The March 11, 2011, earthquake off the Pacific coast of Japan, known as the Tohoku or the Great East Japan Earthquake, registered 9.0 on the moment magnitude scale, with the epicenter approximately 80 miles east of Sendai, Japan. The focal depth was approximately 20 miles. It was the most powerful known earthquake to have hit Japan. The earthquake triggered extremely destructive tsunami waves up to 133 feet high at locations along the northeast coast of Japan. The tsunami caused a number of accidents at the Fukushima Daiichi nuclear power plant, which Japan still needs to recover from. The focus of this article will be on structural damage caused by the earthquake, particularly to concrete structures, rather than structural damage caused by the tsunami.
I had the opportunity to survey damage caused by the earthquake (and the tsunami) in late June as a member of an earthquake investigation team assembled by the Precast/Prestressed Concrete Institute. I met with four officials of the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) of the Government of Japan, who are directly involved in the development of the Building Standard Law of Japan (BSLJ), the rough equivalent of our International Building Code (IBC). Tomohiro Hasegawa, one of the officials I met with, shared with me a presentation he had made on the findings of field survey teams jointly organized by the National Institute for Land and Infrastructure Management (NILIM) and the Building Research Institute (BRI) of Japan. His conclusions:
- Structural damage to buildings was not so significant on the whole, even in areas where seismic intensity of more than VI was recorded.
- Thus the current seismic design in BSLJ is generally appropriate for earthquake-related damage mitigation.
- Most of the buildings suffering from seismic damage were found to be designed by the old seismic design method, which had been valid until 1981.
The observations of the PCI investigation team coincided with the first conclusion above.
The document usually referred to as the Japanese Code is the Building Standard Law, including its enforcement order, which contains requirements enabling implementation of the Building Standard Law.
While the BSLJ specifies loads and allowable stresses, and certain minimal requirements for detailing of members, details of structural design (such as methods of structural analysis and the proportioning of members) are specified in Structural Standards issued by the Architectural Institute of Japan (AIJ). These standards, prepared separately for each structural material, serve as supplements to the law. The AIJ Standard for reinforced concrete bears roughly the same relationship to the Building Standard Law as the ACI 318 Building Code Requirements does to the IBC.
The 1968 Tokachi-oki earthquake caused significant damage to buildings designed in accordance with building regulations then in force. The Building Standard Law underwent a partial revision as a result. More importantly, in 1971, a large-scale revision of AIJ Standards ensued, incorporating ultimate strength design in shear of reinforced concrete beams and columns, including much more stringent shear reinforcement requirements. Post-1971 concrete structures performed much better in the 6.9 Kobe earthquake of 1995 and the Great East Japan earthquake than their pre-1971 counterparts, primarily due to the improved shear design of columns.
The Miyagiken-oki earthquake of 1978 caused damage as severe as the 1968 earthquake. This led to a revision of the enforcement order of the Building Standard Law, which began to be enforced from June 1981. The revision introduced a requirement of two-phase design for most buildings.
The purpose of the first phase design (essentially the same design as prescribed by the previous Building Standard Law) is to protect a building against loss of function in earthquakes that can occur several times during its life. Such earthquake motions may cause peak ground accelerations of 0.08 to 0.10 g. This design objective is assumed to be achieved by adoption of the traditional level of seismic force and the traditional allowable stress design method.
The second phase design is intended to ensure safety against an earthquake that could occur once in the lifetime of a building. Such an earthquake motion may cause ground accelerations of 0.3 to 0.4 g. Traditional seismic design assumed that buildings would survive severe earthquakes as a result of built-in overstrength and ductility. However, this was not expressly required to be confirmed.
Post-1981 structures designed by the two-phase procedure performed well in the 1995 Kobe and the Great East Japan earthquakes; severe damage in such structures was relatively rare.
The Disaster Prevention Research Institute of Kyoto University issued some revealing statistics following the Kobe earthquake.Figure 1 shows that the strongest correlation of damage was with the age of the structure. The relative lack of earthquake damage in the Tohoku earthquake was largely because most structures in affected areas were of newer vintage.
Unlike the U.S., Japan has not been a believer in 3-year code updates. After 1981, the framework of Building Standard Law was significantly revised for the first time in 1998, introducing performance-based regulations wherever feasible. In June 2007, the latest amended Building Standard Law came into force in Japan. This change was prompted by the discovery in November 2005 of problems resulting from the falsification of structural calculations. Structural design provisions did not change.
Both the 1995 Kobe experience and the 2011 Great East Japan experience have shown significant changes in the Building Standard Law in 1971 and 1981 to have been impressively effective in preventing damage to newer buildings.
S.K. Ghosh Associates Inc. is a seismic and code consulting firm located in Palatine, Ill., and Aliso Viejo, Calif. President S. K. Ghosh, Ph.D., is active in the development and interpretation of national structural code provisions. He can be contacted at email@example.com, or at www.skghoshassociates.com.