otto glogau (1975) (1)0043 - code philosophy behind nzs4203

7/22/2019 Otto Glogau (1975) (1)0043 - Code Philosophy Behind NZS4203

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C O D E P H I L O S O P H YO . A . G l o g a u *

SYNOPSISThis lecture presen ts backgr ound in forma tion on the philoso phica lframew ork within which some of the mor e important de cisions were mad eand which led to the final proposal for the seismic provisions for NewZealand NZS 4203 "Code of Practice for General Structural Design andDesign Loadin gs". Aspects of ductility and seismic design coefficientsfor vari ous types of structu re are consi dere d with illustrati ons toshow how the new code covers the nece ssary desig n requi remen ts. Thedesign principles lying behind the code requirements are discussed,part icul arly for concre te frame and shear wal l type struc tures. Theinter acti on between eart hquake motio n of the subsoil and moti on of thestruc ture is also discusse d. A number on unreso lved problems are hig h

lighted, particularly those concerned with short period structures,long acceleration seismic pulses , the damping-amplification relationshi pand math emat ical mode llin g. Overs eas codes are compared showing latestdeve lopm ents and illustrating the wide rang e of values used for the basi crequ irem ent. The lecture conclu des with a consid erati on of likely futuredevel opmen t and the manner in which the code is likely to be updated inthe future.

1.0 INTRODUCTIONThe objectives of the code are given inthe model buil ding by- law portion of thecode and need not be repeate d here. Theydiffer little from those set by the codesof other advanced countries ( 1 / 2 , 3 ) : and

indeed from those given in the (little read)comment ary on the presen t New Zealand Code(4).Setting desirable general objectivesis thus not particularly difficult for acode committee, but quantifying requirementsthat will achieve the se objectiv es is not aneasy task. The committ ee must produce a setof man dato ry clause s and guidel ines thatwill achie ve a reaso nable level of protecti onfor life and property at an acceptable cost.They should take into account the relativeseismicity of New Zealand as compared withother coun trie s, its gene ral standard ofliving, locally established buildingpracti ces, available material s, and designmethods.The depth of detail with which a codeshould spell out its requirements issubject to considerable difference of opinionamongst engi nee rs. Some wish to see nomore than a statement of objectives, andundo ubted ly, giv en a capa ble desig ner and aprogressive client the result can beexce llen t. Unfo rtun ate ly, in a real lifesituat ion the probab ility of both these twoconditions occurring together is small andgeneral requirements on their own, such as"buildi ngs shall have duct ility " areineff ectiv e in compe titiv e design, and evenless so in "design and build" situations.Accordi ngly, it has been the co mmi tte e 1smajority view, based on past experience, that

* Chief Struct ural Engine er, Ministry ofWorks and Development, New Zealand.

the objectives of the code will be achievedonly it it spells out its min imum crite riain consid erabl e deta il. It is also impo rtant that a code does not crea te cond itio nsunder which competent and consc ient iou sdesigners are forced to choose between thelimited interest of their clients andthose of the genera l public at larg e. Anexample of this is the situat ion thatnearly always arises if the re qui reme ntsfor protection against nons tructu raldamag e are not made suffi cient ly spec ific .

Providing adequate restraints whileavoiding overly restrictive clauses thatwill stifle sophisticated and innovativemet hod s is one of the most di ffi cul t tasksfor code commit tees. One of the fea turesof the code that eased this task was themanner of presenting mandatory andcommenta ry clauses on opp osi te pages inclose associa tion. This appr oach isparticularly useful when dealing with asubject, such as earth quake engi neer ing,in whic h developments a re rapid but many"grey" areas remain. Guid ance gi ven in thecomment should be helpf ul to the busydesigner but allow some scope for alternativesto those willing to spend the time andeffort to investigate in depth.It has been recognised th at engi neer smust make design decisions in situationswhere the facts are imperfectly known andthe committee has not shunned its responsibility in avoiding vagueness where verpossibl e. The conventional decisio n-makin gproc ess in areas wher e there is lack ofsufficient information from earthquakedamage and/or research includes that time

honou red approach used by all commi ttee s -review of overseas codes and drafts, intuition,and finally committee consensus followingcareful review of all comme nts. Invariablysome requirements are fairly arbitrary buthopefully - notwithstanding the standard

B U L L E T I N O F T H E NE W Z E A L A N D N A T I O N A L S O C IE T Y F O R E A R T H Q U A K E E N G I N E E R I N G , V O L . 9 , N 0 . 1 . M A R C H 1 9 76


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4 4joke about a camel being a horse d esigne dby a committee - these arbitrary requi remen tsshould be better and mor e re pres entat ivethan were they based on one m a n 1 s opiniononly. Camels after all are better th anhorses for the purpos e they were design edfor

Sinc e it is out sid e the scope of thislecture to discuss detailed aspects of thecode, this is done only wher e they high ligh tsome of the more unu sual and importantfeatures.2.0 DUCTILITY

The 1965 New Zealand code incl udedsome vagu e refer ences to ducti lity. Howthis was to be achiev ed was far from cle arand the only available guidance was aslightly modified version of the SEAOCrequir ement cont ained in the New ZealandConcrete Code. Very few non public buildi ngsof less than four sto reys in hei ght w eredetailed with the aim of achiev ing ducti lity,wher eas the lessons from San Fern ando (1971)and Toka chi- oki (1968) are clear - allbuildings regardless of height must bedesigned to posse ss some degre e of duct ilit y.The code distinguishes between the degree ofductility that should be available inductile structures designed to low S factors(.8 to 1.2) and that ava ila ble for o therstructures. Ductile structures, whichinclude both frames and shear walls, capableof dissip ating seismic energy in a flexura lmode at location s and in a mann er specifie dby the code , should possess "adequateducti lity" . Some guid ance is given in thecode with regard to what is mean t by"adequate" by specifying the number andmagn itud e of excurs ions into the inelast icrange whic h a structu re should be capa bleof witho ut exc eeding a given loss ofstrength during the pro cess . In the caseof conventional designs these requirementsmay be assumed to have been met by compl iancewith the detailing rules given in themate rial code s, their pendin g revision s orin suitable reference publications.

The term ductility without the qualification " adequa te" is used to descr ibe themuch less well understoo d requireme nt forsystems that dissipate energy in a shearm o d e . These systems must be design ed forloadings derive d from S = 1.6 or mor e. Insevere ear thqu ake s, damage is expecte d but,becaus e of the geomet ry of these sy stems ,total instabi lity is unlikel y to result.Resea rch is being underta ken into thebehaviour of these systems but progress isfraught wit h diffi cultie s because thetesting of even scaled down simplis ticrepresenta tions of real buildings requireslarge capacity equipment and expensivemodels3.0 TOTA L HORIZ ONTAL SEISMIC FO RCE

The new code introduces the concept ofa multi-term evaluation of the horizont alseismic design co-e fficie nt to be appliedto vari ous structu res. Just how manyseparat e facto rs should be given was one ofthe more diffi cult decis ions that thecommittee had to make . Obviously many morepara meter s affect the perf orma nce of astructu re and if known with a dequa teprecision the committee would probably have

favoured their inclus ion. More factorstend to give mor e choice to d esig nerswithout any significant increase in designeffort . After all, factors can readily becombined into simpler expres sions. However,it was consider ed that at pres ent adequat edata was lacking and in fact severa lco-ef fici ents g iven are no more than a broadreflection of the committee 1s combined viewof expected earthq uake perfo rman ce and thereis unavoidable overlap of several parame ters.If for instance a presum ed di ffer ence of 3%in damping bet ween a structural steel frameand a reinforce d concre te frame bui ldingrepres ents a 1.3 factor in peak respo nse,then the perf orman ce of the structur al steelbuilding designed using U - 0.8 would needto be 1.7 times that of a reinf orced concreteframe using M = 1.0. Diff ere nces in unde r-capacity factors tend to increase the gapby a further 10% . In prac tice the requireddiffe rence in ducti lity is usua lly much lessbeca use stiffn ess criter ia have to be metand in many instance s are the critical desig nconsideration. Neverthele ss the commentmade with regard to the requ irem ents forreinforced concrete detailing also appliesto structural steel.

The committee is well awa re thatultimately all factors including thosespecified in the mate rial code s are reflect edin the desi gn. It was not overl ooked forinstance that reinforced masonry wouldgenerall y be designed for hori zont al l oadingsthat were 1.6 x 1.2 / 0.8 = 2.4 times theloading for ducti le reinfo rced c oncre teframes and additionall y tha t lower mater ialunder capaci ty factors would apply . At thesame time this large difference was basedon, and made condit ional to, signif icantimproveme nts being made to curre nt desig nand detaili ng of the reinfor ced concret eductile shearwalls and frames ( 6 , 1 3 ) 9It does not appear to be gener allyapprecia ted just how wide the ran ge of themulti-term derived C^ values is, partic ularlywhen several of the factors differ by nomore than 10 or 20%. In the following on lythe range of factors commonly in use havebeen included - extreme va lues of risk andunusual structure types have been exclud ed:

ZONING Short period range= 0.15/.1 = 1.5Long period rang e= 0.075/0.05 = 1.5

SOIL FACTOR Short period range = 1Long period range= 0.08 25/ 0.05 = 1.65IMPORTANCE 1.6/1 = 1.6STRUCTURE ( Ca te go ry 1 - 6 ) 1.6/0.8 = 2MATERIALS 1.2/0 . 8 = 1. 5RISK (Categories 1 and 2 only)1.1/1 = 1.1DYN MI ANALYSIS 1/0.9 = 1.11TOTAL RANGE (Short perio d) = 8.8

Reference has already been made to thefact that many of the terms are uncertain inmagn itu de. In Japan the zoning range (3zones) is only 1.0/0.8 = 1.25 but in U .S.A .it is 4. Japa n uses at prese nt a combinedsoil and construction factor with a rangeof1.5/0.6 = 2.5. In Cali forn ia the range


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4 5of stru ctu re fac tors is 2 (or 2.86 if shea rand diagonal tension requirements areincluded).

Recently severe (provisional) requiremen ts for hospi tals have been i ntroduce din Cal ifo rni a, see Figure 2, wher eas SEAOCin their 1974 cod e specif y an I fact or of1.5 for esse ntia l facilit ies.The value of C d in the equat ion V =C W t whe re C^ = CISM R is one of theimportant parameters in providing satisfactoryearthquake resistance and some general comments on its level for vari ous st ructur es ma ybe useful.

3.1 Ductile FramesIn suitab ly designed and detail edducti le fram es the level of has animportant effect on the area of the hysteresisloop avai labl e for energy diss ipati on. Low

C^ value s result in large inelastic resp onseand hence large ductili ty demand. Give nthe present state of the art of detailingreinfo rced con crete (6) or struct ural steelthe level of C^ required by thi s code may bemarginally too low.For purposes of this discussion we mayacce pt the valid ity of elastic respo nsespectra as giving a broad in dicat ion ofducti lity dema nd. Let us consi der furthera severe but by no mean s maxi mum credi bleearth quake of the Skinner Spectru m type(scaled to l/3g maximum ground acceleration).If subjected to this earthquake, a class

III, category 2 reinforced concrete b uildingwit h 0.4 second fundame ntal period andposs essi ng say 5% damping will be subjec tto an overall dis placement ductility demandof 0-33 x^2.33 = 5. Loca l ductil ity dem andsare significantly greater than overallductility de mand so that most buildingsare likely to suffer some structural damagein an earthquake of this size unlessdetai ling is improved ^ # T n e majority ofreinforced c oncrete buildings will bedesign ed for C^ value s 20% lower (category1) and whe re a dynamic an alysi s has bee ncarried out the value could be even l ower.Desig n to lower C^ factors incre ases d ucti litydemand b ut is compensated for by the factthat many buildings will be at least 20%stron ger than indicated by C^ valu es be caus ethe probable yield strength of the steel isgrea ter than the mini mum assumed andBausch inger effects produce increased stresslevels beyon d that at first yield duri ngsubsequent loading cycles.

The 1965 New Zealand code envisagedthat red uct ion fact ors of only 4 woul d berequir ed as it was thou ght , and that wasprob ably cor rect for the types of st ructu resthen buil t, that 10% damping could beexpected during intense ground motion s.The re are a numbe r of reason s why the ba siclevel of seis mic co-effi cient s for ducti leframe s has been somewhat reduce d:(1) Giv en the desi gn and detai ling pri nci plesof the code the performance of ductileframes should be better than that ofthose designed and detailed under thepresen t provisions for a higher C 3v a l u e , with much less attention giv en

to , and knowledge of other aspects of

the design.(2) No direct evid ence from eart hqua kedamage is avail able with regar d to thelack of effectiveness of the type ofprovisions contained within this code.(3) Well propo rtion ed struc ture s mayrespond less intensely than is predictedfrom elastic respon se spectra der ivedfor single mass syst ems.(4) Improved struc tural deta ilin g proc edur esare in advanced stages of devel opme nt.(5) Reduced requ ireme nts for the horizon talloadings of ductile frames would helpto pay for urgent ly neede d upgra dingof other aspects of the 1965 New Zealandcode provisions.3.2 Partially Ductile Shear Walls

The 1965 New Zealand cod e con sider edthat squat shear wal l buil ding s would pos sesssuffici ent damping to comp ensa te for theincreased resp onse due to other ca use s.Histor ically the perf orma nce of shear wallbuildi ngs has been very good at least fromthe point of view of prote ctio n of life andnonstructural compon ents. This good performance was partiall y due to the bulk of the sebuildings being low structures with wallsperfora ted by relat ively small open ings andposses sing stren gths greatly in excess ofthe minim um require d by the cod es.

Mode rn structure s are mor e likely tohave walls that are deep mem ber ed fr ames orisolated squat pier s of no mor e than co delevel strength, and these struc tures cannotgenerally be expected to perform satisfactorily if designed us ing the low C 3 valuesspecified for ductile frames . Systemsdissipating most of the seismic energy in ashear mode are stiffn ess degrad ing system swhich have much greater re spons e thanductile flexural systems exhibiting relativelystable hysteretic behavio ur. Detailingmeasu res known at pres ent can only delay thedegradation process and therefor e satisfactoryperformance requires desigrrsto a higherloading. Further more, the behaviou r of manyof the shear faili ng wal l syste ms is dif ficult to predi ct and may vary fro m du ctil eflexural, to shear failu re or roc king . Thebulk of structures in this category are ofshort period whic h adds to the u ncert aintyof the behaviour O f 1 0 ' .Direct eviden ce that the 1965 New Zealandcode loading levels are insufficient for thistype of structure comes from in vest igati onsinto the failure or severe damage tc manystructures during the 1968 Tokachi-oki earthquake ( I D . These conventionall y designedand detailed structures of less than 4storeys were built to resist 0.2g a t workingstress es and were of a cons truc tio n standar dat least equal to that in New Zealan d. Itis notewo rthy tha t the eart hqua ke had itsepice ntre at a dis tanc e of some 250 km andrecorded ground accelerations were only

0 225g maximum. Very extensive in vestigationscarried out in Japan in dicat ed th at onlythose structures capable of resisting theequiv alent of a hori zont al stat ic load oflg remained completely undamage d.

For partially ductile shear wal ls, thecode specif ies a level for Cd of twice th atfor ducti le buil ding s in categ ory 1, thusgiving a result ant ultim ate lev el of C 3 fora class 111, reinforced c oncrete structure


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4 6of only .24g and for reinforced mas onry 029g.These load levels are lower than those forwhich the Tokachi-oki structures wer edesigne d, but are probab ly the maxi mumacceptable at present. To minimis e damageduring earthquake atta ck, the coderecommends detailing measures that willpromo te the formati on of a distr ibute dsystem of cracking of controlled width.

(12)SEAOC pract ice has for many yearsrequired higher loadings for "box systems"the equivalent of our category 4 structures.These systems not only have to be des ignedto twice the values for ductile frames butadditi onally the load factor for sheardes ign has to be increa sed from 1.4 to 2.0as wel l. Refer figure 1.3.3 Ductile Shear Walls

The value of specif ied for thevarious types of shear walls which qualifyfor inclusion into categ ory 3 or 4 isroughly comparable to that for ductile frames.Refe rence (13) contains most of the background material that led to this decision.3.4 Other Structural Systems

Apart from the diag onal bracing systemcapa ble of plasti c defo rmat ion in tensiononly which has been the subject of limitedresear ch only d ^ ) , the code make s someprovi sion in the small buil ding s section fora few structural types that do not properlyfit one or the other cat ego rie s. The perform ance of these types of structure is lesspredictabl e than that of others, but established prac tice , parti cula rly in the presum edseismic ally less active nor the rn regions ofthis country made it necessary that the codeshould make some prov isio n for them. Wheth eror not the loading specified by the code andthe built-in restraints are adequate is notclear at this stage, therefore the skilland integrity of the desi gner s and thevigilance and competence of the vettingautho rity may play an ( uncomforta bly) largerole in ensuring a satis facto ry d esign .4.0 DESIGN P RINCI PLES

This sect ion is the mos t importantaddition to the original draft of the codewhic h was circul ated for commen t. It hadbecome clear that the information in theexisting material codes or their proposeddraft revisions was inadequate and thatdesigners required some guida nce. Becauseof its importance clause 2.3.1.1 of thesection on Energy Dissipation and CapacityDesign, will be repeated in its entiretyhere:

Design shall be in accordance with therelevant New Zealand Standard for thestructural material concerned provided thatthe gene ral prin cipl es of desig n set outbelow shall be f ollowed :(a) Buil ding s shall be desi gned to dissi pate

significant amounts of energy i nelastic-ally under earthquake attack.(b) Buil ding s designe d for flexu ral ductileyielding, or for yielding in diagonalb r a c e s , shall be the subject of capacitydesign. In the capacity design of earthquake-resistant structures, energy-dissipati ng elements or mechan isms are

chosen and suitably designed anddetail ed, and all other structu ralelements are then provided with sufficient reserve strength capacity toensur e that the chosen ener gy-di ssipa tingmechan isms are maintained throughoutthe defo rmat ions that may occu r.Columns or walls which are part of atwo-way horizont al force resistin g systemshall be desig ned for the concur rent effec tresulting from the simultaneous yielding ofall beams framing into the column or wallfrom all directions.The code then proceeds to give detailedrecom mendat ions for frames and walls ai medat achieving the above objectives.The state of the art is such, however,that conside rabl e caution is required if weare to be reaso nably succ esful . The respons eof a structure itself is uncertain and will

be discus sed later, but so is the dis trib ution of loads with in the struc ture. Studi esusing numerical integration responseanalysis techniques into the post-elasticrange have indicated that even in regularbuildings with conservatively designedcolumns current methods of analysis anddesign cannot exclude the possi bilit y thatcolumn hinges will form, possibly at bothends with in a single storey. The dist ribution of beam capaci ty moments into c olum nsis also very uncertain as the number ofstoreys that wil l simultane ously form beamhinges and hence the appropriate columnaxial loads is no more than an educatedg u e s s . Provided stringent detailing measur esare taken, forma tion of a column hinge fora short period of time is probably notdisas terou s even in a RC column but anycondition that could result in non-ductilefailure is serious, and therefore to avoidthis type of failure conserv ative assum ption sare nece ssar y. Assum ed axial loads have animportant bearing on flexural capacitiesand these in turn on column shears Theleast resea rched para meter is the verti calearthquake effect. Measured amplificationof the vert ical comp onents of ground mot ion shave been very high at San Fernando, butjust how dama ging these are when occurr ingover a short interval of time in combinationwith horizon tal motio n effects is not knownat prese nt. Theor etic ally the ductili tydemand on upper storey columns and beams issignif icantl y increas ed. Some arbitr ary,modes t provis ion was made in the orig inaldraf t of thgls cod e but des ign er s, alre adystruggling to preven t columns from liftingdue to the axial tension induce d by sever alstoreys of hingeing beams, found the provisionsvery diff icult to meet in prac tice . Unti lmore factual information becomes availablethe committ ee decided to releg ate ver tica learthquake eff ects, excepting those onhorizontal cantilevers, to a non-specificcommentary clause.

Column hinge mechanisms as distinctfrom other column hinges are not permittedexcept in structures of less than two storeysand in top stor eys. This requir ement isintended to avoid the extremely highductility demands that result from sidesway mech anis ms. Recent research into thebehaviour of buildings with soft firststoreys (15) revealed the extreme complexityof the who le subject of inelastic r espo nse.


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4 7Perha ps the mos t interestin g result of thisstudy was not the very la rge (expected) f irststorey drifts but the unrel iabil ity of thesoft first storey as a safety fuse for theprote ction of upper stor eys. Even when theyield strength of the first storey wasreduced to l/10th of that of a normalstructu re it transmi tted shear fo rcesgreatly in excess of its yield value, thusresulting in large ducti lity dema nd in thestoreys abo ve. (The elasti c stiffn ess wasnot v a r i e d ) . Only when the first storeyprovided essentially an elastie-perfectlyplastic mechanism and in addition had agreatly reduc ed yield streng th, were thetransmitted forces limited. The particularstudy also highl ighted the need to conside rearthquake requirements o n a statisticalbasis as the authors who had used twentydifferent motions in their study conclu ded:

Analysi s of response d ue to 1 or toeven several earthquakes is likelyto be quite mislea ding .The subject of concurrency has beenadequately discussed in reference (18).The uncer taint ies of seismic loaddistribution are compounded by the uncertainties in the behaviour of joints andmembers The detailing of cantilever

shear wall str ucture s is far from resolve d.One of the most signific ant para meter s,the magnit ude of the shear forces corresponding to the yield capaci ty mome nt is not wel lestablishe d, but a recent study 0-7) indicatesthat they have been great ly under esti matedin the pas t. How damag ing these relativel yshort high shear stress peaks are is notknown at present.

A number of modificati ons we re made tothe origina l draft to cope wit h the desig nof squat shear wal ls. If the requir emen tsof capacity design for ductile walls wereapplied to these systems several an omali eswould ari se. It seemed unre ason able torequi re the shear strengt h of these wall s,which are presumed to fail in shear toexceed that of their flexura l yield capa city.In the case of walls posses sing relat ivelylow shear strength as for example reinforcedmasonry wal ls , the minimu m reinforcementintrodu ced for other purpos es such as faceloadings, shrinkage e t c is sufficient togiv e these walls yield capa citie s thatexceed any reliable shear strength theycan be expec ted to pos ses s. In fact wal lgeometry usually precludes ductile behaviour.In secti on 3.1.2 refer ence has been mad e tothe rather more severe SEAOC r equir ement s(see also fig. 1 ) . Flexural reinforceme ntrequir ed in squat wall s for purp oses ofresis ting in-pl ane stres ses is as a rule noprob lem. One reason for not lowering theS factor to that used for flexu re forstru cture s of catego ries 1 to 5 is that th ebeha viou r of many catego ry 6 struc ture s israther unpre dicta ble. Even if some sort offlexural behavio ur were exhibited by astru cture in this cla ss , or some of itsm e m b e r s , this would not automaticallyconstitu te adequate ductili ty .

Fou ndat ion desig n for squat shear wall swas anoth er difficulty that led to modi fica tions to the original draft. Rocking ofshear wal ls, although little investigatedto dat e, appe ared to be an accept able

alternative to dissipating seismic energyby shear , provide d it did not take plac eat too low levels of res pon se. Acco rdin glyrocking of shear category 6 wal ls ispermitted at loadings corresponding toappr oxim atel y S = 1.3 and for any s yste mat a level of SM = 2. Mor e resear ch isurgently needed particularly with regardto the likely move men ts that wi ll takeplace during rocking to enab le the design erto evaluate the consequences.5.0 SUBSOIL EFFECTS

Qualitative considera tion of subsoileffects on the response of structures hasbeen recommended since 1965 ( 4 ) f hencemore definite requirements contained inthe new code. Some of the reas ons as towhy this was done only in very br oad te rms ,are given in refer ence (7) and t heseinclude: lack of uniqueness of site periodand damping characteristics wh ich aredepe ndent on ampli tude of shaki ng. Ofparti cular in terest in this rega rd is acomprehensive set of recordings made todeter mine the respo nse of the earth fillSANNOK IA Dam (Okamoto et al, 1966) Theratio of maximum cre st accelerati on tomaximum base acceleration varied from about5 for base motions of 0 03g to about 2 formotions of lg.

Interesting too are simu ltan eousrecor dings tak en in Japa n on and below theground surface (1). In an eart hqua ke wit haccelerations approx. 0.lg at groundsurface near the Earthquake Researc hInstitu te, Tokyo, only half the value ofaccelerations were measured at basem entlevel. Similarly acceleratio ns measuredon the ground in another l ocati on in Tokyower e 3 to 6 times gre ater than t hose 15 munderg round in an earth quake with p eakaccel erati ons of about O.Olg. Fromresp onse spectr a of the mot ion s it hasbeen determined that the large surfac eamplifications predominantly applied toshort period waves but not to those withlonger periods for whic h the rat io t endedtowar ds 1. Umem ura sugg ests that asmuch as a 50% reduction in design co-efficientfor short period buildings with basemen tscould be appro priat e (0.2g) and su pport sthis by lack of damag e to rigid buil ding swith basemen ts in Japa n.

Some designers may have hoped thecode would contain recomme ndation s for codecurves relating building period to siteperiod as has been done in the 1974 SEAOCprovisions . Perhaps a reference to fig. 1compar ing SEAOC flexibl e soil and AlJ stiffsoil curves is in order to put the m atte rin perspective.The following example discusse d inreference (5) will highlight the di fficu lties Figu res 4A and 4B are re lat ivevelocity spectra of ground motion recordstaken during the San Fernando earthqua keof February 1971. Figure 4A shows relative

velocity spectra less than h km apa rt (fig.3) on similar depth s of allu vium . Fig ure4B shows that the maxi mum res pon ses onrock (SL site) were the same as those on900 ft of alluv ium (ATH site).Hudso n ^ revie wing reco rds from 20

ground stations during the San Fernando


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4 8

earthq uake concluded that it appeared thatlocal surface and subsurface topographymight overshadow the influence of localsite conditions and that another earthquakemight be equally complicated but quite different at any par ticu lar site. Some evidenceof a similar nature has also been observedin N.Z. ( 2 2 ) b Earthquake motions particularlythose of distan t earthqu akes are s ignificant lymodifi ed by subs oils . The effect of surfacelayers has been compared to that of afrequency filter and power ampli fier. Asa result some ground motio ns may be amplifiedto a much greater degree ^h an o thers. Itis thought that the serio us damage to lowmodern reinforced concrete structures atTokac hi-ok i (8) 1968 could be attributed inpart to the spectru m modi fyin g charac teris ticsof the surface layer.6.0 SOME UNRE SOLV ED PR OBLE MS

Almost every aspect of aseismic designand analysis contains elements of uncertaintysome of whic h will be discuss ed in otherpapers at this seminar. A number ofaspects are of part icul ar imp ortance to aloading code . For instance , one of thequestions that could not be answered withany degree of certainty was whether thebasic design co-efficients of the coderesult in a reason ably co nsist ent deg reeof prot ecti on for structures of varyingfundamental period but of otherwise similartype and mat eri al. Regio ns of parti cularuncertainty appear to be the level of thebasic co-efficient for structures of lessthan 0.4 sec per iod an d that for thosegreater than say 2 sec.Even if one accepts that earthqu akerespo nse spec tra for buil ding s on stiffsoils are reaons ably wel l mode lled by theshape of the "Skinner" spectrum envelope,a scaled down versio n of this shape may notbe a satisfactory guide for the design ofstructures on which overall displacementducti lity d eman ds of 4 or more may be mad e.At first sight it migh t appea r that the flatportion of the code curve for periods lessthan 0.45 sec is ove r-co nser vati ve sincedamped elastic acceleration response spectraon stiff soil have a falling branch in thisregion. Surprisingly the question concernsthe exact opp osit e - namely wh ethe r for

buildings of less than 0.4 sec designed tocode levels excessively high ductilitydemand could result (9 ). There is considerable uncertainty about the low frequencycontent of earthquake ground motions and itwould be very impr udent to design longerperiod structures to a scaled down ElCentro curv e. SEAOC recogn ising this

have over the course of years lifted andflattened their curve in this region andtheir 1974 curve for average condi tionswhere site resonance is not specifi callyinvest igated is now well above all N. Z.curves betwe en 3/4 - 2 sec (fig. 1 ) .A source of particular concern in thedesign of structures relying on significantinelastic behaviour for their survival isour lack of knowledge of the number andchara cteri stics of large and long acce lera tionp u l s e s . The effect of these puls es iscompara ble to applying a horiz ontal staticforce exceeding the capacity of a structure,render ing even the best hyste retic beha viou rinef fect ive. Some of the damag e at the OliveView Hospital complex during the 1971 earthquake has been attributed to an early longpulse only half as intense as the laterp e a k ( 2 3 ) # if a long pul se occur s at astage during an earthquake when a str uct ure 1selastic resistance has already been exhausted,

then obviously the intensity of a long pulseleading to failure need not be very great.If the prescence of such long pulsesis a reason ably high probabil ity in earthquake motions consider able caution wouldneed to be exercised with regard to r elatin gcode desig n curves to linear elastic resp onsespectra.Desi gners wishing to inves tigate thebehaviour of their structures using anumerical integration response analysiswill need to use the results within thelimit ation s imposed by other an alys ismeth ods specified by the code. This method

is a valu able re search tool but the va lidi tyof the result depends on the character ofthe ground mot ion used, the chose n d ampingfactor and the mathe matic al mode l of thestru ctur e. Becau se the committ ee was unab leto mak e recom mendat ions for several of thesevital parameters limitations had to beimposed to avoid large diff eren ces instrengths and stiffness resulting from varying assumpti ons. Difficulties with regardto specif ying an appro priat e suite of eart hquake mot ion s has already been d iscu ssedabove and under subsoil effects but evenif the ground motion were of a less randomnature the problem of response would be farfrom resolved.The table below compares re comme ndeddamping values by two authoritativeorganis ations . The difficulties associatedwith deter minin g suitable damping value sare due t c the fact that the valu es depen dnot only on the structure but also its nonstructural components and further on the

R E C O M M E N D E D D A M P I N G R A T I O S

B.R.I.Japan Los AngelesCity Draft CommentsSteel frame 0.01 0.02 - 0.05 L.A. values vary withfabrication and cladding.Reinforcedconcrete frame 0.03 0.05 - 0.10 L.A. dep end ing onstructure type claddingetc.


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4 9intensity of mot ion . Few if any recordshave been obtained from buildings dissipatingenergy inelastically during an earthquake.

Dampi ng has a signi ficant eff ect onampli ficat ion a s is shown in Fig. 5 wher evalues given by several authors arecompar ed. The range of valu es for a give n% of damping during elasti c respon se isalmost 100 %. Obviously the error introducedby unce rtai ntie s in dampin g varie s with thefundamental period of the structure and thespectral intensity of the moti on. Equivalen tviscous damping is most effective for motionsresemb ling harm onic mot ions and at peakresp onse. Mu ch less error resu lts for lowregions of response i.e. a 60 storey structuresubjected to an El Centro 1940 NS mot io n,and for inelastic responses during which alarge amount of hysteretic damping is prese nt.Another important parameter, the mathemati cal mode l of the buil ding is also very

imprecisely know n, particularly the forcerestoring characteri stics of the system(shape of hyst eres is loops) whic h ge nera llyvari es with each cycl e. Even the mos tambitious computer programs use radicallysimplifi ed mod els of real 3-dimen sionalstructur es, particularly for RC. Little isknown for instance of the extent to whichfloor slabs and secondary beams participateat high levels of concu rrent inelas ticresponse.The code as a mat ter of pr acti calneces sity does not require foun datio ns tobe design ed for forces grea ter than tho seresulti ng fro m SM = 2. For zone A class

111 structures no foundation needs to bedesigned to a level greater than 0.15 x 2= 0.3g. The implic ations of this provisi onparticularly with regard to rocking are notwell understood.7.0 COMPAR ISON WITH RECENT OVERSEAS CODES

Figu res 1 and 2 are composite s relati ngthe seismic loading provis ions of NZS 4203to that of some Cali forn ian and Jap anes ecodes. An adequate comparison requiresnorm alis ing of curves to allow for diffe renc esin load factors and whet her the eart hqua keload is der ive d from dead load onl y (as isgenerally SEAOC practice) or includes aport ion of the live load. Sign ific ant alsois whether the negative beam momen ts aredete rmin ed from a comb inat ion of loadfactored dead and live loads and earthquakeand if so wha t load factors are u sed.Califor nian and N. Z. practice differ considerably in this respect because N.Z. provisionsinclude some live loads in the derivationof the eart hqua ke load but unity load fact orfor dead load is used in the load c ombi nati onof gravity and earthquak e. For purposes ofthis comparison the two opposing effects ofthese va ryin g appro aches betw een t he N. Z.and Califo rnian requireme nts have beenconsidered as self cancelling.

The Japanese provisions shown are thosewidely applied to high rise co nstructionwhi ch is subje ct to appro val by a high risebuilding committee . They are much moresevere than NZS 4203 but the seismicity ofJap an is 10 times that of N. Z. , at least inthe terminolo gy used by seismo logis ts.Figur e 6 shows that many Japane se buildin gshave been constructed to these high loadings

including 50 and 60 storey bui ldi ngs . Ofparti cular in terest is the fact that theminimum values is0 05g and is not reacheduntil the fundamental period becomes 3h toover 7 seconds. Until recently designswer e carried out using seismic steel str essesof only lh times thos e allowed for gravityloadings, but curre ntly the allowab le sei smicstresses for structural steel are close toyield level.

The current Japanese code for ordinarystruct ures place s a pena lty of l%g forevery 4 metr es in height ov er 16 met re s.Subject to adjustments by a soil andconstruction factor of from 0.6 to 1.5and normalised for allowab le stresse s theminimum design co-efficien t in the highestJapanese seismic zone would be about0 25gat ultimate load.Composites of code curves such as thoseshown in Fig. 1 mus t have a sobering effect

on any thoughts one mig ht harbou r that earthquake enginee ring is an exac t, even reas onably exact science. It is likely thatinternational agreement on disarmamentswould be easier to reach than agreement ona code desig n cur ve. If it wer e merel y amatter of scale this could readily beexplained by difference s in regional seismicitybut other features such as the wide plateauof 0.2g in the Japa nese cu rves for bu ildin gsup to 0.9 sec eve n for stiff so ils may hav eother reasons. Tradition? May be.8.0 LIKELY FUTURE DEVELO PMENTS

The weal th of unre solv ed prob lems andthe flood of new information naturallyraise questions as to the future of thecode. The writer advocates a gradualdevelopment of the code rather than thepresent practice of a traumatic 10 yearlycomple te over haul . The new format of thec o d e , which minimi ses proble ms of adoptionby local bodies is well suited to such anapproach. The princi ple of more frequentand therefore mor e gradual upda ting is inline with overseas trends e.g. AC 1.Obvio usly chang es wou ld and should not bemade unless there is solid evidencepreferably from earthquake damage that arequi remen t in the code is inadequ ate,otherwise inconvenie nce to designer s andclients will result.The committee has endeavoured toconvince SANZ that updating of the codeshould be on a regul ar yearly b asis such asis done for instan ce wit h the UniformBuilding Code . Althou gh unable to obtainSANZ assurance that this could be doneunti l there is a sign ific ant impr oveme nt intechnical and support staff levels of thatorganisation, the committee will pressahead with efforts of its own to completereview of the code in the light of designersexperience by mid 1976. The present ed itionshould be available from SANZ early 1976.In many in stanc es updati ng may take

the form of alternative approaches tosupplement existing requi remen ts. Thereis for instan ce no pro vis ion for relia bilit ydesign proced ures. There are good prospectsthat the next code will con tain recommen dationsfor the design of membe rs s ubject principallyto gravity loadings at least on a semi-probabilistic basi s. Seismic considerations


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5 0however increase the comple xities enormo uslyand prog ress is unce rtai n. Even in highlyseismic areas with records avai lable for along period rati onal predict ion of sequenceof earth quake shakings that a buildi ng wi llexper ience during its lifeti me is at presentextremely uncertain. Presuming that adequatedata on seismicity of a regio n is availa blewe must still estima te attenu atio n ofintensity ove r the dist ance betwe en thehypocentre and the site.

Figu re 7 shows the deri vati on of anexpression relating ground accelerationwith focal dist ance . Note that the scatteris over 100 times in the region of g reate stintere st. The strongly elong ated patte rnof isoseis mal li nes in figure 8 areevidence of the same complexity in therelationship between distance and earthquakee f f e c t s . For rock moti ons other authors20 ) are less pess imis tic.Lack of statis tical da ta as well asreliable data is of course no ground forentirely rejecting a probabilistic basisfor desi gn. The minim um inform ation requiredfor the evaluation of safety and for designis the expected va lue (e.g. the average) ofeach design varia ble plus a mea sure ofuncer taint y (e.g. its co-ef ficie nt ofv a r i a t i o n ) . Indeed it is true of pro bab ilistic models that the significance of databased objective information, or lack thereof,can be reflec ted properl y in combin ation swith necessary engineering judgement.Expe rien ce has howev er shown that where themea sur e of uncer taint y in data is high,probabilis tic approac hes lead to morestringent requirements than the safetyrequirements obtained by traditionalapproaches which are apparently acceptableto society.

9.0 CONCLUSIONThe wri ter beli eves that this code isan advanced and practical standard whichprovides a reasonable level of protectionto life and property at an economic levelin cost taking into accou nt the rel ativeseismic ity of N. Z. as compared wi th oth ercountries, and the total cost of a building'sconstruction.

10.0 REFERENCES(1) Rose nbl uet h, E. and Esteva, L. :Poye cto de Regl amen to de lasConstruc ciones en el Distrito Fede ral:Folleto Complementario : Vol. 2 , Mexico1962.(2) Design Ess entials in EarthquakeResistant Buildin gs, Archite cture Inst,of Japan.(3) Basic Desi gn Criter ia of the Re commend edLateral Force Requiremen ts and CommentaryA S C E . ST9, Sept. 197 2.(4) MP 12 , Comme ntary on Ch. 8, NZSS 19 00,1965, CI. 5.3, CI. 10.(5) Hudson, D . E.: Local Distri bution ofStrong Earthquake Ground Motions.B u l l . S. Soc. of Amer ica, V ol . 62,N o . 6, 12/72.(6) Blakeley, Megget and Priestley: SeismicBeha viou r of Two Full Size RC Beam/Column Joint Units, Bull, of the N.Z.N.S.E.E., March, 1975.(7) Taylor, P. W.: Earthq uake DesignFormula Considering Local Soil Consid

erati on. Discu ssion ASCE ST6 June1972, pp. 1 336-1 338.(8) Yama hara , H.: The Interrel ationBetween Frequency Characteristics ofGround and Earth quake Damag e toStru ctur es. Japan Society of SoilMech. Vol. X, No. 1, 1970.(9) Skinner, R. I. and McVerry, G. H.:Base Isolation for Increased EarthquakeResistance of Build ings. South PacificRegional Conference on E.E., We llingt on1975.(10) Umemura, H.: Earthquake-R esistantDesign of Structures, University ofTokyo, p. 4 7.(11) Gene ral Report on the Tokach i-ok iEart hquak e in 1968 .(12) Recommen ded Lateral Force Requi reme ntsand Commentary. Structural EngineersAssocia tion of California 197 4 .(13) Paula y, T.: Report on Design Aspe ctsof Shear Walls for Seismic Areas,University of Canterbury, 1974 , p.2 9.

(14) Bazan and Rosenblueth: TechnicalN o t e , Journal of the ASCE, Feb. 1974,ST2, p p . 4 8 9 - 4 9 3 .(15) Chopra, A. K.; Clough, R. W. andClough, D. P.: Earthquake Resistan ceof Buildi ngs with a Soft First S torey .Earthquake and Structural Dynamics,V o l . 1, 1973.(16) Park, R. and Paulay, T.: Duct ileRC Frame s. Bulletin of the N.Z. N.S. E.E. ,Vol. 8, No. 1, Mar ch, 1 975.(17) Blakele y, R. W. G.: Cooney , R. C. andMegg et, L. M.: Seismic Shear Loadin gat Flexural Capacity in Cantilever WallStructu res. South Pacific RegionalConference on E.E., Wellingto n 19 75.(18) Arms tron g, I. C.: Capaci ty Desig n ofReinforced Concret e Frames for Duct ileEarthquake Performance, Bull, of theN . Z . N . S . E . E . , Vol. 5, No. 4, 1972.(19) Gloga u, O. A.: The Obje ctiv e of theN . Z . Seismic Design Code, Bull, of theN . Z . N . S . E . E . , Vol. 5, No. 4, 1972.(20) Whitman, B iggs, Brennan, Co rnell , deNeufvil le, Vanmar cke: Seismic DesignDecision Analysis, MIT S tructuresPublication No. 385, 1974.(21) Schnab el and Seed: Acce lera tion s inRock for Eart hquak es in the Weste rnUnited S tates (to be published).(22) Steph enson , W. R.: Eart hqua ke InducedResonant Motion of Alluvium, Bul l.N . Z . N . S . E . E . , Vol . 7, No. 3, 1974 .(23) Bert ero, Et al: "Design Implic ation sof Damage Observed in the Oli ve Vie wMedical Centre", Proceedings 5WCEE,Rome 1973, Vol . 1.(24) NZS 4 203 N.Z . Standard Code of P racticefor Gener al Struct ural Design and Desig nLoadings.


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51

35

3 0H

SeismicOtttgn

2 0

OS

S E A O CShear S Oog.TensionK 33 S I 5U 2 0

NZS 4203 Z O N E AVX| 1 0 1 6 M 1 2

| dandNZS 4203 ZONl l S-0-8 M*0-8

1 0 1-5 t o 2 5 3 0 3 -5 4 0

m\o InSteondsN.Z.. S.E.A.O.C. and A IJStjtmte Ptrtqf* Coefficients

FIGURE1 5 5 n

Period in S e c o n d sN.Z.. S.E.A.O.C, Essenttol Focilities and TifSe 17 California Hotpl tol t) Seismic Design Coefficients

FIGURE2


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118*12 118*10 118*08 118*06

I KILOMETER

34 I?

34 t

34*oe

Sof lanV MAP B A S E D ON G U T E N B E R G 1 * 5 7 )

EPICENTER

SYLMAR

L O C A T I O N MAPP A S A D E N A AND THE

SAN FER N AN D OE A R T H Q U A K E E P I C E N T E R

PASADENA , tO MILES ,1 KILOMETERS

STRONG MOTIONGROUND MEASUREMENTSS A N F E RN A N D O E A R T H Q U A K E

F E B R U A R Y 9, 1971 S E IS M O S C O P E S T A T IO N S E IS M O S C O P E- G U T E N B E R G

O A C C E L E R O G R A P H ) A C C E L E R O G RA P H S E I S MO S C O P E

VT

F I G U R E3 : L O C A T I O N MAP ANDM A X I M U M R E L A T I VE V E L O C I T YRESPONSE VALUES.


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F I G U R E 4 : C O MP A R I S O N O F V E LO C I T Y S P E C T R A(A ) M I L L I K A N L IB R A R Y A N D A T H E N A E U M(B ) S E I S M O L O G I C A L L A B O R A T O R Y A N D A T H E N A E U M(C ) S E IS M O L O G I C AL L A B O R A T O R Y A N D M I L L I K A NL I B R A R Y , A N D( D ) J E T P R O P U LS IO N LA B O R A T O R Y A N D S E I S M O LO G I C A LL A B O R A T O R Y .


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SINUSOIDAL 5 DAMPING

1 0 2 0 SE CP E R I O D

F I G U R E 5 : C O M P A R IS O N O F A M P LI F I C A T I O N O F G R O U N DA C C E L L E R A T I O N A C C O R D I N G T O 3 A U T H O R S

7",(sec) (Dr.T.Hisida)

FIGU RE 6 : RE LA TIO N BETWEEN DESIGN BASE SHEARC O E F F IC I E N T A N D F U N D A M E N T A L N A T U R A LP E R I O D O F H IG H -R I S E B U I LD I N G S


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F IG U R E 7 : V A R I A T I O N O F M A X I M U M G R O U N D A C C E L E R A T I O NW I T H F O C A L D I S T A N C E . A F T E R E S T E V A ( 1 969) .

F I G U R E 8 : I S O S E I S MA L L I N E S I N T H E S A L I N A S V A LL E Y ,C A L I F O R N I A E A R T H Q U A K E , 1 9 06 . A F T E RLAWSON (1908 AN D 19 10) , REPROD UCED BYD A V I S O N ( 1 93 6) .

otto glogau (1975) (1)0043 - code philosophy behind nzs4203

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