Environmental Effects Of Kobe Earthquake
The Kobe Earthquake, also known as The Great Hanshin Earthquake occurred January 17th, 1995, resulting in the second most deadly and most damaging Japanese earthquake of the 20th century behind The Great Kanto Earthquake of 1923. The epicenter was 20 km southwest of Kobe, with a moment magnitude of 6.9. Figure 1 shows the location of the epicenter with respect to the island of japan. The main ground shaking lasted about 20 seconds as a result of a right lateral strike-slip faulting system.
A considerable portion of the areas infrastructure, which at the time was considered to be able to withstand a major earthquake was destroyed. The worst damage occurred in heavily developed regions built upon poor soils and artificial fill (loose and soft soil) leading to amplified ground motions and longer durations. (EQE) Kobe city was the most populous city in the region which suffered most infrastructure destruction and loss of life. Other densely populated regions affected include the towns of Ashiya and Nishinomiya, which lie east of Kobe. The event resulted in an estimated 6,000 casualties and about 41,500 people were injured (NIST). The majority of damage, both in loss of life and monetary loss was seen in building structures (89% of all …show more content…
casualties and 59% of the costs were due to building damage or collapse). (EEFIT)
Traditional Japanese timber frame construction before 1971 was a major contributor to damage and loss of life due to the Earthquake.
These were usually non-seismically engineered residential structures, in which the walls were not designed to resist the significant lateral loads created by the heavy tile roofs during shaking (EQE). The more engineered building structures, particularly reinforced concrete and some steel reinforced concrete (SRC) construction will be examined in this report. There was also damage to steel structures; they generally performed better due to their flexibility, but failure and collapse of steel buildings coincided with lack of code development as will be discussed regarding concrete buildings.
(NIST)
The development of building codes and regulations played a critical role in which buildings experienced most damage or total collapse. Much of the damage was seen in construction prior to lessons learned in previous Japanese earthquakes and certain code changes that are common with code developments for seismically prone areas around the world. The damage seen was not unique or surprising compared to building damage seen in other countries. Typical failures seen in reinforced concrete buildings due to lack of ductility such as classic short column shearing due to infill wall stiffening, poor detailing of column ties and rebar type, and structural irregularities causing soft first stories or torsional effects were common in causing building damage and providing lessons to be learned from this earthquake.
Building code and construction practices:
The key lessons to be taken from the 1995 Kobe Earthquake show the importance of advancements in building code provisions and construction practices in which buildings are to designed and detailed to the best of the engineering communities knowledge. The Kobe Earthquake showed that damage patterns closely paralleled the improvements of the seismic code over time. In general, concrete buildings were seen to have less damage as their age decreased.
Buildings constructed before 1981 accounted for the majority of structures that were damaged or collapsed (NIST). Building code development in Japan has historically been seen to follow major seismic events, starting with the first seismic design requirements being implemented after the 1923 Great Kanto earthquake. In 1971 significant changes were made to the Reinforced Concrete code to require closer spacing of column transverse reinforcement in response to the 1968 Tokachika, Japan and the 1971 San Fernando, California Earthquakes. In 1981 new design prcedures were inplemented to account for various structural periods and the addition of procedures to check for strength and ductility in a maximum expected event. Other key changes included provisions to limit or account for stiffness irregularities and torsional eccentricities, as well as minimum story stiffnesses to limit interstory drift.(NRCR)
In downtown Kobe, more than 60% of buildings had substantial damage, 20% of which had some type of partial or complete collapse. When surveying the RC building damage in the cities of Kobe and Ashiya, the following distribution of collapsed and damaged buildings was developed as seen in figure 2. (NIST) The distribution shows how the code developments reduced the number of damaged and collapsed buildings. The decreased transverse reinforcement spacing requirements and other improvement up to the 1971 code reduced the amount of collapsed and severely damaged buildings substantially. Buildings constructed after the significant code changes in 1981, saw little or no damage, collapse being completely avoided.
Damage to key structural systems:
Shear Walls:
Concrete shear walls generally performed well. It was found the buildings that avoided much damage relied on extensive concrete shear walls. (NIST) There were no shear wall collapses in larger structures, but smaller buildings that contained shear walls did see collapse. In the examples of shear walls that did fail, it was found that the walls were very lightly reinforced, allowing soft story failures to occur on the first level of many buildings a common deficiency seem in much of the RC construction examined.
Reinforced Concrete Columns and soft stories:
RC columns in buildings constructed before 1971 suffered the most damage. The failure mechanism observed most frequently was brittle shear failure of columns due to insufficient transverse reinforcement and bad detailing practices in general. While this type of failure occurred throughout the height of buildings, it was commonly seen in the first story where the building geometry led to soft story collapses. This was the most common failure mode of RC buildings. The first stories of these buildings were usually used as store fronts or parking garages, therefore having more open space and a reduced stiffness because of too few columns and often and increased story height. This stiffness irregularity causes the upper portion of the building to twist and move with respect to the first floor columns, leading to the critical damage and collapses observed. Figure 3 shows and example of McDonald’s restaurant that collapsed due to a soft story mechanism at the first floor. Other column failures involved “short column” failures, where partial heights of the columns were stiffened by masonry infill walls resulting in damaging high shearing forces from the increased stiffness (EQE) .
These brittle failure shear deficiencies were a consequence of the lack of code requirements and poor reinforcement detailing of the columns. Small and widely spaced ties with no additional crossties, failed to provide shear strength and proper confinement under significant levels of reverse cyclic loading. Tie spacing requirements were a main revision of the 1971 Building Standard Law, where minimum tie spacing was reduced from 300mm to 100mm in the plastic hinge regions of the columns and to 150mm in the rest of the column (NRCR). The ties were also usually only closed with 90° hooks rather than 135° hooks, which led to opening of the hoops after cover spalling, therefore unable to provide the confinement needed to achieve any level of ductility. Smooth, undeformed reinforcement was also found in many of these failures. This type of reinforcement was known to be used up to the 1960’s (NIST). All aspects of RC performance and behavior suffer from the lack of bond that the smooth bars fail to provide. The most harmful results of the decreased bond resulted in buckling of longitudinal reinforcement and reduced confinement from transverse ties. Figure 3 shows an example of a first story column brittle shear failure, notice the small size and wide spacing use of transverse reinforcement.
Mid-Hieght Story Collapse and SRC:
Partial or full collapse of stories within the height of the building (above the first story) were also seen. It is common in Japanese architecture to have building configurations that are highly irregular, these configurations usually pose a challenge when designing for seismic forces. (EQE) Common irregularities include the story plan layout, story mass differences, shear wall distribution, and changes in the number and size of openings in walls within a story and between successive stories, which lead to unsymmetrical stiffnesses and torsional eccentricities. Much of the time, failure came from brittle shear failure of the columns due to similar detailing issues previously discussed.
Abrupt changes in stiffness along the elevation of buildings also lead to mid-height collapse. These were seen in buildings with discontinuous shear walls and SRC columns. While both were causes of mid height collapse, the later was most prevalent. Steel reinforced concrete (SRC) construction was commonly seen in structures taller than 7 stories. SRC is a type of construction that consists of structural steel shapes encased in concrete, including transverse reinforcement. Earlier and more problematic versions of this construction consisted of latticework of interconnected steel plates encased in concrete.
It was common to use SRC columns up to the 7th story and transition to conventionally RC or steel shapes alone for the columns in the remaining stories above. Severe damage or collapse was seen in buildings with this type of construction used prior to 1981. Most often, the failures were seen at the transition of SRC columns and conventionally reinforced concrete columns (NRCR). This caused stiffness irregularities that were especially vulnerable to shaking. Early SRC construction also followed the same provisions for transverse reinforcement detailing as those of columns built prior to 1971. This meant the ties were widely spaced, so these columns were poorly confined.
A good example of a mid-story collapse was seen in the Kobe City Hall Annex. The Annex consisted of two buildings, one built in the 1980’s and an older building constructed in 1957, both connected by walkways. The newer and taller building sustained very little damage, but the 6th story columns and shear walls of the older building had failed in shear. The complete 6th story collapsed and came to rest with a permanent offset of 2 meters. Failure of this building was caused by increased shear due to pounding forces from interconnected buildings, but more importantly from the abrupt change in strength and stiffness of the columns at the sixth floor. This is where column construction was transitioned from SRC to conventionally reinforced concrete.
Although the older SRC construction methods did cause problems leading to severe damage or collapse, SRC construction did prove to prevent some collapses that would have surely occurred if the entire building was constructed with conventional RC. In general SRC buildings performed better than RC buildings, a main reason being that the RC buildings were commonly made of more dated, non-ductile concrete fames, while the newer buildings, were built with the more modern SRC techniques with more recent code provisions in mind. This meant hot rolled steel sections were used rather that steel plate latticework, and adequate confining reinforcement was provided. This is yet another example of how code progression improved the building performance in this earthquake.
Conclusions:
The effects of the 1995 Kobe earthquake presented need for more research into the assessment of seismic vulnerability of RC structures as well retrofit and rehabilitation of older, code deficient buildings with non-ductile detailing of lateral load resisting wall and moment frame systems. The necessity of ductility requirements in building seismic codes were prominently demonstrated as a result of the Kobe earthquake. In both steel and concrete structures, brittle failures resulted in severely damaged or collapsed buildings, with deadly consequences.
Ductility in seismic design of structures was first introduced into the Japanese building code in 1981, buildings constructed before this as well as before other major code revisions in 1971 accounted for most all of the extreme damage and collapse. As with main lessons learned in most earthquakes around the world, advancement and updating of building codes is the most important factor in limiting damage buildings will be subjected to during an earthquake.
A considerable portion of the areas infrastructure, which at the time was considered to be able to withstand a major earthquake was destroyed. The worst damage occurred in heavily developed regions built upon poor soils and artificial fill (loose and soft soil) leading to amplified ground motions and longer durations. (EQE) Kobe city was the most populous city in the region which suffered most infrastructure destruction and loss of life. Other densely populated regions affected include the towns of Ashiya and Nishinomiya, which lie east of Kobe. The event resulted in an estimated 6,000 casualties and about 41,500 people were injured (NIST). The majority of damage, both in loss of life and monetary loss was seen in building structures (89% of all …show more content…
casualties and 59% of the costs were due to building damage or collapse). (EEFIT)
Traditional Japanese timber frame construction before 1971 was a major contributor to damage and loss of life due to the Earthquake.
These were usually non-seismically engineered residential structures, in which the walls were not designed to resist the significant lateral loads created by the heavy tile roofs during shaking (EQE). The more engineered building structures, particularly reinforced concrete and some steel reinforced concrete (SRC) construction will be examined in this report. There was also damage to steel structures; they generally performed better due to their flexibility, but failure and collapse of steel buildings coincided with lack of code development as will be discussed regarding concrete buildings.
(NIST)
The development of building codes and regulations played a critical role in which buildings experienced most damage or total collapse. Much of the damage was seen in construction prior to lessons learned in previous Japanese earthquakes and certain code changes that are common with code developments for seismically prone areas around the world. The damage seen was not unique or surprising compared to building damage seen in other countries. Typical failures seen in reinforced concrete buildings due to lack of ductility such as classic short column shearing due to infill wall stiffening, poor detailing of column ties and rebar type, and structural irregularities causing soft first stories or torsional effects were common in causing building damage and providing lessons to be learned from this earthquake.
Building code and construction practices:
The key lessons to be taken from the 1995 Kobe Earthquake show the importance of advancements in building code provisions and construction practices in which buildings are to designed and detailed to the best of the engineering communities knowledge. The Kobe Earthquake showed that damage patterns closely paralleled the improvements of the seismic code over time. In general, concrete buildings were seen to have less damage as their age decreased.
Buildings constructed before 1981 accounted for the majority of structures that were damaged or collapsed (NIST). Building code development in Japan has historically been seen to follow major seismic events, starting with the first seismic design requirements being implemented after the 1923 Great Kanto earthquake. In 1971 significant changes were made to the Reinforced Concrete code to require closer spacing of column transverse reinforcement in response to the 1968 Tokachika, Japan and the 1971 San Fernando, California Earthquakes. In 1981 new design prcedures were inplemented to account for various structural periods and the addition of procedures to check for strength and ductility in a maximum expected event. Other key changes included provisions to limit or account for stiffness irregularities and torsional eccentricities, as well as minimum story stiffnesses to limit interstory drift.(NRCR)
In downtown Kobe, more than 60% of buildings had substantial damage, 20% of which had some type of partial or complete collapse. When surveying the RC building damage in the cities of Kobe and Ashiya, the following distribution of collapsed and damaged buildings was developed as seen in figure 2. (NIST) The distribution shows how the code developments reduced the number of damaged and collapsed buildings. The decreased transverse reinforcement spacing requirements and other improvement up to the 1971 code reduced the amount of collapsed and severely damaged buildings substantially. Buildings constructed after the significant code changes in 1981, saw little or no damage, collapse being completely avoided.
Damage to key structural systems:
Shear Walls:
Concrete shear walls generally performed well. It was found the buildings that avoided much damage relied on extensive concrete shear walls. (NIST) There were no shear wall collapses in larger structures, but smaller buildings that contained shear walls did see collapse. In the examples of shear walls that did fail, it was found that the walls were very lightly reinforced, allowing soft story failures to occur on the first level of many buildings a common deficiency seem in much of the RC construction examined.
Reinforced Concrete Columns and soft stories:
RC columns in buildings constructed before 1971 suffered the most damage. The failure mechanism observed most frequently was brittle shear failure of columns due to insufficient transverse reinforcement and bad detailing practices in general. While this type of failure occurred throughout the height of buildings, it was commonly seen in the first story where the building geometry led to soft story collapses. This was the most common failure mode of RC buildings. The first stories of these buildings were usually used as store fronts or parking garages, therefore having more open space and a reduced stiffness because of too few columns and often and increased story height. This stiffness irregularity causes the upper portion of the building to twist and move with respect to the first floor columns, leading to the critical damage and collapses observed. Figure 3 shows and example of McDonald’s restaurant that collapsed due to a soft story mechanism at the first floor. Other column failures involved “short column” failures, where partial heights of the columns were stiffened by masonry infill walls resulting in damaging high shearing forces from the increased stiffness (EQE) .
These brittle failure shear deficiencies were a consequence of the lack of code requirements and poor reinforcement detailing of the columns. Small and widely spaced ties with no additional crossties, failed to provide shear strength and proper confinement under significant levels of reverse cyclic loading. Tie spacing requirements were a main revision of the 1971 Building Standard Law, where minimum tie spacing was reduced from 300mm to 100mm in the plastic hinge regions of the columns and to 150mm in the rest of the column (NRCR). The ties were also usually only closed with 90° hooks rather than 135° hooks, which led to opening of the hoops after cover spalling, therefore unable to provide the confinement needed to achieve any level of ductility. Smooth, undeformed reinforcement was also found in many of these failures. This type of reinforcement was known to be used up to the 1960’s (NIST). All aspects of RC performance and behavior suffer from the lack of bond that the smooth bars fail to provide. The most harmful results of the decreased bond resulted in buckling of longitudinal reinforcement and reduced confinement from transverse ties. Figure 3 shows an example of a first story column brittle shear failure, notice the small size and wide spacing use of transverse reinforcement.
Mid-Hieght Story Collapse and SRC:
Partial or full collapse of stories within the height of the building (above the first story) were also seen. It is common in Japanese architecture to have building configurations that are highly irregular, these configurations usually pose a challenge when designing for seismic forces. (EQE) Common irregularities include the story plan layout, story mass differences, shear wall distribution, and changes in the number and size of openings in walls within a story and between successive stories, which lead to unsymmetrical stiffnesses and torsional eccentricities. Much of the time, failure came from brittle shear failure of the columns due to similar detailing issues previously discussed.
Abrupt changes in stiffness along the elevation of buildings also lead to mid-height collapse. These were seen in buildings with discontinuous shear walls and SRC columns. While both were causes of mid height collapse, the later was most prevalent. Steel reinforced concrete (SRC) construction was commonly seen in structures taller than 7 stories. SRC is a type of construction that consists of structural steel shapes encased in concrete, including transverse reinforcement. Earlier and more problematic versions of this construction consisted of latticework of interconnected steel plates encased in concrete.
It was common to use SRC columns up to the 7th story and transition to conventionally RC or steel shapes alone for the columns in the remaining stories above. Severe damage or collapse was seen in buildings with this type of construction used prior to 1981. Most often, the failures were seen at the transition of SRC columns and conventionally reinforced concrete columns (NRCR). This caused stiffness irregularities that were especially vulnerable to shaking. Early SRC construction also followed the same provisions for transverse reinforcement detailing as those of columns built prior to 1971. This meant the ties were widely spaced, so these columns were poorly confined.
A good example of a mid-story collapse was seen in the Kobe City Hall Annex. The Annex consisted of two buildings, one built in the 1980’s and an older building constructed in 1957, both connected by walkways. The newer and taller building sustained very little damage, but the 6th story columns and shear walls of the older building had failed in shear. The complete 6th story collapsed and came to rest with a permanent offset of 2 meters. Failure of this building was caused by increased shear due to pounding forces from interconnected buildings, but more importantly from the abrupt change in strength and stiffness of the columns at the sixth floor. This is where column construction was transitioned from SRC to conventionally reinforced concrete.
Although the older SRC construction methods did cause problems leading to severe damage or collapse, SRC construction did prove to prevent some collapses that would have surely occurred if the entire building was constructed with conventional RC. In general SRC buildings performed better than RC buildings, a main reason being that the RC buildings were commonly made of more dated, non-ductile concrete fames, while the newer buildings, were built with the more modern SRC techniques with more recent code provisions in mind. This meant hot rolled steel sections were used rather that steel plate latticework, and adequate confining reinforcement was provided. This is yet another example of how code progression improved the building performance in this earthquake.
Conclusions:
The effects of the 1995 Kobe earthquake presented need for more research into the assessment of seismic vulnerability of RC structures as well retrofit and rehabilitation of older, code deficient buildings with non-ductile detailing of lateral load resisting wall and moment frame systems. The necessity of ductility requirements in building seismic codes were prominently demonstrated as a result of the Kobe earthquake. In both steel and concrete structures, brittle failures resulted in severely damaged or collapsed buildings, with deadly consequences.
Ductility in seismic design of structures was first introduced into the Japanese building code in 1981, buildings constructed before this as well as before other major code revisions in 1971 accounted for most all of the extreme damage and collapse. As with main lessons learned in most earthquakes around the world, advancement and updating of building codes is the most important factor in limiting damage buildings will be subjected to during an earthquake.