Kobe Earthquake Essay, Research Paper
The January 17, 1995 Kobe Earthquake
An EQE Summary Report, April 1995
Executive Summary
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On the first anniversary of the moment magnitude (MW) 6.7 1994 Northridge Earthquake, Kobe, Japan was struck by an MW6.9 earthquake. Both earthquakes struck in the pre-dawn hours, both ruptured beneath densely populated areas, and both caused horrible damage. Yet in Kobe there were many more deaths, financial losses dwarfed those in Northridge, and the amount of destroyed building stock and infrastructure was far worse in Kobe than in Northridge.
The reasons for these differences are many, but it would be incorrect to issue a blanket condemnation of current Japanese seismic engineering practice. While engineered structures did fail due to design flaws, they were predominantly older structures built before the current Japanese building code became effective; or they frequently failed due to problems revealed to be deficiencies in California design practices by the Northridge Earthquake. Japanese seismic engineering expertise has justifiably been considered among the best in the world, and a careful examination of the damage in Kobe does not change that conclusion.
Despite differences in design and construction practices, the same general principles frequently came into play: highway collapses were often primarily due to insufficient lateral ties in the concrete columns, nonductile concrete frame buildings did much worse than ductile design, shear walls typically helped to lessen catastrophic damage, and soft soils resulted in greater damage to the structures constructed on them.
The most important lesson in both earthquakes is that the knowledge to significantly improve structures to resist earthquake damage and thereby avoid most of the deaths and financial losses exists; what is lacking is a consistent willingness to marshall the resources necessary to put that knowledge to work on the scale necessary to prevent disasters. It is an odd paradox, for time and time again it is demonstrated that it usually costs less to prepare for earthquakes in advance than to repair the damage afterwards.
Differences in Kobe and Northridge
While there are more similarities than differences in structural performance in the Kobe and Northridge earthquakes, there are important differences that explain why the Kobe Earthquake was so much more damaging. Some of the lessons from these differences apply only to Japan, others apply to all areas of the world at risk from earthquakes.
The vast majority of deaths in Kobe occurred in the collapse of housing built using traditional Japanese methods. Traditional Japanese housing construction is based on a post-and-beam method with little lateral resistance. Exacerbating the problem is the practice of using thick mud and heavy tile for roofing, resulting in a structure with a very heavy roof and little resistance to the horizontal forces of earthquakes. U.S.-style frame housing with light-weight roofs is now coming into use in Japan and newer housing constructed using these methods had little or no damage from the earthquake.
Another significant difference between the Kobe area and the Northridge area is the quality of the soils. Because of a severe shortage of available land, much of modern urban Japan, including Tokyo, is built on the worst soil possible for earthquakes. Much of the newer construction in Kobe, particularly larger buildings, is built on very soft, recent alluvial soil and on recently constructed near-shore islands. Most of the serious damage to larger commercial and industrial buildings and infrastructure occurred in areas of soft soils and reclaimed land. The worst industrial damage occurred at or near the waterfront due to ground failures-liquefaction, lateral spreading, and settlement.
The Port of Kobe was an extreme example of the problems associated with poor soils in areas prone to earthquakes. The port is built almost entirely on fill. The engineering profession has tried hard to develop methods for strengthening filled areas to resist failures during earthquakes, but most of these methods have been put into practice without the benefit of being adequately tested in strong earthquakes. The results were decidedly mixed, but the failures costly_most retaining walls along the port failed, and the related ground settlement pulled buildings and other structures apart.
Buildings
The large commercial and industrial buildings in the Kobe area, particularly those built with steel or concrete framing, are similar to buildings of the same vintage in California. The Japanese building code had a major revision for concrete-frame buildings and a more limited revision for steel-frame buildings in 1981. The Uniform Building Code, as used in California, had major changes in 1973, 1975, and several times since then. The current Japanese code requires that buildings in Japan be designed for somewhat higher force levels than does the Uniform Building Code. Both areas require design for much higher forces than most other earthquake regions of the world.
Typically, pre-1981 concrete-frame buildings performed very poorly in Kobe, with many collapses. Post-1981 buildings performed much better_some were extensively damaged, but most had light damage. The buildings that fared best, and those without significant damage, had extensive concrete shear walls.
As in other earthquakes, large commercial and industrial steel-frame buildings performed better than any other type. However, major damage and a few collapses were observed. Pre-1981 steel buildings had most of the serious known damage. Certain innovative types of steel buildings, including high-rises, had very serious damage, and collapses might have occurred if the duration of the earthquake had been a few seconds longer.
Building owners usually do not understand that the earthquake provisions of even the strictest building codes do not necessarily have reasonable performance criteria for larger and stronger earthquakes. The current regulations, including those for all of California, are typically written with the expectation that in a strong earthquake a building will be severely damaged_in fact, it is assumed the building may need to be torn down, but it should not collapse. In California, higher performance criteria are mandated for certain types of structures_schools, hospitals, police and emergency response buildings, and certain power facilities. An informed building owner can choose to use these higher criteria, and thus avoid having their high-value, heavily occupied commercial building designed, in effect, to the same earthquake performance level as a low-value farm building.
Transportation
A number of major expressways, rail lines, and bridges, some of very modern design, were severely damaged. There are no significant new lessons from the collapse and damage of the older unretrofitted bridges and elevated structures. The structural and foundation details that typically caused damage to the expressways and rail lines have been observed in numerous earthquakes, and the damage was predictable. Some of the upgrade details observed in older retrofitted structures, such as steel column jacketing, are now widely used in California for strengthening. The apparent good performance of these details in Kobe is important to ongoing U.S. programs and needs to be studied in detail.
Many bridges and bridge approaches were severely damaged. The performance of large new bridges, including cable-tied arch, braced arch, and cable-stayed bridges, should be studied extensively because this is the strongest earthquake to affect such bridges.
The Port of Kobe, much of which was new, was devastated by widespread and severe liquefaction and/or permanent ground deformation, which destroyed more than 90% of the port’s 187 berths and damaged or destroyed most large cranes. Shipping will be disrupted for many months, and some shipping business will probably never return to Kobe, resulting in significant losses to the local economy.
Other Infrastructure
The electrical and telecommunications systems in Kobe and surrounding areas performed as expected based on experience from previous earthquakes. Long term power outages were isolated to the most heavily damaged areas. Facilities near the epicenter sustained damage while resiliency of the systems prevented widespread service interruption. Most of the major transmission lines skirt the heavily damaged region of Kobe the results may have been substantially different if the epicenter was located closer to the 500 kV transmission system. There were substantial financial losses to the electrical utilities, however, because expensive specialized equipment must be replaced and the distribution network must essentially be rebuilt within heavily damaged areas of Kobe.
During the earthquake, Kobe’s water system sustained approximately 2,000 breaks. Generally, ground or building failure was the cause of the severe damage to Kobe’s water systems. The resulting lack of water contributed significantly to the fire problem and will be a major hardship on the population for several months. The gas system had major damage, generally caused by ground or building failure, which also contributed significantly to the fire problem.
Fire
More than 150 fires occurred in Kobe and surrounding areas in the hours after the earthquake. These resulted in several large fires, and fire fighters were for the most part unable to combat them because of streets being blocked by collapsed buildings and building debris, traffic congestion, and severe water system damage. Calm wind conditions prevented conflagrations. The United States and Japan have both sustained the largest peacetime urban conflagrations in this century’s history_because of earthquakes. Fire following earthquake is a potential major agent of damage, and needs to be recognized as such by planners.
Conclusion
The Kobe Earthquake dramatically illustrates the damage that can be expected from earthquakes to modern industrialized society. Most of what happened could have been predicted and much of the damage was preventable. Hopefully, the disaster will spur building owners to continue, and to increase where needed, their efforts to improve the earthquake resistance of their properties.
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The January 17, 1995 Kobe Earthquake
An EQE Summary Report, April 1995
Introduction
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An entire city block destroyed by fire, Chuo Ward.
On Tuesday, January 17, at 5:46 a.m. local time, an earthquake of magnitude 7.2 (Mj)1 struck the region of Kobe and Osaka in south-central Japan. This region is Japan s second-most populated and industrialized area, after Tokyo, with a total population of about 10 million. The shock occurred at a shallow depth on a fault running from Awaji Island through the city of Kobe, which in itself has a population of about 1.5 million. Strong ground shaking lasted for about 20 seconds and caused severe damage over a large area.
Nearly 5,500 deaths have been confirmed, with the number of injured people reaching about 35,000. Nearly 180,000 buildings were badly damaged or destroyed, and officials estimate that more than 300,000 people were homeless on the night of the earthquake.
The life loss caused by the earthquake was the worst in Japan since the 1923 Great Kanto Earthquake, when about 140,000 people were killed, mostly by the post-earthquake conflagration. The economic loss from the 1995 earthquake may be the largest ever caused by a natural disaster in modern times. The direct damage caused by the shaking is estimated at over 13 trillion (about U.S.$147 billion). This does not include indirect economic effects from loss of life, business interruption, and loss of production.
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Left: Map of the Kobe area.
Right: One of hundreds of collapsed buildings throughout central Kobe.
Damage was recorded over a 100-kilometer radius from the epicenter, including the cities of Kobe, Osaka, and Kyoto, but Kobe and its immediate region were the areas most severely affected. Damage was particularly severe in central Kobe, in an area roughly 5 kilometers by 20 kilometers parallel to the Port of Kobe. This coastal area is composed primarily of soft alluvial soils and artificial fills. Severe damage extended well northeast and east of Kobe into the outskirts of Osaka and its port.
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Left: Collapsed portion of the Hanshin Expressway.
Right: Search party investigating a collapsed residential wood-frame building, Nada Ward.
Our experience with many past earthquakes in developed, industrial areas is that the media, particularly television, can present an exaggerated image of the damage by concentrating on the few spectacular collapses that occurred. The actual damage in Kobe and the surrounding region, however, was much worse than the media could convey, because it is very difficult to show more than local damage at one time. For example, images of the main, 550-meter-long collapsed section of Kobe s elevated Hanshin Expressway were ever-present throughout the media, but that collapse was only a small fraction of the losses to the area s highway system.
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Self-defense troops performing a search and rescue operation at a collapse site.
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Left: Many streets were blocked by collapsed buildings, hindering emergency response.
Right: This man is hauling water. Nine days after the earthquake, 367,000 households were still without water.
Central Kobe, according to many older residents and our investigators, presented the image of a war zone, with a large percentage of both commercial and residential buildings destroyed.
All of this happened in about 20 seconds.
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The January 17, 1995 Kobe Earthquake
An EQE Summary Report, April 1995
Earth Science
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The causative plate action.
Southwestern Japan is located on the southeastern margin of the Eurasian Plate, where the Philippine Sea Plate is being thrust (subducted) beneath the Eurasian Plate in a northwest direction along the Nankai Trough. A portion of this relative plate motion is taken up by right-lateral strike-slip faulting along a major east-northeast-trending fault known as the Median Tectonic Line (MTL), located immediately south of Awaji Island and Osaka Bay.
The main shock occurred along a northwest-trending branch of the MTL called the Arima-Takatsuki Tectonic Line (ATTL). This fault system, like the MTL, has a predominantly right-lateral strike-slip sense of displacement. Historically, this region has seen somewhat lesser seismicity than in the Tokyo area and some other parts of Japan, but has had magnitude 7 or greater events in historical times (e.g., in 1596). In 1916, a magnitude 6.1 earthquake occurred at almost the same epicentral location as the 1995 event.
In the Kobe area, cretaceous granites are overlain by a relatively thick Plio-Pleistocene sedimentary unit called the Osaka group, which consists of alluvium interbedded with marine clays. Relatively thin terrace deposits and recent alluvium overlie the Osaka group. Fill material has been placed along much of the waterfront and comprises human-made islands, such as Port and Rokko islands.
Preliminary reports from the Japanese Earthquake Research Institute indicate that the hypocenter of the Mj7.2 (equivalent to Mw6.9) main shock occurred at a depth of approximately 15 to 20 kilometers. The main shock s focal mechanism indicates predominantly strike-slip movement along a plane that dips 80. to 90. to the southwest. The aftershock sequence (and, by inference, the faulting below the surface) is approximately 60 kilometers long, extending from the northern part of Awaji Island along the Nojima Fault to northeast of Kobe along the Rokko Fault zone.
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Japanese earthquakes, 1961-1994.
An approximately 9-kilometer-long surface fault rupture was identified along the Nojima Fault, which is on the northwestern coast of Awaji Island and southwest of Kobe. The fault strikes N40.W, dips steeply to the southeast, and has a predominantly right-lateral strike-slip sense of displacement consistent with the mechanism of the main shock and the trend of the aftershocks. Geomatrix Consultants (a geotechnical firm) measured local displacements at two locations along the northern part of the fault from the recent earthquake: Vertical displacements were 1.2 meters, and right-lateral displacements were 1.5 meters. These displacements are in good agreement with measurements by others, who reported maximum vertical displacements of about 1.2 meters and right-lateral displacements of 2.1 meters. Past surface-faulting events, which are probably similar to the most recent event, were evidenced by the 6- to 7-meter-high fault scarp along the fault. Given a long-term slip rate of 1 millimeter per year for the ATTL, as listed in Active Faults in Japan: Sheet Maps and Inventory by the Research Group of Active Faults, and an average displacement of about 1 to 1.5 meters, as suggested from observed displacement on the Nojima Fault, it appears that an earthquake roughly the size of the Kobe shock occurs on average once every 1,000 to 1,500 years along this portion of the ATTL.
It is unknown whether the surface fault rupture extended to the northeast across the Akashi Strait and onland to connect with faults in the Kobe-Nishinomiya area. Equivocal evidence of surface faulting has been described in this area and apparently is consistent with the aftershock sequence, which is approximately 60 kilometers long and extends northeast of Kobe. Based on empirical data of earthquake magnitude versus surface fault length, a 9-kilometer-long surface rupture should yield only an Mw6.2 earthquake, whereas a 60-kilometer-long rupture should yield an Mw7.1 earthquake, which is more consistent with the observed magnitude for this earthquake.
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Ground motion map.
A shaking intensity of up to 7 on the JMA intensity scale [equivalent to X to XI on the Modified Mercalli Intensity (MMI) scale] has been assigned to the coastal strip extending from the Suma Ward to Nishi-nomiya and in the Ichinomiya area on Awaji Island; JMA 5 (MMI VII to VIII) to Iwakuni, Hikone, Kyoto, and Toyooka; and JMA 4 (MMI VI) to Nara, Okayama, Osaka, Takamatsu, Shikoku, and Wakayama. The distribution of maximum horizontal ground accelerations and velocities recorded in the Kansai area is shown on page 8. This figure was modified from a map provided by the Earthquake Research Institute, University of Tokyo. The map has been augmented with additional acceleration and velocity recordings reported by the Committee of Earthquake Observation and Research in the Kansai Area. The maximum horizontal accelerations are those reported by several different agencies and represent either the maximum of the two peak horizontal accelerations or the vectoral combination of the two horizontal components. A maximum acceleration of 0.84g (g equals 981 cm/s/s) was reported in central Kobe, and several recordings in the range of 0.5g to 0.8g were reported in the heavily damaged Kobe-Ashiya-Nishinomiya area.
A preliminary estimate of the 250 cm/s/s (0.25g) and 500 cm/s/s (0.51g) iso-acceleration contours is overlain on the map on page 8. The contours show a distinct bulge toward the northeast, indicating that ground motions were higher northeast of the epicenter in the direction of rupture propagation principally because of source directivity (i.e., focusing). The 250 cm/s/s contour does not extend as far as Osaka, which is consistent with the lower intensity (JMA 4) reported for this area. It is interesting to note that the maximum accelerations in the Kyoto area are similar to those in the Osaka area, even though the former was reported to have a JMA intensity of 1 unit higher.
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Left: Comparison of Kobe Earthquake strong ground motion data with predictions from Campbell and Bozorgnia (1994) indicates that the Kobe strong motion was typical.
Right: Generalized Modified Mercalli Intensity (MMI) map for the January 17 event.
A comparison of the recorded maximum accelerations with predictions for an Mw6.9 strike-slip earthquake (page 9) indicates that the accelerations recorded during the earthquake are generally consistent with, though possibly slightly higher than, those recorded worldwide during other major strike-slip earthquakes of similar magnitude. The maximum accelerations are also similar on average to those recorded during the 1994 Mw6.7 Northridge, California, Earthquake. This comparison, along with other structural and geotechnical information that is available, would seem to suggest that the greater damage and the larger numbers of deaths, casualties, and homeless sustained during the Kobe Earthquake were likely caused by the aggregated effects of an extremely dense population, an older building stock, and the predominance of poor soils in the strongly shaken area.
Liquefaction and Other Ground Failures
The earthquake caused extensive ground failures, which affected buildings, underground infrastructure, the port, highways, all types of other facilities on soft or filled ground, and recovery efforts.
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Parking lot on reclaimed land near Ashiya. Sand covering the lot is evidence of large-scale liquefaction and sand ejection.
Ground failures occurred primarily because of liquefaction, the result of loose, water-saturated sand being shaken during an earthquake and assuming a semiliquid state. The areas affected by liquefaction were more heavily developed than any other earthquake-stricken region to date. Therefore, the lessons are valuable and will enhance our knowledge of liquefaction for both natural soils and reclaimed lands with high water tables.
The affected areas were located primarily along the coastline and the numerous watercourses in the general area of Kobe and the valleys between Kobe and Osaka. Widespread liquefaction, over many square kilometers, occurred around Kobe, Ashiya, Nishinomiya, Amagasaki, Osaka, Sakai, Izumiotsu, Kishiwada, and other areas around Osaka Bay. Massive liquefaction and lateral spreading took place in areas of reclaimed land and on the many artificial islands in the city of Kobe and Nishinomiya. Ejected sand from liquefaction covered much of the islands and interfered with rescue and recovery operations.
Similar effects were observed throughout the Kobe mainland along the coast, including parts of downtown. Typically, as in downtown Kobe, settlement and liquefaction of less than 50 centimeters were observed. That increased to as much as 3 meters along the coastline. The settlement caused severe damage to underground utilities, severing all services (gas, water, sewage) to large parts of the mainland and to all reclaimed islands, including the largest islands Rokko and Port. A month after the earthquake, these services had largely been restored to Rokko and Port islands.
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Failed quay wall in Nishinomiya. Lateral spreading and settlement of fill material have pushed the wall to the right. Note the backhoe for scale.
The most obvious and destructive liquefaction and related lateral spreading of soils and settlement occurred along the dozens of kilometers of seawalls along the port. Lateral spreading on the order of 3 (or more) meters and vertical settlement of 2 to 3 meters were observed along the seawalls of numerous islands, including Port and Rokko islands, and throughout the Port of Kobe. The largest settlements, and worst damage, seemed to be associated with the older reclaimed lands, such as the older parts of the port. The newer, engineered fills performed somewhat better than did the old fills, but with less than adequate results.
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Human-made island in Nishinomiya showing evidence of large-scale liquefaction, settlement, and lateral spreading
Numerous buildings on reclaimed land tilted because of ground settlement. These were primarily older, heavy concrete, industrial buildings, probably on mat foundations. The majority of industrial and other buildings on fill were supported on piles (most of these were lighter steel buildings). Most pile-supported buildings appeared to perform well; many multistory or large pile-supported buildings in areas where extensive liquefaction (and limited lateral spreading) occurred had little or no damage. Typically, the sidewalks of such buildings would settle 50 centimeters or more, but there would be no apparent damage to the buildings themselves. The same was generally true for newer highway structures supported on piles. However, the strong shaking may have exceeded the capacity of many pile foundations supporting elevated expressway and bridge piers, causing tilting or lateral movements (observed to be as much as 2 meters) of the piers. This often contributed to damage or collapse of the superstructures.
References
1. Pacheco, J. F., and L. R. Sykes. 1992. Seismic Moment Catalog of Large Shallow Earthquakes, 1900 to 1989. Bulletin of the Seismological Society of America, Vol. 82: 1306-1349.
2. Ellsworth, W. L. 1990. Earthquake History, 1769 – 1989. In The San Andreas Fault System, California. R. E. Wallace, ed. U.S. Geological Survey Professional Paper 1515: 153-187.
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The January 17, 1995 Kobe Earthquake
An EQE Summary Report, April 1995
Buildings
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This collapsed concrete building in Kobe completely blocked the street.
The number of buildings destroyed by the earthquake exceeds 100,000, or approximately one in five buildings in the strongly shaken area. An additional 80,000 buildings were badly damaged. The large numbers of damaged traditional-style Japanese residences and small, traditional commercial buildings of three stories or less account for a great deal of the damage. In sections where these buildings were concentrated in the outlying areas of Kobe, entire blocks of collapsed buildings were common. Several thousand buildings were also destroyed by the fires following the earthquake.
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Left: Badly damaged concrete shear wall building.
Right: Ground settlement in central Kobe.
Mid-rise commercial buildings, generally 6 to 12 stories high, make up a substantial portion of the buildings in the Kobe business district. The highest concentration of damaged mid-rise buildings was observed in the Sannomiya area of Kobe’s central business district. In this area, most of the commercial buildings had some structural damage, and a large number of buildings collapsed on virtually every block. Most collapses were toward the north, which was evidently the result of a long-period velocity pulse perpendicular to the fault. This effect has also been observed in other earthquakes. Failures of major commercial and residential buildings were noted as far away as Ashiya, Nishinomiya, and Takarazuka. In general, many newer structures performed quite well and withstood the earthquake with little or no damage.
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Mid-height collapse of a mixed-use building (built circa 1977) in Nishinomiya. This type of collapse was very common in this earthquake.
In the heavily damaged central sections of downtown Kobe, approximately 60% of the buildings had significant structural damage, and about 20% completely or partially collapsed. One survey of a 120,000-square-meter area in downtown Kobe (the Sannomiya area) found that 21 out of 116 buildings, or 18%, were visibly destroyed. Another report indicated that 22% of office buildings in a portion of the Kobe city center were unusable, while an additional 66% may need more than six months for complete restoration. City inspectors declared approximately 50% of the multifamily dwellings in Kobe as unsafe to enter or unfit for habitation, leaving more than 300,000 people homeless.
Age of construction, soil and foundation condition, proximity to the fault, and type of structural system were major determining factors in the performance of structures. Damage was worst in the areas bordering the port or streams and rivers-where soils were either poorly consolidated alluvial deposits or fill-and tended to be relatively minor in the foothills of Rokko Mountain, where either soils are very shallow or there are rock outcroppings. Loose and soft soils amplify ground motions in comparison to bedrock, especially ground motions within a certain frequency range. The duration of shaking also tends to be longer on such soils.
Structural damage directly resulting from soil failures was observed for smaller buildings without pile-supported foundations, but it did not appear to be the dominant problem for mid- and high-rise structures supported on piles that extended into dense soils or rock. Although hidden damage may be discovered at a later date, the performance of piles appeared to be good as long as substantial lateral soil displacement did not occur.
A survey of 24 commercial buildings being demolished in the central Sannomiya area of Kobe two months after the earthquake found the following breakdown of building types: 70% were frame type, 20% were shear wall type, and 10% were braced frame type. The breakdown of the frame-type structures included 50% nonductile concrete frame, 35% steel reinforced concrete (SRC) frame, 10% moment-resisting steel frame, and 5% steel frame with masonry infill. Of the shear wall buildings being demolished, 75% were concrete and one was unreinforced masonry. Several of the buildings being demolished were of multiple construction types.
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The ruins of the Ginza after the 1923 Great Kanto (Tokyo) Earthquake and fire.
Building Code
The first building code in Japan was introduced in 1926 after the 1923 Great Kanto Earthquake and ensuing fire devastated Tokyo. The regulations have been reviewed and amended several times over the years as the result of damage during subsequent strong-motion earthquakes. Bridge codes and codes for civil-engineering-type structures (e.g., quay walls) have undergone similar changes over the years.
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Buildings in central Kobe (Chuo Ward). In the foreground is the complete collapse of a two- or three-story traditional Japanese wood-frame building with a heavy tile roof. On the right is a six- or seven-story office building of 1960s’ or 1970s’ vintage. This reinforced concrete building is a typical example of a mid-height story collapse. The high rise to the left is a post-1981 office building that has no apparent damage. Ground settlement in the vicinity of these buildings was between 30 and 60 centimeters.
Since the 1926 code, Japan’s seismic codes have typically been as advanced as any in the world. Japanese engineers upgraded their standards after the 1968 Tokachi-oki Earthquake in northern Japan and California’s 1971 San Fernando Earthquake. In the early 1980s, laws and orders concerning seismic design methods for buildings were extensively revised. The current Japanese seismic provisions are specified in the Building Standard Law Enforcement Order by the Ministry of Construction (1981), and in the Standards for Seismic Civil Engineering Construction in Japan (1980). During the period between 1971 and 1980, some lessons learned in previous earthquakes were included in the design of major buildings, even though the requirements were not yet codified.
In the last several years, U.S. and Japanese professionals have been working together to understand seismic performance and to upgrade codes. Direct comparison of the codes for the two countries is difficult because of their different formats; however, comparative studies have suggested th
The current design philosophy in Japan is to keep seismic stresses within the elastic (non-damaging) range for earthquakes that can be expected to occur once or twice (moderate earthquakes) during a building’s life span, and to prevent collapse for larger, less frequent earthquakes. This means that for a moderate earthquake, the building is expected to have little or no damage. A similar philosophy is used in the United States, although, in general, more damage is considered acceptable for moderate-sized earthquakes.
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Concrete-frame structure with a mid-story collapse (Flower Road, Kobe).
Buildings are divided into four general types in the current Japanese code. In general, the guideline is: the larger the building, the more engineering and attention to quality of the seismic-force-resisting system required.
· Small buildings
For small, one- or two-story wood buildings and one-story buildings of other construction types, prescriptive construction requirements apply, and no explicit design is required. A similar practice is applied to wood-frame houses in the United States.
· Buildings less than approximately 30 meters high
For buildings with a regular configuration, prescriptive requirements apply. Additionally, a comparison of calculated and permissible stresses for the loads associated with a moderate earthquake (0.2g peak ground acceleration) must be made.
Irregularly shaped buildings are checked using the same requirements as for regular buildings. In addition, calculated drift (horizontal deflections) must be compared with allowable drifts, and the engineer must either (1) limit configurational irregularities, and meet minimum member size and detailing requirements that vary with construction material type, or (2) check the ultimate strength at each floor level versus the demands for a severe earthquake (1.0g peak ground acceleration). The demands for a severe earthquake are amplified for structures with large configurational irregularities, and reductions in demand are made to account for the ductility of the construction type. Note: Many Japanese buildings are quite irregular in their configurations when compared to U.S. buildings, which makes them much more difficult to design for earthquakes.
Steel buildings less than 13 meters high can be checked as regular buildings if the assumed moderate earthquake forces are amplified by 50%, and if the connections for the braces and the frames are designed to be stronger than the braces, columns, and beams.
Concrete buildings less than 20 meters high can be checked as regular buildings if they have a minimum combined shear wall and column area at each story. (For larger buildings, the minimum combined areas must be checked for each story in each direction.)
· Buildings between the approximate heights of 30 and 60 meters
These buildings are treated in the same manner as are irregular buildings under 30 meters high, except that an ultimate strength check at each floor level for a severe earthquake is required.
· Buildings more than approximately 60 meters high
These buildings require special permission from the Ministry of Construction, and a dynamic (computer) analysis must be performed for the severe earthquake scenario. In practice, these buildings are subjected to nonlinear analysis tech-niques. Peer review is also required.
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Undamaged reinforced concrete school in the Rokkomichi area. The building was used as a refuge center in the weeks following the earthquake.
It appears that, in general, buildings (other than smaller buildings) constructed using the above provisions of the current code performed well in the earthquake and protected life safety. However, a number of newer buildings, including high rises, were severely damaged and more damage may be uncovered as buildings are carefully evaluated. Structures that did poorly included older houses and smaller commercial buildings (both concrete and steel), and mid-rise concrete structures designed and constructed prior to the early 1980s using the same nonductile details that had been employed in high-seismic U.S. regions up until the early 1970s.
Reinforced Concrete-frame Buildings
Many of the mid-rise structures in Kobe were reinforced concrete-frame buildings of two types: The older ones were of nonductile concrete frame and the newer ones were of SRC frame.
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Modern parking garage in central Kobe (Chuo Ward). The building was undamaged. The structural system includes steel moment-resisting frames and concrete shear walls.
Dozens of reinforced concrete commercial buildings partially or completely collapsed at one or more floor levels. Typically, the buildings were 6 to 12 stories tall, and the failure often occurred within the middle third of the building height. One possible contributing factor was that the period of the strong ground motion pulses may have been in a range that generally coincided with higher vibration modes for these buildings. This would have tended to amplify stresses in the middle portion of the buildings.
Another possible factor was that there were changes in building strength or stiffness at these levels. For example, if shear walls or the steel columns encased in concrete that extend up from the foundation discontinue at a floor level, the strength and/or stiffness of the structure above that floor may be significantly less than at the floor below.
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Kobe City Hall, Sannomiya District. In the foreground is the old City Hall with a mid-story collapse. Behind it is the new City Hall, which exhibits signs of only minor damage.
The pre-1981 code required that a concrete-frame building exceeding six stories in height have SRC construction for the lower six stories as a minimum, although those buildings for which EQE engineers reviewed drawings always used SRC throughout the building height. The older code also specified design lateral loading that is more uniform over the height of the building, instead of having amplified forces near the top and reduced forces at the bottom, as is currently the practice in Japan and the United States. The older code’s practice results in weaker upper stories.
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Left: Mid-height collapse of a concrete-frame building.
Right: Soft story collapse of a restaurant in Kobe.
Instances of concrete structures with collapses or failures in the bottom (ground) floor were also fairly common. These failures typically resulted from soft or weak stories created by the need for garages and the desire to have numerous large open windows for storefronts at the bottom floor. The high land costs and general congestion in Japan exacerbate this problem. Very narrow multistory buildings with open storefronts are very common. Irregular distribution of shear walls or concrete frames resulted in substantial torsion, causing the structure to twist as well as sway due to earthquake loading.
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Left: Severely damaged reinforced concrete building with shear walls at Sannomiya Station, central Kobe. The building consists of a relatively simple (structurally and architecturally) upper portion on top of a complex lower portion
Right: Detail of the shear wall damage at the setback level.
The damage mode most commonly observed was a brittle shear failure of concrete column elements, leading to a pancake collapse of the floor level above. The brittle failures resulted from inadequate reinforcing details. In general, damaged columns were observed to have lateral reinforcing (referred to as ties) with relatively large spacings. These ties typically had hooks at their ends that were bent only 90o. Consequently, when the earthquake struck and the concrete cover outside the ties spalled or fell off, ties opened up and could not provide the confinement to the central concrete core. Complete failure quickly followed. Many of the damaged buildings in Kobe were also constructed with undeformed reinforcing bars.
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Diagram of typical detailing of ductile versus nonductile reinforced concrete columns.
Similar nonductile concrete construction has been the source of building and elevated highway or overpass collapses in past earthquakes, such as Southern California’s 1971 San Fernando and 1994 Northridge earthquakes. Current code requirements include closer and larger ties of deformed steel, 135o hooks that extend into the confined concrete, and cross-ties to supplement the rectangular ties around the perimeter bars. In addition, ties must be closely spaced and extend through the joint created by the beams and columns. Buildings possessing these enhanced detailing features are referred to as ductile moment frames. “Ductile” refers to a building’s ability to dissipate energy and deform without having brittle or sudden failure. In general, designs produced using the Japanese code tend to result in stronger columns and beams that are detailed in such a way that they have less ductility than do typical U.S. buildings in high seismic zones.
Hundreds of thousands of existing buildings of similar nonductile construction are present in seismically active areas throughout the world. Unless these buildings are retrofitted, many lives will be needlessly lost in future major earthquakes.
Reinforced Concrete Shear Wall Buildings
Many concrete shear wall buildings were severely damaged, and some had partial collapses. Many of these were multifamily residential structures where the shear walls had severe cracking, and horizontal displacements occurred at construction joints. One mid-rise concrete shear wall structure overturned and fell into the street. Some of the damaged structures had concrete walls in one direction only, and it appeared that the concrete frames had initially failed and allowed deformations, which caused damage to the shear walls in their weak or out-of-plane directions.
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Left: Mid-story collapse of one wing of a Kobe hospital.
Right: Interior of a badly damaged reinforced concrete building.
Failures of shear walls often led to permanent offsets of one floor relative to the next. This, in turn, led to damage of the frame columns. It is not clear whether the walls in these buildings were intended to function as the primary lateral-load-resisting elements, or whether they were intended to share this function with the reinforced concrete frames.
Again, the most severely damaged buildings generally appeared to be of older construction, dating from about 1950 to 1980. Newer structures with configurations that were not too irregular and did not have soft stories appeared to perform relatively well, generally ensuring the life safety of occupants.
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Badly damaged older reinforced concrete building in Sannomiya. Much of the damage is concentrated at structural discontinuities.
Many severely damaged shear wall buildings, including newer buildings, had unusual configurations by U.S. standards. These included dramatically varied architectural details, such as many irregular wall openings for windows, in the lower floors. Such architecture makes it much more difficult (and expensive) to properly design the structural system for earthquakes. Many severely damaged large commercial buildings had mixed-use occupancies-for example, stores in the lowest three floors and offices above. Typically, the failures occurred in the lower stories where the structural framing was more irregular in order to accommodate large, clear spaces.
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Badly damaged reinforced concrete column. Note the heavy longitudinal reinforcement, with scant shear reinforcement.
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Left: The two buildings in the foreground look similar, but their diverse performance indicates that they are probably structurally dissimilar. They may have been built at different times.
Right: The collapse of this building was so complete that it was impossible to deduce an obvious failure mode.
Reinforced Concrete-encased Steel-frame Buildings
As previously mentioned, a popular construction type in Japan for the last 25 years is a structural steel-frame building encased in reinforced concrete, termed steel reinforced concrete (SRC). Older SRC buildings commonly had solid structural steel elements in the frame connections, but used trusses constructed of smaller rolled steel shapes and plates in the center portions of the members. It appears that SRC construction generally performed better than did the older reinforced concrete-frame buildings; however, story collapses were noted in several SRC buildings. Some of the collapsed buildings thought to be concrete frame may actually be partially SRC. This is due to the requirement in the old code that a building exceeding six stories in height must use SRC in the lower six stories, but can use reinforced concrete framing in the upper stories. That results in a large stiffness and strength irregularity at the seventh floor. In newer construction of this type, the horizontal ties in the concrete encasement around the steel shapes are generally spaced closer together, and the newer structures tended to perform better.
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A reinforced concrete building (shear walls in the transverse direction) in Nagata Ward, Kobe. The bottom floor of this building collapsed.
Steel-frame Buildings
Generally, two types of steel-frame structures were observed, moment frames and concentric braced frames. Many smaller steel-frame structures in the central business district had severe damage or collapsed. In general, such structures appeared to have been minimally engineered. In many cases, these damaged buildings contained relatively light, flat-bar diagonal bracing members within the side walls, which buckled or were fractured at connections. In some cases, light steel moment frames in the front of the building were permanently distorted up to a few meters, causing the buildings to lean dangerously. Fracture of welded connections was observed in several steel-frame buildings in downtown Kobe.
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Left: Overview of the Ashiyama Seaside Town, consisting of steel-frame buildings along the shoreline of Ashiya, across from Rokko Island. The complex was built between 1975 and 1979.
Right: Typical units at the Ashiyama Seaside Town. Note the steel-truss elements forming a moment frame for the lateral-load-resisting structural system.
At the Ashiyama Seaside Town, 21 of 52 mid- and high-rise condominium structures built between 1975 and 1979 had severe damage to the structural steel framing. This innovative and unconventional structural system consisted of macro-steel moment frames in which the column and girder members were large steel trusses. Girders were typically located at every fifth floor. Housing units consisted of precast concrete assemblies that had been brought to the site by barge. Damage observed included the brittle fracture of square, tubular columns up to 50 centimeters wide with 5-centimeter-thick walls, and fracturing of steel wide-flange diagonal bracing elements. Residual horizontal offsets in column elements were observed to be as large as 2 centimeters in some cases. In general, it appeared that the brittle fractures had occurred in framing elements subjected to high combined tensile and shear stresses. In one of the units, six of the eight main steel columns forming the lateral-load-resisting system had fractured.
Despite the serious damage to the steel frames, the other elements (including windows) of the buildings did not appear to have significant damage. The steel framing in these modularly constructed buildings was located on the exterior of the building and was highly visible. In most high-rise steel structures in Japan, however, the framing is hidden by architectural elements and fireproofing. Consequently, there may be many other steel-frame structures where similar damage is present but hidden from view. That is what was observed with more than 140 modern steel-frame buildings in the Los Angeles area after the 1994 Northridge Earthquake. This may have been the reason that several steel-frame buildings with no obvious major structural steel damage were being demolished two months after the Kobe Earthquake.
A common Japanese method of constructing steel moment-frame buildings incorporates shop welding of beam stubs to the columns and field bolting of beam splices, away from zones of large strength demands. This practice has the advantage of allowing improved quality control at critical locations. However, this does not eliminate all the vulnerabilities inherent in beam-column connections, and some fractures like those observed following the Northridge Earthquake were reported. Although this method undoubtedly results in better-quality welds, it does not preclude the type of moment connection damage observed after the Northridge Earthquake.
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Left: Detail of the truss elements at the Ashiyama Seaside Town. Note the minimal damage to the concrete units. This photo shows the intersection of the vertical and horizontal frames.
Right: Fractured web in a diagonal truss element, Ashiyama Seaside Town.
In general, it appears that design philosophies and techniques used in steel construction in Japan result in structures with higher degrees of redundancy than in the United States. In typical Japanese new steel construction, all of the steel frames in buildings are included in the lateral-load-resisting system, whereas only a selected small number of frames in many structures in the United States have been detailed to resist seismic loads. Similarly, many braced-frame structures in Japan appear to have a large number of smaller braces, whereas in the United States it is common to see a smaller number of large braces. The redundancy provided by the frames and braces results in more locations where energy can be dissipated in a major earthquake. It is expected that such redundancy provides added resiliency for the buildings so constructed, and may have been a contributing factor to the relatively good performance of modern steel structures in the Kobe area.
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Fractured building column at Ashiyama Seaside Town.
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Left: Severe racking of a steel building in Sannomiya.
Right: Buckled diagonal brace in a parking garage.
The January 17, 1995 Kobe Earthquake
An EQE Summary Report, April 1995
Industrial Facilities
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Left: An example of a heavy concrete structure supported on a slab-on-grade. When the underlying soils liquefied and settled, the building settled and rotated. Buildings on piles typically performed much better.
Right:: Typical damage to Port of Kobe facilities. The large warehouse in the center was damaged when the interior slab settled. The center of the roof was supported by a column on the slab and was pulled in when the settlement occurred. Also note the severe displacement of the quay wall to the right.
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A damaged port crane on Rokko Island. The damage occurred when the quay wall moved to the left from the overall lateral spreading and settlement of the island. There was differential lateral movement of the two crane rails, pulling the crane legs apart. This phenomenon continued to occur for days after the earthquake.
Somewhere between 3% and 5% of Japan s industry is located in the area of strong ground shaking in and around Kobe. This includes most types of industry from light manufacturing to high-technology and heavy industry. As in most of Japan, and particularly Tokyo, much of the industry is concentrated near the port on landfill or very recent, soft soils. Due to strong ground motion amplification on soft soils and the extensive ground failures (caused by settlement and liquefaction) in these areas, damage to industry in the Kobe area was severe. Observed failures included extensive damage to large building foundations; all types of industrial buildings, equipment, and equipment systems; fire protection systems; racks; and inventory. The reduced ability to transport raw materials and finished goods to, from, and within the region will also greatly impact industry in the Kobe area. Industries affected include shipbuilding, steel plants, breweries, pharmaceutical firms, computer component manufacturing plants, and consumer goods production facilities.
Structural Damage
Access to industrial facilities in the region was very limited. The EQE team did have access to the facilities of some U.S. and European multinational companies, of which there is a large percentage in Kobe. In those facilities, structural damage was generally minor. Other damage, however, was not.
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Damage to unanchored laboratory equipment
The most severe structural damage, as well as associated damage to exterior storage areas and tank farms, occurred to industrial structures immediately adjacent to wharves and other retaining structures at navigable watercourses and other coastal facilities. Severe damage to industrial structures along shorelines was observed from Nishinomiya to western Kobe. Numerous structures settled more than 2 or 3 meters and were partially or fully submerged in water. Other structures partially collapsed or tilted severely because of foundation failures. Most tilted structures were probably buildings on mat foundations (usually pre-1980s vintage) without supporting piles.
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A heavily damaged steel plant in the Nishinomiya Port area. The most apparent damage was caused when the top third of the concrete stack sheared off. The top portion of the stack plummeted into a neighboring portion of the facility.
Wherever there was lateral spreading of soils and retaining structures along the shoreline, extensive damage was observed to tank farms (tank tilting), silos (tilting and collapses), cranes, stacks, and other such structures. Several tall, industrial, reinforced concrete stacks were leaning, and at least one collapsed. The collapsed stack was observed at a steel facility along the waterfront in Nishinomiya. The upper one-third of the stack broke off and embedded into the adjacent building.
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Left: These unanchored tanks fell off their supports.
Right:: Rocking and displacement of the quay wall was caused by lateral spreading of reclaimed land. This area, on the Nishinomiya Port, was flat before the earthquake.
Away from the shoreline, structural collapse of industrial facilities was relatively rare compared to the collapses in the housing, transportation, and commercial sectors. However, ground settlement caused extensive damage to the interiors of buildings, as well as to the infrastructure that is routed into the buildings. One case involved a pile-supported industrial building with a floating floor slab (or slab-on-grade). Although the peripheral piles successfully supported the structure, the floor slab failed when the underlying soil settled; this failure pulled down the columns that had supported the roof and caused it to collapse. Loading platforms, roads, storage and parking areas, various utilities, and other appurtenant structures were often observed to be severely damaged. Such damage was particularly severe on the numerous recently engineered islands in Osaka Bay. Many square kilometers of such land were observed to be affected.
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Severe damage to piers and warehousing along damaged quay walls. The quay walls have rotated toward the water, pulling the structures with them. The partially collapsed Hanshin Expressway can be seen in the background.
Steel manufacturers in the Kobe area were severely affected by the earthquake. One steel company Japan s fifth largest estimated that it would take months to resume full operations in its Kobe plant, while another steel company was unable a week after the earthquake to provide an estimate of restoration time. Both firms Kobe headquarters buildings were declared unsafe structures and could not be occupied. It was reported that four buildings at the first steel company collapsed, and the company was considering closing its Kobe industrial facilities and shifting operations north to its Kakogawa plant.
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A damaged concrete plant on reclaimed land east of Rokko Island. The conveyors are severely damaged, and several tanks have toppled.
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Left: Silos and port facilities on reclaimed land next to Rokko Island. Tanks in the foreground are leaning. The silos in the background are severely damaged.
Right:: Damage along the Port of Kobe shore. The severe rotation, lateral spreading, and settlement of the quay walls and fill material are typical of almost the entire developed border of the port. The building in the foreground has split into two parts.
Very large, multistory, reinforced concrete shear wall warehouses on Rokko Island and in central Kobe had very little damage. These buildings appeared to have superior lateral strength and were evidently designed considering the contribution of heavy storage to the design earthquake loads. The observed damage to contents resulted primarily from toppling of stacked goods or unanchored storage racks.
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Tanks in the port area. The ground shows signs of massive liquefaction and settlement. The tanks appear to be on pile-supported foundations.
Small- and medium-sized manufacturing firms were heavily damaged. Structural, fire, or contents damage affected more than 40% of the local knitted goods manufacturers and more than 90% of the synthetic leather shoe manufacturing facilities. City officials worry that production will now be moved to low-wage countries like China.
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Damage to facilities on reclaimed land, Nishinomiya Port. The quay walls have rotated and displaced, with the full surface dropping as much as 3 meters in some areas. In the background is a badly damaged bridge.
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These sake tanks appear to have survived the complete collapse of the traditional wood-frame building that housed them.
Heavy damage to the numerous liquor (sake) production facilities also occurred in the area stretching over Kobe s Higashi Nada and Nada wards and Nishinomiya. About one-third of the country s entire liquor production takes place in Kobe. Traditional wooden plants and storehouses collapsed, and some reinforced concrete structures had severe damage. At many of the major facilities, modern reinforced concrete buildings appeared to be undamaged. High-technology equipment housed in these structures, however, may have been severely damaged or destroyed, compounding business interruption losses.
Nonstructural Damage
Differential settlement and tilting of ground-supported slabs within buildings damaged equipment. In one case, the slab-on-grade in a pile-supported structure settled differentially between the pile caps. While not structurally significant, this resulted in extensive misalignment of manufacturing equipment. Re-leveling of the machinery was expected to take several weeks.
The shaking itself also caused damage to more sensitive equipment and equipment that was not properly anchored. For some plants, short-term fixes to equipment that had been affected by settlement involved jacking up machinery as much as 30 centimeters in order to achieve proper alignment. This procedure often caused significant delays in resuming production.
Breakage and leakage of fire sprinkler lines in manufacturing facilities were observed from Akashi to Osaka, resulting in extensive damage to manufactured goods, stock, and machinery. Virtually all of the leakage can be attributed to the failure of unbraced or inadequately braced piping. Fortunately, there were no fires reported at these facilities. Had proper bracing been in place, considerable damage and business interruption could have been avoided. It should be noted that it was not sufficient to simply clean up the water damage to resume operations. Repairs to the fire suppression systems also had to be completed.
One research facility located about 20 kilometers northeast of central Kobe had only minor structural damage to most of its buildings, and breakage of water and wastewater lines caused by minor ground settlement. Extensive damage to the contents, however, was noted. Unanchored lab hoods shifted, bookshelves and cabinets toppled onto desks, and computer equipment fell to the floor. There was extensive breakage of glass jars containing a variety of chemicals. Most of this damage could have been easily prevented with simple anchorage.
Other Causes of Business Interruption
Many of the industries affected by the earthquake are suppliers of parts for industries outside the affected area. Since much of Japanese industry relies on just-in-time delivery, damage to industry located in Kobe and the breakdown of the transportation system in the area are causing business interruptions to a variety of industries not directly affected by the earthquake. Business interruption insurance is typically not available in Japan, which will add significantly to the overall industrial losses.
One report stated that by January 21, at least four major electronics plants, six steel or heavy industrial plants, and three beverage plants had been shut down because of the earthquake. In some cases, facilities were closed because employees were unable to get to work, rather than because of severe physical damage to the facility itself. By Monday, January 23, nearly one week after the earthquake, many of these plants had reportedly resumed at least partial production. In some cases, the availability of water, gas, and power determined whether or not a business reopened.
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: Damage along the Port of Kobe shore. The severe rotation, lateral spreading, and settlement of the quay walls and fill material are typical of almost the entire developed border of the port. The building in the foreground has split into two parts
A reduction in work force availability is an important factor in industrial operations. Personal tragedy, loss of housing, and the debilitation of mass transit meant that many employees were unable to work right after the earthquake. This, in turn, means that many businesses will be unable to recover from the disaster in a timely manner, which may bankrupt some industrial concerns.
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The January 17, 1995 Kobe Earthquake
An EQE Summary Report, April 1995
Transportation
One of the most far-reaching and disturbing aspects of the earthquake was the severe and extensive damage to the transportation system. Kobe sits astride the principal transportation corridor between the central and southwestern parts of Japan s main island, Honshu. The corridor is less than 5 kilometers wide between Osaka Bay and the mountainous terrain on the north side of Kobe. Earthquake damage to highways, bridges, and rail systems left Kobe s city streets as the only land access along this corridor, resulting in major congestion and greatly impeded relief efforts. Many of these surface streets were also unusable, blocked by debris from collapsed structures and damaged by ground settlement. Use of alternative road or rail lines added hours to normally short