RECENT DEVELOPMENT IN SEISMIC DESIGN OF REINFORCED CONCRETE BUILDINGS IN JAPAN

Japan experienced a quick development of highrise reinforced concrete frame-type apartment building construction, about 30 stories high, in the last decade. Outline of this development is first introduced in terms of planning of buildings, materials, construction methods, earthquake resistant design and dynamic response analysis. This quick development was made possible by, among others, the available high strength concrete and steel. In an attempt to further promote development of new and advanced reinforced concrete building structures, a five-year national project was started in 1988 in Japan, promoted by the Building Research Institute, Ministry of Construction. Outline of this project is introduced in the second part of this paper. It aims at the development and use of concrete up to 120 MPa, and steel up to 1200 MPa.


INTRODUCTION
The Building Standard Law in Japan provides design seismic loadings and principal design procedures for buildings up to 60 m in height.Structural design of any building with the height in excess of 60 m is subjected to the review of the Structural Review Committee for Highrise Buildings of the Building center of Japan, and subsequently a special permit by the Minister of Construction is issued.
As far as reinforced concrete (RC) buildings are concerned, the height had been limited to about 20 m in practice by means of administrative guidance.Any building taller than, say, seven stories had to be constructed by steel structure or composite steel and reinforced concrete (SRC) structure.This administrative guidance was a traditional one, stemming out from public distrust on the seismic resistance of concrete structures ever since 1925 Kanto Earthquake.
In the recent ten years or so, this trend has changed rapidly.There are currently various movements towards the higher RC construction.
Among them, the most remarkable is the increase of highrise RC frame construction.Kajima Construction co.broke out the movement.They completed the first highrise RC, an 18-story apartment building, in 1974, followed by another 25-story apartment building in 1980.These 1 Professor, University of Tokyo highrise buildings were realized after a long and extensive effort in research and development of the company.Other construction companies followed, and the number of concrete buildings increased together with the increase of building construction.
Figure 1 shows the amount of annual highrise construction in Japan, which is the number of annual reception by the Building Center for the review of the Committee for Highrise Buildings.The figure also shows number of SRC and RC buildings in each year.Total number in each year varies from less than 10 to more than 100, according to economic fall and rise.Concrete construction takes about 23 percent on the average, however more than half of it is taken up by RC in recent years.
The quick development of highrise RC construction owes to many things, but availability of high strength materials was evidently the most fundamental factor.In an attempt to further promote development of new and advanced type of RC construction, the Ministry of Construction started in 1988 a national five year research project entitled "Development of Advanced Reinforced Concrete Buildings using High-strength Concrete and Reinforcement" (usually referred to as "New RC") • This is a very ambitious project, which will probably lead to the realization of highrise RC buildings up to 60 stories, and buildings with wider spans, allowing for use in greater variety.

Floor Plan and Elevation
Highrise RC frame construction, currently, is used exclusively for apartment houses, because of better habitability provided by concrete.Floor plan of these buildings is generally regular, and symmetric with respect to one or two axes.In almost all cases, all frames in both directions are designed as moment resisting frames.Span length is around 5 m, which is shorter than SRC or steel buildings.The small span is adopted in order to limit the axial load on a column, and thereby reduce the seismic force acting on a Column.
The number of stories of the highrise RC buildings ranges from 20 to 40 stories.The story height is about 3 m, which is also very small, permissible only for residential buildings.The frame elevation is generally quite regular, avoiding sudden change or discontinuity of stiffness in the vertical direction.Most buildings have one-story basement, and the foundation is supported, in most cases, by bearing piles of cast-in-place concrete.
Most of these highrise RC buildings consist of frames only, in two directions.Very few of them have shear walls in one or two directions.The main reason for not using shear walls is to avoid complexity in analysis, design and construction by the introduction of shear walls. •

Framing Members
Column section is usually square, with the maximum dimension of about 90 cm at the base of buildings.Axial reinforcement ratio is about 2 to 3 %.elastic fundamental natural period.The range shown by dotted curves corresponds to most highrise construction in Japan, either steel or SRC construction.It seems highrise RC buildings have slightly lower base shear, as long as they are compared on the basis of elastic natural period.Probably it would be a more fair comparison to take natural period based on cracked sections, although it is not a common practice to do so in Japan.structural analysis is carried out for permanent loading as well as design earthquake loading.Computer analysis is normally performed using displacement method, based on the uncracked section, considering flexural, shear and axial deformation of members, and rigid zones at member ends.
When the structure is susceptible to torsional deformation, three-dimensional frame analysis is carried out.
Moment redistribution is applied in some cases, although it is not widely used.The amount of moment redistribution is usually modest, and its appropriateness is demonstrated in the subsequent nonlinear frame analysis, so that no yield hinges would occur under the action of design seismic loads.The ultimate load carrying capacity may be evaluated by limit analysis.However, nonlinear incremental frame analysis is usually performed, which gives not only ultimate capacity but also the primary load-displacement relation for each story for use in the dynamic earthquake response analysis.
For the calculated member forces associated with the mechanism, ultimate strength of each member is investigated whether the assumed mechanism would be actually formed.This consists of the following three points: (1) Beam ductility.(2) Column strength and ductility.Except where yield hinges are expected to occur, columns should be protected against flexure and shear.A practical problem in this respect is how to determine design forces.Forces determined in the inelastic frame analysis correspond to predetermined load profile, but forces during dynamic excitation are subjected to much fluctuation due to ratio of upper and lower story drift, usually referred to as higher mode effect.Furthermore columns must be protected against forces coming from beams in two directions.
The Guidelines (2] is serving for practice in this regard also. (3) Beam-column joints.
Prevention of premature joint failure is achieved by restricting shear stress in the connection, and by restricting bond stress along the beam bars passing through the joint.
For exterior beam-column joints, beam bar anchorage is checked and is carefully detailed.

Earthquake Response Analysis
Reinforced concrete structures start cracking at a relatively low level of loading.Hence the elastic linear analysis based on the uncracked section serves little in predicting actual behaviour.

As a
simplified analytical model for nonlinear analysis, a lumped mass shear model is almost exclusively used in the time history dynamic response analysis for both level 1 and level 2 earthquake ground motions.
The restoring force characteristics of stories are defined by simplifying the load-displacement relation from incremental In some cases, so-called flexural shear model is used, in which flexural deformation of overall structure due to overturning moment is separately evaluated and added to the shear deformation which is the frame deformation.The flexural deformation is evaluated on the basis of linear elasticity.

When
the building is susceptible to torsional vibration, a dynamic quasi-three-dimensional model is used in the response analysis, which consists of many shear models, or flexural shear models, corresponding to each frame interconnected by rigid floor diaphragms.
One of the serious drawbacks of shear models is that it cannot predict member ductility factor.Usually it is evaluated indirectly by equating dynamic story drift to the static one in the incremental frame analysis.However, some engineers opt to carry out dynamic frame analysis where inelastic deformation of constituent frame members are directly accounted for in the time history earthquake response analysis.
As for the input earthquake ground motions, the Building Center of Japan recommends the use of waveforms in the following three categories (3] for any highrise buildings: (1) Well known "standard" motions, e.g.El Centro 1940 NS and Taft 1952 EW.
(2) Records taken at nearby stations, e.g.Tokyo 101 1956 NS for buildings in Tokyo.
Earthqu~ke motions are normalized in terms of maximum velocity to the levels as prescribed in Table 1.In many cases, design criteria for story drift angle in Table 1 are found to be the governing criteria.

Range of Material strength
The quick development of highrise RC construction owes to many things, but development of the use of high strength concrete and high strength, large size reinforcing bars was evidently the most fundamental factor.In an attempt to further promote development of new and advanced RC construction, a national project lasting five years was started by the Ministry of Construction in 1988 (1).The project was officially named as "The Project on Development of Advanced Reinforced Concrete Buildings using High-strength Concrete and Reinforcement", but it is usually referred to as "New RC Project".
The range of material strength set out as the target of this project is shown in In contrast, the ranges of strength for concrete and steel are much larger.Concrete from 30 to 120 MPa and steel from 400 to 1200 MPa are included.Comparing the zones for these range of materials to zones A and B, it is obviously unrealistic to assume that structural behaviour of New RC structures can be understood simply by extrapolating the knowledge of ordinary RC structures.The area in Figure 2 for the New RC is further divided into four zones, namely zones I, II-1, II-2, and III.
Structures in these zones will be studied and developed by somewhat different tactics.Experimental approach is indispensable, but in general, theoretical examination of experimental data will be emphasized in this project.current technical knowledge on RC structures will also be re-examined.
In some zones in Figure 2, particularly in zone III, basic problems will have to be re-examined, and hence the project may not yield much practical results.Most practical results are expected in zones I and II-1, because these zones are relatively close to the boundary of the current technology, and simple extrapolation will be effective at least partially.

Objectives of Research and Development and Final Expected Results
The objectives of research and development and the corresponding final results expected in the project are summarized in Table 2.
Results will be partly available to refine the current RC technology.In the table under the third objective, the word "guidelines" for structural design and construction do not mean a type of the guidelines that will give full details of technology, but it will give only basic principles for design and construction practice.Such a soft type of the guidelines is preferred at this stage of the game, as definite and detailed specification-type guidelines often tend to impede the development of relevant technology.(2) Physical properties of high-strength and super high-strength concrete.
(3) Mix proportion design, casting and curing works and quality control.
For the Reinforcement Committee: (1) Development of high-strength and super high-strength steel bars.Mechanical properties of steel bars.
properties of confined (3) Constitutive equations for RC elements and application of finite element method.
(4) Bond between concrete and steel bars.Anchorage and arrangement of steel bars.
For the Structural Element Committee: (1) Mechanical properties of beams and columns.
(3) Effect of shear force on beams, columns, and shear walls.
( 4) Mechanical properties of beam-column joints and frames.

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For the Structural Design Committee: (1) Methods for modelling and analysis of structural frames in each zone. (2) Practically structures.

feasible types of
(3) Design seismic loads and requirements for structural performance.
For Another problem related to the ductility of high-strength reinforced concrete is the property of high-strength steel.At present super high-strength steel bars of 1300 MPa specified yield strength are used very frequently in practice for lateral shear reinforcement, but they have never been used as longitudinal reinforcement.In order for high-strength steel bars to be used as longitudinal reinforcement, their quality in terms of stress-strain relationship should be improved.The most important practical problem is how much improvement can be specified and realized by steel manufacturers.
In the long run, the ductility of New RC members will have to be lower than ordinary RC members, and hence it will be necessary to develop design philosophy which depends more on strength, but less on ductility, of constituent materials.
Bond between concrete and reinforcing bars is another basic issue in New RC.Demand on bond increases in proportion to the increase of yield strength of steel bars, but the bond capacity does not increase in proportion to the increase of concrete strength.Thus the bond becomes one of the critical problems of New RC structures.In particular, resistance against bond splitting failure is an important subject for bar development.Use of high strength materials usually results in a reduction of cross section of members, thus reduction of stiffness of members and structures.Among structural design criteria listed in Table 1, that for the drift angle governs in most highrise RC buildings even at present, and it is expected to be more so for New RC buildings if the same design criteria as in Table 1 are maintained.Since the design criterion for drift was established not on any rational basis, it is not easy to challenge for its revision by a rational discussion.But the author believes that we will have to start discussing on it seriously, sooner or later.

CONCLUSION
In this paper the author first reviewed the present state-of-the-art of the seismic design of highrise RC buildings in Japan.Architectural Institute of Japan (AIJ).1990.Design guidelines for earthquake resistant reinforced concrete buildings based on ultimate strength concrete.Tokyo.
Building Center of Japan (BCJ), 1986.On the earthquake motions for use in dynamic response analysis of highrise buildings, Building Letter, 6, 49-50.Tokyo.
FIGURE 1 ANNUAL HIGHRISE CONSTRUCTION IN JAPAN

FIGUREFigure 2 .
FIGURE 3 STREHG'l'B OF MATERIALS AND FIELDS OF RESEARCH AND DEVELOPMENT