TRADITIONAL WOODEN BUILDINGS IN A RURAL DISTRICT TOWN CALLED KIRAGAWA-CHO

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1 TRADITIONAL WOODEN BUILDINGS IN A RURAL DISTRICT TOWN CALLED KIRAGAWA-CHO Yasuhiro Hayashi, Chiaki Watanabe 2, Noriko Takiyama 3, Toshiyuki Tai 4, Yu Hasebe 5 ABSTRACT: We have proposed a procedure to specify houses with poor seismic performance from many houses in historic districts in Japan easily and efficiently. In order to verify our methods, traditional wooden houses in district located in Kochi Prefecture are investigated. Traditional town houses in three districts,,, and, are chosen for comparative study with those in. First, we performed ambient vibration measurements of the eleven houses in to identify the natural frequencies. Next, a rigorous seismic evaluation was conducted. Finally, two types of approximation methods to evaluate yield base shear coefficients of traditional wooden houses proposed by the authors are proved to be applicable to houses in four historic districts. KEYWORDS: Historic houses, ambient vibration measurements, seismic performance evaluation INTRODUCTION 23 In Japan, there is a growing importance in preservation of historic districts and streetscapes of the Important Preservation District for Groups of Historic Buildings (IPDGHB), gathering attention from all around the nation as a sustainable method for town planning respecting the characteristics of each district. However, most of fatalities come from the collapse of old wooden houses according to the recent earthquake damage statistics in Japan. Therefore, the seismic retrofit of old town houses to mitigate earthquake damage is very important as well as the preservation of townscape and compliance with the resident s demand. This paper deals with methods to evaluate of seismic performance Yasuhiro Hayashi, Professor, Department of Architecture and Architectural Engineering, University, C2-38, -Daigaku Katsura, Nishikyo-ku,, Japan. hayashi@archi.kyoto-u.ac.jp 2 Chiaki Watanabe, Assoc. Professor, Institute of Wood Technology, Akita Prefectural University, - Kaieisaka Noshiro Japan. p.chiaki@iwt.akita-pu.ac.jp 3 Noriko Takiyama, Research Assoc., Department of Architecture and Architectural Engineering, University, C2-36, -Daigaku Katsura, Nishikyo-ku,, Japan. noriko@archi.kyoto-u.ac.jp 4 Toshiyuki Tai, Graduate student, Department of Architecture and Architectural Engineering, University, C2-36, -Daigaku Katsura, Nishikyo-ku,, Japan. rp-tai@archi.kyoto-u.ac.jp 5 Yu Hasebe, Graduate student, Department of Architecture and Architectural Engineering, University, C2-36, -Daigaku Katsura, Nishikyo-ku,, Japan. is2.hasebe@archi.kyoto-u.ac.jp of wooden houses in an Important Preservation District for Groups of Historic Buildings. In this paper, traditional wooden houses in historic district located in Kochi Prefecture, Japan, are investigated. There is high concern about building damage by strong seismic ground motions and the tsunami due to a big scenario earthquake called the Nankai earthquake with very high occurrence probability within tens of years. Furthermore, earthquake disaster vulnerability in this district is increasing due to the population reduction and aging. In order to specify houses with poor seismic performance from many houses in a historic district easily and efficiently, we proposed two types of approximate evaluation method of yield base shear coefficients of traditional wooden houses. One method is based on the ambient vibration measurements, and the other is based on a simple floor plan. In order to verify our methods, traditional wooden houses in district located in Kochi Prefecture are investigated comparing with those in the other three districts,,, and. First, is introduced briefly from the viewpoint of the history, feature, and disaster vulnerability of the district. Then, the structural features of traditional wooden houses in are introduced comparing with those in the other three districts. Furthermore, the results of a rigorous seismic performance evaluation and ambient vibration measurements are introduced. Finally, proposed methods are proved to be applicable to houses in four historic districts including.

2 2 INVESTIGATED DISTRICT An amendment to the Law of the Protection of Cultural Properties in 975 introduced a new category of cultural properties, under the name of Groups of Traditional Buildings, extending protection to historic cities, towns and villages including castle towns, post-station towns, towns built around shrines and temples-and other areas of historic importance throughout Japan. According to this system, municipalities designate certain areas as Preservation Districts for Groups of Traditional Buildings based on regulations and formulate a preservation plan in accordance with the Preservation Ordinance in order to execute the preservation project systematically. Upon receiving a proposal from a municipal government, the national government selects those of high value as Important Preservation Districts for Groups of Traditional Buildings (IPDGHB). Acknowledgement of the value of the district in question by the act of selection enables the Agency of Cultural Affairs and Prefectural Board of Education to provide guidance and advice to municipal preservation projects. They also support municipalities by providing financial assistance to their projects for repairing listed Traditional Buildings and Structures and improving non-listed structures and structures to harmonize the latter with surrounding historic and natural features, for installing disaster prevention facilities, and for setting up guide boards. Besides, support is also given through preferential tax treatment. As of December 8, 29, 86 districts in 74 cities, towns and villages have been classified as the IPDGNB. The town located in Muroto city and in the vicinity of the Kochi Prefecture is formed along the beach highway from Kochi to Muroto in Edo period. The town has prospered from the Meiji era to the beginning of the Showa era especially as a distributing point of good quality charcoal. In, there are many traditional wooden buildings such as merchant residences with plaster walls and warehouses with mizukirikawara, a kind of roof tiles at gable walls as shown in Fig.. There are some houses with brick masonry gable walls as shown in Fig. 2. We can also find the various types of stone masonry walls at the border line of lot called ishiguro as shown in Fig. 3. Plaster walls and stone masonry wall are the protection against strong rainfall and wind because this area is frequently hit by typhoons. These features show a regional and historic townscape in the modern days very well. On the other hand, in Kochi Prefecture, there is concern about building damage by strong seismic ground motions and the tsunami due to the Nankai earthquake of about magnitude 8.4 with very high occurrence probability as shown in Fig. 4. Seismic intensity of 5 upper or 6 by instrumental seismic intensity used in Japan lower and flood depth of 5 m are predicted in town during the scenario earthquake. Figure : Typical wooden houses in the investigated area. Figure 2: Traditional houses with brick walls. Figure 3: Stone masonry walls at the border line of lot called ishiguro.

3 Figure 4: Investigated districts and their exceedance probability of seismic intensity 6 lower within the next 3 years. Exceedance probability over t years.. t= PGV v Figure 5: Exceedance probability of PGV. Figures 6, 7 and 8 show the change in the population and the elderly population ratio of Kochi Prefecture and Muroto City. While population is decreasing, the elderly population ratio has begun to increase. Especially, the elderly population ratio in Muroto city is 32.9% and is higher than 25.9%, which is that in Kochi Prefecture. At town, a population is decreasing by 59 in these four years. Moreover, the size of household a family is decreasing and becomes 2.2 persons. It turns out that earthquake disaster vulnerability is increasing in Muroto city. Traditional town houses in three districts, in Prefecture, in Mie Prefecture, and in Nagano Prefecture, are chosen for comparative study with those in. Investigations similar to that introduced in this paper have already been conducted for town houses in three districts by authors []-[3]. flourished as the post station town of the old highway called Nakasendo, which connected Tokyo to during the Edo era and is designated as an IPDGNB in 978. Namely, the houses were originally constructed in the 7th Century. The houses stand in a row for about km from south to north on both sides of the Nakasendo. is an old capital of Japan. Many traditional town houses remain in. But the whole city area of except for the north-western part was attacked by a big fire at the end of Edo era. Most of town houses called machiya that remain in the city were built since the Meiji Era and were less than years old. city prospered for many years as a temple town of the Shrine famous for relocating the sacred symbol at intervals of 2 years.,, and are located in the central part of the Honshu Island. However, the exceedance probability of high seismic intensity is high in district as well as compared with those of and. This is because the occurrence of the Tonankai earthquake is predicted within tens of years and district is located near the source area. Population,,,, Kochi pref. Muroto city S55 S6 H2 H7 H2 H7 Figure 6: Change in population Population Ratio of aging population (%) 2,8 2,75 2,7 2,65 2,6 2,55 2, , Kochi pref. Muroto city 2,79 2,65 Figure 7: Population in town ,62 Figure 8: Change in elderly population ratio

4 3 STRUCTURAL FEATURES Traditional town houses in have gable roofs and their ridges are in the direction parallel to their frontal road. Since a roof tile deteriorates by briny air, replacement is required once in about 5 years. A column stands on a foundation stone. Main load-bearing elements are timber frame and mud walls, and no diagonal brace is used. Before 932 when wood can be purchased from the Local Forestry Offices, the hemlock fir which grows wild on the outskirts was used for structure timber material. For termite control, the Japan cedar, the hemlock fir, and the cypress are used for the column. Although the pine is used for the beam, many parts of them are attacked by termites. The cement plaster wall made by tosashikkui and mizukirikawara at an outer wall are raised as a typical feature of the town houses. The tosashikkui is slaked lime with very high purity. And since paste is not included, tosashikkui is strong in water and has the feature that thick coating is possible. The mizukirikawara is a kind of tiles tiered in several rows to prevent the rain charge to the surface of gable walls. Plaster on the surface of the outside wall is painted on the layer repeatedly. Outer wall thickness of five town houses is over 2 mm (see Fig. 9) among six with mizukirikawara. Brick walls are used for outer walls in three houses built from the last stage of Meiji to Taisho. Figure shows the detail of a brick wall. There is a space of about 3mm between the brick and the mud wall inside. To prevent rain sinking in, columns and mud walls are covered by cedar bark. The brick wall is connected with columns using nails at intervals of to 2m. Though there are four rooms in the first floor, it is possible to use them as a big room by removing the sliding screen used for partition as shown in Fig.. A rest room and a bathroom are behind the main building and are connected by a roofed passage. In the houses built until the end of Meiji era, the ceiling of the front rooms of the second floor was lowered by about m, and the second floor was used as a lumber room. The wooden boards are paved in the second floor, and a luggage can be stored and also be taken out by removing some floor boards. Inner walls are mud walls, and their thickness is about 8mm. The section of columns is a square, 2mm on a side as shown in Table. On the other hand, the traditional town houses in,, and have narrow entries, and are long in depth. Three or four rooms are located in a line in the depth direction beside toriniwa, which is an earth-floored area and provides access from the front to rear of the house. There are few walls in the ridge direction, which is parallel to the frontal street. On the other hand, there are two outer walls in the span direction, perpendicular to the frontal street. The roofs of houses in are tin roofing but those in,, and are tile roofing. The side walls in are made by board or mud wall and those in and are always mud wall. But thickness of mud walls in and is much thinner than thickness of outer walls in. Figure 9: Detail of a plaster wall Figure : Detail of a brick outer wall Figure : Plan of a house 4 SEISMIC PERFORMANCE EVALUATION The yield base shear coefficients C y of selected wooden houses are evaluated in the ridge and span directions respectively, based on the capacity spectrum method for traditional wooden structures [4]. The yield base shear coefficient C y is calculated by dividing the horizontal

5 restoring force of the st-story at the /3 rad of deformation angle (Q y ) by the total weight W. The force-deformation angle relationship (capacity curve) of the whole wooden house is calculated by summing the standard force-deformation angle relationships of each structural element, such as a frame, a full wall, a upper or lower partial wall etc. The standard force deformation angle relationships for typical structural elements are specified depending on the type of wall and frame or material of wall etc., in the capacity spectrum method. On the other hand, the total weight W is determined as the sum of the fixed and live loads. The fixed and live loads are calculated from the weight per unit area stipulated in the Article 84 of the enforcement ordinance of the Building Standards Law [5]. The rules and assumption of summation of the bearing capacity and weight are explained as follows: [] A restroom, a bathroom, and roofed passage behind the main building are disregarded. [2] The full walls: The bearing capacity of full walls is calculated by dividing the column span by the standard column span at,82 mm and multiplying the quotient by the standard resisting force. For example, the standard resisting force of full walls with 6 cm thickness at /3rad, mud wall is 9kN, wood siding wall is 4kN, and plaster board is 4kN. Although a gable plaster wall is thick, only the portion pinched by columns with 8 mm in thickness is considered to evaluate the resisting force. But the resisting force by brick walls is ignored. [3] The upper and lower partial walls: The bearing capacities of upper and lower partial walls are calculated by multiplying the number of column spans independently of the wall material. For example, the standard resisting force of upper walls at /3rad, mud wall is 4kN, wood siding wall is.7kn, and plaster board is.7kn. [4] The frame of tenon joints: The bearing capacities of tenon joints in the timber framing are corrected by dividing the reference height (2,73 mm) by the column length on each floor. In this paper, the frame of tenon joints is assumed as short tenon joints, the standard resisting force is.5kn. [5] The fixed loads: The fixed loads such as the weight of the roof, ceiling, floor and wall etc. are considered. The weight per unit area is different due to the type or material of each factor. Main factor of fixed loads are roof and wall for wooden houses. The weight per unit area for tile roofing is 64 N/m 2. On the other hand, in the case of wall, weight per unit area for mud wall is 83 N/m 2, and that for wood siding wall or plasterboard is 35 N/m 2. Additionally, weight per unit volume is 6 N/m 3 for mud outer wall and is 8.6 N/m 3 for brick outer wall. [6] The live loads: The 6N/m 2 of weight per unit floor area is assumed as the live loads during earthquakes. The seismic performance of eleven houses in is evaluated using the above-mentioned calculation method. The relationship between the restoring force divided by the total weight and story drift angle for the HS s house is shown in Fig. 2. The total area of the first floor A, story height of i-th story H i, equivalent height of SDOF system H e, W/A, Q y /A, N c /A, W/N c and C y for investigated houses are listed in Table, where W is the total weight, Q y is yield base shear, and N c is the number of columns in the first floor. The C y values for investigated houses are also shown in Fig. 3. Frequent distribution curves for C y in the weak direction of houses in four districts are regressed using a lognormal distribution as shown in Fig. 4. In addition, the average and the standard deviation of W/A, Q y /A, N c /A, W/N c and C y for houses in are listed in Table 2 comparing those in,, and. From these figures, the followings can be pointed out for the structural properties from the statistical point of view. [] The yield base shear coefficients in are not so high among four districts, even if the seismic risk is very high. Houses whose C y are nearly equal to. are not few. [2] The average of Q y /A in is not so large compared with that of other districts even if outer wall is especially thick in. [3] Since N c /A in is the smallest among the four districts, the W/N c is the largest. Table : Summary of investigated houses Name Thickness Dimension of Age Number Natural freq. [Hz] Story height[m] W/A Nc/A W/Nc Cy of gable Nc column [mm] A [yrs] of stories [m 2 ] Qy/A [kn/m 2 ] wall [mm] STD MAX H H2 He [kn/m 2 ] [/m 2 ] [kn] TT OY MJ MN HS HT KY 9 2 4, NY TY , TD HK 7 2 2, Ave STD

6 Base shear coef. C y Base shear coef. C y /6 /3 /2 /5 /2 / Story drift angle (rad) Figure 2: Estimated restoring force characteristics of HS s house TT OY MJ MN HS HT KY NY TY TD HK Figure 3: Yield base shear coefficients of investigated houses Base shear coef. C y Figure 4: Distribution of yield base shear coefficients Table 2: Stocastic characteristics of traditional wooden houses in four districts W/A (kn/m 2 ) Q y/a (kn/m 2 ) C y W /N c (kn) weak axis strong axis N c /A (/m 2 ) weak axis strong axis (.84) (.85) (.4) (.59) (.397) (.357) (.55) (.36) (.84) (.43) (.426) (.374) (.2) (.69) (.65) (.442) (.7) (3.29) (2.24) (.9) (.94) (.7) (.9) (.6) (.82) (.4) (.47) (.84) Ave(SD) 5 AMBIENT VIBRATION MEASUREMENTS Ambient vibration measurements were conducted for eleven town houses. Each accelerometer has three channels, one vertical and two horizontal directions simultaneously. More than two accelerometers were set on the second floor of each house to identify torsional mode and one accelerometer was set on the soil surface. All accelerometers are synchronized by using GPS signals. Sampling frequency was Hz. The duration time of each record was usually 6 seconds. The measured data were processed in the frequency domain, as follows. Each time series data were divided into segments with duration times of 4.96s. To minimize errors of noise, the ensemble mean was calculated and the fast Fourier transform (FFT) was applied to the ensemble mean. The Fourier spectral ratio of the second floor to the ground level is calculated. Identified natural frequencies from the Fourier spectral ratio are listed in Table.. As shown in Fig. 5, natural frequencies in the ridge and span directions are 3. Hz and 5.3 Hz, respectively. In addition, torsion motion can be observed from 5.3 Hz to 6.2Hz due to the eccentricity of the plan view as shown in Fig.. However, if the main building and roofed passage are separated structurally, such torsion motion is not observed. Spectral Ratio A/G B/G 3.Hz 5.3Hz 6.2Hz 7.6Hz Frequency(Hz) (a) Ridge direction Spectral Ratio A/G B/G 5.3Hz 6.2Hz 8.9Hz Frequency(Hz) (b) Span direction Figure 5: Fourier spectrum ratio (HS s house)

7 Frequent distribution curves for natural frequencies f in the weak direction of houses in four districts are regressed assuming lognormal distribution as shown in Fig. 6. Natural frequency distribution of houses in is almost identical with that in. But natural frequencies of town houses in and are smaller than those in and Frequency (Hz) Figure 6: Distribution of natural frequency 6 APPROXIMATE EVALUATION OF C y Figure 7 shows the relation between yield base shear coefficient C y and the natural frequency f identified from ambient vibration measurements. Figures 7(a), (b), (c), (d) are the results for town houses in [], [2],, and [3], respectively. Solid and open symbols are results for houses in the span and ridge directions, respectively. Furthermore, solid lines indicate an approximate evaluation formula as C 2 y f /6, () In which we assume the strength of a wooden house is proportional to the initial stiffness [6]. Broken lines show another evaluation formula, which is developed for the simple calculation as Cy ( f )/. (2) The C y - f relation of the town houses in four districts can be approximately expressed by these two lines as shown in Fig. 7. Namely, yield base shear coefficient C y can be roughly estimated from natural frequency obtained from ambient vibration measurements. On the other hand, since the yield base shear coefficient C y is the yield base shear force Q y divided by total weight W, we proposed an another approximate evaluation method [3] as Cy Qy / W, (3) where Qy f Lfull pn and part. W A. (4) f It is assumed that the total weight W of a house is proportional to the total floor area A f and base-shear force Q y is evaluated from the total length of full walls divided by,82 mm L full and the number of partial wall spans N part. The f and p are the factors of L full and N part., respectively. These factors can be determined considering the specification of walls is determined. For example, f =9kN/m and f =4kN for mud walls. Figure 8 shows the relationship between A f and W. The relationship is classified whether the principal specification of walls are board or mud wall in. It is necessary to determine the factor should be determined for every district by performing a sampling investigation as performed in this paper. It is thought that the rigorously evaluated C y can be approximated successfully by using equations () to (3). 7 CONCLUSIONS Structural characteristics of traditional wooden houses in a historic town called are investigated and summarized in this paper. Based on the results of the ambient vibration measurements and seismic performance evaluation of the traditional town houses in and the other three historic districts in Japan, the following conclusions are drawn. [] The yield base shear coefficients in are not so high even if the seismic risk is very high. [2] Structural characteristics, yield base shear coefficients and natural frequencies are compared among four districts. [3] Two types of approximation methods, which is based on the results of ambient vibration measurements or simple floor plan proposed by the authors, to evaluate yield base shear coefficient for the traditional houses has been proved to be applicable to four historic districts. By evaluating yield base shear coefficients using either of the proposed methods, the houses with poor seismic performance in a region can be specified. But the specification accuracy can be improved by applying both methods. Specified houses should be investigated in detail preferentially whether seismic retrofit is needed or not considering the seismic hazard or expected loss. ACKNOWLEDGEMENT A part of this research was supported by the Asahi Glass Foundation. We would like to show our appreciation to Dr. T.Morii of former Assistant Professor of University for his great contribution to the field investigation. REFERENCES [] Kiso K., Saratani A., Morii T., Sawada K., Aono F., Watanabe C. And Hayashi Y.: Study on reduction of seismic damage for traditional wooden house in Mie Prefecture local environment, AIJ J. Technol. Des., No.24, pp ,26 (in Japanese). [2] Ida S., Morii T., Nii A. and Hayashi Y.: Simple method for seismic evaluation of traditional houses in based on ambient vibration measurements, Part 2: Proposal of seismic evaluation method, Summaries of technical papers of annual meeting architectural institute of Japan, Struc-tures Ⅲ C-2, pp.23-24, 28 (in Japanese).

8 [3] Hayashi Y., Higa S., Hasebe Y., Morii T., Matsuda M., and Koshihara M.: Seismic Performance Evaluation for Groups of Traditional wooden houses, ICOSSAR'9, pp , 29. [4] Suzuki Y., Saito Y., Katagihara K., Ikago K and Nojima C.: Method of Evaluating Seismic Performance of Wooden Frames Limit Bearing Capacity Analysis in Wide Range of Deformation-, The th Japan Earthquake Engineering Symposium, pp , 22. (in Japanese) [5] Ministry of Land, Infrastructure, Transport and Tourism (MLIT): Article 84 of the enforcement ordinance of the Building Standards Law., 24 (in Japanese) [6] Hayashi Y., Nii A. and Morii T.: Application of vibration measurements for timber-framed houses, Proceedings of the 4th Annual meeting of Japan Association for Earth-quake Engineering, pp.4-5, 25. (in Japanese). Yield base shear coef. C y Yield base shear coef. C y Yield base shear coef. C y S-Dir. f 2 /6 (f-)/ Frequency f (Hz) f 2 /6 (f-)/ Frequency f (Hz) f 2 /6 (f-)/ (a) (b) Frequency f (Hz) (c) Total weight W(kN) Total weight W(kN) Total weight W(kN) W=2.5*A f (R=.64) Gross floor area A (m 2 ) f (a) : W=2.26*A f (R=.9) Gross floor area A (m 2 ) f (b) W=3.37*A f (R=.83) Gross floor area A (m 2 ) f (c) (a) (b) (c) Yield base shear coef. C y R-Dir. f 2 /6 (f-)/ Frequency f (Hz) (d) Figure 7: Relation between natural frequency and yield shear coefficient Total weight W(kN) Boad wall Mud wall : W=.36*A f (R=.96) : W=.93*A f (R=.8) Gross floor area A (m 2 ) f (d) Figure 8: Relation between gross floor area and total weight (d) Figure 9: Typical houses in four districts