SYSTEM IDENTIFICATION AND RESPONSEANALYSIS OF RC HIGH-RISE BUILDINGSUNDER SUCCESSIVE EARTHQUAKES
Muhammad Rusli ∗MEE10514

Supervisor: Taiki SAITO ∗∗ABSTRACT
System identification was performed to two RC high-rise buildings located in Tokyo, Japan, one was
37 stories and the other was 32 stories. Response analysis was conducted to the 37-story building forwhich there was structural data available. The strong motion data of the buildings that recorded thesuccessive earthquakes: before, during, and after the mainshock of the 2011 Off the Pacific Coast of
Tohoku Earthquake were used as input in system identification to obtain dynamic parameters. The
parameters as results of system identification were then used as input data in response analysis toobserve the behavior of the MDOF model under some earthquake motions and to determine theadequate damping type that governs the behavior. Health monitoring was conducted as a combinationof system identification and response analysis to monitor the more specific part of the building. The
dynamic parameters of the buildings after the mainshock were changed compared to that before themainshock, the frequency decreased to 24%, the period increased to 32%, and the damping factor
increased to 50%. The adequate damping types of the MDOF model before the mainshock were modal
damping and Rayleigh damping, while that during the mainshock were proportional damping to initialstiffness and proportional damping to nonlinear stiffness. The structural members were in a goodcondition before the mainshock, but the stiffness degradation occurred when they were struck by themainshock, the story shear force exceeded its cracking capacity but still below the yielding capacity,with maximum ductility 0.4, it happened to almost the entire structure except the 4 top stories. The
decreases of frequency on the upper and lower part of the building were 15% and 18% respectively
that indicates the stiffness degradation on the lower part was higher than that on the upper part.
Keywords: Reinforced Concrete, High-rise, Earthquake, System Identification, Response Analysis.

1. INTRODUCTION
The existence of high-rise buildings nowadays becomes a common sight in many cities, both indeveloped and developing countries. Various reasons stand behind its construction: the increasing
demand of housing unit, limited land in urban areas, or by the reason of community prestige due to theexistence of a highrise building considered to reflect economic and technological development of aregion. Reinforced concrete (RC), as a popular building material is widely used due to someadvantages such as low construction cost, simple treatment, and easily shaped for aesthetic purposes.
In earthquake prone areas, it was necessary to ensure the safety of highrise building
structures against the damage due to earthquakes, although rare of highrise buildings were damaged
when earthquake struck, until the Mexico earthquake in 1985. That earthquake destroyed many
highrise buildings because of its characteristic of a long period which resonated with high-risebuildings that had the same characteristic of period. One of the methods to ensure the structural safety∗∗∗

Researcher, Research Institute for Human Settlements (Puskim), Ministry of Public Works (PU), Indonesia.
Chief Research Engineer, International Institute of Seismology and Earthquake Engineering (IISEE), Building
Research Institute (BRI), Japan.

is to identify the dynamic parameters and monitor the behavior of building, to find out some tendencyof the decreasing quality of structure that will lead to the fatal failure anytime when earthquake strikes.
The big earthquake of magnitude M9.0 struck Japan on March 11, 2011 at 14:46 JST, theepicenter was at off-shore of Sanriku (38.103ºN, 142.860ºE), in 24 Km depth. The magnitude of themain shock was the largest in Japan up to the present (2011). According to the report of The
Headquarter for Earthquake Research Promotion (2011), this event had a maximum seismic intensity 7
JMA observed in Kurihara City, Miyagi prefecture. The maximum aftershock was a M7.7 earthquakethat occurred at 15:15 on March 11, as of April 11. There were more than 60 aftershocks of M6.0 orover. On April 7, there was a M7.1 earthquake with a seismic intensity 6 Upper observed in Miyagi
prefecture. The aftershock area extends approximately 500km in an N-S direction. There is fear thatlarge aftershocks will occur from now on, and there is a possibility that the area will be hit by strong
shaking and high tsunami. The severe damages on buildings were mostly caused by the tsunami, thus
concentrated along the eastern coast of Honshu Island (pacific coast), causing collapse or dragged
away of buildings. The earthquake itself did not significantly affect structural damages of newbuildings (designed by the building code after 1981) or retrofitted buildings. Although only minor
structural damages were found on some buildings, the successive earthquakes after the mainshock had
led to fears of increasing levels of damage.
The K-Tower and the S-Tower are the target RC high-rise buildings in this study, both
located in Tokyo, Japan. The K-Tower is 119 m height which has 37 stories, three strong motion
sensors were planted in this building, located on the first floor (01F), middle floor (18F) and top floor(37F). The S-Tower is 111.99 m height which has 32 floors, two strong motion sensors were plantedin the building i.e. on the first floor (01F) and the top floor (32F), one sensor planted underground.
The strong motion data
records of the K-Tower to be analyzed
in this study are 127, while that of the
S-Tower are 60. Those buildings haveregular shapes in vertical as well as inhorizontal, the appearance and generalplan view of the buildings can be seen
Figure 1. The K-Tower
Figure 2. The S-Tower
briefly in Figure 1 and 2.

2. OBJECTIVES AND METHOD
The strong motion data of the Target RC high-rise buildings that recorded the successive earthquakes:
before, during and after the mainshock of the 2011 Off the Pacific Coast of Tohoku Earthquake wereused as input data in system identification to obtain dynamic parameters of the RC high-rise buildings
in each earthquake event. The dynamic parameters as results of system identification were then used
as input data in response analysis to observe the behavior of the structural model in the multy degreeof freedom system (MDOF) and to determine the adequate damping type of the structural model undersome earthquake motions.
The health monitoring conducted as combination of system identification and response
analysis to monitor the condition of the more specific part of the building. The results of those analysis
stages were studied to draw some conclusions. Analysis of the strong motion data was performed byusing the ViewWave software (developed by DR. T. Kashima, BRI), while the response analysis of the
MDOF model was performed by using the MDOF OS software (developed by DR. T. Saito, BRI).

3. SYSTEM IDENTIFICATIONThe strong motion data records of the K-Tower and the S-Tower are analyzed in this chapter. The
dynamic parameters that will be obtained are frequency and period which are identified by Fourier
spectral ratio, and damping factor which is calculated by using half power method. The scatters of theFrequency, F(Hz)modal frequency of the K-Tower and S-Tower are shown in Figure 3 and 4 respectively, the reddashed line represent the mainshock of the 2011 Off the Pacific Coast of Tohoku earthquake.
5
4
3
2
1
0

The mainshock

1

7

Mode 1Mode 3Mode 2Mode 413 19 25 31 37 43 49 55 61 67 73 79 85 91 97 103 109 115 121 127
Data Number

Frequency, F(Hz)Figure 3. Frequency of the K-Tower
5
4
3
2
1
0

Mode 1Mode 3The mainshock

Mode 2Mode 41 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59
Data Number

Figure 4. Frequency of the S-Tower
All of modal dynamic parameters of the K-Tower and S-Tower were analyzed up to the 4thmode and the average value before and after the mainshock were calculated, the shift of thoseparameters before and after the mainshock can be seen in Table 1.
Table 1. The average of dynamic parameters before and after the mainshock
Building
K-Tower

S-Tower

Mode
1
2
3
4
1
2
3
4

Before the mainshock
F(Hz) T(sec)
h(%)
0.528
1.894
2.96
1.643
0.609
1.48
2.931
0.341
1.26
4.138
0.242
0.64
0.475
2.105
4.444
1.519
0.658
1.271
2.661
0.376
0.434
3.793
0.264
0.407

After the mainshock
F(Hz) T(sec)
h(%)
0.432
2.318
4.45
1.355
0.739
2.21
2.417
0.414
1.13
3.422
0.293
0.64
0.359
2.792
5.121
1.139
0.880
2.849
2.028
0.494
1.073
2.921
0.343
0.754

F
-18.15-17.49-17.53-17.31-24.47-25.02-23.79-23.00Shift (%)
T
h
22.34
50.03
21.36
49.95
21.39
-10.36
21.04
-0.18
32.63
15.24
33.73
124.11
31.39
147.22
29.93
85.48

4. RESPONSE ANALYSIS
Response analysis was conducted to the K-Tower for which there was structural data available. Twostrong motion data will be selected among the successive earthquakes, one is the strong motion data
before the mainshock that occurred on July 16, 2007 (data no.03) which caused the maximum
acceleration on the top floor 50.9 gal, the other is the mainshock of the 2011 Off the Pacific Coast of
Tohoku earthquake (data no.76) which caused the maximum acceleration on the top floor 198.3 gal,occurred on March 11, 2011. The period as an analysis result from the data records is used to compare
the periods that obtained from the response analysis of the MDOF model. There are 4 kinds of analysis
result, each depends on the damping type assumed in the analysis, including: Proportional damping toinitial stiffness, Rayleigh damping, modal damping, and proportional damping to nonlinear stiffness.
The comparisons of periods before and during the mainshock are shown in Table 2.

Table 2. Comparison of period (unit: second)
Event
Before themainshockDuring themainshockMode

Record

1
2
3
4
1
2
3
4

1.974
0.624
0.350
0.238
2.540
0.702
0.432
0.312

Prop. damping to
init. stiffness (h1)1.974 100%0.745 119%0.460 131%
0.325 137%
2.825
111%
0.694
99%
0.476
110%
0.353
113%

Rayleigh
damping (h2)1.974 100%0.741 119%
0.451 129%0.325 137%
3.020 119%0.752 107%
0.476 110%0.337 108%

Modal
Prop. damping toDamping (h3) NL Stiffness (h4)1.974 100%
1.974 100%0.741 119%0.745 119%
0.451 129%0.460 131%
0.325 136%
0.325 136%
3.020 119% 3.062
121%
0.752 107% 0.694
99%
0.476 110% 0.476
110%
0.353 113% 0.353
113%

In Table 2, before the mainshock, all of the damping types give the same similarity in the
1st and 2nd mode, but in the 3rd mode the Rayleigh and modal damping give the closest period, while inthe 4th mode it is closed by the modal damping and the proportional damping to nonlinear stiffness.
During the mainshock, the proportional damping to initial stiffness gives the closest period to that ofthe data record in the 1st mode, in the 2nd mode the proportional damping to initial stiffness and to
nonlinear stiffness give the same similarity, in the 3rd mode all of the damping types give the same
similarity, while in the 4th mode the Rayleigh damping gives the closest value.
The comparisons of maximum acceleration before and during the mainshock are shown in
Table 3. Before the mainshock, the Rayleigh and modal damping give the closest maximum
acceleration to the data record on the 18th floor, while the proportional damping give the closest
maximum acceleration on the 37th floor. During the mainshock, the proportional damping to nonlinear
stiffness give the closest maximum acceleration to the data record on the 18th floor, while on the 37th
floor the Rayleigh and proportional damping to nonlinear stiffness give the same similarity.
Table 3. Comparison of maximum acceleration (unit: gal)
Event

Story

RecordBefore themainshockDuring themainshock

18
37
18
37

23.94
50.92
140.69198.27

Prop. damping toRayleigh
init. stiffness (h1) damping (h2)22.78 95%
23.59 99%79%
39.42 77%40.20
98.22 70%
162.16 115%
156.56 79%
226.60 114%

Modal
Prop. damping todamping (h3) NL stiffness (h4)22.78 95%
23.59 99%39.42 77%40.20 79%170.92 121% 125.06 89%
229.12 116% 170.96 86%

37
31
25
19
13
7
1
0
0.04 0.08
a. Max. ductility

Story

Story

The structural performance under the strong motion before and during the mainshock arerepresented in maximum ductility and hysteresis curve as shown in Figure 5 and 6.

b. Hysteresis curve

Figure 5. The performance before the mainshock

37
31
25
19
13
7
1
0
0.25 0.5
a. Max. ductility

Crackb. Hysteresis curve

Figure 6. The performance during the mainshock

5. HEALTH MONITORING
In the K-Tower, the strong motion sensors installed on the first floor (01F), middle floor (18F) and top
floor (37F), thus identification of suspected damage can only monitor the two parts of the building,which is the lower part (1st floor to 18th floor) and upper part (18th floor to 37th floor), while the detailsof each floor cannot be done due to limited number of installed sensors.

Frequency, F(Hz)The method is to observe the changes offrequency along the occurrence time of earthquakes(2007 to 2011) and try to find out which part has beenchanged or dominantly changed by the earthquake
events. By applying Fourier spectral ratio to both thelower part (18F/01F) and the upper part (37F/18F),
the frequency and period of those parts can beobtained. Illustration of the method represent in
Figure 7. The scatters of frequency are shown in
Figure 8, and the values are shown in Table 4.

Data record
at the 37th floor
Data record
at the 18th floor
Data record
at the 1st floor

Figure 7. Health monitoring

1.25

37F/18F
18F/01F

The mainshock

1.00
0.75
0.50
0.25
1

7

13 19 25 31 37 43 49 55 61 67 73 79 85 91 97 103 109 115 121 127
Data Number

Figure 8. The scatters of frequency in 37F/18F and 18F/01F

2

2

1
0

1
03
0

76
1

3

18F/01F

2

2

1
0

1
03
0

76
1

Frequency(Hz)37F/01F

Frequency(Hz)Frequency(Hz)

3

3

2

2

37F/18F

1
0

1
03
0

76
1

40

Story

Before the Mainshock After the Mainshock Shift
Min Max Average Min Max Average (%)
37F/01F 0.493 0.562 0.528 0.383 0.476 0.432 -18.15
37F/18F 0.806 0.928 0.866 0.673 0.861 0.735 -15.17
18F/01F 0.493 0.562 0.528 0.383 0.470 0.431 -18.38
Position

Frequency shift before and during themainshock is represented for the whole part
(37F/01F), the lower, and the upper part as in
Figure 9. Mode shape in the 1st and the 2nd modecan be seen in Figure 10. Data no.03 represents
the strong motion before the mainshock, whiledata no.76 represents the mainshock.
Story

Table 4. Frequency of the K-Tower

03
20
76

0
-30 -15 0

Figure 9. The shift of frequency

15 30

40
03
20
76

0
-30 -15 0

15 30

Figure 10. Mode shape

To evaluate the building condition according to the change of vibration characteristics i.e.
frequency and mode shape, several cases were arranged to simulate different damage scenarios ofstructure and tried to find the similar characteristics between the monitored building and the simulated
structural model. The frequency shift of the K-Tower is similar to that of the structural model indamage scenarios of case 4 (damage on the top and bottom) and case 5 (damage on the entire
structure) where the frequency obtained in the smaller earthquake reduces when the structure is struckby the bigger earthquake. The mode shape of the K-Tower is similar to the graph of mode shape ofstructural model in damage scenario case 5, although there was a different tendency in the 2nd mode
because of the damage level in the simulation was much higher than that in the observed building.

6. CONCLUSION
The natural frequency of the RC high-rise buildings after the mainshock was decreased, comparing tothat before the mainshock. The frequency shift of the K-Tower reached 18%, while that of theS-Tower was 24%. The natural period of the K-Tower observed before the mainshock was 1.9 sec andincreased after the mainshock to 2.3 sec, while that of the S-Tower observed before the mainshock
was 2.1 sec and increased after the mainshock to 2.8 sec. The damping factor of the K-Tower before
the mainshock was 2.9% and increased after the mainshock to 4.4%, while that of the S-Tower before
the mainshock was 4.4% and increased after the mainshock to 5.1%. The decrease of frequency
indicates the stiffness degradation that could be caused by the structural damage.
Modal damping and Rayleigh damping in the MDOF model before the mainshock, gave
the closest parameters to the data record from the real building, but during the mainshock, the closest
parameters was given by the proportional damping to initial stiffness and to nonlinear stiffness. The
strong motion data before the mainshock did not affect the structural members, the maximum ductility
was 0.06, hence no stiffness degradation occurred, but when it was struck by the mainshock, almost
the entire story exceeded the cracking capacity except the 4 top stories, the maximum ductility was
0.42. Thus, the stiffness degradation was caused although that did not reach the yielding capacity,
hence the structure reached the inelastic condition and supposed there was a minor damage occurred.
The shift of frequency on the upper and the lower parts were 15% and 18% respectively.
By conforming the vibration characteristics (frequency shift and mode shape) of the RC high-risebuilding with the damage simulation of the MDOF model under the El-Centro earthquake 1940, thesimilar characteristics were found to the simulation of case 5 when the structure damage was on theentire story. The maximum ductility of the MDOF model under the El-Centro was 1.41 which
occurred on the 34th story.

7. ACTION PLAN
The method in this study can be applied to other RC high-rise buildings to monitor thecondition of the structure when experiencing successive earthquakes. The analysis only used thestrong motion data on the building in horizontal direction by neglecting vertical direction and the dataof the underground. It is recommended to consider the effect of vertical ground motion andsoil-structure interaction to get more satisfactory results. The identification of the vibration
characteristics to recognize the mode shape was only performed up to the 2nd mode, because the strong
motion sensors were installed only on the 3 locations. It is recommended to install more strong motion
sensors to obtain the vibration characteristics in the higher modes. The same method can be tried to be
applied to other types of structures to identify the behavior of the different types of high-rise buildings.

ACKNOWLEDGEMENT
I would like to express my gratitude to Dr. K. Morita for useful advices and sharing the valuable dataand information, and to Dr. T. Kashima to provide the useful software for processing the data wave.

REFERENCES
Aoyama, H., 2001, In A.S. Elnashai & P.J. Dowling (Eds.), London: Imperial college press.
BRI (Building Research Institute), 2007, K-Tower installation of strong motion seismograph.
Chopra, A.K., 1995, New Jersey: Prentice-hall.
Jishin (Headquarters for Earthquake Research Promotion), 2009
Kashima, T., 2010, 14th European Conference on Earthquake Engineering.
Koh, C.G., and Perry, M.J., 2010, Leiden: CRC press.
Mari, J.L., Glangeaud, F., and Coppens, F., 1999, Paris: Editions technip.
Morita, K., 2010-2011, Lecture notes on system identification in vibration analysis, IISEE/BRI.
Shibata, A., 2010, Sendai: Tohoku university press.
Web site: Jishin, Headquarters for Earthquake Research Promotion, http://www.jishin.go.jp/