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Overview of Structural Damage
By Charles Scawthorn
|Kobe is situated to the west of Tokyo on the southern
of Honshu Island.
Kobe is the western most part of the greater urban expanse of the Yodogawa Basin centered on Osaka and bordering on Osaka Bay. The entire regional population is in excess of 10 million. Kobe (population 1.5 million) lies on the bay margin and lower slopes of Rokko Mountain, in a 2~4 Kilometer wide densely built-up urban corridor between Osaka and the main routes to southern Honshu. Several smaller cities (Ashiya, Nishinomiya) lie between Kobe and Osaka. The Kobe corridor is the main transportation corridor between central and southern Honshu, with several major highways and four railroads. Kobe is one of the great ports of the world which, in recent years, has seen major development of offshore manmade islands (Port Island, Rokko Island) in the Osaka Bay margins. Heavily damaged areas include most of Kobe, Ashiya and Nishinomiya, particularly in the lower elevations, and Awaji Island.
While the official name of the event is the Hyogoken Nambu earthquake, the disaster is increasingly being referred to in Japan as the Hanshin Daishinsai (Osaka-Kobe Great Earthquake Disaster). This region is not as seismically active as the Tokyo area and some other parts of Japan, but has experienced magnitude 7 or greater events in historical times (e.g., 1596). A 1916 M6.1 event occurred at almost the same epicentral location.
The most recent major revision of the Building Code was in 1981, and design requirements for buildings in this area are similar to those for Tokyo. Approximately 75,000 buildings were reported destroyed by the earthquake.
The great majority of the destroyed buildings are nonengineered wood frame traditional Japanese buildings, which are typically unbraced or lightly braced one-and two-story post and beam construction with heavy tile roofs. As in past earthquakes, this event resulted in the complete collapse of the first story of these houses and light commercial structures, if not both stories. The 1981 code introduced more stringent seismic requirements, so that newer buildings performed much better. Most recently, U.S. -style stud-wall construction has been entering the Japanese market, although it is not yet a major influence.
|The sixth floor of the old
Kobe City Hall collapsed.
(Photo courtesy of C.
Well in excess of 100 midrise buildings constructed during the 1960's and 1970's from reinforced concrete were observed to have failed, in many cases catastrophically. Most failures appear to have been shear failures of columns, which were observed to have very light transverse reinforcement. A number of mid-height single story pancake collapses were observed. An example was the 1960's vintage eight-story Kobe City Hall, which sustained a complete collapse of the sixth floor, while the neighboring 1980's vintage 16-story New City Hall was undamaged.
Relatively few steel buildings were observed to have significant damage, with the exception of the 51 building Ashiyahama Seaside Town complex. These buildings, of late 1970's vintage, range in height from about 10 to 29 stories, and are of nonconventional construction. That is, the structure consists of large trussed steel column-beam frames, with typically a two-legged bent per building. A number of 500 mm square steel box columns (50 mm wall thickness) were observed to have totally fractured.
A major impact of the earthquake was damage to the transportation system. Kobe is the main transportation corridor between central and southern Honshu. Due to damage to roads and other land transportation, the only land access along this corridor was via city streets, which resulted in major congestion, greatly exacerbating relief efforts. Bypassing by alternative road or rail lines added hours to travel times.
|A portion of the Shinkansen (Bullet Train) that collapsed
during the earthquake. (Photo courtesy of C. Scawthorn.)
Highways and Roads
Only two limited access highways exist in the narrow transportation corridor. The
Hanshin Expressway is carried on single large hammerhead reinforced concrete piers, many
of the concrete sections failing in shear and/or flexure over a 20 kilometer length. Along
the harbor shore is the Wangan (Harbor) Expressway, a newly elevated highway generally of
steel and crossing a number of navigable waterways on major crossings. Many of the steel
girders appear to have jumped from their beam seats and were askew, although few had
|A detailed view of a sheared
column which supports the
Shinkansen. (Photo courtesy
of C. Scawthorn.)
Rail facilities were particularly hard hit in this earthquake (for further information on damage to underground rapid transit facilities, see article by Thomas O'Rourke on page 6). Three main lines (JR West, Hankyu and Hanshin) run through the corridor, in general on elevated embankments, and all sustained embankment failures, overpass collapses, distorted rails and other severe damage. In Kobe, the Shinkansen (Bullet Train) is generally in a tunnel through Rokko Mountain. No information was available regarding the tunnel. At the east portal of the tunnel, the line is carried on an elevated viaduct, built in the 1960's. For a length of three kilometers, this viaduct was severely damaged, with a number of the longer spans collapsing. In general, these collapses were due to shear failure of the supporting columns.
Ports and Harbors
The Port of Kobe, one of the largest container facilities in the world, sustained major damage. Rokko and Port Islands experienced widespread liquefaction and, on Rokko Island, a number of gantry container cranes failed due to crane rail distortions, with one collapse. Shipping in general had to be diverted to other ports (Osaka, Nagoya, Yokohama), while passenger and some vehicle ferry service was gradually established in the days following the earthquake. The Port of Osaka experienced almost no damage.
Kansai International Airport was only recently completed (1994) on a manmade island some 30 kilometers to the southeast of the epicenter. Itami is the former international airport for Osaka, and now handles much of the domestic traffic. It lies about 10 kilometers east of the heavily damaged area. Neither airport appears to have sustained significant damage.
Performance of lifelines was varied in this event, with electric power and telecommunications maintaining system functionality, but water, wastewater and gas utilities losing service to most of Kobe. (For specific statistics on lifeline restoration, see article by Masanobu Shinozuka on page 1.)
Electric power performed very well in the earthquake, with very little reduction in service. Electric distribution was available to all parts of the cities, except where overhead distribution poles were damaged by collapsing buildings.
Telecommunications also performed very well in the earthquake, with very little reduction in service. No information was available regarding damage to telephone exchange buildings. Telephone service was available in the most heavily damaged areas the day following the earthquake.
Underground water pipelines sustained severe damage in the earthquake, with numerous breaks resulting in general lack of service in Kobe, Ashiya and Nishinomiya.
The gas system sustained numerous breaks in its underground distribution system, with general curtailment of service. The population in the heavily impacted areas were informed to plan for no gas service for about two months. Information on other gas system facilities was not available, although a large gas holder in Kobe, near the port, was observed to have no obvious structural damage.
The Kobe Fire Department (KFD) had minimal staffing on duty at the time of the earthquake. Initial actions included recall of off-duty personnel, and responding to fire calls. Approximately 100 fires broke out within minutes, primarily in densely built-up lowrise areas of the central city (Nada, Higashinada, Hyogo and Suma wards) which are comprised of mixed residential-commercial occupancies, predominantly of wood construction. The total number of fires for January 17 is 142. Water for firefighting purposes was available for two to three hours, including use of underground cisterns. Subsequently, water was available only from tanker trucks. Wind was calm, and fire advance was relatively slow. In a number of cases, fires are observed to have stopped at relatively narrow fire breaks (eg., 10 m) or, in at least one case, at a high-rise apartment building (concrete walls but well fenestrated, so that the reason for this building not being consumed is unclear). Final burnt area in Kobe is estimated at 1 million sq. meters, with 50% of this in Nagata ward.
Some of the material reported herein is based upon work supported in whole or in part by the National Science Foundation, tbe New York State Science and Technology Foundation, the U.S. Department of Transportation and other sponsors. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of MCEER or its sponsors.