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.