DAMAGE TO WOOD BUILDINGS DURING THE GREAT TOHOKU EARTHQUAKE PART2- DAMAGE DUE TO GROUND MOTION

Transcription

1 DAMAGE TO WOOD BUILDINGS DURING THE GREAT TOHOKU EARTHQUAKE PART2- DAMAGE DUE TO GROUND MOTION Naohito Kawai 1, Hiroshi Isoda 2, Takahiro Tsuchimoto 3, Mikio Koshihara 4 ABSTRACT: The damage to wood buildings occurred over a widespread from Tohoku down to northern Kanto. The cause of damage was either by ground motion or by tsunami. In a series of damage reconnaissance reports, outline of damage and analytical study including damage statistics, earthquake, and strong motion characteristics as a part 1 and damage due to ground motion as a part 2 are described. The damage due to ground motion is categorized by the primary damage, which were the ground deformation including the liquefaction or the vibration of the superstructure. In this paper, the result of the reconnaissance on timber buildings damaged due to the ground motion is described. Then, the result of the time history analysis is conducted to clarify the relationship between the characteristics of ground motion and the damage of wood houses. Last of all, results from the reconnaissance on timber buildings damaged due to the tsunami are also discussed. Many wood houses got swept away by tsunami in areas of deep inundation. However, there were a few cases where wood houses with high grade structural specifications and shield against tsunami impact remained in place area. KEYWORDS: Reconnaissance, wood house, Warehouse, Traditional building 1 OVERVIEW OF INVESTIGATION / INTRODUCTION 123 The Managing Committee on Timber Structures of the Architectural Institute of Japan (AIJ), in cooperation with Tohoku and Kanto Chapters of AIJ, conducted earthquake damage reconnaissance on Timber buildings, mainly in the prefectures of Iwate, Miyagi, Fukushima, Ibaraki and Tochigi. This paper describes the damage of wood buildings due to ground motion mainly and summarize the damage them due to tsunami briefly. In the part 1 of this series, analytical study including damage statistics, earthquake, and strong motion characteristics are described. Damages were classified either by the cause of ground deformation or by the vibration of the superstructure. The damage of timber buildings including collapse occurred in a wide area along with from Tohoku to northern Kanto, although severe damage is dotted about 1 Naohito Kawai, Kogakuin University, Nishi-Shinjuku Shinjuku Tokyo, Japan. kawai-nk@cc.kogakuin.ac.jp 2 Hiroshi Isoda, Shinshu University, Wakasato Nagano City Nagano, Japan. hisoda@shinshu-u.ac.jp 3 Takahiro Tsuchimoto, National Institute for Land and Infrastructure Management, 1 Asahi Tsukuba, Japan. tsuchimoto-t92ta@nilim.go.jp 4 Mikio Koshihara, Institute of Industrial Science, The University of Tokyo, Tokyo, Japan. kos@iis.utokyo.ac.jp the area and the numbers were limited. The types of damage are extremely diverse due to differences in site amplification of the ground motion characteristics. Typical examples were the occurrence of collapse or other severe damage as a result of landslides or failure of retaining wall on slopes, and tilting or sinking of entire structures as a result of the liquefaction of sandy soils. Many wood houses got swept away by tsunami in areas of deep inundation. However, there were cases where wood houses were shielded from direct tsunami impacts by relatively large surviving structures which remained intact. There were also a few cases where wood houses presumably having excellent structural specifications remained in place, although they suffered severe damage to their walls or frames.overall, the main cause to the damages in residential houses and dwelling shops are caused by the lack of seismic resisting elements, and moreover the deterioration of materials due to natural decay and termite problem are also believed to be another important factor to the seismic damage. However, reports show that modern houses in Nasu, Tochigi Prefecture where deterioration was not an issue also suffered from serious seismic damage.

2 2 DAMEGE DUE TO GROUDN MOTION The cause of damage is divided by either the damage due to ground deformation or the damage due to the vibration of the superstructure. In this chapter, the damage due to the vibration of the superstructure is categorized as minor damage (damage to finishing materials only), moderate damage (structural damage), and collapse. The use of timber buildings are houses, stores, apartments, barns, warehouses, school buildings, public buildings, shrines, gymnasiums, and other large buildings. Their construction method, number of floors, size, and shape are different. The earthquake caused extensive damage including collapse throughout northeastern Kanto to Tohoku in broad area. In areas such as river basins where the ground is soft, timber buildings are damaged due to the resonance phenomenon. This is consistent with the results of the time history analyses using strong motion records (See part1). In addition to serious structural damage, the minor damage in roof tile and finishing exterior and interior wall occurred in extensively throughout northern Kanto to the Tohoku area. In addition to vibrational damage, a large amount of damage occurred due to ground deformation. Houses on slopes collapsed or were heavily damaged due to landslides and the destruction of retaining walls. Prominent damage throughout the area was due to slope or land subsidence due to liquefaction of the upper structure of the sandy ground. The whole superstructure was often tilted by the liquefaction but did not collapse. The cause of the damage is not clear due to ground deformation or vibration in the area where damage occurred on soft soil Vibrational Damage to Houses Serious damage to houses has reported in many communities where are Ishinomaki, Osaki, Kurihara, Kesennuma, Sendai and Misato, in Miyagi Prefecture, Sukagawa in Fukushima Prefecture, Nasu Karasuyama, Nasu, Takanezawa and Haga in Tochigi Prefecture and Mito, Ibaraki, Sakuragawa, Naka, and Hitachiota in Ibaraki Prefecture. In addition, the cases of sporadic collapse in dilapidated housing have been reported in other cities such as Katori, Chiba Prefecture and Kounosu Saitama Prefecture. Figure1-8 illustrate some of this damage. Most of the extensive damage occurred in relatively old houses, often obsolete buildings, with decay and termite damage. In particular, the building members in Figure1 showed extensive termite damage. The houses typically have large openings facing the street, which is inadequate for earthquake resistance. Exterior walls often showed chipped mortar and glass as well as termites in the wood sheathing, which sometimes spread into the structural members. Therefore, the damage of most dwellings was amplified due to the termite damage. In contrast, vibration damage built in one distinct typical construction is found in Figure 8, which shows the partial collapse of the high stone foundation near the city of Nasu Karasuyama and the town of Ichigai in Tochigi Prefecture. This vibrational failure of the foundation was often found and would lead to the collapse of the wood superstructure. Figure 9 shows an example of damage in storehouse of wood frame and stone cover. Figure 1: Collapse of wood house in Osaki-city Figure 2: Partial collapse of wood house in Kurihara-city Figure 3: Collapse of wood house with store attached in Misato-town Figure 4: Severe damage of wood house with store in Osaki-city

3 foundation Figure 5: Severe damage of wood house with store in Sendai-city Figure 9: Damage of storehouse by wood frame and stone in Nasukarasuyama-city Figure 6: Severe damage of wood house with store in Kurihara-city Figure7: Severe damage of wood house with store in Sendai-city 2.2 Vibrational Damage to Storehouses A wide array of damage to storehouse buildings was observed from Kanto to Tohoku, many of which have attached dwellings. Examples of storehouse damage are in Figure 10 through Figure. 12. The primary type of damage was in the form of fallen roof tiles or chipped exterior wall coatings. Major damage, demonstrated in Figure 10, was evident in the form of residual inclination of the building. The group of storehouses in Osaki, as seen in Figure 11, was over 200 years old and had obvious structural deficiencies. Although no deformation to the columns and other structural members is observed, the termite decay is extensive within the members and would be difficult to repair. Figure 12 shows a building on the same property as the warehouses. A retrofit was performed on the interior wooden shear walls and reinforced concrete masonry foundation inside. The structural timbers were still intact but the roof collapsed. Figure 13 through Figure 15 show the damage of residential storehouses. Residual deformations were small and the damage was primarily in the form of cracks and spalling of the mud coatings on the wall as well as falling roof tiles. Figure 10: Permanent inclination of a storehouse Figure 8: Collapse of wood house with high stone

4 Figure 11: Damage of older storehouses in Osaki-city Figure 15: Damage of three storehouses with heavy clay walls in Tuskuba-city Vibrational Damage to Barns In residential areas, many structures such as barns and small storage buildings collapsed. Adequate seismic structural design was not considered in their buildings. Examples of such damage are in Figure 16 through 18. Figure 17 shows the story collapse of shed with the concrete block first story. The concrete block was broken. Figure 12: Retrofitted storehouse in Osaki-city Figure 16: Collapse of shed in Kurihara-city-1 Figure 13: Damage of storehouse with heavy clay walls in Tsukuba-city-1 Figure 17: Collapse of shed in Kurihara-city-2 Figure 14: Damage of house with heavy clay walls in Tsukuba-city-2

5 Figure 18: Collapse of shed in Osaki-city 2.4 Vibrational Damage to Schools There is a two construction method in school buildings made of timber. One is old timber construction methods and the other is newly constructed of a glued laminated timber. A school building using older timber construction method was reported to have a partial second floor collapse and large deformations in Osaki city. Traces of sand boiling in the schoolyard were observed because of liquefaction of the sandy ground. Figure 19 shows the markings indicating sand boiling. Figure 20 also shows the residual story drift of the same school building. School buildings built using a glued laminated timber, for example, an elementary school gymnasium in Nakacity, only experienced minor damage to the concrete foundation and timber braces. Figure 20: Story drift of school building in Osaki-city 2.5 Vibrational Damage to Temples and Shrines Damage to shrines is widely reported across the Northeast of Japan with a notable collapse of the main building of a temple in Takanezewa town, seen in Figure 21. In addition, Figure 22 shows the severe damage of the gate to a shrine in Osaki-city. Marks around the site seem to indicate sand boiling caused by the liquefaction of the sandy ground. These shrines were constructed by traditional methods and were located on soft soils. Figure 21: Collapse of the main building of temple in Takanezawa-town Figure 19: Damage of school building and markings of sand boiling in Osaki-city Figure 22: Severe damage to the gate of a shrine in Osaki-city

6 Figure 23 show a residual story drift of the building in a site of temple. The foot of this building was out of the foundation. 2.7 Building Damage due to Landslides This earthquake caused many landslides and destruction of retaining walls, leading to the destruction of many houses built on the slopes. Areas of intense damage were found in Aoba-ku Sendaicity, Izumi-ku Sendai-city, Wakabayashi-ku Sendailcity, Fukushima-city, Kagamiishi town, and Nasukarasuyama-city in Tochigi Prefecture. Examples of such damage are found in Figure 26 through Figure 33. Figure 23: Severe damage of temple building in Osakicity 2.6 Vibrational Damage to Other Buildings In addition to other building types, a renovated theater in Kurihara-city experienced large residual deformation, evident in Figure Other examples of damaged timber buildings included houses that used logs assembly. Although this is a rare construction method, there are reports of damage at the intersection of the logs in houses in Nasushiobara-city. Figure 26: Collapse of hotel due to landslide in Sendaicity Figure 27: Severe damage of house by landslide in Sendai-city Figure 24: Severe damage of old cinema theatre in Kurihara-city Figure 28: Severe of a house due to landslide in Sendaicity Figure 25: Inside of the same building

7 Figure29: Collapse of retaining wall in Sendai-city Figure 30: Severe damage of a house due to landslide in Sendai-city-1 Figure 33: Severe damage of house due to collapse of retaining wall in Osaki-city 2.8 Damage due to Liquefaction Soil liquefaction occurred rapidly in a wide range of places due to the sandy soil. Ground subsidence leads to serious damage or a tilt of the superstructure. This damage was observed in Kashima, Kamisu, Itako, Future city, Inashiki, Kawachi-machi, in in Ibaraki Prefecture, Asahi city, Urayasu, Funabashi, Narashino, Chiba, Abiko, Katori in Chiba Prefecture and Kuki city, Saitama Prefecture. The structural damage to the superstructure due to liquefaction was rather small if it used a reinforced concrete foundation. Damage was typically in the form of structural inclination and subsidence. 2.9 Ground Deformation and Other Damage In flat ground, even though liquefaction does not occur, the soft ground results in land and soil cracks. Damage to the superstructure from differential settlement is reported in Sakura, and Narita in Chiba Prefecture. Similar damage happened in the other regions. This type of the damage occurred to old houses, so the cause of the damage may not be clearly due to ground deformation or vibration. Figure 31: Severe damage of a house due to landslide in Sendai-city-2 3 Damage due to Tsunami In this paper, damage due to ground motion was described mainly. Damage to tsunami wave was extremely severe as shown in Figure 34. Most of wood buildings near seaside area completely were washed away. There were a few timber buildings after tsunami attack in seaside area. In this chapter, these buildings are introduced briefly. Figure 32: Severe damage of house due to landslide -3 Figure 34: Overview in Arahama area after tsunami attack

8 In Wakabayashi ward, Sendai-city, Miyagi prefecture, with a few exceptions, most wood houses were completely washed away by the tsunami. Figure 34 demonstrates one of the few wood houses left standing, due to a location downstream of a reinforced concrete building. Inundation depth estimated from remaining reinforced concrete buildings reached 6 to 8 meters. In an area approximately 1.5 km away from the shoreline in Arahama district, many low-rise buildings were washed away. In this area, some houses situated in a line the direction of the water flow withstood damage, as evident in Figure 35. Inundation depth in this area was estimated to be about 4 to 5 meters, from the remaining houses being low-rise and non-wooden. On the other hand, there were several cases identified that no buildings were left standing in the direction of the tsunami waves. Figure 36 demonstrates one of the few timber buildings left standing, with damage limited to the walls in the direction of floodwater. The method of construction was relatively new with adequate structural fasteners. Figure 34: Wood house remained downstream of RC building Figure 35: Wood houses remained in a line 4 SUMMARY AND CONCLUSISONS Damages were classified either by the cause of ground deformation or by the vibration of the superstructure. The damage of timber buildings including collapse occurred in a wide area along with from Tohoku to northern Kanto, although severe damage is dotted about the area and the numbers were limited. The types of damage are extremely diverse due to differences in site amplification of the ground motion characteristics. Typical examples were the occurrence of collapse or other severe damage as a result of landslides or failure of retaining wall on slopes, and tilting or sinking of entire structures as a result of the liquefaction of sandy soils. Many wood houses got swept away by tsunami in areas of deep inundation. However, there were cases where wood houses were shielded from direct tsunami impacts by relatively large surviving structures which remained intact. There were also a few cases where wood houses presumably having excellent structural specifications remained in place, although they suffered severe damage to their walls or frames.overall, the main cause to the damages in residential houses and dwelling shops are caused by the lack of seismic resisting elements, and moreover the deterioration of materials due to natural decay and termite problem are also believed to be another important factor to the seismic damage. However, reports show that modern houses in Nasu, Tochigi Prefecture where deterioration was not an issue also suffered from serious seismic damage. ACKNOWLEDGEMENT This paper is based on the reconnaissance report by the Managing Committee on Timber Structures of the Architectural Institute of Japan (AIJ)[1], in cooperation with Tohoku and Kanto Chapters of AIJ. Contribution of the reconnaissance team members, listed below, is greatly appreciated: Naoyuki Itagaki (Akita Prefectural Univ.); Masahiro Inayama (Univ. of Tokyo); and Takafumi Nakagawa, Yasuhiro Araki (Building Research Institute). REFERENCES [1] Preliminary Reconnaissance Report of the 2011 Tohoku-Chiho Taiheiyo-Oki Earthquake, Architectural Institute of Japan, Figure 36: Timber building escaping complete devastation