kN)

Wi (kN)

∑Wi (kN)

Wi (kN)

∑Wi(kN)

9 3.80 17,252 17,252 17,876 17,876 35,128 35,1288 3.85 10,536 27,788 10,187 28,063 20,723 55,8517 3.85 28,306 38,842 28,316 38,503 56,622 77,3456 3.90 22,143 50,449 20,904 49,219 43,047 99,6695 3.90 40,522 62,665 39,235 60,138 79,756 122,8034 3.95 34,996 75,517 32,282 71,517 67,278 147,0343 4.40 55,432 90,428 53,135 85,418 108,567 175,8452 4.30 56,987 112,419 47,928 101,064 104,915 213,4821 4.25 79,640 136,626 68,589 116,517 148,229 253,144

B1 - 93,473 173,113 73,991 142,580 167,465 315,693Note: “Wi” for B1F is the weight of the seismic isolation layer

over that of seismic isolation devices. “Hi” is the story height.

Table 6. Natural period by eigenvalue analysis and microtremor 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

Table 7. Bilinear loop parameters in seismic isolation layer Standard Hard case Soft case

Initial stiffness Kb1 (*) 974,875 1,178,659 840,554Post-yield stiffness Kb2 (*) 80,638 97,411 69,591

Yield displacement Dy (mm) 15.0 14.6 13.6Yield force Qy (kN) 14,576 17,710 11,417

Unit: * in N/mm2

-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 in seismic isolation layer

Fig. 18. Three-dimensional frame model

Building No.1

Building No.2

5.1.3. Seismic Response Analysis Model

A seismic response analysis model was made based on static analysis results and time-history seismic response analysis was conducted. Fig. 20 shows the analysis model. A parallel multi-lumped mass model was made by concentrating masses on individual floors. Shear force-displacement relation of the superstructure was modeled by the tri-linear equivalent shear model that substituted relationships between story-shear force and relative story displacement of each story based on the static analysis. A damping factor of the superstructure was assumed to be 3% in proportion to an initial stiffness. For the seismic isolation layer, only a hysteresis damping based on the restoring force characteristics was considered. Each multi-lumped mass model in seismic response analysis assumed at the center of gravity in each building. Parallel lumped masses were connected basically by a rigid spring in the horizontal direction. One-third of the elastic stiffness of the slab in axial and shear force was also analyzed as a case of insufficient stiffness of the connection. To prevent torsion in the seismic isolation layer, the eccentricity ratio in horizontal maximum response displacement of the seismic isolation layer was minimized to less than 1% and permissible deformation of seismic isolation devices included a margin of 10% for maximum response displacement. Minimum clearance in building circumference was ensured at 600 mm.

5.1.4. Structural Integration of Building

The following considerations were taken for new establishment slabs using prestressing cables:

a) For in-plane force, stress acting on a new slab was calculated in both translational modes of the two buildings based on seismic response analysis for the parallel multi-lumped mass model and the other mode that flexes via the joint between the two buildings based on static and modal analysis of a three-dimensional model, and it was confirmed that this is below allowable slab stress.

b) For out-of-plane force, slab bending resistance is not expected in design, shown above, but it was confirmed that the amount of allowable strain in prestressing cable set based on yield strain of the cable had sufficient margin for strain increments originating in bending deformation caused at slab edges by the relative displacement of the two buildings. Working bending moment was also calculated for connection when the prestressing cable was extended to the maximum, and it was also confirmed that margin was sufficient for bending strength when axial force determined in seismic response analysis acted on the connection.

5.2. Seismic Performance Targets

Table 8 shows seismic performance targets for the upper structure, seismic isolation layer, and foundation used in response analysis. To maintain the upper structure to be elastic range at each building layer, story drift angle of main building frames was configured at 1/250 radian or less. A seismic performance of seismic isolation devices was

desired to be less than safety deformation and allowable tensile stress, and foundation structure was desired to be less than allowable stresses. In all target values, an importance factor (I=1.25) was configured as a safety margin for seismic performance.

5.3. Evaluation Results

Seismic retrofitted building performance was evaluated based on static and dynamic analysis. Fig. 21 shows seismic response analysis results for the upper structure and seismic isolation layer after retrofitting for hard case stiffness variations as the severest case. Figures at left show results in the longitudinal direction and those at right results in the transverse direction. Each direction shows maximum story drift angle, displacement, story-shearing force, and floor response acceleration. The result of the response analysis, which was conducted under the 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 displacement of seismic isolation layer are 0.188 and 419.3 mm, respectively. Response results were confirmed to satisfy seismic performance targets for both upper structure and seismic isolation layers, and that floor response acceleration was roughly 300 gal or less. From these analysis results, retrofitting for maintaining building function and emergency medical activities in large earthquakes was thus evaluated as effective.

Table 8. Seismic performance targets for site-specific ground 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

Fig. 20. Seismic response analysis model

RF

9F

8F7F6F5F

4F

3F

2F

1F Seismic isolation layer

Building No.1Building No.2

Connecting element

6. Conclusions

We have developed a methodology of seismic isolation retrofit by integrating a couple of adjacent buildings, and actually applied it to the two large-scale buildings at the Hamamatsu Medical Center. This is the first hospital retrofitting using seismic isolation in Japan. We have detailed the seismic retrofit scheme integrating the two buildings using prestressed concrete slabs. From the microtremor measurements and evaluated building vibration before and after integration, confirming that integration was successful. During retrofitting, we used temporary support with steel brackets and prestressing cables to install seismic isolation equipment safely and economically. In the seismic design phase, we simulated broadband input earthquake ground motion for a hypothetical Magnitude 8 earthquake near the site, and confirmed structural safety and functionality by evaluating seismic building performance based on time-history seismic response analysis.

Acknowledgements We thank the staffs of Hamamatsu City and Hamamatsu Medical Center for their generous understanding and cooperation during design and construction phases. Overall building renovation was designed by Yokogawa Architects and Engineers, Inc. We thank Messrs. Takashi Yamada and

Eiji Yoshikawa for their encouraging support in project design and supervision. We thank Dr. Takumi Toshinawa of Meisei University for microtremor measurement.

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

Prefectural Government Office Building”, Proc. of the 13th World Conference on Earthquake Engineering, CD-ROM, 2004.

[2] Y. Hisada, “A Hybrid Method for Predicting Strong Ground Motions at Broad-frequencies Near M8 Earthquakes in Subduction Zones”, Proc. of the 12th World Conference on Earthquake Engineering, CD-ROM, 2000.

[3] “Report on the Damage Investigation of the October 23, 2004 Mid Niigata 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, Summaries of Technical Papers of Annual Meeting, Architectural Institute 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, Summaries of Technical Papers of Annual Meeting, Architectural Institute of Japan, B-2, pp. 157-158, 2008.9. (in Japanese)

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

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

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

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

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

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

Stor

y

1/250/1.259

8

7

6

5

4

3

2

1

0 200 400 600 800Acceleration (gal)

Floo

r

9

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R

0 10 20 30 40 50 60Displacement (cm)

Floo

r

9

8

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R

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

Stor

y

Force of firstshear failure/1.25

9

8

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6

5

4

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2

1

●：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

1/250/1.259

8

7

6

5

4

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1

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

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3

2

1

R

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