seismic response analyses of an asymmetric base-isolated building during … · 2018-02-23 ·...

STRUCTURAL CONTROL AND HEALTH MONITORINGStruct. Control Health Monit. 2015; 22:71–90Published online 11 April 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/stc.1661

Seismic response analyses of an asymmetric base-isolated buildingduring the 2011 Great East Japan (Tohoku) Earthquake

Dionysius M. Siringoringo1,*,† and Yozo Fujino2

1Department of Civil Engineering, University of Tokyo, Tokyo, Japan2Institute of Engineering Innovations, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

SUMMARY

Analysis of strong motion recordings of a base-isolated building during the March 11, 2011, Great EastJapan (Tohoku) Earthquake is reported in this paper. The building, located in Tokyo Bay area, is an asymmetricL-shaped structure consisting of seven-story and 14-story building with vertical opening. Vibration monitoringsystem was installed on the building in 2010, and seismic responses were recorded including the strongest shaking(peak ground acceleration 0.80–1.40m/s2) experienced during the main shock of March 11, 2011, Great EastJapan Earthquake. The building survived the earthquake without structural damage. The study in this paperincludes response analysis, system identification, and seismic performance evaluation of the structure, especiallyperformance of base-isolation system. The study shows that despite considerable shift in effective frequency ofthe building due to the increase in flexibility of isolation system during the main shock, large acceleration wasrecorded on the superstructure with the peak floor accelerations of nearly 300 cm/s2. Two factors contribute to thiscause: one is the characteristics of the building where torsional modes dominate the seismic response of upperstories and the other is resonance, where dominant frequencies of ground motions coincide with the naturalfrequencies of torsional modes. Moreover, analysis shows that torsional modes were not significantly influencedby performance of base isolation, so that even though the base isolation has functioned properly, the upper storiesstill experienced large floor accelerations. The paper also discusses long-term observation of seismic responsesduring aftershocks and various levels of earthquakes between 2010 and 2012. Copyright © 2014 John Wiley &Sons, Ltd.

Received 17 October 2013; Revised 4 March 2014; Accepted 10 March 2014

KEY WORDS: building seismic monitoring; asymmetric base-isolated building; building torsional response; 2011Great East Japan Earthquake; system identification

1. INTRODUCTION

Base isolation as a seismic mitigation technique for building and bridge has gained popularity inJapan especially after the 1995 Hyogo-ken Earthquake. The favorable response during anearthquake, ability to limit structural damage to a low and repairable level, and ability to maintainfunctionality after a large earthquake are some advantages that have increased the acceptance ofbase-isolation technology among engineers and building owners. Nowadays in Japan, base-isolation system is widely used for hospital, school, university, and office buildings [1]. Thesystem usually consists of seismic isolators such as elastomeric or sliding type combined withenergy absorbing dampers. The isolator creates a structure with longer natural period, whereasthe damper provides higher damping to reduce the structural response. According to a recentsurvey conducted in various locations in Japan, seismically isolated buildings generally showed

*Correspondence to: Dionysius M. Siringoringo, Department of Civil Engineering, University of Tokyo, Tokyo, Japan.†E-mail: [email protected]

Copyright © 2014 John Wiley & Sons, Ltd.

72 D. M. SIRINGORINGO AND Y. FUJINO

good performances during the 2011 Great East Japan (Tohoku) Earthquake, in that accelerationresponses of superstructures were less than the input ground motions as a result of isolationeffect [2,3].

There are two main concerns regarding the application of base-isolation system on an asymmetricbuilding, that is, asymmetricity of isolation system and asymmetricity of superstructure system. In bothcases, the asymmetricity could be caused by eccentricity in either superstructure or isolation system.When eccentricity exists in a superstructure system, coupled lateral–torsional motions occur as theydo in the asymmetric nonisolated buildings. The effect of torsional motion can be significant when abuilding has large eccentricity. Outer isolators will experience larger force than inner ones whentorsional motion is dominant. Large building rotation also contributes to significant corner deformationsuch that outer columns will experience larger shear force as a result of combined shear and momentfrom floor eccentricity.

The behavior of asymmetric base-isolated buildings and the effects of torsional motion on baseisolation performance have been extensively studied analytically, numerically, and experimentallyby many researchers. Lee [4], who defined the dynamic torque amplification factor as a ratio ofdynamic torque to the static torque at the center of stiffness, concluded that when eccentricity ofisolation systems is small (<0.2L, L is the longest plan dimension), displacements of the basedue to rotational motions remain small even if eccentricity of the structure is large. However, asshown by Eisenberger [5], this conclusion is valid only for a certain type of ground motion. Panand Kelly [6] show analytically the importance of ratio between torsional and lateral frequencyon the rotational motion and corner deformation. Nagarajaiah [7] investigated the effects ofeccentricity of isolation, eccentricity of superstructure, and frequency ratio between superstructureand base, and concluded that the main source of torsional motion in base-isolated buildings withelastomeric bearings is the eccentricity of isolation system, in that increasing the eccentricity ofisolation system leads to the increase in torque amplification.

Experimental studies on a scaled model building in the shaking table tests have been performed toinvestigate performance of asymmetrical base-isolated buildings. Employing models with variouseccentricities, Nakamura et al. [8] showed that the rotational motion of the structure can be minimizedby reducing the eccentricity of the isolation system. Hwang and Hsu [9], who tested asymmetricsuperstructure with various eccentricities on the isolation system, concluded that increasing theeccentricity will amplify the rotational response of the structure. Furthermore, corner displacementwas amplified by the rotational motion; with the contribution of the rotational motion to the maximumcorner, displacement can be as high as 30%.

Strong motion recordings of instrumented buildings have also been used to study the torsionalresponse of base-isolated buildings. The records from the 1985 Redlands earthquake (ML 4.8) onthe Foothill Community Law and Justice Center [10] revealed that the transverse motion of thebuilding is more dominant than the longitudinal motion. The rotational motion of thesuperstructure due to spatial variability of ground motion and the extreme length of the structureare considered as the main reason. Nagarajaiah and Xiahong [11] observed nominal torsionalresponse of the University of South California hospital building during the 1994 Northridgeearthquake. The building has an eccentricity ratio of 5%, and the ground motion had significantenergy in the higher mode.

The aforementioned studies, experiments, and strong motion observations provide valuableinsights on the behavior of asymmetrical base-isolated buildings and conditions that influencethe performance of isolation system. Recently, many base-isolation buildings, especially in anearthquake-prone country such as Japan, are instrumented with seismic monitoring systems. Themonitoring systems provide seismic responses that are essential for seismic performanceevaluation. This paper describes a study on strong motion recordings from such building. Thestudy includes the following: (i) seismic response evaluation of base-isolated building duringthe main shock of March 11, 2011, Great East Japan Earthquake using time-invariant and time-variant system identification; (ii) investigation on the seismic response characteristics consideringthe building asymmetricity; and (iii) evaluation of the long-term seismic response characteristicsof the building for over a year that includes the foreshock, main shock, aftershocks, and variousamplitude earthquakes in 2010–2012.

Copyright © 2014 John Wiley & Sons, Ltd. Struct. Control Health Monit. 2015; 22:71–90DOI: 10.1002/stc

ASYMMETRIC BASE-ISOLATED BUILDING IN 2011 GREAT EAST JAPAN EARTHQUAKE 73

2. DESCRIPTION OF BASE-ISOLATED BUILDING AND SEISMIC ISOLATORS

The monitored structure is the SIT Building located in Tokyo Bay area. The building consists of twoparts: 14-story main building (M) and seven-story annex building (A) (Figure 1). The main building is97.2m long, 43.2m wide, and 67.5m high with 19 bays in longitudinal direction and six bays intransverse direction. The annex building is 81m long, 21.6m wide, and 31.2m high with 16 bays inlongitudinal direction and three bays in transverse direction. Both buildings are of braced steel framesand connected at the corner by elevator shaft to form an L-shaped asymmetric structure. The 14-storymain building has vertical opening in the middle starting from the second floor to the seventh floor. Theopening divides the main building into the west section and east section, named hereafter as sectionsMW and ME, respectively. Meanwhile, the seven-story annex building has some voids on floors toprovide access for escalator. Both buildings are connected by concrete slab at the basement level, ontop of which isolation systems are placed.

The isolation system consists of 59 natural rubber bearing (NRB) and 26 sliding bearing. There aretwo types of NRB, namely, the 100 cm diameter with initial stiffness 11.9 kN/cm (43 units) and the110 cm diameter with initial stiffness 12.9 kN/cm (16 units). The NRBs are made up of 26 layers ofrubber with 0.75 and 0.83mm thickness for diameter 100 and 110 cm, respectively, and steel shimsof 0.45 cm. The design maximum deformation of NRB is set to 250% of the shear strain or about49 cm. Meanwhile, the ultimate deformation is approximately 350% of the shear strain or about69 cm. The sliding bearings’ surface is made of polytetrafluoroethylene with the initial stiffness12.3, 17.5, and 22.9 kN/m for diameter 56.4, 71.4, and 94.4 cm, respectively. The thickness ofthe sliding surface is 2mm with the friction coefficient of 0.013. To limit the horizontal motion,dampers are added to the isolation system. Two types of dampers are used, namely the leaddamper (28 units) and the U-shaped steel dampers (33 units). The layout of isolators and dampersare given in Figure 1(b).

The eccentricity ratios of the building and isolator system are listed in Table I. The ratio is defined asthe eccentricity in the x- and y-directions with respect to building length in the corresponding directionand presented separately according to building sections and floors. The table shows that the eccentricity

Figure 1. (a) SIT Building, (b) layout of isolation system, (c) sensor locations, and (d) architecture of the monitor-ing system.


Table I. Eccentricity ratio of the SIT Building.

Eccentricity ratio (%)

X-direction Y-direction

Annex building1st Floor �0.72 �1.562nd Floor �1.58 �9.073rd Floor 0.64 3.414th Floor �0.47 �7.865th Floor 0.49 �3.696th Floor �1.94 �5.767th Floor �1.91 5.50

Main building1st Floor 3.00 �0.532nd Floor 7.32 4.238th Floor 2.60 1.949th Floor 3.67 1.8210th Floor 2.48 2.1611th Floor 2.68 2.1212th Floor 4.14 2.0413th Floor �1.70 2.7714th Floor 0.46 0.10

Main building section MW3rd Floor 7.52 11.394th Floor 7.61 11.735th Floor 7.72 11.816th Floor 7.74 11.977th Floor 5.82 �23.86

Main building section ME3rd Floor �3.24 �9.904th Floor �3.05 �9.645th Floor �2.96 �9.696th Floor �2.85 �9.867th Floor �2.83 �9.98

Isolators 3.41 �8.83

Note: Eccentricity ratio = eccentricity/building length × 100%


in the y-direction is generally larger than in the x-direction for annex and main building, and the ratiosin some floors are larger than 10%, indicating significant asymmetricity of the building.

3. DESCRIPTION OF SEISMIC MONITORING SYSTEM

The building has a seismic monitoring system that consists of 21 triaxial accelerometers (18 accelerometerson the structure and three accelerometers on the ground) and four triaxial displacement-meters measuringrelative displacement between basement and the first floor (Figure 1(c)). Vibration sensors are placed onthe basement (below the isolators) and the first, fourth, seventh, ninth, and fourteenth floors.

Acceleration responses are recorded by small servo-type accelerometers SQ-32 with resolution of0.01 cm/s2 and measurement range of ±2000 cm/s2. The sensors are connected through local areanetwork, and the clock on each sensor is precisely synchronized with the Network Time Protocol server.Furthermore, a GPS-connected controller unit is assigned to provide a global reference position andsynchronized time recording among the sensors. With this system, the delay time among sensors can bereduced to the maximum of 4ms [12]. The controller is connected to three accelerometers on the groundlevel that act as trigger. Once these accelerometers record ground acceleration larger than the threshold0.5 cm/s2, the controller will activate the monitoring system to record building responses. The responseswere sampled at 100Hz and stored in a server for further analysis. It should be mentioned that horizontalaccelerations were oriented in the x- and y-directions according to the building orientation as opposed to



the commonly used north–south and east–west directions. The sensors nomenclature is organized asfollows: the first index denotes sensor type (A, accelerometer; D, displacement-meter), the second indexrepresents the building (M, main; A, annex building), the third index represents story level, and the lastindex shows location of the sensor in the floor (i.e., S, south; E, east; W, west). The type of sensors and theirlocations are shown in Figure 1(c).

4. DESCRIPTION OF RECORDED SEISMIC RESPONSES

At 02:46 PM Japan Standard Time (JST) on March 11, 2011, northeastern Japan was struck by the mostdevastating earthquake in 130 years. The Great East Japan earthquake has the moment magnitude of9.0, the largest ever recorded in Japan. The Japan Meteorological Agency (JMA) seismic intensity5� (lower 5), out of the maximum scale of 7, was recorded on the building location. This seismicintensity is equivalent to scale VII in the Modified Mercalli Intensity scale. Seismic responsesdescribed in this study consist of the main shock event on March 11, 2011, at 02:47 PM, foreshockevent, and several aftershocks on the same days and few days afterward with the JMA seismic intensityequal to or larger than 3. In the beginning, the discussion will focus on the building seismic responsesduring the main shock event on March 11, 2011, at 02:47 PM JST. Afterward, responses due to severalsmall earthquakes in 2010, foreshock to the March 11, 2011, earthquake, aftershocks, and variouslevels of earthquake between 2011 and 2012 are discussed to provide comparison of the responsecharacteristics. Table II provides detailed information on five largest earthquakes that give the largestexcitation recorded on the building in the period between 2010 and 2012.

5. CHARACTERISTICS OF RECORDED MAIN SHOCK GROUND MOTIONS

Figure 2 shows time histories of ground acceleration obtained from free-field AF0E sensorduring the main shock. The excitation lasted for about 10 min, in which the significant dura-tion (i.e., 95% energy of the excitation) lasts for about 6 min since the arrival of the primarywave. The maximum ground acceleration in the X, Y, and vertical directions were 0.16, 0.17,and 0.08 g, respectively. The peak acceleration occurred at 143 and 130 s for the X and Y directions,respectively. The ground accelerations have dominant frequency content between 0.7 and 1.2Hz in bothXand Y directions.

In Figure 3, the response spectra of two ground accelerations, namely AM0W (basement level) andAF0E (free-field sensor), are shown. The spectra are compared with the design response spectrafor level 2 earthquake with soil condition classes 2 and 3, as specified in the Japan’s BuildingCode. As can be seen in the figure, the spectra of the main shock have large amplitude in theperiod range of 0.7–1.2 s and at about 2.5 s. For the period range below 2.5 s, which is the periodrange of the building, the spectra amplitudes of the present earthquake are still well below thedesign spectra.

Table II. Five earthquakes with the largest excitation recorded on the SIT Building between 2010 and 2012.

Max recorded groundacceleration (cm/s2)

Max recordedacceleration (cm/s2)

Earthquake(date-month-year), JST Mw

JMA seismicintensity AM0W-Y AM0W-X AA7S-Y AM11E-X

11-03-2011, 02:47 PM (main shock) 9.0 5� 110.53 80.46 297.42 204.1211-03-2011, 03:16 PM (after shock 1) 7.7 4 53.03 38.24 239.99 87.9929-05-2012, 01:36 AM 5.2 4 29.58 28.20 32.83 29.4824-11-2012, 05:59 PM 4.8 4 27.09 18.49 44.58 15.9807-12-2012, 05:19 PM 7.3 4 29.20 26.37 49.79 38.78


(b)(a)

Figure 2. (a) Recorded ground acceleration time histories (AF0E: free-field) during the main shock; (b) Fourierspectra of ground acceleration.


6. CHARACTERISTICS OF RECORDED BUILDING RESPONSES

The characteristics of accelerations recorded on the top floor of the annex and main buildings areshown in Figure 4. The responses have similar characteristics with the accelerations on the lower floorswith one dominant peak. Similar to the accelerations on the lower and ground floors, the peakaccelerations occur at about 143 and 130 s for the X and Y directions, respectively, indicating nosignificant time lag between the response of the ground floor and the upper floors. Table III lists thepeak accelerations recorded by all sensors during the main shock. Strong amplification was recordedon the annex building in the Y direction. The peak acceleration on the south section of seventh floor(AA7S) is nearly 300 cm/s2 (Figure 5) or about three times of the basement acceleration. Large accel-eration was also recorded on the fifth floor (AA7S), with peak acceleration of 265 cm/s2. This large ac-celeration can be considered exceptional considering that the peak acceleration on the basement levelof the southeast section of the annex building is only 96 cm/s2. The transfer functions of accelerationsof the annex building in the Y direction (Figure 6) reveal that the responses were dominated by fre-quency peak at 0.86Hz.

0 0.5 1 1.5 2 2.5 30

0.2

0.4

0.6

0.8

1

1.2

1.4

Spe

ctra

Acc

eler

atio

n(g)

Period(sec)

DS-L2-S2DS-L2-S3

AM0W-Y

AM0W-X

AF0E-YAF0E-X

Figure 3. Acceleration response spectra at surface level for 5% damping and comparison with design spectra atsurface level (note: DS, design spectra; L2, level 2 earthquake; S2/S3, site class 2/3 for Tokyo area).


Figure 4. Recorded horizontal accelerations at the top floor of the building: (a) seventh floor of the annex building,(b) 14th floor of the main building section NW, and (c) 14th floor of the main building section NE. Fourier spectraof horizontal acceleration on the (d) seventh floor of the annex building, (e) 14th floor of the main building section

NW, and (f) 14th floor of the main building section NE.

Table III. Summary of the SIT Building peak responses during the main shock of March 11, 2011, Great EastJapan Earthquake (units: acceleration in cm/s2; displacement in cm).

Mainbuilding

Peak responseAnnexbuilding

Peak response

X-direction Y-direction Vertical X-direction Y-direction vertical

AM0W 80.46 110.53 86.88 AA4S 69.61 265.15 78.94AM4W 66.75 84.09 107.07 AA7S 85.31 297.43 99.32AM7W 69.67 81.36 122.62 AA0S 80.72 96.03 76.54AM9W 87.93 80.92 149.88 AA0W 111.53 108.82 87.35AM14W 236.52 141.41 178.92 AA1S 92.53 89.41 84.38AM4E 129.07 92.23 72.10 DA0S 5.60 4.82 0.29AM7E 112.36 82.36 90.27 DA0W 7.88 5.06 0.24AM9E 116.29 74.91 103.74AM14E 204.12 137.38 117.43AM1W 85.35 118.04 49.67AM1E 120.66 94.68 46.09AM0E 82.39 110.62 88.83DM0W 8.04 4.78 0.47DM0E 9.90 5.12 0.57


Response amplification of the upper stories was also observed on the main building in the Xdirection. The peak accelerations on the fourteenth floor of the east and west sections of the mainbuilding were about 204 and 236 cm/s2, respectively, or about 2.5 times of the ground acceleration.Figure 7 shows the transfer functions of accelerations of the main building in the X direction, and itcan be seen that large acceleration responses were mainly due to frequency component at 0.58Hz,whereas responses in the Y direction were dominated by frequency component at 0.44Hz.

The significant acceleration amplification of the upper stories is rather unexpected for a base-isolated building. A common assumption in the design of seismically isolated building is that lateraldeformation is concentrated on isolator level and the upper structure behaves rigidly without significantdynamic amplification. In Japan, although not explicitly prescribed in the code, on the basis ofexperiments on medical facilities and electronic appliances, the common design practice is to limit


0.5 1 1.5 2 2.50

5

10

15

Frequency (Hz)

Tra

nsfe

r R

atio AA4S

AA7S0.86Hz

Figure 6. Transfer functions of recorded accelerations on the annex building in the Y direction.

50 100 150 200 250 3000123456789

1011121314

Max Acc X-Dir (cm/s2) Max Acc Y-Dir (cm/s2)

Flo

or

50 100 150 200 250 300

Max Acc Vertical (cm/s2)50 100 150 200 250 300

0123456789

1011121314

0123456789

1011121314

AA7S

AA4S AA4S

AA7S

Figure 5. Recorded maximum acceleration amplitude for each floor in each direction (note: 0 = basement level).

0.5 1 1.5 2 2.50

2

4

6

8

Frequency (Hz)

Tra

nsfe

r R

atio

0.5 1 1.5 2 2.50

2

4

6

8

Frequency (Hz)

Tra

nsfe

r R

atio

0.5 1 1.5 2 2.50

2

4

6

8

Frequency (Hz)

Tra

nsfe

r R

atio

0.5 1 1.5 2 2.50

2

4

6

8

Frequency (Hz)

Tra

nsfe

r R

atio AM9W-Y

AM14W-Y

AM9W-XAM14W-X

AM9E-YAM14E-Y

AM9E-XAM14E-X

0.44Hz 0.58Hz

0.44Hz

(b)

(c)

(d)

(a)0.58Hz

Figure 7. Transfer functions of recorded accelerations on the main building in the (a,b) Y and (c,d) X directions



.

the horizontal floor acceleration to maximum 300 cm/s2 [13]. Avoiding large floor acceleration isnecessary to protect nonstructural components, highly sensitive equipment, and computer facilitiesinside the building. In the following sections, we shall discuss the reason behind significant dynamicamplification of the upper stories.

7. SYSTEM IDENTIFICATION TECHNIQUES

The seismic behavior of the building is examined thoroughly by evaluating vibration modes pertinentto the seismic responses using system identification techniques. The general approach of earthquake-induced system identification is to use the input–output relationship to recreate structural models that


are capable of reproducing the actual responses, for example, via output-error minimization of parametrictime-invariant model [14], MIMO state-space with system realization model [15–17], or time-variantadaptive least square technique and autoregressive exogenous (ARX) model [18,19]. In this study, twotypes of nonparametric system identification procedures were employed, namely the time-invariantsystem using moving window and the time-variant system using recursive least square (RLS).

7.1. Time-invariant system identification

The first system identification, a time-invariant MIMO system, is based on the system realization usinginformation matrix (SRIM) [20]. The SRIM algorithm utilizes correlation functions between input andoutput data to realize a state-space model and to estimate the modal parameters. The identificationprocedure starts by estimating the observability matrix Op from the so-called information matrix thatis composed by the correlation functions of input and output data. To determine the observabilitymatrix Op, one can start by obtaining the matrices of input–output correlation data:

Rhh¼OpR̂xxOTp (1)

where the quantities Rhh and R̂xx are defined as follows:

Rhh¼Ryy�RyzR�1zz R

Tyz (2a)

R̂xx¼Rxx�RxzR�1zz R

Txz: (2b)

The quantity Rhh is determined from the input (z) autocorrelation matrix Rzz, the input–outputcorrelation matrix Ryz, and the output (y) autocorrelation matrix Ryy, and it exists only if the inputautocorrelation matrix Rzz is a nonsingular matrix. To obtain the solution for matrix Op,factorization of Equation (1) is required. In this factorization, the observability matrix is divided intothree matrices using the singular value decomposition as follows:

Rhh :; 1 : p� 1ð Þm½ � ¼ OpR̂xxOTp :; 1 : p� 1ð Þm½ � ¼ H2NΣ2

2NVT2N : (3)

Finally, following the identity in Equation (3), the observability matrix can be obtained as follows:

Op ¼ H2N and R̂xxOTp :; 1 : p� 1ð Þm½ � ¼ Σ2

2NVT2N : (4)

Given the observability matrix, the system matrix A can be estimated as follows:

A ¼ O�p 1 : p� 1ð Þm; :ð ÞOp mþ 1 : pm; :ð Þ (5)

where the asterisk (*) denotes the pseudo inverse matrix. The integer p should be chosen such thatmatrix Op(m + 1 : pm, :) of dimension (p� 1)m × 2N has rank larger than or equal to 2N; hence,p⩾ 2N/m+ 1.

The modal parameters of the structural system can be estimated by solving the eigenvalues problemof matrix A as follows:

AΦ̂¼eΛΦ̂: (6)

Matrices eΛ and Φ̂ denote the eigenvalues and eigenvectors of matrix A, respectively. Theeigenvalues and eigenvectors can be real or complex, where in the latter case, they appear in complex

conjugate pairs. The eigenvalueseλi are actually expressed in z-domain and, therefore, can be related tothe modal characteristics of dynamics system using the following transformation:

λi ¼ ln eλi� �

=Δt: (7)

After transformation, the natural frequency (ωi) and modal damping ratio (ξ i) can now be estimatedas follows:



ωi ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiRe λið Þ2 þ Im λið Þ2

q; ξ i ¼ �Re λið Þ=ωi: (8)

Mode shapes matrix in coordinate system is obtained by transforming the eigenvectors in z-domaininto coordinate domain using the output transformation matrix R,

Φ¼RΦ̂: (9)

Detailed information on the system identification algorithm and example of application is givenin [20,21].

To implement the system identification effectively, a set of input–output data was selected.Responses from triaxial accelerometers located at the basement, the same level as the isolators (i.e.,AA0S, AA0W, AM0S, and AM0W), were selected as inputs. Meanwhile, accelerations from 13sensors on the upper stories (i.e., AA1S, AM1W until AM14E) were selected as outputs. Note thatthe input and output data are derived from multiaxial accelerations, so that the identified mode shapeshave three-dimensional shape. It should be mentioned that in the time-invariant assumption, the modalparameters remain constant during a specific time window in which the input–output data wereanalyzed. However, considering that the response of base-isolated building may enter nonlinear regionduring large excitation, this assumption may not be satisfied throughout the whole responses.Therefore, a piecewise linear analysis was conducted using moving time window. A shorter timewindow was selected for analysis, during which the modal parameters were assumed to remainconstant. For this reason, the total time history responses were divided into several time windowsconsisting of 50 s of input and output data. Each time window generates a set of modal parameters,from which the change of parameters with respect to time can be evaluated. By applying thisprocedure, the 600 s main shock response was divided into 12 time windows of input–output data.

7.2. Time-variant system identification

The second system identification is the RLS method using the ARX model. In this technique, themodal parameters are considered as time-variant quantities. Consider the ARX model given by thefollowing equation:

A qð Þy kð Þ ¼ B qð Þz kð Þ þ w kð Þ (10)

where y(k) and z(k) are the output and input sequences, respectively, and w(k) denotes the white noisesignal. A(q) and B(q) represent the polynomial functions that include the autoregressive and movingaverage coefficients {ai}, {bi} and are defined as follows:

A qð Þ ¼ 1þ a1q�1 þ⋯þ anaq

�na (11a)

B qð Þ ¼ b1q�1 þ⋯þ bnbq

�nb: (11b)

The quantity q� j describes a backward shift operator q� jy(k) = y(k� j), whereas na and nb are theorders of the system output and input, respectively.

The autoregressive and moving average coefficients {ai}, {bi} are evaluated using input and outputsequences z(k) and y(k) recorded on the building. The input is the acceleration response located at thebasement, whereas the outputs are the accelerations on the upper floors. The model parameters areassumed to be estimated by the following equation:

θ Nð Þ ¼ R Nð Þ�1Ψ Nð Þ; (12)

where

θ Nð Þ ¼ a1; a2;⋯ana; b1; b2;⋯bnb½ �T (13)



R Nð Þ ¼ 1N

XNk¼1

λN�kφ kð ÞφT kð Þ (14)

Ψ Nð Þ ¼ 1N

XNk¼1

λN�ky kð ÞφT kð Þ: (15)

The quantity λ denotes the coefficient of forgetting factor, whereas φ(k) is given by the following:

φ kð Þ ¼ �y k � 1ð Þ;⋯;�y k � nað Þ; z k � 1ð Þ;⋯; z k � nbð Þ½ �T : (16)

In the ARX model, the natural frequencies ωj and damping ratios ξ j are evaluated as modulus rj andargument pj of the poles of polynomial functions by employing the following equations [22,23]:

ωj ¼ln 1=rj� �

2πξ jΔtand (17a)

ξ j ¼ln 1=rj� �

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffipj� �2 þ ln 1=rj

� �� 2q : (17b)

In this study, the RLS–ARX is implemented as a SISO system. The response from the basement(AM0W) was selected as the input, whereas the accelerations from the upper stories (i.e. AA7S,AM14W, and AM14E) were used as outputs. Accordingly, three separate SISO systems wereemployed, namely AM0W–AA7S system for the annex building, AM0W–AM14W system for themain building west section, and AM0W–AM14E system for the main building east section. In thecalculation, 5 s of response delay was selected, and λ= 0.99 was chosen as the forgetting factor.

8. RESULTS OF SYSTEM IDENTIFICATION AND COMPARISON WITH ANALYTICALMODEL

The MIMO time-invariant SRIM system identification results in three dominant modes: the firsttranslation mode identified at 0.44–0.55Hz, the first torsional modes identified at 0.58–0.68Hz, andthe second torsional mode at 0.85–0.95Hz. The identified frequencies differ slightly for each timewindow as listed in Table IV. And, as illustrated in Figure 8, the first mode has characteristic of largemodal displacement at the isolator layer and small modal displacement of the upper stories resembling

Table IV. Comparison between identified and estimated frequencies of the SIT Building.

Estimated by FEMTranslational

modeTorsional mode(main building)

Torsional mode(annex building)

Assumed initial stiffness of isolator (Hz) 0.50 0.64 1.05Assumed large deformation of isolators (Hz) 0.16–0.18 — —

Identified from main shock ω1 (Hz) ξ1 (%) ω2 (Hz) ξ2 (%) ω3 (Hz) ξ3 (%)

Frame 1 (t= 1–50s) 0.61 2.83 0.68 3.79 0.96 3.20Frame 2 (t= 51–100 s) 0.49 8.87 0.68 5.81 0.97 3.51Frame 3 (t= 101–150 s) 0.50 11.32 0.64 5.90 0.84 8.70Frame 4 (t= 151–200 s) 0.44 14.60 0.58 7.22 0.85 13.47Frame 5 (t= 201–250 s) 0.45 11.62 0.58 11.60 0.90 3.91Frame 6 (t= 251–300 s) 0.48 8.10 0.63 5.04 0.82 3.93Frame 7 (t= 301–350 s) 0.49 5.69 0.63 4.99 0.94 3.30Frame 8 (t= 351–400 s) 0.50 4.19 0.63 5.52 0.94 3.19Frame 9 (t= 401–450 s) 0.50 3.26 0.65 4.21 0.96 4.64Frame 10 (t= 451–500 s) 0.51 2.82 0.67 4.39 0.98 3.70Frame 11 (t= 501–550 s) 0.51 3.28 0.67 3.96 0.97 4.73Frame 12 (t= 551–600 s) 0.52 3.56 0.67 3.70 0.99 4.49


Figure 8. First three modes identified from system identification of the main shock (note: results are obtained fromframe 4, and the blue lines are from sensors on the east side of the main building).


the rigid body motion of the superstructure. This is a typical characteristic of the fundamental base-isolation mode. The second is the torsional mode with large modal displacement concentrated on thetop corner of the main building. In this mode, the modal displacements of the superstructure of theannex building are smaller. On the other hand, the third mode is the torsional mode with large modaldisplacements concentrated on the corner of the annex building and small modal displacements on theupper floors of the main building. In both torsional modes, the modal displacements on isolation levelare significantly smaller than they are in the first translation mode.

In design, two scenarios of isolator behavior and their influence on modal parameters were consid-ered. One is the case of small and medium earthquake in which the initial stiffness of isolators areconsidered, and the other one is during large earthquake, where the isolators enter the secondarystiffness. For the small and moderate earthquake, the isolators are expected to remain on the initialstiffness, and the estimated natural frequencies are as shown in Table IV. In the design for largeearthquake (maximum input larger than 500 cm/s2), substantial deformation of isolation up to 40 cmis assumed, and as a result, the first translation mode dominates the building vibration. In such condi-tion, the first natural frequency of the building could reach as low as 0.16Hz, and the structure movesas rigid body where the main deformation concentrates on isolator layer. When large deformation oc-curs on the isolators during large earthquake, it is expected that the torsional modes of the upper struc-ture become insignificant and translation mode due to the flexibility of isolation dominates vibration ofthe building. Comparing the identified natural frequencies with the estimates from two scenarios, onecan see that the identified frequencies are closer to the response estimates for small and moderateearthquakes. This goes to say that the present condition represents a case of intermediate level ofground motion (i.e., maximum input between 100 and 500 cm/s2), where the effect of torsional motionis still dominant, and this dominant torsional motion results in large peak horizontal acceleration on theupper structure.

8.1. Variation of natural frequencies and damping ratios during the main shock

A variation of natural frequencies and damping ratios over time during the main shock can be observedfrom the results of RLS–ARX and SRIM as described in Figure 9. Note that the results from the time-


Figure 9. Time variation of natural frequency and damping ratio of the first three modes identified from the mainshock by RLS–ARX model and SRIM algorithm.


invariant identification agree well with the results of the time-variant identification for the three modesindicating that time variation of the modal parameters can be captured reasonably by time-invariantidentification when moving time window analysis is applied. The general trend shows that strongshaking causes a significant reduction in the natural frequency. The natural frequency of the first modedecreases from about 0.6 Hz in the beginning of the response to about 0.44 Hz during the largestexcitation (t = 150–250 s). A similar reduction in the natural frequencies was also evident fromthe second and third modes, although to a lesser extent. In the SRIM identification, reductionsof the natural frequencies were identified during the largest excitation ( frames 3 and 4). Thesecond and third modes also show the reduction from 0.68 and 0.95 Hz in the beginning ofthe response to 0.58 and 0.85 Hz, respectively, during the largest excitation. The frequenciesincrease slightly near the end of excitation when excitation amplitude becomes smaller but donot recover to their conditions prior to the arrival of a seismic wave. The results suggest thateffective linear vibration properties of the isolated structure are strongly dependent on the inten-sity of the excitation.

While reducing the natural frequency, the strong shaking increases the effective damping ratio.The increases in damping ratio were observed in the first three modes. The result of RLS–ARXshows that the damping ratio of the first mode increases from about 5% in the beginning of theresponse to about 20% during the largest excitation. Similarly, the damping ratio of the secondand third modes also increases from about 3% and 4% during the beginning of the responses toabout 10% and 15%, respectively, during the largest excitation. Note, however, that the dampingestimates are generally decreasing nearly to their original values at the end of the responses.The increase in damping was also evident from the results of the time-invariant SRIMidentification, where significant damping for the first mode about 15% was identified from the timewindow (t = 150–200 s), which corresponds to the largest excitation. Similar results can be observedfrom other modes (Table IV). The increase in damping is related to the performance of isolators anddampers that become fully engaged during the largest excitation, as shown by the largest isolatordeformation occurring during t = 130–170 s. Large damping in the present earthquake is inagreement with common practice in Japan, that is, to design a number of dampers so that theequivalent damping is about 15–20% [13].



8.2. Variation of mode shapes during main shock

As previously mentioned, the SRIM identification is a MIMO system, whereas RLS–ARX is a SISOsystem, so only the SRIM provides mode shapes information. Figure 8 shows the first three modeshapes estimated by SRIM from the main shock (time window t= 150–200 s). As shown in the figure,only the first mode is significantly affected by flexibility of isolators in that the relative modaldisplacement between the basement and the first story is large. The other two modes are more affectedby the upper stories modal displacement in that the second mode at 0.58Hz is dominated by thetorsional response of the main building upper story, whereas the third mode at 0.85Hz is dominatedby the torsional response of the corner of the annex building.

Figure 10 shows the first mode shapes for the main shock generated from SRIM analysis duringseveral time frames. One can observe a significant isolator flexibility in the Y direction during therelatively strong shaking (i.e., frames 3 and 4) and the time-varying characteristics of the modeshape. Results are shown for frame 2 (t = 51–100 s) when the initial shear waves have arrived andaccelerations are steadily increasing, frame 3 (t = 101–150 s) and frame 4 (t = 151–200 s) whenaccelerations are steadily increasing and reaching the maximum amplitude, and frame 6 (t=251–300 s)after the principal body waves have passed the site. Results for frames 3 and 4 clearly indicateincreasing relative isolator flexibility as the amplitude of shaking increases. Near the end of theresponse, that is, when the amplitude of shaking has decayed, the mode shape resembles that ofthe beginning of the response. These results indicate that the modal displacement ratio betweenthe basement and the first floor is strongly dependent on the amplitude of shaking as a result ofnonlinear isolator response.

9. RELATIONSHIP BETWEEN ISOLATOR DEFORMATION, MODE SHAPES, AND GROUNDEXCITATION

The influences of excitation intensity on the isolator’s relative displacement for the first three dominantmodes during the main shock are shown in Figure 11. In this figure, abscissa quantifies the ground

0 0.2 0.4 0.6 0.8 1

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Figure 10. Time variation of the first mode during the main shock.


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Figure 11. Influence of the shaking intensity on the isolator’s relative displacement during the main shock for the(a) first mode, (b) second mode, and (c) third mode.


motion amplitude in terms of response spectra of acceleration AM0W for 5% damping. The responsespectra of horizontal ground acceleration were calculated at the frequencies that correspond to the firstthree modes (i.e., 0.45, 0.58, and 0.85Hz). In such a way, we can quantify the frequency componentsof the ground motion that are relevant to the natural frequencies of the structure. It is noted, however,that the response spectra amplitudes have linear relationship with the root mean square (RMS) ofground motion in that large accelerations (in terms of RMS) result in large acceleration spectra,although the linear trend of each mode is not necessarily the same.

To measure the influence of shaking intensity on isolator displacement, an index called relativeisolator modal displacement (RIMD) is utilized. For each mode shape, the RIMD measures therelative modal displacement at isolator level with respect to the modal displacement at the rooflevel. The mode shapes used here are derived from the time-invariant system identification ofthe main shock using moving window scheme as explained earlier. Note that when the isolatordisplacement is large, the RIMD value will become closer to unity, and for an ideal rigidsuperstructure, the RIMD value is equal to one. The RIMD index also confirms the influenceof isolator displacement on the global modal displacement for each mode. As can be seen inthe figure, only the first mode shows a clear trend in which the RIMD values increase as thespectra acceleration increase for that mode increases. The other modes, on the other hand, donot share the same tendency. These results indicate that only the first translation mode issignificantly affected by the isolators’ deformation.

10. ISOLATOR’S BEHAVIOR DURING LARGE EARTHQUAKE

Figure 12(a) shows the orbit displacement of the base slab above the isolation layer relative to the slabbelow the isolation layer. The maximum relative displacement about 10 cm was recorded in the Xdirection by sensor DM0E located near the east corner of the main building. The maximumdisplacement in the X direction is about twice than the displacement in the y direction. The design

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Figure 12. (a) Orbit motion of isolator displacements shows relative motion between the basement and thefirst floor recorded by displacement meter at DM0E; cumulative isolator displacement measured at (b) DM0E-X

direction and (c) DM0E-Y direction.



maximum deformation of NRB is set to about 250% of the shear strain or equal to 49 cm, which meansthat the maximum recorded displacement corresponds to 20.4% of the design maximum shear strain.Figure 12(b,c) shows the cumulative story displacement of the isolated layer recorded by sensorDM0E. The maximum cumulative displacement is 415 cm, which is considerably large consideringthat the maximum recorded shear strain is only 20.4% of maximum design. The long duration of theearthquake is thought to be the reason behind this large cumulative story displacement.

To evaluate the time variation of the isolator during the main shock, a force–deformationrelationship of the isolator system is approximated. The force is estimated from the total accelerationimmediately above the isolator by filtering out the frequency components larger than 1Hz that is thefrequency range that corresponds to the frequency contents of the first three dominant modes. Thefiltered total acceleration is assumed to be proportional to the total shear force imparted to the isolators,whereas the relative story displacement recorded by the displacement sensor provides the deformationof the isolator system. By employing the two quantities, one can plot an ‘equivalent hysteresis loops’ ofthe isolator system from which the time variation of the isolator effective stiffness can be inferred.

An example of variations of hysteresis characteristics is shown in Figure 13 for several timewindows. The straight line on each figure connects the maximum and minimum loads to show theapproximate effective stiffness of the isolator layer. As illustrated in the figure, the effective stiffnessis relatively high before the passage of the shear wave (t= 100–130 s) and becomes smaller with thearrival of the largest amplitude of shear wave that causes the increase in the relative displacement ofthe isolator layer. The effective stiffness remains small even after the passage of the shear wave after200 s, indicating that the system has been softened from the initial condition. The results are consistentwith the time variation of the natural frequencies estimated by the system identification. Moreover, asshown in the figure, the largest isolator deformation occurs during t= 130–170 s, which is the timewhen the hysteresis loop reaches the maximum area signifying the increase of energy dissipation. Thiscondition is consistent with the fact during this period damping ratio also reaches the maximum valueas shown by the results of system identifications.

11. LONG-TERM VARIATION OF MODAL PARAMETERS

Since being installed in September 2010, more than 140 seismic events have been recorded by themonitoring system until December 2012. Most of the earthquakes were of small and moderate scale(JMA seismic intensity of 3 or less), and only five events including the March 11, 2011, main shockhave seismic intensity equal or larger than 4. With relatively large earthquake database, we can eval-uate the behavior of the building and observe the trend in the modal parameters with respect to the levelof seismic excitation and their long-term variations. For this purpose, the linear time-invariant system

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t:100-170s

Figure 13. Variation of hysteresis characteristics of isolator during the main shock recorded from displacementmeter DM0E in the X direction along the time of the main shock.



identification (SRIM) was utilized. The identification was performed separately for each input–outputdata set, and the results were evaluated with respect to the RMS of the input amplitude, which is theacceleration from sensor AM0W. In the implementation of the SRIM algorithm, the moving timewindow of 50 s was employed in order to capture the variation of the modal parameters in a singleseismic event.

Figure 14 shows the sequential long-term variation of the first mode. Note that we focus thediscussion on the first mode, because it is the mode that is significantly affected by the isolatorbehavior. As shown in the figure, for one seismic event, the natural frequency reaches the minimumvalue in the beginning of the record that is during the largest excitation and increases gradually towardthe end. A similar behavior is repeated in the next earthquake events. These repetitions result in thesharp turns in alternating directions on the natural frequency as time progresses. The minimumfrequency of the first mode for the entire monitoring record is 0.44Hz, observed during the main shockof the March 11, 2011, earthquake. Prior to that event, the lowest observed frequency was slightlyhigher 0.55Hz, recorded during the foreshock event on February 9, 2011. Note that the naturalfrequency of the first mode was higher before the foreshock event with the average of 0.6Hz. The mainshock of the March 11, 2011, earthquake was followed by several aftershocks with moderate level ofexcitations, and in the course of 2months after the main shock, the building experienced frequentearthquakes with JMA seismic intensity 3 or 4. One can note from the figure that it takes about2months (from March 2011 to May 2011) for the natural frequency to return to its pre-main shockvalue (i.e., the lowest frequency recorded during the foreshock event).

Figure 15(a,b) shows the trend of the natural frequency of the first mode plotted with respect to theexcitation amplitude. As expected, the frequency decreases as the input increases indicating theincrease in flexibility as a result of the isolation system. The decrease in the natural frequency signifies

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Figure 14. Long-term time sequential variation of the first mode: (a) natural frequency from September 2010 toDecember 2011, (b) natural frequency from January 2012 to December 2012, (c) damping ratio from September2010 to December 2011, and (d) damping ratio from January 2012 to December 2012. Note the circle in (a)

showing the variation of frequency in one earthquake event.


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Figure 15. Long-term variation of natural frequency and damping ratio of the first mode with respect to input am-plitude. (a) Natural frequency for earthquakes before March 11, 2011, during the main shock, and aftershocks, (b)natural frequency for earthquakes in 2011 and 2012, (c) damping ratio for earthquakes before March 11, 2011,during the main shock, and aftershocks, and (d) damping ratio for earthquakes in 2011 and 2012 (note: aftershocksin (a) and (c) are all earthquakes that come from Tohoku area (plate boundary area) that occur between March 11

and March 31, 2011).


the increase in flexibility of the structure as a direct consequence of base-isolation function duringearthquakes. An interesting feature observed in these figures is that for the same level of excitation,the natural frequencies of the first mode during aftershocks were lower than those observed beforethe main shock.

The prolonged effect of reduction in the natural frequency is thought to be caused by the stiffnessreduction of the isolator system as results of the property known as the Mullins’ effect [24], whichis a temporary and recoverable reduction in modulus due to cyclic straining history on the rubberproperty of isolators. The reduction in effective modulus with cycling of elastomeric isolator systemdepends on the material properties of elastomer and the strain history. Considering the high amplitudeand long duration of excitation experienced during the main shock of March 11, 2011, earthquake, andmany large aftershocks that follow, it is understandable that recovery of effective modulus takes longertime since all events occurring within a short time span affect the strain history of the isolator system.

Damping values obtained from system identification analyses suggest a wide range between 1%and 15%. From Figure 14(c,d), it can be seen that damping reaches the maximum value in thebeginning of the record, that is, during the largest excitation, and decreases gradually toward theend of the record, as opposed to the trend in the natural frequency. The largest modal dampingof about 15% was observed during the main shock. The dependence of damping to excitationintensity can be observed clearly from Figure 15(c,d), where the damping ratio increases as theinput acceleration increases. The increase in damping is attributed to the base-isolation system thatbecomes fully engaged during large excitation. Moreover, energy dissipation mechanism in baseisolation also includes vibration absorption devices such as u-shaped steel dampers and leaddamper, which increase the overall structural damping when the isolation system begins to functioncompletely during large earthquakes.

12. CONCLUSIONS

Strong motion recordings of an asymmetric base-isolated building located in Tokyo Bay area duringthe March 11, 2011, Great East Japan (Tohoku) Earthquake and various levels of earthquakes thatfollow are investigated in this paper. On the basis of response analysis and system identification, thefollowing conclusions are drawn:



Co

1. The recorded input excitations in the 2011 Great East Japan (Tohoku) Earthquake are still belowthe seismic design ground motions. The building experienced intense shaking for about 10minwith peak ground acceleration 0.80–1.40m/s2 but survived the earthquake without structuraldamage.

2. Seismic response of the building is dominated by three vibration modes: (i) the first translationalmode characterized by large modal displacement at the isolator layer and rigid body movementof the upper stories; (ii) torsional mode with dominant modal displacement at the corner of themain building; and (iii) torsional mode with dominant modal displacement at the corner of theannex building. During the main shock, the first mode shows large lateral modal displacementat the isolator layer indicating that base-isolation system has functioned. This result is supportedby recorded relative displacement between isolation and the first story that reached themaximum amplitude of 10 cm.

3. The building experienced frequency reduction and increase of damping during the main shock.Frequency reduction indicates the increase in flexibility due to base-isolation system, asdemonstrated by reduction of the first mode from about 0.6Hz at the beginning of the responseto about 0.44Hz during the largest excitation. Meanwhile, the increase in damping is aconsequence of shear deformation in the isolation system and functioning of the damper systemduring the large excitation. By employing time-variant and time-invariant system identificationsin an intra-event or using various levels of earthquakes, analysis shows that frequency reductionand increase in damping are strongly related to the excitation intensity.

4. Despite the functioning of base-isolation system, strong response amplifications on the upperstories were recorded during the main shock. Peak acceleration on the top of the annex buildingis nearly 300 cm/s2, or about three times of the basement acceleration. Similarly, a strongamplification was also observed on the main building, with peak acceleration on the 14th floorof the west corner of about 236 cm/s2, which is 2.5 times of the ground acceleration. Results offrequency analysis and system identification of a superstructure reveal that unlike a typical base-isolated building where the first sliding mode dominates the response, the torsional modesdominate the horizontal accelerations of the superstructure, and they cause amplification of up-per stories. Analysis also shows that the torsional modes were not significantly influenced byperformance of the isolation system. Moreover, natural frequencies of the torsional modes coin-cided with the frequencies of ground motions that have the largest energy. Combinations ofthese factors are thought to be the reason behind strong amplification of the superstructure.

5. Significant response amplification could be critical to nonstructural elements or sensitiveequipment on the building and should be carefully considered in the design. At the current state,the torsional effect is not very harmful to the building, and no nonstructural components weredamaged during the earthquake. However, further study on the possibility of larger and moresignificant consequence in case of a larger earthquake is needed. And in an extreme case,countermeasures such as additional damping on the upper floor could be considered to suppressexcessive vibration.

6. Using over 140 recorded earthquakes between 2010 and 2012, we observe that for the samelevel of excitation, the natural frequencies of the first mode remain low for about 2months afterthe main shock. The prolonged effect of reduction in the natural frequency is thought to becaused by the stiffness reduction of the isolator system as results of the temporary andrecoverable reduction in modulus due to cyclic straining history on a rubber. The high leveland long duration of excitation experienced during the main shock of the March 11, 2011,earthquake and occurrence of many moderate level aftershocks that follow closely after the mainshock are considered to influence the strain history of the isolator system and prolong itsstiffness reduction.

ACKNOWLEDGEMENTS

This research is financially supported by Strategic Research Program of Japan Science and Technology Agency(JST) in the area of advanced integrated sensing technology (CREST) under the title ‘Risk Monitoring andDisaster Management of Urban Infrastructure’ (principle investigator: Yozo Fujino). The authors gratefully

pyright © 2014 John Wiley & Sons, Ltd. Struct. Control Health Monit. 2015; 22:71–90DOI: 10.1002/stc


acknowledge this support. The authors also gratefully acknowledge the valuable discussions on monitoring systemwith Dr. Katsuaki Konno of Shibaura Institute of Technology and Dr. Takafumi Nishikawa of NagasakiUniversity. The assistance from NS Consultant Ltd. on the initial design documents and drawing and fromMr. Hisashi Yamasaki, former graduate student on data analysis, is also highly appreciated. The opinions, findings,and conclusions expressed in this material are those of the authors and do not necessarily reflect those of theabovementioned institutions.

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