京 都 大 学 防 災 研 究 所 年 報 第47号 C 平成16年3月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 47 C, 2004

Evaluation of Seismic Performance of Wooden Houses during the Northern Miyagi
Earthquakes of July 26, 2003

Hidemaru SHIMIZU*, Yasuhiro HAYASHI**, Yoshiyuki SUZUKI***,
Takeshi MORII**** and Kyousuke MUKAIBOU***** COE Researcher, DPRI, Kyoto University** Associate Professor, DPRI Kyoto University*** Professor, DPRI Kyoto University**** Graduate School of Engineering, Kyoto University

Synopsis
This paper presents the results of our investigation on wooden house damages during theearthquakes in Northern Miyagi, Japan. First, earthquake observation records are
summarized. Next, the peak ground velocities in the area of severe damages are estimatedfrom the toppling ratios or displacements of tombstones. Then, from a survey of thedamages in three towns, it is found that the damage ratio of one-story houses is muchhigher than that of two-story houses. This damage trend is analyzed and explained byusing the method of equivalent-performance response spectrum. Finally, seismic
performances of 21 typical houses are compared based on the response limit strength
design method.
Keywords: Northern Miyagi earthquakes, traditional wooden houses, peak ground
velocity, damage, height of hanging wall, response limit strength design method

1. Introduction
2. Earthquake observation record
The Northern Miyagi earthquakes occurred on
July 26, 2003 in the Miyagi Prefecture, northern Japan,
as shown in Fig. 1. The main shock of the earthquakes
occurred at 7:13 AM with magnitude M6.2. The
epicenter is located at latitude N38.402, longitude
E141.175, with focal depth 12km. The foreshock andthe biggest aftershock occurred at 0:13 and 16:56 in
the same day, respectively. Very high JMA intensities
of more than 6- were observed at many stations during
three earthquakes, and thousands of buildings wereseverely damaged. Especially, the earthquakes caused
collapses of many wooden houses. The objective of
this paper is to evaluate seismic performances of thewooden houses in the areas with most severe damages.

Due to the strong-motion networks developed very
densely in this area, a large number of ground motion
recordings were acquired. Many records wereobtained by local town offices. But only the JMA
seismic intensities and peak ground accelerations wereprinted out, and most of the digital data were lost
unfortunately because of being overwritten by many
aftershock data.
Fig. 1(a) shows the peak ground velocity (PGV)
distributions calculated using accelerograms obtainedat the K-NET, KiK-net, and JMA stations during themain shock. A solid line indicates an active fault
called as “Asahiyama flexure”, which has

A (cm/s/s)

1000

Fe=3Hz

100
10

1.5Hz

0.75HzV (cm/s)

100

Fig. 2 Predominant frequencies(b) Overturning ratio of tombstones of observed ground motions(a) Observed PGVs
Fig. 1 Observed PGV distribution and overturning ratio distribution of tombstones

Table 1 Representative earthquake recordings obtained from three earthquakes
JMA
Time magnitude
MJMA

0:13

5.5

7:13

6.2

16:56

5.4

Town

Peak
JMA
Peak
Predominant
Epicentral Predominant
Groundseismic
Groundfreq. of ground
Organization intensity
distance freq. of soil
Acc.
Vel.
motions(km)
Fg (Hz)IJMA
Fe (Hz)A (cm/s2) V (cm/s)

Naruse Town office
5.9
2005
Yamoto Town office
5.5
476
NaruseNILIM
440
Yamoto Town office
6.2
850
Nango
Town office
6.0
491
Naruse Town office
6.0
756
Kashimadai Town office
5.91606
Wakuya
JMA
5.8
555
Kanan
Town office
5.7
337
KogotaTown office
5.5
585
Momoo Town office
5.5
383
Matsuyama Town office
5.4
340
Ishinomaki
NILIM
250
Kanan
Town office
5.7
649
Nango
Town office
5.4
276
WakuyaJMA
5.2
342
WakuyaNILIM
351
NILIM：V=SI、 Town office：IJMA=1.40+0.98log(AV)North-South strike as shown in Fig. 1. Three starmarks show the epicenter positions of the foreshock,main shock, and the biggest aftershock, respectively.
The records with PGVs higher than 15 cm/s wereobserved in the Wakuya station and Ishinomaki
station deployed by the JMA. These two JMA stations
are 15km and 11km away from the epicenter of themain shock, respectively. Predominant period ranges
from 0.3 to 0.6 sec at the Wakuya station and is lessthan 0.3 sec at the Ishinomaki station.
Table 1 shows representative records obtained
from three earthquakes. Relationship between thepeak ground accelerations (PGAs) and PGVs is shownin Fig. 2. An extremely high PGA (2005 cm/s2) wasrecorded in the foreshock event at the NARUSE town
office station 3.5km away from the epicenter. A very
high PGA (1606 cm/s2) was also obtained in the main
shock event at the Kashimadai town office station
10.5km away from the epicenter. Excluding these tworecords, the predominant frequency Fe at moststations ranges from 0.75 Hz to 3.0Hz as shown in Fig.
2. The estimated PGV reaches about 100 cm/s at the
Nango town office station and about 90 cm/s at the
Yamoto town office station. However, some of these

20
32
51
93
101
65
24
42

73
26
40
36
26
38
44
22

21

3.5
4.5
2.6
4.2
9.9
1.0
10.5
15.2
11.3
17.5
19.7
16.0
11.1
1.1
4.9
6.9
6.6

3.8
2.6
−
2.6
1.1
3.8
3.6
0.9
1.1
−
−
−
−
1.1
1.1
0.9
−

16.0
2.4
1.4
1.5
0.8
1.9
10.6
2.1
0.7
3.6
1.5
1.5
1.5
2.7
1.0
2.5
2.7
Fe=A/V/2p

values might be affected by amplification effects ofsurface soils. The predominant frequencies Fg of thesoils around the seismic observation stations are also
listed in Table 1. The predominant frequency Fg isestimated as the peak frequency of H/V spectrum
obtained from the microtremor measurements. The Fg
is about 1.1Hz at the Nango town office and the Kanan
town office where the Fe’s are small. Therefore, thesmallness of Fe is due to the soft surface soil condition.
In other locations where the soils are not soft, thepredominant frequencies are estimated to be greaterthan 1.5Hz.
3. Outline of investigation
Since the areas with most severe damages ofwooden houses are near the epicenters where thenumber of observation stations is very limited, wewere unable to obtain the ground motion intensities
directly in these areas. Therefore, preliminary
investigation was conducted on July 28 and 29 toevaluate the peak ground velocity distribution in termof the overturning ratio of tombstones in these areas.
From August 9 to 12, we performed detailed

B(cm)
Kaneko & Hayashi (2000)
Observed

80

Nango

60

H(cm)

Top
stone

Naruse40
200Kashimadai Wakuya

Upper stone

Naruse-zeki
Middle stone

Ishinomaki

Lower stone

050

PGV(cm/s)

100

150

Horizontal displacement (cm)Overturning ratio (%)

100

80
70
50
40
30
20
10
0

Average size of a tombstone
H=74cm, B=29cm
Fig.3 Relation of PGV and overturning ratio of tombstones
investigation to grasp structural features and damage
characteristics, including microtremor measurementsof soils and buildings and simplified surveys to obtaindamage statistics. The overturning ratios oftombstones were investigated at 34 sites where
marked by as shown in Fig. 1(b). The investigatedwooden houses are in the Yamoto town, Kanan town,
and Nango town, located in the area bordered bydashed lines in Fig. 1(a) and zoomed in as Fig. 1(b).
4. PGV estimated from toppling of tombstones
The observed overturning ratio of tombstones in anarea may be used to estimate the PGV of the area.
Kaneko and Hayashi (2000) developed the following
relation

P =Φ( {ln(V )-ln(V0 )} /0.4)(1)where F is the standard normal distribution function, P
and V denote the overturning ratio and the PGV,
respectively, and V0=10(B/H0.5)(1+B/H)2.5. Here B and
H are the width and height of the top stone of atombstone, respectively (see Fig. 3). Eq. (1) showsthat the overturning ratio of tombstones is affected by
the shape and size of the rigid top stones, and slender
tombstones (small B/H) tend to topple down easily. In
this study, B and H are assumed to be 29cm and 74cm,
respectively, according to the average size of themeasured tombstones. The relationship (1) between
the PGV and the overturning ratio P is shown by asolid line in Fig. 3. Also depicted in the figure by solid
square marks is the relationship of the maximum PGVvalue during three earthquakes at a seismic
observation station listed in Table 1 and theoverturning ratio of tombstones at the nearest grave tothe station. The two relationships show good
agreement as seen in Fig. 3. Therefore, we canestimate the PGV in terms of the overturning ratio in
the same area. Consequently, in the areas with severe
damages near the Asahiyama flexure, the PGVs are
estimated to be 85 to 100cm/s according to theoverturning ratios of 60-75%.

Fe=0.75
Fe=1.5Fe=2.25

60

0

50

100

150

PGV(cm/s)

Fig.4 Relation of PGV and horizontal
displacement of tombstones

However, Eq. (1) is not applicable to those
tombstones which have been strengthened using
dowels to connect the top, upper, and lower stones
together to prevent it from toppling. The dowels are
made of steel bars or cylindrical stones. In these cases,
the tombstones slide during earthquakes and may
finally fall down. Fig. 3 shows a tombstone that slid
and fell down during the Northern Miyagi earthquakes.
It is interesting that the material of the top and upper
stones was different from that of the middle and lower
stones. Probably, this tombstone was damaged during
the Miyagi-Oki earthquake in 1978, and reinforced byusing doweling.
The widths of the lower stone and middle stone areabout 80 cm and 64 cm, respectively. Therefore, inorder for the jointed tombstone to fall down, it isnecessary for the lower part to slide more thanhalf-width (40cm or 32cm). The PGV required tomake the jointed part to fall down can be estimated byusing the following formula developed by Kaneko &Hayashi (1999)

D = 0.02 µ -0.3 Fe-0.5 (V - V1 )1.56, (V > V1 ) , (2)where D denotes the sliding displacement, m is thefriction coefficient, V1=mg/(2πFe) is the minimum
velocity needed for sliding. In our estimation, m isassumed to be 0.05 in order to avoid overestimation of
the PGV. In addition, considering the predominantfrequency of recordings shown in Fig. 2, Fe is varied
parametrically as 0.75, 1.5, and 2.25 Hz. Fig. 4 shows
the sliding displacement with respect to the PGV forthree different values of Fe. It is seen that thedisplacement D increases as Fe becomes smaller. For
most cases in which the soils are not soft, thepredominant frequencies are estimated to be greaterthan 1.5Hz. If a tombstone moved more than 40cm,
the PGV is estimated to be more than 90 cm/s. It turns
out that PGVs estimated from the toppling ratios ordisplacements of tombstones are more than 90cm/s inthe areas where the wooden house suffered serious
damages.

Tabel.2 Summary of damage survey of wooden houses

Minor

Slight or no

Moderate to colapse

1

1

0.8

0.8
Damage ratio

Damage ratio

Slight or no

0.6
0.4

Natural freq. of house
Trans. Long. (Hz)
3.8
3.8
3.5
3.0
2.7
3.3
3.2
3.8
3.3
3.0
2.5
2.1
2.0
2.6
3.8
4.7
5.0
4.4
2.7
2.9
5.8
6.8
4.7
3.5
2.3
2.6
5.5
4.4
4.0
3.7
3.9
3.2
3.7
4.0
3.5
2.0
Minor

Moderate to colapse

0.6
0.4
0.2

0.2
YamotoKanan
NangouInvestigated town(a) Investigatedtowns

Fig.6 Hanging walls (NMH)8

One-storyTwo-story
Numberof stories of(b) Number

stories

5. Evaluation of seismic performance of wooden
houses
Structural properties and damages have been
investigated for wooden houses in the Yamoto town,
Kanan town, and Nango town, where severe damages
occurred to many houses. We selected 21 typicalhouses in the regions without serious damages in order
to find why they avoided serious damages under thesame strong ground motions. These houses are listed
in Table 2. Two different types of roofs were found in
the houses. The old houses built 100 years ago are
one-story houses or one-story houses partiallyextended to two stories. Roofing of these houses was
originally thatch, but some have changed to tiles or
steel sheets. The relatively new houses are covered bysteel sheets. Fig. 5 shows a typical one-story housewith thatch roofing. Beams in the roof truss were very
thick, and consequently the weight of the roof isheavy.
An important feature in the one-story houses is thatthey usually have hanging walls as shown in Fig. 6,

One-storyTwo-story

6
4
2
0

0

0

Fig. 5 A typical one-story
house house (KII)

Longitudinal direction (Hz)

Soil.
(Hz)
5.6
7.6
10.8
6.3
5.8
5.2
4.3
2.3
4.8
3.1
4.7
2.0
0.9
0.8
0.8
1.0
0.7
-

No. of Const.
Roofing* Damage
stories Year
YYT
2
1993
TL
SlightYAM
2
TL
Slight～1900
YAS
1
Slight～1800 TH+SS
Yamoto YSH
1SS
Slight
～1850
YST
2
1979
TL
Slight
YIY
1
1904
Tile
Moderate
YOM
1No～1800 TH+SS
KST
2
1971
SS
Slight
KUB
2
1998
TL
Slight
KII
1
TH
Minor
～1840
Kanan KUM
1
1950
TL
Minor
KSH
1
1992
TL
Minor
KTM
1
1941
TL
Minor
KKM
2
1945
TL
Moderate
NNH
2
2000
SL
Slight
NTK
1
1995
TL
Slight
NKT
2
1953
TL
SlightNango NSH
1
1950
TL
Minor
NYM
1
1960
TL
Minor
NMH
1
～1900 TH+SS Moderate
NOH
2
1965
TL
Moderate* TH:Thatch, TL:Tile, SS:Steel Sheet, SL:Slate
Town Name

0

2
4
6Transvers direction (Hz)8

Fig. 8 Fundamental frequencies
of one- and two-story houses

and the heights of the story and the hanging walls arequite high. The story height ranges from 2.6m to 3.9m,
mostly higher than modern houses with heights about
2.7m.
To catch a general trend of the damages, surveys
were carried out for houses in the three towns to obtaindamage statistics. Fig. 7(a) shows damage ratios of thethree towns. As the distance from the epicenter of themain shock increases in the order of the Yamoto town,
Kanan town, and Nango town, the damage ratio
reduces. The ratio of moderate damage to collapse isabout 20% in the Nango town, and about 28% in the
Yamoto town. The damage ratio is clearly related to
the number of stories, as shown in Fig. 7(b). The ratioof one-story houses with moderate damage to collapse
is about 50%, 2.5 times as high as that of two-story
houses. While Fig. 7(a) gives an expected trend, theresult shown in Fig. 7(b) is surprising, and needs to beinvestigated.
Fig. 8 shows fundamental frequencies of thewooden houses in the transverse and longitudinal
directions. They were obtained from micro-tremor
measurements, and therefore, only corresponding to3000

h=0.05

1000

0
0.1

One-story houseTwo-story house

40
60
80

Wakuya NS

100
120
0.2

1

Period(s)

80

120

0.7

0.2

OthersHanging wall

OthersMud wall00

YIY
YST YAM YAS YSH YYT YOM(Moderate)(Slight) (Slight) (Slight) (Slight) (Slight) (No)

Cy

Cy

0.25

0

NMH
NOH
NYM
NSH
NKT
NTK
(Moderate)(Moderate)(Minor) (Minor) (Slight) (Slight)c) Kanan town
longitudinal directione) Nangou town
longitudinal direction
1

1
OthersHanging wall

OthersMud wall

0.25

0

0

YIY
YST YAM YAS YSH YYT YOM(Moderate)(Slight) (Slight) (Slight) (Slight) (Slight) (No)

Cy

Cy

0.5

0.25

Hanging wall

Mud wall

0.75

0.75

0.5

0.5

KKM
KUM
KTM
KSH
KUB
KST
(Moderate) (Minor) (Minor) (Minor) (Slight) (Slight)

Mud wall

0.75

Mud wall

0.25a) Yamoto town
longitudinal direction
1

Hanging wall

0.75

0.5

0.25

Hanging wall

0.7

y

0.75

Others

0.3
0.4
0.5
0.6
Yield base shear coef. C

1

Mud wall

0.5

Wakuya EW

100

1
Hanging wall

0.75
Cy

60

Fig. 10 Maximum deformation angles of one- and two-story houses

1
Others

One-story houseTwo-story house40yFig.9 Acceleration response
spectra during the main shock

Cy

0.3
0.4
0.5
0.6
Yield base shear coef. C

20

Max deformation angle rad

2000

20

Max deformation angle rad

NS
EW
UD

2

Sa(cm/s )Wakuya

0.5

0.25

0
KKM
KUM
KTM
KSH
KUB
KST
(Moderate) (Minor) (Minor) (Minor) (Slight) (Slight)b) Yamoto town
transverse direction

NMH
NOH
NYM
NSH
NKT
NTK
(Moderate)(Moderate)(Minor) (Minor) (Slight) (Slight)d) Kanan town
transverse directionf) Nangou town
transverse direction

Fig. 11 Yield base shear coefficient
small deformations. The solid and hollow symbols
correspond to the one-story and two-story houses,
respectively. It is interesting that the fundamental
frequencies of the one-story houses are lower thanthose of two-story houses. This may be due to thedifferences in roof weights and/or heights of stories,
and may be related to the difference in the damage
ratios. However, the natural frequencies of wooden
houses that are highly nonlinear vary substantially as
their deformations increase; thus we would explain thephenomenon in terms of the deformation drift angle.
A method called equivalent-performance response
spectrum was developed by Hayashi (2002) toestimate the maximum drift angle based on theknowledge of the ground motion and the wooden
house structure. Let a house be modeled by anequivalent SDOF system. An equivalent period Te fora specified maximum drift angle R is given by

Te = 2π {( M e / M ) H e R / C y g}

0.5

(3)

where Me and He are the effective mass and height of
the SDOF system, respectively, and M, Cy, and g are
the total mass, yield base shear coefficient, andgravitational acceleration, respectively. For thisSDOF system, an equivalent acceleration response
spectrum corresponding to the specified drift angle R
is estimated as
S ae = Qy /( M e Fh ) = C y g /(( M e / M ) Fh ) (4)where Fh is the reduction factor of acceleration
spectrum depending on the damping ratio h as follows

Fh = 1.5 /(1 + 10h){h = 0.2 1 − (100 R )−0.5} + 0.05(5)

The maximum drift angle R can then be estimated byequating the equivalent acceleration response
spectrum given by Eqs. (3)-(5) to the response spectraof the measured accelerations such as those in Fig. 9.
In this study, we assume that Me/M=1.0, He=3.3m fora typical one-story house, and Me/M=0.9, He=5.5m
for a typical two-story house, and Cy varies between
0.3 and 0.6. Fig. 10 shows the estimated maximum
drift angles corresponding to the recorded groundaccelerations at the Wakuya station during the main
shock. It is seen that the maximum drift angle of atypical one-story house is larger than that of a typicaltwo-story house. Since the large deformation is the

0.9

0.9
Moderate
Minor
Slight
No

0.6

Cy

Cy

0.6

Moderate
Minor
Slight
No

0.3

0.3

0

0.15
0.3
Eccentricity ratio

0.45

0

0.15
0.3
Eccentricity ratio

0.45

Fig.13 Cracks inhanging wall (YIY)a) Longitudinal directionb) Transverse direction
Fig. 12 Yield base shear coefficients and eccentricity ratios of houses

4 mattatami

Closet14 mattatami●●▲

Closet12.5 mattatami▲

7.5 mattatami10 mattatami8 mattatami▲a) YIY in Yamoto town

17.5 mattatami

▲

Room

Front

▲▲▲▲ ▲
Veranda▲15 mattatami10 mattatami▲▲▲●15 mattatamiKitchenUp

Kitchen

▲

▲

8 mat
tatami

ClosetPassageway
Closet▲Dressing▲

8 mat
tatamiBathroom

14 mattatami

▲

N

▲

9 mat
tatami

7.5 mattatami

Closet10 mattatami

W.C

W.C

▲

ClosetW.C
Closet▲ClosetFig.16 Break of column (NMH)

Fig.15 separation of
hanging wall (NMH)

Fig.14 Separation of
lintel-column joint (YIY)

▲

0

▲

0

Front

Transverse▲：Separation of hanging wall
Longitudinal●：Break of frameb) NMH in Nangou town
Fig.17 Ground plans of houses

direct cause of the damages, the maximum drift angles
R less than 1/30rad correspond to a low ratio of severe
damages at the location. This result is consistent withthe statistics shown in Fig. 7(b).
To evaluate the seismic performance of the 21
selected houses, the response limit strength design
method has also been applied. It has been modified by
Suzuki et. al. (2003) to be suitable for wooden houses.
As one of the major modifications, several unit
structural elements are identified for wooden houses,
and their force-displacement relationships are
determined by tests. These elements are thenintegrated to constitute a specific wooden house, andits force-displacement curve is calculated analytically
based on those of the elements.
Applying the response limit strength design
method, we have calculated contributions of different
structural elements toward the yield base shear
coefficient Cy. Since Cy is defined at the deformation
angle of 1/30rad that is considered to be a typical yield
point, it reflects the strength of the structure. In

general, Cy ranges from 0.4 to 0.8 in both thelongitudinal direction and transverse direction. Fig. 11
shows the contributions of three different structuralelements to Cy for the 21 houses in two directions.
One third to half of the total yield shear is loaded to
mud walls, and another one third to half is loaded tohanging walls. Thus, the hanging walls play very
important role in the seismic performance.
Besides the strength represented by the yield baseshear coefficient Cy, the structural asymmetry of thehouses may be also an important factor for thedamages. Fig. 12 shows both the Cy and eccentricity
ratio of the first story in the transverse andlongitudinal direction, respectively, for the selectedwooden houses. In calculating the eccentricity ratio,
the center of gravity of a wooden house wasdetermined by taking the maldistribution of the
2-story into consideration. Although not very clear,Fig. 12 does give a general trend that a wooden housewith a large eccentricity ratio and a low yield baseshear coefficient is more likely to be damaged during

earthquakes.
Among the four houses with moderate damages,
the YIY and NMH houses had frame damage. In bothhouses, frame damages occurred at the lower joint oflintels and columns. Since frame damages are crucial
for a wooden house, these two houses need to beinvestigated in more details.
Fig. 13 shows cracks in a hanging wall in YIY
house, while Fig. 14 shows the separation of a lintel
from its connecting column. Since site conditions of
the seven houses in the Yamoto town are similar,
differences in damage level are mainly due to thedifferences in their seismic performance. This issubstantiated by the analytical results shown in Fig.
11(a), that the strength of the YIY house is the second
lowest in the Yamoto town in the longitudinal
direction. Fig. 17(a) shows the ground plan of the YIY
house, where most cracks in the hanging walls and thedamage in a lintel-column joint were observed in thelongitudinal direction. The damages wereconcentrated in the southwest part of the house dueto the eccentricity and also indicated insufficient wallsin this part.
For the NMH house, the separation of the hanging
walls and the break of a column are shown in Figs. 15
and 16, respectively. The damages can also explainedby the weak seismic performance of the structure. The
strength of the NMH house in the transverse directionis the lowest in the Nangou town, as shown in Fig.
11(f), and about 70% of the total strength in thelongitudinal direction was provided by hanging walls,
as shown in Fig. 11(e). The ground plan of the NMH
house is shown in Fig. 17(b), where most of damages
took place in the south part and west part due to theasymmetry of the house.
The damages at the joints of the hanging walls(lintels) and the columns in both YIY and NMH
houses point out that the over concentration of thestrength in the hanging walls may be a reason for theframe damages.
In applying the method of response limit strength
design, the force-deformation curve for the hanging
wall was obtained from test specimens in the lab.
However, the heights of the hanging walls in the 21
houses are much larger than that used in the tests.
Thus, the shear failure shown in Fig. 6 will not occur
in the lab. To improve the method for evaluating theseismic performance of existing buildings, theforce-deformation relationship of the unit elements
should be obtained through tests on real scalespecimens.
6. Conclusions
Damages of wooden houses during the Northern
Miyagi earthquakes have been investigated. First,
earthquake observation records are summarized. The
PGVs in the areas where the wooden house sufferedserious damages are estimated based on the toppling
ratios or displacements of tombstones. Surveys wereconducted in three towns to obtain statistics of thedamages. It is found unexpectedly that the damage
ratios of one-story houses are much higher than thoseof two-story houses in all three towns. This damage
trend is analyzed and explained by using the methodof equivalent performance response spectrum. Two
factors are found to be critical for the seismic
performance of a house: one is its strength, andanother is its eccentricity. To determine the strength,
the response limit strength design method is employed.
It is found that a wooden house with a large
eccentricity ratio and a low yield base shear
coefficient is vulnerable during earthquakes. Twohouses with frame damages are investigated in moredetails. It is found that while high hanging walls
contribute considerably to the total strength, the jointsof the hanging wall and the columns may be damaged
due to the over concentration of the strength.
Acknowledgments
This research was supported by the Japanese Ministry of
Education, Culture, Sports, Science and Technology(MEXT) 21st Century COE Program for DPRI, Kyoto
University (No.14219301, Program Leader: Prof. Yoshiaki
Kawata) as well as the Grant-in-Aid for Scientific Research(B) (1) (No. 15999999, Principal Investigator: S. Ikebuchi,
Kyoto University). We thank the following members of
Kinki branch in The Architectural Institute of Japan for theircooperation to carry out the damage investigation.: Prof. Y.
Saito of Hiroshima International University, Prof. M. Gotoof Kanazawa Institute of Technology, Prof. J. Nakamura of
Osaka City University, Prof. H. Murakami of Yamaguchi
University, Prof. I. Iwamoto of Osaka Prefectural College ofTechnology, Dr. M Nagano of Nippon Risk Management
Corporation, Dr. N. Kojima of Non-Life Insurance Rating
Organization of Japan, and Mr. T. Okuda and Mr. T. Suda ofKishirou Architectural Design. We are also grateful forusing the valuable seismic observation recordings such as
the K-NET and KiK-net distributed by the NIED, NILIM,
and Japan Meteorological Agency in Japan.

References
1) Kaneko, M. and Y. Hayashi (2000) : Proposal of a Curve
to Describe Overturning Ratios of Rigid Boies, AIJ, No.
536. pp.55-62 (in Japanese).
2) Kaneko, M., Y. Hayashi, and K. Tamura : Evaluationof Sliding Displacement of Furniture during Earthquake-By using Revised Formula to Estimate Sliding
Displacement of Furniture, Trans. of AIJ, B-2,
pp.527-528, 1999 (in Japanese).
3) Hayashi, Y, :Evaluation of Seismic Design Load basedon Equivalent-Performance Response Spectra, Proc. of
11th JEES, pp. 651-656, 2002.11 (in Japanese).
4) Suzuki Y., Y. Saito, K. Katagihara, K. Ikago, and C.
Nojima: Method of Evaluating Seismic Performance ofWooden Frames - Limit Bearing Capacity Analysis in
Wide Range of Deformation -, Proc. of 11th JEES,
pp.1523-1528, 2002.11 (in Japanese).
5) AIJ, Damege survey in northern Miyagi, Japan,
earthquakes of July 26, 2003 preliminary report, 2003.8(in Japanese).
6) http://www.kik.bosai.go.jp/kik/index_en.shtml
7) http://www.k-net.bosai.go.jp/
8) http://www.nilim.go.jp/japanese/database/nwdb/

要 旨
2003年宮城県北部の地震で被害を受けた木造建物の被災調査を実施し、耐震性能評価を行った。先ず、震源域の地
震動強さの考察を実施し、墓石転倒率、墓石滑動移動量から推定される最大地動速度は90cm/s以上である事を示した。
次に、平屋建ての建物が２階建てより被害率が高かった事を、等価１自由度系の解析から考察した。そして、被災建物の耐震性能を限界耐力計算手法から評価し、耐震性能は比較的高く、背の高い小壁が重要な耐震要素である事を示した。
キーワード: 宮城県北部の地震，木造建物，最大地動加速度，被害程度，背の高い小壁, 限界耐力設計法

COE31.

2003 年 7 月 26 日宮城県北部の地震における被災木造建物の耐震性能評価○ 清水秀丸・鈴木祥之・林康裕
結果より、小壁はせん断による破壊モードが発生

1．はじめに2003 年 7 月 26 日、宮城県北部で発生した一連したと考えられるため、本論文において小壁はの地震では、多くの木造建物が被災した。被害の1/15rad までの耐力と変形性能を有すると仮定し大きかった地域の被災木造建物の構造詳細調査た。

を行った結果、算出される耐震性能評価について

建物重量の算出については調査時に各建物の報告する。また、耐震性能評価と地震被害との整

小屋裏の調査がほとんどの建物で可能だったた

合性についての分析も実施する。

め、屋根材、小屋裏に配置されていた土壁なども2．調査建物概要算出した。茅葺き、壁の重量は、重要文化財(建軸組構法木造建物を対象とした、構造詳細調査

造物)耐震診断指針及び建築基準法に基づき、床を、矢本町小松地区、河南町北村地区、南郷町二

面積当たりの単位重量より算出した。

郷地区で実施した。築 200 年以上から築 3 年まで

1/30rad 時のベースシア係数 C における矢本町、の平屋、2 階建てを各地区で７棟ずつ、計 21 棟の桁行き方向の各耐震要素負担率 Cy を図 1 に示す。

詳細調査より、平面図、耐震要素の抽出、重量算

地震被害との整合性では、無被害の YOM 邸が最も定などを行い、構造的特徴を把握した。矢本町の低い Cy を示した。しかし、７棟すべての結果より、

調査建物一覧を表 1 に示す。

被害程度と耐震性能評価結果は、良い対応関係がこの地域の構造的特徴として、写真 1 に示すよ見られた。また、小壁は耐震要素として有効であ

うな、平屋建物が多く、調査した建物中、12 棟がり、Cy に占める割合が大きいことを確認した。

平屋建てであった。また、写真 2 のように、階高

4． まとめが高く、小壁高さも高い建物が多く見られ、この宮城県北部地震の被災木造建物を対象として地域では、小壁は重要な耐震要素と考えられる。

限界耐力計算法に基づいて、耐震性能評価を実施

3．被災建物の耐震性能評価した。被害程度と耐震性能評価結果は、良い対応

対象建物の詳細調査に基づき、土壁、土壁小壁、関係が得られた。
石膏ボード小壁
石膏ボード

石膏ボード、石膏ボード小壁、貫、柱ほぞを耐震要素として各建物の復元力特性を算出した。算出

0.7

方法は各耐震要素の耐力を加算する限界耐力計

0.6算の手法を用いて算定した。

0.5

限界耐力計算を木造に適応する手法では、隣1/30rad で柱の鴨居部分に生じる曲げにより柱の折損を主体とした破壊モードによって耐震性能を失うと仮定されているが、調査建物では柱の折損を確認することが出来なかった。また詳細調査

小壁
土壁

0.4

Cy

接する全面壁が無い場合、小壁は層間変形角

貫
ほぞ

0.3
0.2
0.1
0

YIY
(被害中)

YAS
(軽微)

YSH
(軽微)

YST
(軽微)

YAM
YYT
(軽微) (軽微)YOM(無)図 1 桁行き方向の耐力負担割合
表 1 矢本町詳細調査建物一覧

写真 1 平屋建物の一例

写真 2 小壁の概要

建物名
YIY
YAS
矢
YSH
本
YST
町
YAM
YYT
YOM

階数
平屋
平屋
平屋
２階
部分２階
２階
平屋

建築年
1904
～1800
～1850
1979
～1900
1993
～1800

屋根材
瓦
鉄板
鉄板+瓦
瓦
瓦
瓦
茅葺+鉄板

被災度
中
軽微
軽微
軽微
軽微
軽微
無