evolution of seismic building design practice in japan

EVOLUTION OF SEISMIC BUILDING DESIGN PRACTICE INJAPAN

ANDREW WHITTAKER,1,* JACK MOEHLE1 AND MASAHIKO HIGASHINO2

1Earthquake Engineering Research Center, University of California, Berkeley, CA 94804, U.S.A.2Research and Development Institute, Takenaka Corporation, Chiba, Japan

SUMMARY

The widespread destruction of the built environment in the Kobe region as a result of the 1995 Hyogo-ken Nanbuearthquake was unexpected by many design professionals, academicians and researchers, both in Japan and theUnited States. Questions raised in the aftermath of the earthquake include: ‘What happened?’, ‘Why did ithappen?’ and ‘What are the implications of the observed damage for seismic design practice in the U.S.—especially those regions of high seismic hazard?’ The goals of this paper are three-fold, namely: (a) to trace thedevelopment of design and construction practices in Japan so that the observed damage can be better understood;(b) to briefly to compare seismic design practice in regions of high seismicity in Japan and the U.S. at the time ofthe Hyogo-ken Nanbu earthquake; and (c) to assess the implications of this devastating earthquake on designpractice in the U.S. Construction practice in Japan and the U.S. differs substantially. As such, it is difficult to makedirect comparisons between design practice in the two countries. However, simple comparisons between currentseismic codes in Japan and the U.S. suggest that Japanese buildings are likely to be stronger and stiffer than similarbuildings in the U.S. The implication of the behavior of older buildings in Kobe during the Hyogoken-Nanbuearthquake is that severe damage to similar constructions in the U.S. must be expected in a design earthquake.Further, unexpected severe damage and collapse of modern construction in Kobe suggest that modernconstruction on the West Coast of the U.S. would be likely to suffer damage in a severe earthquake. 1998 JohnWiley & Sons, Ltd.

1. DEVELOPMENT OF JAPANESE CODES AND BUILDING CONSTRUCTION

1.1. Seismic code development

The modern era of Japanese building construction can be considered to have had its origin with theend of the Edo Period and the Meiji Restoration in 1868. At this time, Japan was eager to embraceforeign culture and sciences, including the adoption of some foreign building construction practices.The Architectural Institute of Japan (AIJ) was formed in 1886, and by 1888 the first city planninglegislation was in place. Formation of the Earthquake Investigation Committee following the 1891Nobi earthquake was an early step toward developing modern earthquake resistant constructiontechnology in Japan1 (see Table I).

THE STRUCTURAL DESIGN OF TALL BUILDINGSStruct. Design Tall Build.7, 93–111 (1998)

CCC 1062-8002/98/020093–19 $17.50 1998 John Wiley & Sons, Ltd.

Received August 1997Revised September 1997

* Correspondence to: Andrew Whittaker, Earthquake Engineering Research Center, University of California-Berkeley,1301 South 46th Street, Richmond, CA 94804-4698, U.S.A.

TableI. Someeventsin the historyof Japaneseseismiccodedevelopment

Date Event Codechange

1891 Nobi earthquake(M7.9) causesextensivedamage

1892 EarthquakeInvestigationCommitteefounded

1915 Sanoproposesconceptof seismiccoefficient

1919 City PlanningLaw andUrbanBuilding Law enactedfor six major cities.First structuralstandards

1923 GreatKantoearthquake(M7.9) andfire

1924 UrbanBuilding Law addsarticle to requirea seismiccoefficientof 0⋅1; heightlimit of 100 ft unchanged

1925 EarthquakeInvestigationCommitteerecharteredastheEarthquakeResearchInstitute

1933 Muto proposesD-valuemethodtocalculatestressesunderhorizontalforces

1937 Japan–ChinaWar putsJapanunderwarconditions

1943 UrbanBuilding Law suspendedexceptfor somefirerestrictions;relaxationof allowablestressesduetowartimeshortageof constructionmaterials;dualallowablestressesintroduced.

1948 UrbanBuilding Law restored.

1950 Constitutionof Japan Building SeismicLaw replacesUrbanBuilding Law;seismiccoefficientincreasedto 0⋅2, allowablestressesundertemporaryloadssetat twice theallowablestressesunderpermanentloads

1958 First standardfor SRC

1963 Height limit of 100 ft abolished;Ministry ofConstructionrecommendsuseof SRCfor buildingsover6 storiesin height

1964 Nigataearthquake(M7.5)

1968 Tokachiearthquake(M7.9) causesheavydamageto RC buildings;firsthigh-risebuilding in Japan

1970 Steelcoderevised

1971 SanFernandoearthquake(M6.4) Concretecoderevisedto requirecloserspacingoftransversereinforcement

1972–77 Ministry of Constructionprojectfordevelopmentof a newseismicdesignmethod

1978 Miyagi-ken Oki earthquake(M7.5)

1981 Building StandardLaw changed;main featuresincludeseismiccoefficientthat varieswith periodandtwo-level design

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1998JohnWiley & Sons, Ltd. Struct.DesignTall Build. 7, 93–111(1998)

Observationson building performance in earthquakes during this period strongly influencedthedevelopmentof modernearthquake engineeringconstruction.2 Masonry construction, introduced inthe British form to reduce fire hazardsassociatedwith traditional wood construction, suffered manycollapsesin the1891and1894earthquakes,slowingdevelopmentof thisconstruction form.Followingthe1923GreatKanto earthquake,engineersnoted thatsteel frameswithstoodearthquakeloadsbutdidnotwithstand thesubsequentfires,contributing to thedevelopmentof steelreinforcedconcrete (SRC)construction in which reinforcedconcrete enclosesstructuralsteel sections.In the sameearthquake,observationsof poor performance of reinforcedconcrete construction without hooksat the endsofreinforcementled to the abandonment of this practice.The generally good performanceof wallbuildingsin this same earthquake contributedto the developmentof this construction form.

In 1915,ToshikataSano introducedthe concept of the seismic coefficient, which definesa lateralforce for seismicdesign(V) astheproduct of a seismic coefficient (C) andthestructureweight (W):

V � CW �1�At this time,no specific guidancewasprovidedon thevaluefor theseismic coefficient.2 The1923

GreatKantoearthquake led to theaddition of anarticle in theUrbanBuilding Law to requireseismicdesign of all buildings for a seismiccoefficient of 0⋅1. The useof a seismic coefficient of 0⋅1, inconjunctionwith amaterialssafety factorof 3 onultimatestress,wasdeemedsufficient for abuildingto withstand a major earthquake suchasthe 1923event.TheUrbanBuilding Law, which appliedtocertain urbandistricts, wasa landmark in seismic codedevelopment worldwide. Introduction of theMuto D-valuemethodin 1933provideda means of calculating internal stressesfor buildingsunderhorizontalforces.3 The D-valuemethodis similar to theportal methodof analysiswidely usedin theUnited Statesprior to the adventof computers for structural analysis.

Wartime activities in the first half of the twentieth centuryhadan important impact on buildingconstruction in all of Japan.3 WhereasWorld War I provided opportunities for rapid economicdevelopmentin Japan,war conditionsin the 1930sand1940shadmoreadverseeffects.

During partof this period,becauseof a shortageof constructionmaterials, allowable stresseswererelaxed. In 1943,the UrbanBuilding Law wassuspendedentirely, except for somefire restrictions.Bombing during World War II took a grim toll on manyJapanesecities, andKobe wasnot sparedsignificant losses.As a consequence, the majority of medium andhigh-risebuilding construction inKobedates from after this war.

Following World War II, theUrbanBuilding Law wasrestoredbriefly, but wasreplaced when the1950 Constitution of Japanstipulated the Building Standard Law. This law applied to buildingconstruction throughoutJapan, as opposed to limit ed districts. Under this new law, the seismiccoefficient wasincreased from 0⋅1 to 0⋅2. Thenet effecton proportioningmaterials wasminimal, astherewasa simultaneousincreasein allowablestressesfor materials.However, theincrease in designlateral forcesrepresenteda significant changein overturning momentrequirements.This effect wasespecially notable in thepostwar building boomwhen manybuildingswereproportionedto bemoreslender thanprewarbuildings.2

Later changesof significanceincluded theabolishmentin 1963of theheightlimit of 100ft. At thesame time, theMinistry of Constructionrecommended thatbuildingsoversix stories in heightshouldbe constructed of SRC.4

Thegrowthin theJapaneseeconomy wasrapidin the1960s.Two keyeventsin thisperiodwerethe1964Olympicsheldin Tokyo,andthe1970World Expositionheldin Osaka.Significantinfrastructureandcommercial construction precededtheseevents. The Tokyo–OsakaExpresswayandShinkansenRailway Line werecompletedbefore the Tokyo Olympics, andthe Hanshin Expressway andmanycommercialbuildingsin the Kansairegion wereopenedbeforethe World Exposition.

A very significant changein the reinforcedconcrete code to require closer spacingof column

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1998JohnWiley & Sons,Ltd. Struct.DesignTall Build. 7, 93–111(1998)

transversereinforcement was imposed in 1971 in responseto heavydamageto reinforcedconcretebuildingsin the 1968Tokachi-Oki earthquake.5

In theperiod 1972–1977, theMinistry of Constructiondirecteda projectfor thedevelopmentof anew seismic designmethod,resulting in a proposalin 1977.3 The 1978Miyagiken-Oki earthquake,causingseriousdamagein Sendai anddemonstrating the effectsof a strongearthquake on an urbanarea,spedadoptionof the proposal in 1981. The new Building Standard Law6 included a seismiccoefficient thatvariedwith structural vibrationperiod,andintroduceda two-phasedesignprocedure.The first phasedesign,which is similar to the design methodusedin earliercodes, is intended asastrengthcheckfor frequent,moderateevents.Thesecond phasedesign,whichhadnotbeenincludedinpreviouscodes,is intended asa checkfor strength andductility for a maximumexpectedevent.Thissamecodewasin effect at the time of the Hyogo-ken Nanbuearthquake.

1.2.Typesof construction in Japan

Aoyama5 discusses types of construction commonin Japanandthe number of storiescommonlyused for each type. Wood frame construction is the predominant form of traditional houseconstruction, commonly being one or two stories in height. Reinforced concrete bearing wallconstruction is themost commonform for apartmenthouseconstruction;typicalheightsarefrom threeto five stories,althoughit maybeusedup to eightstories.Reinforcedconcreteframestructures,withor without shearwalls,areusedfor avarietyof structures,with commonheightsupto sevenstoriesbutwith recent examplesup to 50stories.Steelreinforcedconcrete (SRC)is commonfor awide rangeofconstruction, andis most commonfor structures abovesix stories in height. Steel structures may beusedover the heightrangeandarethe predominantform for modern high-risebuildings.

Eachof theseconstructionformsandmaterials hasits owndevelopmenthistory. In manycases,thehistory is unique to Japan. Somekey aspects of eachare described in the following sections. Thediscussionemphasizesaspectsthatwerepredominantin theperiodfrom theendof World WarII to thepresent, andthereforehadmostsignificancefor construction in Kobe.

1.2.1.Woodframe construction for traditional houses.Traditional wood-frame construction forhouseswasrelatively unchangedfrom thebeginningof thiscentury until the1970s.Two formswereinwidespread use, the older being Shinkabeand the more recent being Ohkabe. More recently,prefabricatedtimber houseshavebecomemorecommon.7

Shinkabe construction usespost-and-beamframing, with lateral resistancebeing providedby amud-filledlattice of bambooin thewalls.Theneedfor additionalbracingwasrecognizedby theAIJ,althoughthespecificationmerely recommended thatsuchbracing, havingdimension not lessthanhalfthe column dimension,be placed wherever it would not impair the external appearance of thedwelling.8 In Ohkabeconstruction, the bamboois replaced with a narrow-plank lathe, preferablyplacedon bothsides of thewall, nailedto the posts,andcoveredwith stucco.Diagonalbracing wasalsorecommended.Connectionsbetweenpostsandbeamsis oftenby woodjoineryratherthannailingor connectors.Roofscommonly areof heavytile, setin a relatively thick mudmortar.

1.2.2. Reinforced concrete bearing wall construction. Observations of building performancefollowing the 1923 Great Kanto earthquakerevealed the value of low-rise buildings that rely onreinforcedconcrete bearing walls. Figure 1 depictsa typical example of bearingwall construction.

As describedby theAIJ,8 thedesignemphasisin bearingwall construction wasonproperlayout andproportioning of the structural system, as opposed to structural calculations. Minimum wallthicknesseswererequiredto rangefrom 120mm(4⋅7 in) or h/25 for one-storyconstruction to 180mm(7⋅1 in) or h/22 for four-story construction. Therequired ratioof wall length(parallel to eachprincipal



direction) to floor areawas15cm mÿ2 (0⋅61 in ftÿ2) for thefirst story in four-storyconstruction,and12cm mÿ2 (0⋅49 in ft2) for all otherfloors.Web reinforcement ratiosof 0⋅15, 0⋅20, and0⋅25%wererequired for the top story, the second-from-the-topstory,andall otherstories,respectively.

Studiesof building damagefollowing the 1968Tokachi-Oki earthquake demonstratethat bearingwall construction providesgoodprotection against severe damage.5 Datafrom this earthquake wereusedto developthe limit s definedin the 1981 Building Standard Law. Regularlyconfigured wallbuildings,with parametersthat fall within the limit s, areexemptfrom the checkfor ultimate lateralload carrying capacityunderthat law.

1.2.3. Reinforced concrete frame and frame-wall construction. Reinforced concrete frameconstruction in Japanis commonly taken to include both pure frame construction and combinedframe-wall construction. Although both types areprevalent,the performance of reinforcedconcreteframeshasledto recommendationsto combineframesandwalls.8 Wall construction mayincludesolidwalls framed by beamsandcolumns, or walledframesthatconsistof deepgirdersandwide columns.Concrete may include normal weight aggregate concrete or lightweight aggregate concrete;8

apparently in the 1960s, the lower mass associated with lightweight aggregates led to someapplicationsin seismic zones, muchasoccurred in the U.S. at the same time. According to the AIJStructural Standardsof the time, the minimum strengths should be not less than 135 kg cmÿ2 for

Figure1. Typical bearingconstructionin Japan8



normalweightaggregateconcrete or 120kg cmÿ2 for lightweight aggregateconcrete. In theOsaka–Kobearea,buildersbeganto usecrushedstonefor aggregatearound 1970,andexclusively since1975.In the transition period,bothcrushedstone andriver gravel wereused.9

Both plain anddeformedreinforcement havebeenusedin Japan. Starting around 1960,builders intheOsaka–Koberegionbeganto usedeformedbarsfor longitudinalreinforcement,andthese becamecompulsory after1970.Deformedtransversereinforcementwasusedaboutsix-to-sevenyearsaftertheintroductionof deformedlongitudinalreinforcement.9 Fivegradesof reinforcementwereidentifiedforusein the 1960s,8 having minimum yield stressesof 24, 30, 35, 40 and50 kg mmÿ2.

Typical details of pre-1972 construction include haunched beamswith maximum transversereinforcement spacingnot to exceed300mm (12 in) and2D/3 or 3D/4 (depending on shearlevels).Columns were required to have hoops with maximum spacingnot exceeding 300mm (12 in),minimum column depth,and 15 times the diameterof the main reinforcement. Rebarin columnstypically consistedof smooth longitudinal rebarwith diametersrangingbetween 22mm (0⋅9 in.) and25mm (1 in), and smoothhoop rebar 10mm (0⋅4 in) diameter. Walls were required to have aminimum thicknessof 120mm (4⋅7 in), andtwo layersof reinforcement wererequired wherewallthicknessexceeded200mm(8 in). Themaximum spacingof wall reinforcementwas300mm (12 in).Themaximum allowableshearstresswasone-eighth of theconcretecompressivestrength.8 Commonpracticehadbeento hooktheendsof all reinforcement,althoughOtsuki2 reported a trendawayfromhooks.In the1960s,hookswereonly requiredfor plain bars,except deformedbarsrequired hooksinhigh stressareasand in stirrupsand hoops.8 Reinforcementsplicescould be either by lapping orwelding. In recentdecades,rebar splicesarealmostalwaysmadeby gaspressurewelding.9 In thisprocess,barsarealigned, buttedtogether, andfused by a combination of heatandpressureappliedbymechanical devices,causing barsto flareout at the splice.

In 1971,followingdamageto reinforcedconcreteconstruction in the1968Tokachi-Oki earthquake,there was a partial revision of the Building Standard Law and a large scale revision of the AIJStandardsincorporating ultimate strengthdesignin shearof reinforcedconcrete.5 Perhapsthe mostnotablechangein reinforcedconcrete construction was the requirement that maximum spacingofhoopsin columnsbe reducedto 150mm (6 in), except in column endregionswhere the maximumspacingwasfurtherreducedto 100mm (4 in); 90° hooksonhoopswerestill permitted,andcross-tiesweretypically not used.

1.2.4.Steelreinforcedconcreteconstruction. Steelreinforcedconcrete(SRC) constructionin Japangrewoutof observationsof performanceduringthe1923GreatKantoearthquake.10 Suchconstructionis commonly termedcompositeconstruction in the U.S. Baresteel frameconstruction,while highlyresistantto earthquake forces,waslessresistantto fire. In steel frames encased in bricks, the brickscommonly failed,exposingthesteel.Steelframeswith concretecasing,or with walls,performedwell.This led to anascendanceof SRCconstruction usingsteelencasedin reinforcedconcrete.Theoriginalform of SRCconstruction, known asthe full web form, usedsteel ‘ I’ or ‘H’ sections.Later, in theinterestof maximizing section modulusandminimizing rolled section consumption, built-up sectionscomposedof small steel sectionsbecamemorecommon.8

Someexamplesof SRCconstruction detailsaregivenin Figures2 (1950s) and3 (1970sand1980s).Theneedfor continuity of thesteelframingmembersat jointswaswell established,andwasprovidedusing weldedor riveted plates. Splice plateswere commonly rivetedor welded.Butt welds wereconsideredunsuitablefor field work at thebuildingsite.8 Two typesof connectionwerecommonatthefoundation level; non-embedment type,whereasteelbaseplatewasanchoredusinganchorbolts,andembedment type, where the steel extendsinto the reinforced concretefoundation with specialdetailing. In 1963, when the 100 ft height limit was eliminated, the Ministry of Constructionrecommendedtheuseof SRCfor buildingsoversix stories in height.While it is unclearwhetherthis



Figure2. Examplesof SRCconstructionfrom the 1950s1



recommendationledto theuseof SRCin thefirst sevenstories,with eitherreinforcedconcreteor steelusedfor the remaining stories, it wasreported that manybuildingsover sevenfloors hadreinforcedconcrete in upperfloorssupported by SRCin the lower sevenstories.9

The1958AIJ Standardfor StructuralCalculation of SRCStructureswasthefirst standardfor SRCin Japan. Until that time, many different types of SRC were built.4 Subsequent revisions werepublished in 1963,1975and1987.Thestandard approachis to calculateallowableloadsasthesumofthe allowable loads for the steelandthe concrete sections, with the reinforcedconcretecomponentdesignedaccording to the reinforcedconcrete standard, andthe steelpart designedaccording to thesteelstandard except local buckling is ignored.Becauseof theobservedsuperior behavior of full webconstruction in the laboratory, the 1975revision to the SRCbuilding coderecommended full steelwebs.9 The1987revisionrecommendeduseof theembedmenttypesteelbase.9 It alsoshowedhowtoevaluate lateral load resisting capacity asrequired by the 1981Building StandardLaw.11

1.2.5.Steelconstruction. Theapplicationsof steelin buildingconstruction grewrapidly in theyearsafter World War II. According to Chiba,12 early steel products were light gaugesteel. The nextdevelopmentstookplacearound 1961whenrolling mills beganto producestandard columnandbeamsections. Developmentsin welding technology enabled the productionof box sections by welding,startingin 1969.Squarehollow steelsectionsbeganto be producedaround 1977.

Figure3. Examplesof SRCconstruction12



With the introduction of higherstrength steels (SS55, SM53andSM58)by themid-1960s,a widerangeof steelswasavailablefor building construction. Allowable stresseswererelatedto a nominalstresslimi t F, definedasthelesserof thenominalyield stressand70%of thenominal ultimatestress.The nominal stresslimi t ranged between2⋅4 tonnescmÿ2 (35 ksi) for normal gradesteel and 4⋅1tonnescmÿ2 (60 ksi) for high strengthsteelfor welding applications.13 Allowable stressesin tension,bending andcompression(with dueallowancefor slendernesseffects)weretypically F/1⋅5, andforshearthey wereF/2⋅6. Width-to-thicknessratios weresimilar to those of the American Institute ofSteel Construction during the same period.

A wide rangeof structural details was usedover the period of interest. Figure 4 showssomecommon connection details from the 1960s.8 In the 1970sand 1980s,steel box sections becameincreasingly popular. Thesewere produceddirectly for useascolumns, starting in 1977, or couldbeconstructedby welding together light gaugesteelsections,rolled parallelflangechannelsections,orthick steelplates;someexamplesareshown in Figure5. It wastypical to shopweld theconnectionsatthe beam–columnjoint andto userivets or bolts in the field away from the joint.

1.3.Lateral force anddesignrequirements

In 1915 Sanointroduced the seismic coefficient concept,which offered an earthquakeresistant

Figure4. Somesteelbeam–columnconnectiondetails8



Figure5. Somesteelbeam–boxcolumnconnectiondetails12



design approach in which the structurewas designedfor a lateral force equal to the product of acoefficientandtheweight.No specific valuewasassignedto theseismiccoefficient,andtheprocedurewasnot part of law, until after the 1923GreatKanto earthquake. In 1924,the UrbanBuilding Lawaddedanarticleto requireaseismiccoefficient of 0⋅1. Theheightlimit of 100ft remainedunchangedfrom earliertimes.This law governedconstruction in majorcitiesuntil 1943,when it wassuspendedbecauseof the war. The law wasreinstated in 1948, andreplacedby the Building Standard Law in1950.2 This law, andthe 1981revision,governedlateral force requirements for the majority of theKoberegionbuildingsthatwerein placeduringthe1995Hyogo-kenNanbuearthquake.Someaspectsof thesetwo laws aredetailedin the following paragraphs.

1.3.1.TheBuildingStandard Law from1950through1980.Thebasicseismiccoefficient during thisperiodwassetat 0.2,or doublethevaluethathadbeenusedsince 1924.As describedby Otsuki,2 thedoubling of theseismiccoefficient caused problems in designrelatedto overturning,andthis wasthesubjectof great debateamongengineers.Theoutcomewasnotadecreasein theoverturningmoment,but instead anincreasein theseismiccoefficientfor portionsof buildingmorethan16m aboveground.TheBuildingStandardLaw wasmodifiedin 1995to includereductionsto accountfor theseismicity ofthe region andsite subsoil.

Accordingto thecode,8 thedesignhorizontal seismiccoefficient K for thesuperstructuremust beatleast equalto

K � K0�� 2�where theproductab must notbelessthan0⋅5, K0 is 0⋅2 for thefirst 16m abovethefoundation andisincreasedby 0⋅01 for eachadditional 4 m, a is a modification factor for groundcondition andtypeofconstruction, andb is a modification factor for seismicity of the region,equalto 1⋅0 for the Koberegion.Valuesof a arelisted in TableII, with simplified descriptionsof relevant conditions.

Allowable unit stressesarereportedby AIJ.8 For steel,theallowablestressesfor seismic loadingare

Figure5. Continued



equalto thenominalyield value.For concrete, theallowablecompressivestressunderseismicloadistwo-thirdsof the designcompressive strength.

In theperiod1972–1977, theMinistry of Constructionconducteda projectaimedat establishinganewseismicdesignmethod.Themethodwasreleasedin 1977,andgenerally accepted by 1978.14 The1978Miyagiken-Okiearthquakeprovidedmomentumfor implementationof thisproposal, whichwasenforcedin June1981 aspart of the Building StandardLaw.5

The design methodfeatures a two-phaseprocedure.The first phasedesignfollows an approachsimilar to that adopted in the NEHRP RecommendedProvisions for the Developmentof SeismicRegulations for New Buildings,15 namely, designat the strength level for steelstructures basedonallowablestressdesignprocedures,with thesteelallowablestressequalto theyield stress,andstrengthdesignfor reinforced concrete with the load factor on the earthquake actions set equalto 1⋅0. Thesecond phasedesignis adirectandexplicit evaluationof strength andductility, andmayberegardedasa checkof whether these aresufficient for severegroundmotions.5 Timber structuresand low-risestructures satisfying rigidity, eccentricity, and detailing limitations neednot be checkedusing thesecond phasedesign.Otherstructures,includingall structuresof height between31m (96ft) and60m(186ft), must becheckedby bothphases.Structuresover60m (186ft) in height aresubjectto specialapproval by the Ministry of Construction.

In thefirst phaseof thedesign,theseismiccoefficientateachlevel Ci is determinedastheproduct offour variables:

Ci � ZRtAiC0 �3�whereZ representstheseismic zone,Rt definesthespectralshapethatvariesasafunctionof soil type,Ai definesthevertical distribution of seismicforce in thebuilding,andC0 representsthepeakgroundacceleration.In theKoberegion,Z is equalto 1⋅0. Exceptfor woodstructuresonsoftsubsoil, C0 is setequalto 0⋅2.VariablesRt andAi varyasshown in Figures6 and7.Theseismicdesignshearforcein theith story,Qi, is calculatedas

Qi � CiW �4�whereWi is the reactiveweight abovethe ith story. For the first phasedesign,seismic actionsarecomputed using unreducedseismicforces.Interstorydrift is limited to 0⋅5%of thestoryheightfor theprescribedseismicforcesunlessit canbedemonstratedthatgreater drift canbetoleratedby thenon-structural components,in which casethe drift limit canbe increasedto 0⋅8% of the storyheight.

In the secondphaseof the design,the engineerchecksplan eccentricity, distribution of lateralstiffness, minimum coderequirements in some cases, andultimate lateral load carrying capacityforeachstory. If checked, the ultimate lateral load capacity is computed using plastic analysis, andultimateseismic demandsareestimatedas

Qu � DsFesQud �5�whereQud is the lateral seismic shearfor severeearthquake motionscalculatedby equation (4) using

TableII. Valuesof a

Soil Wood Steel RC andSRC

Rock,hardsandygravel 0⋅6 0⋅6 0⋅8Sandygravel,sandyhardclay, loam 0⋅8 0⋅8 0⋅9Alluvium 1⋅0 1⋅0 1⋅0Bador soft ground 1⋅5 1⋅0 1⋅0



C0 equalto 1⋅0, Ds is a framingsystem-dependentductility factor(lessthan1⋅0), andFes is ameasureof the regularity of the building, andis calculatedas

Fes� FeFs �6�In equation (6), Fe is ameasureof theplanirregularity of thebuildingandFs reflectstheuniformity

Figure6. Spectralordinatefactor Rt5

Figure7. Vertical distributionfactor Ai5



of thedistributionof lateralstiffnessovertheheightof thebuilding.For reference,Fe rangesin valuebetween 1⋅0 (regular) and 1⋅5 (irregular), and Fs ranges in value between1⋅0 (regular) and 1⋅5(irregular)—the designpenalties associatedwith selecting a highly irregularseismic framing systemareclearly evident.

Theductility factorDs variesasa functionof structural material, typeof framing system, andkeyresponseparameters.Materialsare identified aseither steelor reinforcedconcrete; steelreinforcedconcrete is included underthe headingof reinforcedconcrete.ValuesDs for steelseismicframingsystemsarereproduced from the1981Building StandardLaw in TableIII. For reference,members inductilemomentframeswith excellent ductility havesmallerwidth-to-thickness(or depth-to-thickness)ratios than members in ductile momentframeswith poor ductility. Similarly, members in bracedframes are similarly classifiedon the basisof the braceslendernessratios, with excellent ductilityassociated with stocky braces,and fair ductility associatedwith slenderbraces.

For reinforcedconcreteconstruction,valuesfor Ds variedbetween 0⋅3 and0⋅55 asshown in TableIV; for steelreinforcedconcreteconstruction,valuesfor Dsarereducedfrom thosein thetableby 0⋅05.For a ductile momentframe to be assignedexcellent ductility, columnshaveto be designedto beflexure-critical, havea longitudinalreinforcement ratio lessthan0⋅8%,andhavelow axial (< 0⋅35fc')andshear(< 0⋅1fc') stressesat theformationof themechanism;thelimiting shearstressin beamsin anexcellent-ductility frameis 0⋅15fc'. Poorductility would beassignedto a momentframein which theaxial andshearstressesin the columnsaremuchhigherthanthe limit s notedabove,andfor framesincorporatingshear-critical beamsor columns. For ashearwall to possessexcellentductility, thewallhasto beflexure-critical, andhavealow shearstress(< 0⋅1fc') at theformationof themechanism. Thereaderis referredto theTablesC1 throughC4 (reinforcedconcreteconstruction), andD1 throughD4(steelconstruction) in the 1981Building Standard Law for moredetailedinformation on frameandductility classifications.

Additional information on this innovativedesignprocedureis summarizedby Aoyama.5 The1981

TableIII. StructuralcoefficientDs for steelframedbuildings

Typeof frame

Behaviorof members(1) Ductile moment

frame(2) Concentrically

bracedframe(3) Framesotherthan(1) and(2)

A. Memberswith excellentductility 0⋅25 0⋅35 0⋅30B. Memberswith goodductility 0⋅30 0⋅40 0⋅35C. Memberswith fair ductility 0⋅35 0⋅45 0⋅40D. Memberswith poorductility 0⋅40 0⋅50 0⋅45

TableIV. StructuralcoefficientDs for reinforcedconcreteframedbuildings

Typeof frame

Behaviorof members(1) Ductile moment

frame (2) Shearwalls(3) Framesotherthan

(1) and(2)

A. Memberswith excellentductility 0⋅30 0⋅40 0⋅35B. Memberswith goodductility 0⋅35 0⋅45 0⋅40C. Memberswith fair ductility 0⋅40 0⋅50 0⋅45D. Memberswith poorductility 0⋅45 0⋅55 0⋅50



Building Standard Law was the current edition of the law at the time of the Hyogo-ken Nanbuearthquake.

2. DAMA GE TO ENGINEERED BUILDINGS DURING THE HYOGO-KEN NANBUEARTHQUAKE

TheHyogo-kenNanbuearthquakeoccurreddirectly beneath amajormetropolitanregionin theHyogoprefecture.The earthquake occurred on the north-east trending Nojima fault which extendsfromAwaji IslandacrosstheAkashiStraitsto themain islandof Honshu.16 Between40 and50 km of bi-lateral fault rupturewas reported; the epicenterwasnearthe northern tip of Awaji Island.Groundmotion records from sites in the near-field show high accelerations; at least four sites recordedhorizontalaccelerationsin excess of 0⋅75g.16

Theearthquake resulted in thedeathof morethan5000peopleandtotal economic lossesthatcouldexceed $500billion. The collapse of manyhundredengineeredbuildingsandbridges,the near-totallossof oneof theworld’s largestcontainerports,andthewidespreaddamageto thecivil infrastructuremakes this earthquake and its effects of particular interest to many in the United States. For adescription of building damage and damage to residential construction and bridges,the reader isreferred to EERC.16

The Building ResearchInstitute17 estimate the number of structures in Kobe City at 238000.Although theexactdistribution of building typesis unknown at the time of writing, onepreliminaryreport7 writes‘A pproximately 5 to 10%of thebuildings appearto bethelargercommercial/industrialbuildings(includingalsomid-riseapartmentbuildings),approximately 10to 20%appear to besmallercommercialandmanufacturingbuildings(includingmixedcommercial/residential buildings)andtheremaining 70 to 85%arelow-rise residences…’

As notedpreviously, Kobe wasdevastatedduring World War II, so that much of the engineeredconstruction in the regionfollows from this time. In the late 1940s,1950sand1960s,reconstructionwork was concentratedon both sidesof the JR railway line—a railway line that both definedthetransportation corridor in Kobe following the war andclosely follows the surfaceprojection of thefault rupture.

Damage to engineeredbuildingswaswidespread in the epicentral regionwith severe damageandcollapseobservedin ahighpercentageof olderconstruction. (For thepurposeof thisdiscussion,olderconstructionis assumedto predate theintroduction of the1981BuildingStandardLaw.) Muchof thisolder construction wasconcentratedalong theJRrailway line mentionedpreviouslyandthuslocatedin the regionof mostintenseearthquake shaking.

It is convenient to categorizebuilding damageby construction period. This strategyhas beenadoptedby Japaneseresearchersto assesstheimpactof codechangesontheperformanceof buildings.For reference,the Building Research Institute estimatedthat in Kobe City, 79000 buildings wereconstructedprior to 1981and40000buildingswereconstructedafter 1981.Theconstructionperiodsusedto categorizethedamagearepre-1971,1971–1982,andpost-1982.Theseperiodscorrespond tofundamentalchanges in the Building StandardLaw identifiedearlier in this paper.

From datapublished by AIJ9 it is clearthat the most vulnerable buildings werethoseconstructedbefore 1971. However, there are limited examples of severedamage or collapse of buildingsconstructedfollowing theintroductionof thecurrentBuilding StandardLaw in 1981.Similar dataarereported for the performanceof schoolbuildings.

The poorperformanceof older construction wasnot surprising given similar performanceof suchconstructionin previousearthquakes.Thedamageto modern constructionhasimplicationsfor designpractice in the U.S. and warrants study. Damage to modern reinforced concrete construction isattributed to weak andsoft first stories,torsion and lap splice failures. In steelreinforced concrete



construction, damagewas attributed to anchoragefailures and stiffnessand strength irregularities.Steelbracedandmomentframeconstruction suffered damagesimilar to thatobservedfollowing theNorthridge earthquake, namely bracebuckling, anchorage failure and fracture of beam-to-columnweldedconnections.18 Someof thesefailurescanbeattributed to poordesign details.

3. COMPARISONOF JAPANESEAND UNITED STATESSEISMIC CODES

The rigorouscomparisonof modernJapaneseandU.S. seismic codesat the time of the Hyogo-kenNanbuearthquake requires designand analysisof multiple building typesof differing heightsandframing geometries—with the designsin eachcountry reflecting local construction practice.Thiseffort is beyondthe scopeof this paper.

A simpler (andlessaccurate)comparisonof seismic designpractice in JapanandtheUnited Statescanbemadeby comparingdesignbaseshearsfor regularbuildingsconstructedin Japanin accordancewith the 1981Building StandardLaw, andbuildings constructed in the United Statesin accordancewith the 1994 NEHRP RecommendedProvisions for the Development of Seismic RegulationsforNew Buildings.15

TheNEHRP Provisionspresentguidelines for theseismicdesignof buildings in theU.S.andwillform the basis of the seismic provisions in the proposednational model code, to be called theInternational Building Code,thatis scheduled for publication in theyear2000. Thestaticlateralforceprocedurein the NEHRP Provisionscalculatesa design baseshearat the strength level asfollows:

V � CsW �7�where Cs is a seismic responsecoefficient, and W is the reactive weight. The seismic responsecoefficient is calculatedas

Cs � 1�2Cv

RT2=3� 2�5Ca

R�8�

whereCv andCa areseismiccoefficientsthat vary asa function of soil profile type.Sample dataarepresentedin TableV for thepurposeof comparingthestrength requirementsof the

1981Building StandardLaw andthe1994NEHRPProvisions.Thevaluespresentedin thetablewerecalculatedassuming:(1) a storyheightof 12 ft; (2) that thebuildingsarelocatedon rock sitesin theregionsof highestseismicity (Z = 0⋅4 in theU.S.,Z = 1⋅0 in Japan);and(3) excellentductility for thecalculationof Ds in theBuilding StandardLaw. In TableV, MRF : ductilemomentframe,Wall :structural wall, andBF : concentrically bracedframe.In accordancewith the requirementsof boththe NEHRP Provisionsandthe Building Standard Law, the designbaseshearsfor bracedframes inTableV havebeenincreasedby 50%.Thedesignbaseshearcalculatedin accordancewith theNEHRPProvisionsis computedat thestrength level. In theBuilding StandardLaw, two baseshearforcesarecalculated,namelyQi, which thebuilding is requiredto resistat thestrength level,andQun, which is alower boundon the required ultimate strengthof the building computed usingplasticanalysis.

Thesecond phasedesignin theBuildingStandardLaw is notrequiredfor most engineeredbuildingsin Japan.19 Further,it is unclearif thesecond phasedesign(for forcesQun—seecolumn 9 in TableV)will leadto strongerandstiffer construction thanthat resulting from the first phasedesign.

A direct comparisonof required lateral strengthsof buildingsin theU.S.(column6 in TableV) andJapan(column 8 in TableV) would bemisleadingandwould ignoremanykey factors including theeffectsof gravity loads on member sizesand the substantial differencesin designandconstructionpracticebetweenJapanand the U.S. Nonetheless, it is evident that the force requirements of theBuilding Standard Law substantially exceed those of the NEHRP Provisionsfor reinforcedconcreteframing systems. For steelbracedframing systems, the force requirementsof the Building Standard



Law andtheNEHRPProvisionsaresimilar.For steel momentframes,themember sizeswill likely besimilar after the substantial differencesin drift limit s between the Building StandardLaw and theNEHRPProvisionsareaccounted for.

4. CONCLUSIONS

Someusefulcomparisonsof currentseismic codesin Japanandthe U.S. canbe made. BuildingsinJapanare generally designedfor greaterstrengthand stiffnessthan similar buildings in the U.S.Further,Japaneseframingsystemstendto bemoreredundantthansimilarsystemsin theU.S.As such,assuming similar levelsof seismic hazardfor design,modern Japaneseconstructions will probablyperform betterthanmodernU.S.constructionsin both moderateandsevere earthquakes.

The Building StandardLaw andtheNEHRPProvisionshaveindividual strengthsandweaknesses.The Building StandardLaw would probably be improvedby explicit evaluation of seismic hazard,perhapsthroughtheadditionof seismic responsemaps.TheNEHRPProvisionswouldbesubstantiallyimprovedby the introductionof: (1) a dual-leveldesign proceduresimilar to that incorporatedin theBuilding StandardLaw; (2) a reduction in the valuesassignedto the responsemodificationfactor tolevels more consistentwith thoseusedby theJapanese;and(3) the introduction of significant designpenaltiesfor theconstructionof irregularframingsystems,perhapsusingashapefactorsimilar to thatusedin the Building StandardLaw.

Design andconstruction practicesin JapanandtheU.S.havebeenandremaindistinctly different.Strictly speaking,giventhesedifferences,it is difficult to usetheHyogo-ken Nanbuearthquakedatatopredictthelikely effectsof asubstantialearthquakeonamajormetropolitan areain theU.S.However,some useful information canbegleanedfrom theanalysisof earthquake-damagestatisticsandbroadcomparisonof the development of seismic codesin the two countries.A reasonable parallel canbedrawn betweenchanges introduced in the JapaneseBuilding Standard Law in 1971 and changesintroduced in U.S. construction practiceson the West Coastin the mid-1970s.Another significantconceptualchangeoccurredin Japanwith theimplementationof the1981BuildingStandardLaw. It isreasonableto assumethat pre-1971construction in Japanwill respond to earthquake shakingin asimilar mannerto pre-1975construction on the WestCoast.Further, it is reasonableto assumethatpost-1981construction in Japanwill perform betterthanpost-1975construction on the WestCoast.

In theHyogo-ken Nanbuearthquake,pre-1971buildingssuffered severedamageandcollapse in fargreater proportion than buildings from after this period. Post-1981construction suffered somesignificant damageandisolatedcollapses,anddid not performup to expectation.19 The Hyogo-ken

TableV. Comparisonof designbaseshearsnormalizedby reactiveweight

UnitedStates Japan

Material Stories Frametype T (s) R V/W T (s) Qi/W Qun/W

RC 5 MRF 0⋅64 8 0⋅08 0⋅40 0⋅20 0⋅30RC 10 MRF 1⋅09 8 0⋅06 0⋅80 0⋅16 0⋅24RC 5 Wall 0⋅43 5⋅5 0⋅15 0⋅40 0⋅20 0⋅40RC 10 Wall 0⋅72 5⋅5 0⋅11 0⋅80 0⋅16 0⋅32

Steel 5 MRF 0⋅75 8 0⋅07 0⋅60 0⋅19 0⋅24Steel 10 MRF 1⋅26 8 0⋅05 1⋅20 0⋅11 0⋅14Steel 5 BF 0⋅43 5 0⋅25 0⋅60 0⋅28 0⋅33Steel 10 BF 0⋅72 5 0⋅18 1⋅20 0⋅17 0⋅19



Nanbulessonsfor building seismicdesignpractice in theU.S.arethusclear. In theeventof a severeearthquake in a major urbanarea:

(1) older construction (pre-1975) will suffermuch damageandwidespreadcollapse(2) new construction may not perform as expected, suffering substantial damage and isolated

collapse;(3) substantial lossof life andmajor economic lossesareli kely.

The challengesfacing designprofessionals, researchers, plannersand policy makers are many.Performance-based design procedures are needed. Simple, yet reliable, design methodsmust bedevelopedandcalibrated to producenew buildings with predictableperformance. Efforts areunderway in bothJapanandtheUnited Statesto produce performance-oriented seismicdesignprocedures.Thereaderis referredto ATC20 for adescription of recentlycompletedon-goingactivitesin theUnitedStates.Moreover, cost-effective retrofit designand construction strategiesfor vulnerable buildingsmustbe developedand implemented to mitigatethe seismic risk, both physical andfiscal, in olderconstruction.

REFERENCES

1. AIJ, Illustration of Building Constructionsin Japan, ArchitecturalInstituteof Japan,Tokyo, 1960,85pp.2. Y. Otsuki, ‘Developmentof earthquakeresistingbuilding constructionin Japan’,Proceedingsof the First

World Conferenceon EarthquakeEngineering, Berkeley,California,U.S.A.,1956pp.16.1–16.1.3. Y. IshiyamaandY. Ohashi,‘History of structuralregulationsin Japanesebuildingcodesandoutlineof wind-

resistantand seismic regulations’, N. Raufaste(Ed.), Wind and SeismicEffects, NIST SP 776, NIST,Gaithersburg,Maryland,U.S.A.,January1990,pp. 335–347.

4. T. Naka,M. WakabayashiandJ.Murata,‘Steel-reinforcedconcreteconstruction’,IABSE PreliminaryReport,Amsterdam,TheNetherlands,1972.

5. H. Aoyama, Outline of EarthquakeProvisions in the RecentlyRevisedJapaneseBuilding Code, JapanInternationalCooperationAgency,TsukubaInternationalCenter,Japan,1993,34pp.

6. IAEE, Earthquake Resistant Regulations—AWorld List, International Association for EarthquakeEngineering,Tokyo, Japan,1992.

7. EERI, The Hyogo-ken Nanbu Earthquake: Great Hanshin Earthquake Disaster, 17 January 1995;Preliminary ReconnaissanceReport, EarthquakeEngineering ResearchInstitute, Oakland, California,U.S.A.,1995.

8. AIJ, DesignEssentialsin EarthquakeResistantBuildings (English translationof the 1966 publication inJapanese),ArchitecturalInstituteof Japan,Tokyo, 1970,295pp.

9. AIJ, Preliminary ReconnaissanceReport of the 1995 Hyogo-kenNanbu Earthquake(English edition),ArchitecturalInstituteof Japan,Tokyo, 1995.

10. T. Naka, M. Wakabayashiand S. Takada, ‘Quake resisting design of compositestructuresin Japan’,Proceedingsof the SecondWorld Conferenceon EarthquakeEngineering, Vol. 3, Tokyo, Japan,1960,pp.1811–1826.

11. S.Morino, I. NishiyamaandN. Sakaguchi,‘Hybrid structuresin Japan—researchandpractice’,SSRCAnnualMeeting,Lehigh University,Pennsylvania,U.S.A.,1994.

12. N. Chiba,‘Building in steelin Japan’,Proceedingsof the InternationalSymposium‘Building in Steel—theWay Ahead’,British ConstructionalSteelworkAssociation,London,U.K., 1989.

13. AIJ, Design Standard for Steel Structures (English translation of the 1970 publication in Japanese),ArchitecturalInstituteof Japan,Tokyo, 1979,113pp.

14. Y. Ishiyama,‘Aseismicdesignmethodandits historyin Japan’(Summaryof oral presentation),Proceedingsof the 3rd U.S.–JapanWorkshopon the Improvementof Building Structural Design and ConstructionPractices, ATC-15-2,Applied TechnologyCouncil,RedwoodCity, California,U.S.A.,1989,8pp.

15. BSSC,NEHRPRecommendedProvisionsfor the Developmentof SeismicRegulationsfor NewBuildings,Building SeismicSafetyCouncil,WashingtonDC, U.S.A.,1994.

16. EERC, ‘Preliminary reconnaissancereport on the 1995 Hyogo-kenNanbuearthquake’,ReportNo. UCB/EERC-95/01,EarthquakeEngineeringResearchCenter, University of California, Berkeley, California,U.S.A.,1995.



17. BRI, Materialsdistributedat theU.S.– Japanseminaron thereductionof earthquakedisasterin urbanareas,Building ResearchInstitute,Tsukuba,Japan,1995.

18. AIJ, ReconnaissanceReportonDamageto SteelBuildingsObservedfromtheHyogo-kenNanbuEarthquake,SteelCommitteeof the Kinki Branchof the ArchitecturalInstituteof Japan,Osaka,1995(in Japanese).

19. M. Ozaki,Personalcommunication,1995.20. ATC, ‘A critical reviewof currentapproachesto earthquakeresistantdesign’,ReportNo. ATC-34, Applied

TechnologyCouncil,RedwoodCity, California,U.S.A.,1995.



evolution of seismic building design practice in japan

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