Seismic Design Codes for Buildings in Japan

Review:

Seismic Design Codes for Buildings in Japan

Hiroshi Kuramoto

Department of Architecture and Civil Engineering, Toyohashi University of Technology

1-1 Hibarigaoka, Tempaku, Toyohashi, Aichi 441-8580, Japan

E-mail: kura@tutrp.tut.ac.jp

[Received August 18, 2006; accepted September 27, 2006]

Two revised seismic design codes in the BuildingStandard Law of Japan, which were revised in 1981and 2000, are simply reviewed with the transition ofJapanese seismic design code in this paper. The cen-tral feature of the seismic code revised in 1981 wasthe introduction of a two-phase earthquake design.Allowable stress design was employed for first-phaseearthquake design targeting the safety and serviceabil-ity of buildings during medium-level earthquake ac-tivity. Second-phase earthquake design, which is ul-timate strength design, was added to provide safetyagainst severe earthquake motion. On the other hand,the seismic code revised in 2000 precisely defines per-formance requirements and verification based on ac-curate earthquake response and limit states of a build-ing. The capacity spectrum method is used for evaluat-ing the earthquake response. The code is applicable toany type of material and buildings such as seismic iso-lation systems as long as material properties are welldefined and structural behavior is appropriately esti-mated.

Keywords: the Building Standard Law of Japan, seismicdesign code, two-phase earthquake design, performancebased earthquake design

1. Transition of Seismic Design Code for Build-ings in Japan [1–3]

The Urban Building Law enacted in September 1920was the first regulation in Japan related to structural de-sign for buildings linking allowable stress design consist-ing of design loads, frame stress, and allowable materialstress. Seismic building code design started in 1924 whenthe Urban Building Law was revised as a consequenceof the 1923 great Kanto earthquake disaster. This Lawused structural provisions including a seismic coefficientof 0.1.

In 1950, the Building Standard Law replaced the Ur-ban Building Law with more elaborate provisions forstructural design. The standard seismic coefficient wasraised to 0.2 but seismic design was essentially unchangedbecause this increase in seismic loading accompanied acomparable increase in allowable material stress.

Both the Urban Building Law and the Building Stan-dard Law specified loading, allowable stress, and cer-tain minimum requirements for detailing members. Otherdetails of structural design, such as structural analysisand proportioning of members, are specified in StructuralStandards issued by the Architectural Institute of Japan(AIJ). These standards, prepared separately for each struc-tural material, supplement the law and are revised morefrequently to add new knowledge and provide for new ma-terials as they develop.

The seismic design building code was radicallychanged in 1981 in the largest revision since 1924. Upto this revision, large earthquake events in which manybuildings suffered severe damage had occurred, partic-ularly the 1968 Tokachi-oki earthquake, which signifi-cantly damaged buildings designed based on building reg-ulations then in force. Action undertaken as a conse-quence of this event included partial revision of the Build-ing Standard Law, a large-scale revision of AIJ Standardsincorporating shear design for reinforced concrete build-ings and the establishment of a review procedure for exist-ing buildings for seismic safety. A five-year-project con-ducted from 1972 to 1977 aimed at establishing a newand rational seismic design method. This was released in1977 as a proposal for a new aseismic design method forbuildings. The 1978 Miyagiken-oki earthquake, which re-sulted in damage as severe as the 1968 Tokachi-oki earth-quake, demonstrated the complexity of urban disaster inthe city of Sendai, whose population exceeded 600,000.This triggered the implementation of the Ministry of Con-struction proposal. In July 1980, the Enforcement Orderof the Building Standard Law was revised. It was alsoannounced that this Order, together with supplementarydocuments, would be enacted from June 1, 1981.

The central feature of the revised seismic design codewas the introduction of a two-phase earthquake design.Conventional seismic design was retained, with somemodifications, as first-phase earthquake design targetingthe safety and serviceability of buildings during medium-level earthquake activity. Second-phase earthquake de-sign was added to provide safety against severe earth-quake motion.

The 1995 Hyogoken-nanbu Earthquake caused muchloss of human lives and severe damage or collapse ofbuildings [4]. Many lessons among scientists and engi-neers were learnt about earthquake preparedness, disaster

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Kuramoto, H.

response, seismic design, upgrading of existing buildingsand introduction of new technologies, which assure highsafety levels of buildings against destructive earthquakes.The recognized need for new-generation seismic designled to the development of performance-based engineering[5], which specifically addresses life safety, reparability,and functionality.

Seismic provisions in the building code were signifi-cantly revised in 2000 from prescriptive to performance-based to enlarge choices of structural design, particularlythe application of newly developed materials, structuralelements, structural systems, and construction. It is ex-pected to encourage structural engineers to develop andapply new construction technology. Under the revisedcode, the precise definitions for structural performance re-quirements and verification are specified based on clearresponses and limits. The code is applicable to any mate-rials and buildings including seismic isolation systems onthe condition that material properties are well defined andbuilding behavior properly predicted.

In the following two chapters, a simple review of theabove-mentioned two revised seismic design codes is at-tempted.

2. Seismic Design Revised in 1980 [1� 2]

2.1. General Flowchart

In the general structural design flowchart provided bythe revised Enforcement Order of the Building StandardLaw in 1980 (Fig. 1), buildings are first divided into fourgroups mainly based on height (boxes marked (1)-(4)).

The Building Standard Law provisions do not apply di-rectly to buildings taller than 60 m (box (4)), high-rises tobe designed by special study, usually incorporating time-history nonlinear response analyses. The design is thensubjected to technical review by designated inspection in-stitutions for structural performance. Upon its recommen-dation, a special approval of the structural design is is-sued by the Ministry of Land, Infrastructure and Transport(MLIT).

For buildings not exceeding 60 m, the basic intent of thegeneral flow is two-phase design involving an additionaldesign phase, i.e., second-phase earthquake design, fol-lowing conventional structural design, including seismicdesign, i.e., first-phase earthquake design. First-phase de-sign involves strong earthquakes occurring several timesduring a building’s lifetime. Second-phase design targetssevere or extraordinary earthquakes occurring once in abuilding’s lifetime.

Application of two-phase design is shown in Fig. 1, forthree different groups of buildings up to 60 m in height,in boxes (1)-(3). For these, conventional structural designis conducted first (box (5)), including first-phase seismicdesign involving allowable stress design for permanentand temporary loadings taking maximum strength into ac-count. Major changes in the code related to this designphase are methods in seismic force evaluation.

1. Timber construction

Others, as specified by

Ministry of Construction

2. h m≤ 31

and

other than 1

3. 31 60m h m< ≤

4. h m> 60

5. Conventional structural design including

first phase design for earthquakes

Second phase design for

earthquakes

8. Use of specifications of

Ministry of Construction

11. Special approval

required

10. Approval by building officials of local

government body

6. Check for story drift

7. Check for

rigidity factor and eccentricity

9. Check ultimate capacity for

lateral load

Fig. 1. General seismic design flowchart.

Box (1) is for buildings prevalent in Japan, includinglow-rise reinforced concrete with generous amounts ofshear walls, for which there is ample experience in seis-mic design and evidence of seismic behavior. First-phasedesign, basically unchanged from the previous buildingcode, should provide sufficient seismic resistance to with-stand severe earthquakes, so second-phase design is notapplied to these buildings.

Boxes (2) and (3) involve second-phase design. Themost important step in second-phase design is the evalua-tion of maximum lateral load capacity (box (9)), but con-siderations in boxes (6)-(8) are included in second-phasedesign.

The evaluation of story drift (box (6)) is intendedto eliminate flexible structures experiencing excessivelylarge lateral deflection under seismic loading. In ordinaryreinforced concrete buildings, this check is not critical,but results of calculations are required later for items inbox (7) or (9).

Buildings up to 31 m high (box (2)) involve a choiceof flow into boxes (7) and (8) or into box (9). Box (7)requires a check of the rigidity factor and eccentricity.The rigidity factor refers to the vertical distribution of lat-eral stiffness checked to eliminate buildings with one ormore flexible stories among other stories, such as the softfirst story. Checking for eccentricity provides protectionagainst excessive torsional deformation. These checks arefollowed by the application of a set of additional mini-mum requirements specified by MLIT (box (8)) to ensurecertain strength and ductility. The option of boxes (7) and

342 Journal of Disaster Research Vol.1 No.3, 2006