Post: Friday February 3, 2012
Earthquake resistance of buildings in Japan and risk of earthquake depending on the area
In Japan, Building Standard Act has been strictly reviewed every time we experienced a large earthquake.
All buildings are strictly required to have an earthquake resistance structure. Without rigorous compliance to earthquake proof standard set by a low, one can never get an approval of construction of a building from a public administration office in Japan.
It is said that the collapse ratio of the building as a result of a powerful earthquake is extremely low in Japan compared to any other countries.
Transition of building regulations and earthquake resistance standard by generations
1st generation – buildings before 1971
2nd generation – buildings after 1971 and prior to 1981
3rd generation – buildings after 1981
4th generation – buildings after 2000 (As to wooden structures)
|1971||After the earthquake off the shore of Tokachi in 1968, the standard for tie-hoops of RC structure was tightened.|
|1981||After experiencing the disaster caused by the earthquake off the shore of Miyagi Prefecture in 1978, the Building Standard Act was revised and the New Anti-seismic Design Code came in to effect.
The new standard focuses not only to prevent buildings from collapse caused by earthquakes but also to secure safety of people inside of buildings.
All buildings are expected:
Old standard: to be capable of resisting an impact of JMA seismic scale 5
New standard: not to collapse in an earthquake of JMA seismic scale upper 6 or higher
* The New Anti-seismic Design Standard has been applied to all buildings requesting approval of construction as of June 1st, 1981.
|1995||From the experience of the Hanshin-Awaji earthquake in 1995, Act for Promotion of Renovation for Earthquake-Resistant Structures (an regulation that promotes earthquake-resistance structure on existing buildings) came in to effect.
It required larger buildings than a certain size as specified in this regulation a duty to strive to assess an earthquake resistance level and renovate the structure of buildings in order to secure the same of higher earthquake proof performance level as the New Anti-seismic Design Standard required.
|2000||The Building Standard Act was revised in order to improve safety of wooden buildings and to clarify anti-seismic performance level as well as specifications and forms of building foundation. The ground investigation became virtually mandatory.|
|2009||The Licensed Architect Act was revised as a result of the falsification matter of structural calculation discovered in 2005. The new law required all larger buildings than set on the standard to be structurally designed by a constructional design 1st class registered architect.|
Most of buildings collapsed in the Hanshin-Awaji earthquake in 1995 were built before the New Anti-Seismic Design Standard came in effect. As explained here, the standard of earthquake resistance has been tightened every time the law was revised.
Level of earthquake resistance by structure of building
|Earthquake Resistant Structure||It is the most common structure for detached houses in Japan. All buildings built after 1981 must conform to the New Anti-seismic Structure Standard requiring buildings to have an earthquake resistance structure.
With a seismic resistance structure, main structure of buildings, namely, posts, walls and floors, receive seismic motions. The structure can be divided in to Rigid Structure that is to build ridged structured buildings in order to prevent from collapse and Flexible Structure that main structural parts of which bow flexibly in order to spread the force of seismic motions.
It places damping walls that absorb seismic energy within the building in order to minimize seismic motions. Damping structures can be divided in to Active type that uses energy such as electricity and Passive type that uses physical forces.
|Seismic Isolation Structure||The structure is commonly used for high-rise buildings. It places quake-absorbing devices (isolator) such as laminated lubber that blocks seismic motions from reaching the building, as a part of the foundation of the building. The quake-absorbing devices include laminated lubber, lead, spring, damper, ball bearings, etc. Furthermore, newly invented construction methods use combination of these materials.
The structure reduces the seismic intensity down to 1/3 to 1/5 (less than half) compared to that of seismic resistance structure.
The apartment and office buildings with damping structure or seismic isolation structure are more secure against earthquakes compared to general anti-seismic structure.
Type of building structures used for main part of buildings
|Wooden Structure||It refers to the buildings that primarily use woods as a main material.
With the structure, posts and beams of a building serve as core parts of the building. Many of detached houses in Japan are made of woods.
|Steel Structure (S||It refers to the buildings that primarily use steel materials for the framework of a building. Steel structure is suitable for buildings with large spaces.|
|Reinforced Concrete Structure (RC)||It refers to the buildings that primarily use concrete with iron reinforcing bars inside for the framework of a building.
The structure takes advantage of both reinforcing bars and steel frame. With Reinforced Concrete Structure, “reinforcing bars” that are steel material bars with tolerance against stretching forces reinforces “concrete” that resists compressive forces of the weight of a building.
|Steel Reinforced Concrete Structure (SRC)||It refers to the buildings that primarily use concrete with iron reinforcing bars inside and iron frames for the framework of a building.
The structure takes advantage of both steel structure and reinforced concrete structure and is often referred as “SRC structure”. With SRC structure, Iron frames construct poles and beams, which will be reinforced with iron reinforcing bars, later filled with concrete.
The structure is often applied for high-rise buildings because it provides excellent seismic resistance and is also solid and durable.
Risk of earthquake depending on the area in Tokyo
cf. Bureau of Urban Development Tokyo Metropolitan Government
Risk of disasters differs depending on many different factors such as ground, shape of the land, density of buildings, etc.
In Tokyo, the areas with higher risk of building collapses can be found on the ground that is categorized as alluvial plain or lowland valley such as alluvial fan, natural levee zone or delta. Furthermore, it is said that so-called local town areas along Arakawa River and Sumidagawa River where old wooden houses and light steel structure buildings gather densely have higher risk of fire. More precisely, southern Adachi ward, Arakawa ward, Eastern Taito ward, Western Katsushika ward, Sumida ward and Northern Koto ward areas are considered more dangerous due to the reason described above.
The areas such as Chiyoda ward, Minato ward and Shibuya ward where many foreign companies gather and many foreign expats reside are considered as relatively safe areas.
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