EARTHQUAKE MOTION OBSERVATION AND SSI CHARACTERISTICS OF
AN 8-STORY BUILDING IN BRI
Toshihide KASHIMA
Building Research Institute, Ministry of Land, Infrastructure and Transport
Tachihara 1, Tsukuba, Ibaraki 305-0802, Japankashima@kenken.go.jp
ABSTRACT
Building Research Institute (BRI), Ministry of Land, Infrastructure and Transport, Japan, started
the strong motion observation project in 1957. The observation network has been enriched andenlarged in the past forty years. BRI is now operating several networks with a number of strong
motion instruments.
Urban Disaster Prevention Research Center building (annex) BRI was constructed in 1998 andinstrumented with up-to-date observation system. Sensors are configured in order to grasp building
behavior including the ground during earthquakes. Several valuable records have been obtained inlast two years.
Firstly this paper introduces the observation system that is installed to the annex building of BRI.
Fundamental dynamic characteristics of the ground and building are discussed secondly. Finallythe soil-structure interaction behavior is studied using a simple swaying-rocking model.

Keywords: Strong motion observation, SRC building, Soil-structure interaction, Site effect1.

INTRODUCTION

Building Research Institute (BRI), Ministry of Land, Infrastructure and Transport, Japan, started the strong
motion observation project in 1957. The observation network has been enriched and enlarged in the past forty
years. BRI is now operating several networks with a number of strong motion instruments.
In 1998, BRI has installed the up-to-date strong motion observation system to a newly constructed 8-story steel
reinforced concrete (SRC) building next to the main building in BRI. Totally 22 accelerometers were placed in
the new building, in the main building and in the surrounding ground. We hope the recording system of this kind
will be very directly useful for evaluating the soil-structure interaction (SSI) phenomena. In this paper, we are
introducing a new system that is installed to buildings and underground at BRI, and some observation results.

2.

OUTLINE OF STRONG MOTION OBSERVATION AT BRI ANNEX

2.1. Geological conditions
BRI is located with approximately 60 kilometers to the northeast from Tokyo. The site is situated 30 meter above
sea level on the diluvial heights between the Sakuragawa River flowing into the Kasumigaura Lake and the
Kokaigawa River, a branch flow in the greater the Tonegawa River water system.
The geological investigation shows that clay and fine sand are main contents up to 90 meters depth underground,
inserting sandy gravel. Table 1 indicates surface soil structure at BRI. We confirmed a sandy gravel layer at 88
meters in depth. Its shear wave velocity, however, is not obtained. Transfer functions for shear waves between
ground surface and depths of 42 meters, 68 meters and 88 meters are shown in Figure 1. We assume here that theshear wave velocity of the layer 88 meters deep is about 500 m/s. The figure shows common predominantperiods of 2 to 4 Hz and 9 Hz. Moreover, the transfer function of 88-meter layers has the peak at 1 Hz. Thismight be the peak relevant to the fundamental frequency of the surface soil layers. In addition, the layer between
88 meters deep and approximately 250 meters deep is called upper Kazusa formation group, and the underlying
layer is called lower Kazusa formation group, the Tertiary layers.

1

Table 1. Structure of surface soil layers
VPVSρ
No. H (m) D (m)Soil Type(m/s) (m/s) (t/m3)
1
2.0
2.0
170
110
1.30 Loam
2
6.0
8.0
200
1.30 Sandy Clay & Clayey Sand
1430
3
6.0
14.0
160
1.50 Sandy Clay & Clay4
8.0
22.0 1630
260
1.80 Fire Sand & Clayey Fine Sand5
6.0
28.0 1500
200
1.75 Sandy Clay & Clay6
14.0
42.0 1570
270
7
6.0
48.0 1880460
1.90 Gravel88.0
56.0 1780
340
1.75 Sandy Clay & Clay9
12.0
68.0 1690
290
10
12.0
80.0 1790
380
1.95 Gravel & Fine Sand118.0
88.0 1600280
1.75 Sandy Clay & Clay
12500
2.00 Gravel
H: Thickness, D: Depth, V P: P-wave Velocity, VS: S-wave Velocity, ρ: Density
5Transfer Function

Amplitude Ratio

88m Model
68m Model
42m Model

1

0.5
0.02

0.05

0.1

0.5
1
Frequency (Hz)

5

10

20

Figure 1. Transfer function of surface soil layers
2.2. Building characteristics
The building for accelerometer installation is a newly constructed Urban Disaster Prevention Research Centerbuilding (annex) that was completed in March 1998. The annex building has eight stories with single basement
floor. Total building area is approximately 5,000 m2 and supported by mat foundation on the clayey layer of 8.2
meters underground. The annex is connected to the main building with passages, but is structurally separated.
2.3. Recording system
The sensors are installed with 11 locations (33 channels) in the new building, 7 locations (21 channels) in thesurrounding ground, and 4 locations (12 channels) in the main (older) building. The sensor configuration isshown in Figures 2 and 3. The farthest sensor on the ground is 100 meters away from the annex building, and the
deepest sensor is set up 89 meters in depth. Three sensors are set up at the basement floor and the eighth floor inorder to investigate torsional vibration.
All sensors are connected to the recording equipment at the observation room in the annex. The specifications of
the recording system are shown in Table 2. The system has 66-channel 24-bit A-D converters, a digitalprocessing unit and 40 MB flash memory storage. PC cards are used as storage device and are directly processedby PC. Broad dynamic range and certain operation are ensured by the most up-to-date facilities.

2

8FE

North
100 m
50 m
20 m

BFE

Loam
Sandy Clay &Clayey Sand
Sandy Clay
& ClayA01
GL-14 m
A14

N14

C01

B01

Fine Sand &Clayey Sand

Sandy Clay
& ClayGL-43 mA43

GravelSandy Clay
& ClayGravel &Fine Sand
Sandy Clay
& ClayGL-89 mGravelA89
Figure 2. Vertical sensor configuration

D

D

BFN

9.0 m

9.0 m

8FN

C

C

8.0 m

8.0 m

ELV

BFE

8FE

9.0 m

B

9.0 m

B

BFS

8FS

A

A

B1F

7.5 m
1

7.5 m
2

6.0 m
3

8F

4

7.5 m
1

7.5 m
2

Figure 3. Sensor configuration at basement floor and 8th floor

3

6.0 m
3

4

Table 2. Specifications of recording system
Model
AJE-8200, Akashi Corp.
Sensor
V403BT (negative feedback servo), Akashi Corp.
Number of Channels
66
Frequency Range
DC ~ 30 Hz
Acceleration Range±2G
A/D Converter
24-bit, (Delta-Sigma)
Dynamic Range
114 dB
Recording Medium
ATA Flash Memory Card
Triggering Logic
Disjunction (OR) of Specified Three Components
Functions
Peaks, JMA Seismic Intensity, Spectral Intensity (SI)3.

OBSERVED RECORDS AND DISCUSSION

3.1. Observed records
In the past two years, we obtained several records with moderate acceleration levels. Observed earthquakes andpeak accelerations on the ground (GL, shown as A01 in Fig.2), at the basement floor (BF, shown as BFE in
Fig.3) and at the eighth floor (8F, shown as 8FE in Fig.3) are listed in Table 3.
Horizontal acceleration records on the ground (A01), 89 meters below the ground (A89), at the basement floor(BFE), and at the eighth floor (8FE) for the eathquke#21 are plotted in Figure 4. Dynamic characteristics ofground motions and building response are discussed through the analysis using these records in the following
paragraphs.
Table 3. List of observed records
h
∆
No.
Date
Time
LatitudeLongitude
M
(km)
(km)
1 1998/06/08 08:02 036°06.5’N 139°54.3’E73
4.63
2 1998/06/24 23:52 036°07.3’N 140°06.4’E
67
5.1
58
3 1998/08/29 08:46 035°36.2’N 140°02.7’E
58
4.9
60
4 1998/11/08 21:40 035°36.6’N 140°03.2’E
58
5.1
62
5 1999/03/26 08:31 036°27.3’N 140°37.0’E
54
4.0
19
6 1999/04/25 21:27 036°27.6’N 140°37.8’E
58
4.4
58
7 1999/06/27 19:50 036°06.1’N 139°48.8’E55
4.6
78 1999/07/15 07:56 035°55.4’N 140°27.8’E
48
6.0
77
9 1999/07/23 06:53 036°04.2’N 141°10.3’E49
6.0 101
10 1999/08/09 06:39 035°50.0’N 139°57.6’E
37
5.3 10711 1999/12/10 10:55 036°03.0’N 140°06.2’E73
4.63
12 1999/12/16 22:28 036°37.5’N 139°26.1’E
67
5.1
58
13 1999/12/16 22:47 036°37.9’N 139°26.4’E
58
4.9
60
14 1999/12/27 00:05 036°08.8’N 139°52.0’E
58
5.1
62
15 2000/01/12 11:09 036°27.0’N 140°35.4’E
54
4.0
19
16 2000/04/10 06:30 036°11.2’N 140°04.1’E
58
4.4
58
17 2000/04/15 05:26 036°13.1’N 139°59.5’E55
4.6718 2000/05/08 18:45 036°22.8’N 140°58.3’E
48
6.0
77
19 2000/05/16 19:40 036°26.5’N 140°42.7’E49
6.0 10120 2000/06/03 17:54 035°40.6’N 140°43.1’E
37
5.3 107
21 2000/07/21 03:39 036°31.3’N 141°05.8’E73
4.6322 2000/08/15 03:54 036°12.2’N 140°02.4’E
67
5.1
58
23 2000/08/21 21:27 036°04.4’N 139°57.4’E
58
4.9
60
24 2000/12/05 01:47 035°50.5’N 141°09.6’E
58
5.1
62
h: Focal Depth, M: JMA Magnitude, ∆: Epicentral Distance4

Peak Acc. (cm/s2)
GL
BF
8F
8
3
8
19
14
53
10
6
16
6
3
10
46
26
61
37
25
67
17
12
18
6
4
16
2
1
6
5
4
10
2
1
3
3
2
4
9
6
12
19
10
32
12
7
13
64
32
96
17
8
15
6
4
8
9
5
16
16
11
47
38
26
84
10
5
8
11
7
16
9
8
31

Acc.(cm/s/s)

100

Acceleration
A01(NS) (peak: 31.7 cm/s/s)0Acc.(cm/s/s)

-100
100

A01(EW) (peak: 37.9 cm/s/s)0Acc.(cm/s/s)

-100
100

A89(NS) (peak:- 20.3 cm/s/s)0Acc.(cm/s/s)

-100
100

A89(EW) (peak: 22.5 cm/s/s)0Acc.(cm/s/s)

-100
100

BFE(NS) (peak: 24.9 cm/s/s)0Acc.(cm/s/s)

-100
100

BFE(EW) (peak:- 26.0 cm/s/s)0Acc.(cm/s/s)

-100
100

8FE(NS) (peak:- 50.4 cm/s/s)0Acc.(cm/s/s)

-100
100

8FE(EW) (peak:- 84.3 cm/s/s)

0

-100
0

10

20

30

40

50
Time(sec)

60

70

80

90

100

Figure 4. Acceleration records of the earthquake #21 on July 21, 2000
3.2. Amplification of surface soil layers
Figure 5 shows Fourier amplitude spectral ratios of earthquake motions on the ground (A01) to ones at 89 meters
below the ground (A89). Theoretical ratio calculated from the soil structure shown in Table 1 is also indicated by
the dotted line in the figure. Both spectral ratios, N-S component and E-W components, have common peaks at
0.8 to 1.0 Hz, 2.2 Hz and 3 to 4 Hz. Observed results show good agreement with the theoretical ratio upon thelocation of peaks. However, there are differences in heights of peaks. We assumed damping ratio of 5% for eachsoil layer in the theoretical calculation. Reevaluation of physical constants, especially damping property, of soils
using observed data is necessary for further discussion.

520

Fourier Spectral Ratio of records on 2000/07/21
A01/A89 (NS)A01/A89 (EW)
Theory10

Amplitude Ratio

5

1
0.5

0.2
0.05

0.1

0.5
1
Frequency (Hz)

5

10

20

Figure 5. Fourier spectral ratios of records on the ground (A01) to ones at 89 meters below
the ground (A89) for the earthquake #21
3.3. Dynamic characteristics of annex building
We try to discuss general characteristics of the annex building including the soil-structure interaction effecthereinafter. The left-hand graph in Figure 6 shows ratios of peak accelerations at BF (basement floor, BFE in
Fig.2 and Fig.3) and 8F (eighth floor, 8FE in Fig.2 and Fig.3) to GL (ground level, A01 in Fig.2) in thehorizontal directions. Peak accelerations at BF are 40 to 70 % smaller than at GL. The mean values of the ratios
are 0.61 and 0.59 in the N-S and E-W directions respectively. In contrast, peak acceleration ratios of 8F to GL
widely vary. Average peak velocity ratios are 1.46 and 1.66 in the N-S and E-W directions respectively. The
right-hand graph of Figure 6 compares peak velocities at BF and RF with peak velocities at GL. Similartendencies can be observed with the case of peak accelerations. Non-linearity of amplification does not clearly
appear in ratios in Figure 6.

10Peak Acceleration Ratio

10

BF/GL(NS)BF/GL(EW)
8F/GL(NS)
8F/GL(EW)

BF/GL(NS)BF/GL(EW)
8F/GL(NS)
8F/GL(EW)

5Peak Vel. Ratio

Peak Acc. Ratio

5

Peak Velocity Ratio

1

0.5

1

0.5

0.2
1

5
10
PGA (cm/s/s)

0.2
0.05

50

0.1

0.5
PGV (cm/s)

1

5

Figure 6. Peak acceleration ratio at BF and 8F to A01
Fourier spectral ratios of the records at BF to at GL for all earthquakes in Table 3 are plotted in Figure 7withaverage lines. Average spectral ratios are shown in Figure 9 again. Differences are observed in the higher
frequency range more than 1.7 Hz. Figure 8 shows Fourier spectral ratios of the records at 8F to GL. The spectral
ratio in the N-S component has clear peaks of 1.7 Hz and 2.3 Hz. Natural frequency of the annex building in the
N-S direction can be estimated at 1.7 Hz. The spectral ratio in the E-W direction also has a peak of 1.7 Hz, butdoesn’t have a peak of 2.3 Hz. First natural frequency in the E-W direction is very close to the N-S direction.
The vibration system discussed here includes effect of the soil-structure interaction. Incidentally, damping ratios

6

of the first vibration modes in N-S direction and E-W direction are estimated at 5% and 7%, respectively.
Fourier Spectral Ratio (BF/GL-NS)

20

10

10

5

5
Fourier Spectral Ratio

Fourier Spectral Ratio

20

1
0.5

0.1

Fourier Spectral Ratio (BF/GL-EW)

1
0.5

0.1

0.05
0.05

0.1

0.5
1
Frequency (Hz)

5

10

0.05
0.05

20

0.1

0.5
1
Frequency (Hz)

5

10

20

5

10

20

Figure 7. Fourier spectral ratios of records at BF to A01
Fourier Spectral Ratio (8F/GL-NS)

20

10

10

5

5
Fourier Spectral Ratio

Fourier Spectral Ratio

20

1
0.5

0.1
0.05
0.05

Fourier Spectral Ratio (8F/GL-EW)

1
0.5

0.1

0.1

0.5
1
Frequency (Hz)

5

10

0.05
0.05

20

0.1

0.5
1
Frequency (Hz)Figure 8. Fourier spectral ratios of records at 8F to A01
20

Average Fourier Spectral Ratio

Average of Fourier Spectral Ratio

10

BF/GL (NS)BF/GL (EW)
8F/GL (NS)
8F/GL (EW)

5

1
0.5

0.1
0.05

0.1

0.5
1
Frequency (Hz)

5

10

20

Figure 9. Average Fourier Spectral Ratio of records at BF and 8F to GL
Fourier spectra of records at 8FE, 8FN and 8FS (cf. Fig. 3), and Fourier spectral ratios 8FN/8FE and 8FS/8FE in
the N-S direction are shown in Figure 10 in order to clarify the behavior of the top floor. Although movement of7

western side of the eighth floor (8FN and 8FS) is lager than the eastern side, the phases of both movements aresame at a frequency of 1.7 Hz. At frequencies 2 to 2.3 Hz, spectral ratios quickly reduce and remarkable phasedifferences develop. The peak at 2.3 Hz on the spectral ratio 8F/GL in the N-S direction is caused by thevibration mode with torsional component.
Fourier Spectrum (Parzen: 0.2Hz)

10

8FE(NS)
8FN(NS)
8FS(NS)

100
50
10
5
1
0.5
0.1
0.05
0.05

0.1

Fourier Spectral Ratio [Amplitude] (Parzen: 0.2Hz)
8FN(NS)/8FE(NS)
8FS(NS)/8FE(NS)5
Amplitude Ratio

Fourier Amplitude(cm/s)

500

0.5
1
Frequency(Hz)

5

10

1
0.5

0.1
0.05

20

180

8FN

0.1

0.5
1
Frequency(Hz)

5

10

20

10

20

Fourier Spectral Ratio [Phase] (Parzen: 0.2Hz)

Phase Lag(deg.)

90

ELV

8FE

0

-90

8FS
-180
0.05

0.1

0.5
1
Frequency(Hz)5Figure 10 Fourier spectra and spectral ratios of 8FN and 8FS to 8FE for the earthquake #21
A simple swaying-rocking model shown in Figure 11 is adopted to discuss basic properties of soil-structureinteraction phenomena. Displacement at the top floor includes the swaying component dS and rocking
component dR in addition to building deformation dB. These displacements can be computed from records at five
points as shown in Figure 11.

dS dR dBxRF

H

d S = x BF − xGL

xGL

W
zL

zR
xBF

d R = ( zL − zR )H / W
d B = xRF − xBF − d R

Figure 11. Swaying and rocking model

8

Figure 12 shows ratios of swaying, rocking and building displacements in the horizontal directions for theearthquake #21. Average values of swaying ratios in N-S and E-W directions are 3 % and 2 %, respectively.
Rocking ratios are 11 % and 8 % in average, respectively.
200

Swaying-Rocking Ratio

200

Swaying-Rocking Ratio

Swaying (NS)Rocking (NS)
Building (NS)Swaying (EW)
Rocking (EW)
Building (EW)100
Ratio (%)

Ratio (%)

100

0

0

-100

-100

20

40

60

80

20

Time (s)

40

60

80

Time (s)Figure 12. Swaying and rocking ratios in the displacement at 8F

4.

CONCLUSIONSBRI has started new earthquake motion observation with the dense instrumentation. Amplification effect of
surface geology was examined with good agreement between observed and the theoretical result. Dynamic
characteristics of the annex building are also discussed through analyses of earthquake motion records. First
natural frequencies of annex building in the N-S direction and the E-W direction can be clearly distinguishedfrom the spectral analysis. Results here will be fundamental information for following studies.
Assuming the simple swaying-rocking model, 2 % to 3 % of the displacement at the building top is caused by
the swaying. 8 % and 11 % of the displacement is originated from the rocking. We intend to proceed with studies
on the soil-structure interaction behavior using a number of observation records of the annex building.

5.

REFERENCES

1. Okawa, Izuru, Toshihide Kashima and Shin Koyama, “Dense Instrumentation in BRI Buildings andSurrounding Ground”, UJNR Workshop on Soil-Structure Interaction, Menlo Park, 1998
2. Kashima, Toshihide, Izuru Okawa and Shin Koyama, “Earthquake Motion Observation in and around 8-Story
SRC Building,” NATO Advanced Research Workshop on Strong Motion Instrumentation for Civil
Engineering Structures, Istanbul, 1999

9