Anclaje Del Refuerzo de Confinamiento en Columnas

7/30/2019 Anclaje Del Refuerzo de Confinamiento en Columnas

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165A N C H O R A G E O F T R A N S V E R S E R E IN F O R C E M E N T INR E C T A N G U L A R R E I N F O R C E D C O N C R E T E C O L U M N SI N S E I S M I C D E S I G N

H . T a n a k a 1 , R. Park 2 , B . M c N a m e e 3SYNOPSIS

Four reinf orced concr ete columns with 400 mm (15.7 in) square crosssections were tested under axial compressive load and cyclic flexure tosimulate severe seismic loading. The longitudin al reinforc ement consistedof eight b a r s . The transverse reinforcement consisted of square perimeterhoops surrounding all longitudinal bar s and cross ties betw een theintermediate longitudinal b a r s . The major variable of the study was thetype of anchorage used for the hoops and cross ties. The anchorage detailsinvolved arrangements of perimeter hoops with 135 end hooks, cross tieswith 90 and/or 180 end ho ok s, and cross ties and peri mete r hooks wi thtension splices . Conclusion s were reached with regard to the effectivenes sof the tested anchorage details in columns designed for earthquakeresistance.KEYWORDSC o l u m n s , cross-ties, detailing, ductility, earthquake resistance, rectangularh o o p s , reinforced concrete, reinforcement anchorage, tension splices,transverse reinforcement.

LIST OF SYMBO LS steel stress

ch

"sh

f c

area of reinforcing bararea of concret e core meas ured tooutside of peripheral hooparea of concrete core mea sur ed tooutside of peripheral hoopgross area of column sectiontotal effective area of hoop barsand cross ties in dire ctio n und erconsideration per hoop set

= area of shear reinforcement perhoop set= web width= neutral axis depth, or smaller ofthe distance measured from theconcrete side face to the centre ofbar or one-half of clear spacing ofspliced bar s plus a half b ar

diameter, but not larger than 3d^= distance from extreme com pressionfibre to centroid of the tensionreinforcement= bar diameter= concrete compressive stress= concrete compressive cylinderstrength

Associate Professor, AkashiTechnological College, Japan.Profess or of Civil Engin eering ,Unive rsity of Cante rbury , New Zealand.Professo r of Civil Engin eerin g,Drexel Unive rsity , U.S.A.

h"h cHHuKI

M.AC I

^SlKP

yield strength of transversereinforcementdistance from centre of central stubto hori zont al load pins at ends ofcolumn unitswidth of concret e core measur ed tooutside of periph eral hoopwidth of concret e core measu red tocentres of perip heral hoophorizontal forcetheoretical ultimate horizontal loadgiven by Eq. 16distance from section of maximummoment to section of zero momentdevelopment lengthequivalent plastic hinge lengthmeasur ed mom ent in column at faceof centr al stubmaximu m measur ed moment in columnat face of central stubtheoretical flexural strengthcalculated using ACI concre tecompressive stress blocktheoretical flexural strengthcalculated using the modi fied Kentand Park concrete compr essiv estress distri bution

= axial compressive load on column= centre to centre spacing of hoop

sets= centre to centre spacing of hoopsetsgiven by Eq.given by Eq.

109

BULLETIN OF THE NEW ZEALAND NATIONAL SOCIETY FOR EARTHQUAKE ENGINEERING, Vo l 18, No 2 , JUNE 1985


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16 6V c = given by Eq. 12V = factor ed (ultimate) shear forceuZ m = given by Eq. 17c c = compressive cylinder straine c c = c o m P r e s s i v e cylinder strain atextreme fibre of core concretee = steel strai nsA = horizontal displacement

= given in Fig. 17A F C = given in Fig. 17A = horizontal displacement at firsty yield^ _ curvatur e = rotat ion per unit lengthor strength reduction factor

d> = cur vat ur e at fir st yie ldp = ratio of volume of transverse

reinfor cement to volu me of concret ecorep = A /b dw s w0 = rota tio n of cent ral stub due tounsymmetrical plastic hingingy = nominal displ aceme nt ductility

N factor = A / A Yy R = real displa cemen t ductility factor

= A . / A or A , / A .t y b y

1 . INTRODUCTIONConsiderable efforts have been made

in recen t years to develo p improved seismicdesi gn provis ions for reinforc ed c oncretecolumns in bridge sub structur es andbuildi ng frames. The need for effectivedesig n provisi ons has been emphasised bydamage caused to bridge s and buildin gsduring severe earthq uakes. For examp le,the San Ferna ndo earthqu ake in Souther nCalif ornia in February 1971 causedextensive damage to a number of recentlyconstructed reinforced concrete columns inbridg es and buil dings , mainl y because ofinadequate detailing of those structuralmemb ers for ductility (see Fig. 1 ) .The mos t importan t design conside ration

for ductili ty in the potent ial plastic hingeregion of reinforced concrete columns is theprovision of sufficient transversereinf orceme nt in the form of recta ngulararrange ments of hoo ps, with or wit hout crosst i e s , or circular spirals or circular hoops,in order to confine the compressed concre te,to prevent buckling of the longitudinal b a r s ,and to prev ent shear failure . Anch orag efailure of the transverse reinforcement mustbe preven ted if that reinforce ment is tofunction effecti vely. Seismic design codesnormally spec ify desig n provis ions for thequantity, spacing and anchorage of transversereinf orceme nt in the poten tial plastic hin geregion of columns.The New Zealand conc rete design code[1] specifies that hoops and cross tiereinf orcem ent in reinfor ced concrete columnsshall be anchored either by end hooks formedby a 135 turn around a longitudinal bar plusan extension of at least eight hoop or crosstie bar diameters at the free end of the bar

into the core concrete, or by welding theb ar ends. These anchorage details canresult in a complicat ed reinforce mentfixing job, especial ly on site. This isbec aus e the hoo ps and cross ties need firstto be plac ed over the ends of longi tudin albars and then shifted along thelongitudinal bars to their correct positionin the reinforci ng cage. That is, thehoops and cross ties cannot be inserteddirectly through the side of the cage intotheir correct position.

In order to ease the difficu lty ofplacing transverse reinforcement severalalternative details for cross ties whichsimplify the fabrication of reinforcingcages have been used in the Unit ed State sand other count ries. One alternativedetail involves the use of cross ties with90 and 135 end hooks alternating alongthe mem be r. Such cross ties can beinserted directly into the position fromeach sid e of the cag e (see Fi g. 2b) afte rthe hoops are in plac e. Anothe ralte rnat ive detail invol ves the use of 'J 1bars which have a 135 end hook and areinserted from each side of the cage andlapped in the core conc rete (see Fig . 2 c ) .Such 'J' bar s can be used if the co lumnsize permits development of the tensionsplice. A further alternative detail is touse 'U' bar s whi ch are inserted from eachside of the cage and lapped in the coreconcrete (see Fig. 2d ) . This 'U 1 b ardetail is not recommended for transversebars passing around the longitudinal ba rsin the corners of columns since the tensionsplice will not be effective in the coverconcrete if the cover concrete spalls atlarge column deformation s during severeseismic loading.

It should be emphasised that thereis no great diffi cult y in fabric atingstandard reinforci ng cages incorporatingtransverse reinforcement with 135 endhooks and no tension splices (see Fig. 2 a ) ,providing the fabrication is off site.In factory conditions cranes are readilyavailable and reinforcing cages can befabricated in the most convenie nt posit ions.However, the fabrication of cages on sitein formwork using transvers e re inforceme ntwith 135 end hooks and no splices cancause difficu lty. The alternativetransverse reinforcement details shown inFig. 2b, c and d may be eff ectiv e e noughto be used in a number of cases, forexample where columns need only to bedetailed for limited ductilit y.

This paper describes test resultsobtained from four reinforced concretecolumns which contai ned the variousarrangeme nts of transver se reinfo rcementshown in Fig . 2. The colum ns wer e loadedunder axial compressive load and cyclicflexure in the inelastic range tosimulate severe seismic loading. Theperformance of the columns at variouslevels of displacement ductility factorwas compared.


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167

Fig. 1 Damage Caused to Some Reinforced Concrete Columns in Bridgeand Building Structures During the 19 71 San Fernando Earth quake.

^(a) Standard Hoop and Cross Tie with 135 End Hook

"7(b) Alternating 90

and 135 EndHooks

(c) Overlapping"J" Bars withTension Splices

id) Overlapping "U "Bars with TensionSplices

Minimum Valuesy 0NZS 3101(1) 8d D 135ACI 318 (2) 10d b 135

6db 90%SEAOC (3) 10dD 135CEB-FP(4) 10d D 135AIJ (5) 6d D 135

Where alternated with135 end hooks

Fig. 2 Standard and Alternat ive Details Fig. 3 Anchorage of Transve rse Reinforcementfor Anchoring Transve rse Around a Longitud inal Bar AccordingReinforceme nt for Reinforced to Some Code s.Concrete Columns.


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16 82. COMPARI SON OF NEW ZEALAN D, UNITEDS T A T E S , EUROPEAN AND JAPANESE CODEPROVISIONS FOR THE ANCHORAGE OFTRANSVERSE REINFORCEMENT

It is of interest to compare thedesign provisions for the anchorage ofrectangular transverse reinforcement givenby some current seismic design codes inNew Zealand , United Sta tes, Europ e andJapan. Some code recom menda tion s for theanchorage of the ends of transverse barswhich are bent around longit udinal barsare show n in Fig. 3.The New Zealand cod e, NZS 3101 [1] ,requires at least a 135 turn and an 8d,extension at the free end of the bar intothe core concre te. Fig. 4, which is takenfrom the Commen tary of NZS 3 101, il lustratestypical arrangements of transversereinfo rceme nt in potenti al plastic hinge

regions of rectang ular colum ns. NZS 3101also requires that the longitudinalreinfo rcemen t be placed not further apartbet wee n centre s than 200 mm (7.9 in) andthat the centre to centre spacing acrossthe column section between cross linkedlongitudinal bars be not further apart than200 mm (7.9 i n ) .The American Concrete Institutebuildi ng cod e, ACI 318-83 [2 ], includes inits Commentary the example of transversereinforcement by one hoop and three crossties in the potential plastic hinge regionof a rect angu lar c olum n shown in Fig . 5.Anch orag e of the hoop bars is achieved with

a 135 turn and a 10d, ext ensi on. Thecross ties have a 135 ^ turn and a 10d^extension at one end and a 90 turnand a 6d^ extensi on at the othe r end.Cons ecuti ve cross ties have their 90 hookson opposite sides of the column, evidentlyto counter the possible loss of efficiencyof the 90 hook which is not embedded inthe concrete core whe n the cover concretes p a l l s . Note that the AC I building code,requires the centre to centre spacingacross the column section between crosslinked bars to be not greater than 14 in(365 m m ) .The recommendations of the Structural

E n g i n e e r1

s Asso ciat ion of Califo rnia [3]includes in its Comme ntary the exa mples oftransverse reinforcement provided inpotential plastic hinge regions shown inFig . 6. The use of cross ties wit h tens ionspli ces ('J' bars) is illu str ated in thisfigure.The second draft of the seismicdesi gn appendix to the model code of theEuropean Concrete Committee - InternationalFede rati on of Prestr essing [4] requiresthe ends of transverse bars to be anchoredby at least a 135 turn and a 10d, exte nsio nof the free end of the bar into tne coreconcre te. Typical detai ls of transve rsereinforcement are similar to those inFig. 4.The code of the ArchitecturalIns titu te of Japan [5] req uire s the ends oftransverse bars to be anchored by at leasta 135 turn. In the appen dix of thecommentary a 6d, extension of the free end

of the transverse bar into the coreconcrete is specified.3. PREVIOUS RESEAR CH INTO ANCHOR AGEOF TRANSV ERSE REINFO RCEMENT

There has been some previous researchinto the anchorage of transversereinfo rcemen t in reinforce d concretecolu mns sub jected, to seismic loadi ng.The New Zeal and code specified endanchorage for hoops and cross ties,comprising a 135 turn and an 8d^extension at the free end, has been shownby extensive tests on columns subjected tocompressive load and cyclic flexureconducted at the University of Canterburyto give effective anchorage, even at highdispla cement ductility factors [6] .Moe hle and Cav anag h [7] have te sted

reinforced concrete columns underconcentric monot onic compression withvarious transverse reinforcement detailslinking the intermediate longitudinal b a r s .It was found tha t the use of cross tie swith a 90 turn at one end and a 135 turnat the other end, placed so that consecutivecross ties have their 90 hooks on oppositesides of the col umn s, wer e almost aseffective as cross ties with 135 hooksat both ends. The colu mns of Moeh le andCavanagh were tested under monotoniccompression and it is apparent that cyclicflexure, as in seismic loading, wouldimpose a wors e loading condition.Oes ter le et al [8] hav e tested rein force dconcrete walls with vertical boundarymemb ers containing reinforcement which wasdet ail ed as for col umn s. The use of cros sties with a 90 turn at one end and a 135turn at the other end, alternated end forend over the height of the boundarym e m b e r s , was found to result in satisfactorybehaviour during seismic load reversals inthe inelastic range. The boundary elementswere subjected to cyclic eccentric tension-compress ion loading. It is possible thatfor columns subjected to significantcompressi ve loadin g, as well as cyclicflexure, greater deterioration of thetransver se bar anchorage at the 90 turnmay occ ur. Thi s is beca use the free endof the bar with a 90 turn will be in thespalled cover concrete, rather thanembedded in the core concrete, and thetransverse bar will be required to undergotensile strains well into the plasticrange in orde r to confi ne the compr essedconc rete . It is also of note that somebending e rrors which resulted in hoop ba rhooks having a turn of less than 135 atthe e n d s , due to poor execution of work,was listed as one of the causes for thefailure and collapse of reinforced concretecolumns in the 1968 Tokachi- oki earthqua kein Japan [5,9].

With regard to the tension splicedetail for cross t i e s , such as shown inFig. 6, Oes ter le et al [8] recomm ende dthat it should not be used in the plastichinge regions of vertic al boundary memb ersof wal ls. They state that because ofsevere cracking that can develop inboundary elements under inelastic loadreversals, it is likely that the tension


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16 9

^200'$200 ^200'(a) Single hoop pius two (b) Single hoop plus two

supplementary cross ties supplementary cross tiesbent around longitudinal bars. bent around hoop.

>200 Hoops


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170 Consecutive cross ties shall hovethei r 90 -de gr ee hooks on oppositesides ofco lumn. * 10 dfc E xtension

/-6d|) Extensiont 7 r L n 1 >

Xr / i )X

* Jx S hall not exceed 14inches

Fig. 5 Example of Transverse Reinforcement in Column According toCommentary of ACI 318-83 [2]. (1 in = 2 5. 4 m m ) .

h" FOR A" S H CROSSING X - X A X I S

T E N S I O N S P L I C E

6 M A X . PER A C I - 3 1 8- W H E R E A L T E R N A T E B A R S

A R E T I E D .

h /1 h" / ^1 /L

\k ft^-i1 y i vj

- S U P P L E M E N T A R Y T I E SE N G A G E H O O P , T I ES E C U R E L Y T O L O N G I T.R E I N F O R C E M E N T .1 8 0 B E N D S MAY BEM O R E C O N V E N I E N T F ORP L A C E M E N T T H A N1 3 5 B E N D S P E R M I T T E DB Y C O D E .

H O O P S A N D S U P P L E M E N T A R YC R O S S T I E S C O N T R I B U T ET O A " S h AS R E Q U I R E D B YE . G . 3 0 - 6 OR 3 0 - 7 .

, 6" MAX . PER A C I - 3 1 8W H E R E A L T E R N A T E BARSA R E T I E D .

C O V E R MA Y B ER E D U C E D TO 1/2"F O R E N D S O FS U P P L E M E N T A R YC R O S S T I E S .

" j " B A R S MAY BE U S E DI F C O L U M N S I Z E P E R M I T SD E V E L O P M E N T OF T E N S I O NS P L I C E . W I R E T O G E T H E R ATE N D S .

Fig. 6 Example of Transverse Reinforcement in Columns According tothe Commentary of the SEAOC Recommen dations [3]. (1 in = 2 5. 4 m m ) .


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17 1splices in the cross ties would eventual lybecom e ineffec tive. It is also of notethat the anchorage of transversereinforcement by tension splices in thecover concrete witho ut welding was shownto be a poor det ail in the 1971 SanFernando earthqua ke [10], where circularbridge columns failed due to ineffectiveanchorage of lapped circular hoops whenthe cover concrete spalled.

It is evid ent that mo re tests areneeded on reinforced conc rete col umnssubjected to seismic loading to compare theperform ance of various anchorage detailsfor transverse reinforcement. The importance of good detailing of reinforcement inreinforced concrete structures designed forearthquake resistance cannot be overemphasised . Significant protectionagainst damage will be provided by properlydetailed reinforcement.4. TESTS ON REINFORCED CONCRET E CO LUMNSSUBJECTED TO COMBINED AXIAL LOAD ANDCYCLIC FLEXURE AND WITH VARIOUSANCHOR AGE DETAILS FOR TRANSVE RSEREINFORCEMENT4.1 Principa l Dimension s and LoadingArrangements

Fig. 7 shows the principal dimensi onsand the loading arrangements for the fourreinforced c oncrete column units which w ereconstructed and tested. The column unitshad a total heigh t of 3. 9 m (12 ft 9.5 in)and 400 mm (15.7 in) square cross se ction .The central stub, reinforced so as toprevent failure occurring in that part ofthe test unit, simulated an adjoining beam,footing, or pier cap.During the tests the column axialcompressive load was held cons tant at P =0.2f1A , wher e f 1 = concrete co mpressi vecylindSr strengt h and A = gros s area ofcolumn cross section. ^ A reversib lehorizontal load H was applied at mid-hei ghtto the central stub.

4.2 Deta ils of the Rei nfo rce men t andConcreteIn each of the four column units the

longitudinal reinforcement was eight 2 0 mm(0.79 in) diameter deformed bars of Grade380 steel, arranged symmet rically aroundthe perimet er of the column cross sectionas shown in F i g s . 8 and 9. Thi slongitudinal reinforcement was 1.57% of thegross area of the column cross section.The measu red yield and ultima te strengthsof the longitudinal reinforcement was 474MPa and 721 MPa , respectiv ely. Fig. 10shows the measured stress-s train curve .The transverse reinforcement in eachof the four column units was from 12 mm(0.47 in) diameter deformed bar of Grade275 steel , arran ged as shown in Fig . 9.

The measur ed yield and ultim ate strengthsof the transverse reinforcement was 333 MPaand 481 MP a, respective ly. Fig. 10 showsthe measu red stress-st rain curve . Thecentre to centre spacing of the transverserein forc eme nt set was 80 mm (3.15 in) inthe 400 mm (15.7 in) long regi ons of c olum nadjacent to the central stub and 160 mm

(6.30 mm) else wher e (see Fig. 8 ) .The varia ble in this study was thetype of transverse hoops and cross tiesprese nt in the units (see Fig. 9 ) . Unit 1had hoops with 135 end hooks and cross

ties with 180 end hoo ks. Uni t 2 hadhoo ps with 135 end hooks and cross tieswith 180 and 90 end hooks alternatingfrom side to side along the column.Uni t 3 had lapped 'U 1 bar hoops and lapped'J 1 bar cross ties with 180 end hooks.Uni t 4 had hoops with 135 end hoo ks andlapped 5 J 1 bar cross ties with 180 endhooks. In all columns the extension ofthe end of the hoop or cross ties beyondthe 9 0 , 135 or 180 turn was 6, 8 and5 transverse bar diameters, respectively.All transverse reinfo rcement was deforme db a r .The concrete was cast with the column

units in a horizont al posit ion. Theconcrete was normal weigh t, with a slumpof 7 5 mm (3.0 in) and a max imu m aggr egat esiz e of 20 mm (0.79 in) . All column uniteswere cast at the same time from the sam^" 'batch of concrete . Afte r casting theconcrete was damp cured for 28 days. A tthe test age of abou t 2 mon ths thecompressive strength of the concrete asmeas ure d from 100 mm (3.9 in) diam eter by200 mm (7.9 in) cylind ers was 25.6 MP a(3,770 psi) and the mod ulu s of rupture asmea sur ed from 152 mm (6.0 in) x 152 mm(6.0 in) x 473 mm (18.6 in) pri sm s was3.6 MPa (520 p s i ) .4.3 Compari son of the Trans verseReinfor cement in the Column s WithCode Requirements

The transverse reinforcement providedin the column units is compared with therequ irem ents of the New Zeal and andAmerican Concrete Institute codes below.In all the following calcul ationsthe stre ngt h re du ct io n fac tor (j) isassumed to be 1.0.

(a) The length of the pote ntia l plas tichinge regions in the column units adja centto the central stub in all column unitswas 400 mm (15.7 i n ) .NZS 3101 : Whe n PQ 0. 3f 'A c|>,this le ng this to be not SeSs than thelonger cross section dimension(400 m m ) , and where the m omen texceeds 0.8 of the maximummoment (320 m m ) . Hence 400 mm(15.7 in) governs.AC I 318-8 3 : Wh en P > 0.If1A , this lengthis to Se not lesi than thedepth of the memb er at thejoint face (400 m m ) , one-sixthof the clear span of the member(533 m m ) , and 18 in (457 m m ) .Hen ce 533 mm (21.0 in) gov ern s.Hence only the NZS c ode is complied wit h.(b) The centr e to cen tre spacin g oftransverse reinforcement in potentialplastic hinge regions of all column unitswas 80 mm (3.1 i n ) .


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17 2NZS 3101

ACI 318-83

This sp acing is to be no tgreater than one-fifth of theleast lateral dimension of thecross section (80 m m ) , sixtimes the diameter of thelongitudinal bar to berestrained (120 m m ) , and 200mm. He nc e 80 mm (3.1 in)g o v e r n s .This s pacing is to be notgreater than one-quarter ofthe mini mum membe r dime nsion(100 m m ) , and 4 in (102 m m ) .Hen ce 100 mm (3.9 in) gov ern s.

Hence both codes are complied with.(c) The length of the tension splice in the'J' bar cro ss tie s of Unit s 3 and 4 was288 mm (11.3 in) and in the ! U I b arperi met er ho ops of Uni t 3 wa s 204 mm (8.0 in)NZS 3101 This length is not to besmaller than

3 8 OA,c/f 7"c

(mm) (1)but not less than 300 mm, w hereA, = ba r area and c = smal leror the distanc e measure d fromthe concrete side to the barcentre or one half of clearspacing of spliced bars butnot lar ger than 3d^ whe re d^ =bar diamet er. For the unitsA, = 1 13 mm , c = 36 mm andf^ = 25.6 MP a. Eq. 1 give s26 mm and th eref ore , =300 mm (11.8 i n ) . a

ACI 318-83 : Assum ing a Class C splice,this length is not to besmaller than

1 . 7 d = 1.7 x 0.04Ab (2)

(3)2

but not less than 1.7 x0.0004dKf , (in)b yhFor the uni ts A, = 0.175 inf = 48,30 0 ps i, f = 3,710psi and d b = 0.47 i n 2 . Eq. 3is critical and gives1 . 7 d = 15.4 in (392 m m ) .

Hence both codes are not complied with.(d) The extension of the free endof the cro ss ties wa s 60 mm (2.4 in)bey ond th e 180 turn and 72 mm (2.8 in)beyo nd the 90 tur n. The exte nsio n of thefree end of the square hoop was96 mm (3.8 in) beyond the 135 turn .NZS 3101

ACI 318-83

The extension is to be atle as t 8d, = 96 mm (3.8 in)beyond the 135 and 180 turn(90 tu rns are not p ermi tted) .The extension is to be atleast 10d b = 120 mm (4.7 in)3D and 135 turnseyond 181and 6d,

beyond 90 turns.Hence the extensions were less thanrequired by the codes in most ca ses.(e) The to tal area of hoop bars and crossties per hoop set in the potential plastichinge regions of all column units wasA g h = 3 x f 'Ac g (4)

A , = 0.12s, h" Tsh h f h0.5 +

1.25 (f)fAe g (5)

whe re s^ = centre to cent respacing of hoop sets, h" = widthof concr ete core mea sure d tooutside of peripheral hoop ,A = gross area of columnslction, A c = area of concretecore measured to outside ofperipheral hoop, f f = concretecompressive cylinder strength,f = yield strengt h of tra nsverse reinfor cement, and P =ecompressive load on column.For t he unit s s, = 80 mm , h" =320 mm, A /A = 1 . 5 6 3 , f =25.6 MPa, g f ? = 333 MPa and^ 0 . 2 . E Q . 4/(j)f'A gives^sh c g 249 mm2 and Eq. 5 gives177 mm2 .Henc e A ,shg o v e r n s . 249 m m 2 (0.39 in 2 )ACI 318-83 : This area is not to be lessthan

shandA s h

0.3sh U-2c l A c h

f c

0.12shc f

(6)

(7)yhwhe re s = centre to ce ntrespacing of hoop sets, h =width of concrete coremeasured to centres ofperipheral hoop, = area ofconcrete core measur ed tooutside of peripheral hoop,and the other notation is asfor Eqs. 4 and 5.For the unitA Eq. 6 gives, = 32 0 mm^ and Eq. 7 givesA s n ^ 2 2 7 Hence A , -9 o snn z (0.50 in^) governs.

Hence both codes are complied with.72 mm (2.8 in)


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Pe17 3

150:

1600mm

400

H_2

Note: Loads Hcan bereversed

J600mm

mo:

H

Centralstub

400

400SECTION A-A

H_2

Fig. 7 Principal Dimensions and Loading Arrangements for the Column Units.

600

1800

150

Loadingdirection

200\

Loadingplate

(Wmm 11

TransverseReinf. -D12

LongitudinalReinf. -HD20

Sleeve forpin (50mm 01

Anchor plate(11mm ft I All dimensionsin millimetres

Fig. 8 Longitudinal Section of Column Units Showing Details of Reinforcement.


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17440 0

180hookwith 5dbextension

135 hookwith 8dDextension

m 1 i-90hook

with 6dDextension

STANDARD UNIT WITHHOOPS AND CRO SS TIES

UNIT!

CROSS TIES WITH 180 AND90 HOOKS ALTERNATING FROMSIDE TO SIDE ALONG CO LUMN

UNIT 2180hook with 5d^ extension

^90 turn with21ofo extension

(lido of tension)splice

Loading Direction24 dh of tension

splice

NOTE:Bar lapswere notwelded

LAPPED PERIMETER HOOPS"U" BARS AND 'J " CROSS TIES

UNIT 3

^135 hook with8dfr extension

LAPPED CROSS TIESOF "J " BARS

UNIT 4Fig. 9 Trans verse Sections of Columns Showing Details of Reinforce ment.

AXIAL

All dimension s in millimetres

Fig. 10 Measur ed Stress -Strain Curves forthe Rei nfo rci ng St eel (1 MPa =1 4 5 p s i ) .Fig. 11 Loading Arrange ments andPotentiometer Positions.


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17 5(f) The tota l area of shear rein for cem entper hoop set in the potential p lastic hingeregi ons of all colum n unit s was A = 3 x (tt x 6 2) = 339 m m 2 (0.53 in) ,assuming all three transverse bars areeffective.NZS 3101 This area is to be not smallerthan

V V / f y h ( r a m } ( 8 )wh er e v. = V /b d (MPa) (9)i u w

0.1 (MPa) (10)f'Ac gwhere V factored (ultimate)

ACI 31

shear force, b = web width,w .s = centre to centre spacingof hoop sets, d = distancefrom extreme compressi on fibreto centroid of tensionrein forc emen t, p = A /b d,-t _ ^w _ s wand A = area of tensi onreinforce ment. For the uni ts,V = theoretical ultimatehori zonta l load/ 2= 158 kN(35.4 k i p s ) , b = 400 mm , d =335 mm, p = 9^2/(400 x 335) =0.00703, ' = 25.6 MP a, f , =333 MPa, s c = 80 mm , and yP /f'A = 0.2 .e c gEqs. 8 to 10 giv e A = 27 mm(0.04 in 2) . [Outsicle thepotential plastic hinge regionv is grea ter than give n byEqs. 8 to 10 and the requi redA is even s m a l l e r ] .vThis area is to be not smallerthan

moment in the upper and in the lower halfof each test unit was similar to that ina column between the face of an adjoiningmemb er and a point of contraf lexure . Thehorizonta l load cycles wer e appliedstati cally . Duri ng the tests the axialload applied by the DARTEC machine wasadjusted to compensate for the componentof axial load introduced into the columnby the membe rs of the horizo ntal load framewhich were pinned to the column ends.

During each test , once the requiredlevel of axial load had been applied, thenext step was to det erm ine the "first yiel d"displacem ent of the column. The firstyield displacement was defined as thatobtained assuming elastic cracked sectionbehaviour up to the theoretical ultimatehorizonta l load. The theoretic al ultimatehorizonta l load was computed using themeasured stress-strain relation for thelongitudinal reinforcement, the ACIrectangular compressive stress block withthe measure d concrete cylinde r stren gth,stre ngth red uc tio n fact or of uni ty,and an extreme fibre concrete compressivestrain of 0.003. In the test the elasti ccracked section stiffness was obtainedfrom an initial cycle of horizontal loadingof up to 0.7 5 of the theoretical ultimatehorizonta l load, H , calculated includingthe P-A effect. T^e horizo ntal displacements of the central stub reached in eachdir ecti on at a load of 0.7 5H wer e a verag edand divided by 0.75 to find fee firstyield displaceme nt A .

The subsequent horizontal load cycleswere displacement controlled and consistedof two cycles each to nominal displacementductility factors y N = A/A of 2, 4, 6, 8 and somet imes hig her , where A =maximum horizontal displacement of thecentral stub.4.5 Instrumentation

(V - V ) s 9A v = % J ( i n 2) (IDyhwhe re V = 2(1 + o n n ; L ) Zf^b dc 2 0 0 OA c wg

(lb) (12)For the units^Eqs. 11 and 12 giveA v = 0.004 in (2 m m 2 ) . [Outsidethe potential plast ic hinge -region required A is 0.007 in(5 m m 2 ) ] . v

Hence both codes are complied with.4.4 Testing Procedure

The consta nt axial load of P =0.2f1A = 819 kN (184 kip s) was eapplied to each column unit by a 10 MNDARTEC electro-hydraulic universal testingmachine through cylindrical steel bearingswhich allowed free rotation at the ends ofthe colu mns. The reversib le horiz ontalload was applied to the central stub by adoubl e acting 500 kN MTS electr o-hyd raulicservo jack whi ch could be load ordisplacement controlled. Fig. 11 shows thetest set up. The distri butio n of bending

The horizontal displacement of thecentral stub was measur ed by three linearpotenti ometers mount ed on a rigid frame.The three potentiometers were positionedwith a vert ical spacin g of 150 mm (5.9 in)and hence the rotation of the central stubcould also be calcul ated. The horizontaldisplace ment meas ured by the centrepotenti ometer and the horizon tal loadmeasured by the load cell were used todrive an X-Y plotter durin g testing totrace out the load-displacement hysteresisloops.In order to obtain the distributionof curvature along the col umns , and theconcrete strai ns, a further series of pairsof linear potentiometers were placed at16 0 mm (6.3 in) cent res up and down thelength of each column adjacent to thecentral stub. Thes e potent iomete rs weremou nte d on 8 mm (0.31 in) diam ete r steelrods which passed through the concrete in

the plane of the column section and atright angles to the neutral axis. The rodshad been cast in the concrete but the coverconcrete surrounding the end of each rodwas not present over a depth of 25 mm(1.0 in) and a di am et er of 30 mm (1.2 in)in order to avoid interference by crushed


12/26

176cover concrete. The positions of thepotentio meters are illustrat ed in Fig. 11.

Electrical resistance strain gaugeswere attached at vario us locat ions on theh o o p s , cross ties and longitudinalreinforcement within the potential plastichinge regions.4.6 TEST RESU LTS4.6.1 General Obser vatio ns and HorizontalLoad-Displacement Behaviour

Horizont al load vers us horizo ntaldispla cemen t hyster esis loops measur ed forall column units are shown in F i g s . 12 to15. Also shown in those figures are thetheoretic al ultimate horizon tal loa ds, H ,calculated as described in Section 4.4. uThe slope of the theoretical ultimatehorizontal load lines is due to the P - Aeffect which decreases the horizo ntal loadcarrying capacity of the column withincreasing horizont al displ aceme nt. Themeasured hysteresis loops shown in Figs. 12to 15 for behaviour up to a nominaldispla cement ductili ty factor - 6illustr ate stable beha viou r, good energydissipation and limited reduction instrength . All column unit s reached ahigher flexural strength than the calculatedtheoreti cal strength. Fig. 16 shows thedamage visibl e in the mos t critical regionof the columns at the end of testing.

The plastic rotation occurredunsymm etrica lly either above or below thecent ral stub in mos t of the colum n uni ts.Furth er excursions to great er horizon taldispla cement s led to a concen tratio n ofthe rotation in the plastic hinge which hadformed first. This behav iour could bedetected by the measur ed ro tation of thecentral stub and was visibly obvious at thelast stages in some tes ts. Fig . 17 showsthe implication of unsymmetrical plastichinging which results in a rotati on 6 ofthe cent ral stub. To acc ount for theconcentration of plastic rotation in onlyone plastic hinge in a column unit, thequan tit y 0h has to be added to thehoriz ontal displ acemen t A measured at thecentre of the central stub. The rotation6 was calculated from the differ ence inthe displacements measured by the twolinear potentiometers at the top and bottompositi ons on the central stub. Theduct ilit y factor calc ulat ed from (A + 0h)is referred to as the real displacementductilit y factor, y R , and that calcula tedfrom A/A (ignor ing 9) is ref err ed to asthe nominXl displacement ductility factor.

Table 1 lists the measu red firstyield displ acemen ts (defined in Section4.4), the range of maxim um mo mentsmeasured at the peaks of the loading cyclesto u = 2 , 4 and 6, and the real andnominal displacement ductility factors atsome stages of load ing. The rati o of themaxi mum moment measure d in the second cycleto nomi nal = 6 to the max imu m mom entmeasured for each column unit variedbe twe en 0.88 and 0.96, as liste d in Tab le 1.It can be concluded that for realdisplacement ductility factors up to atleast 6, the behaviour of all column unitswas satisfactory , exc ept perha ps for Unit 3.

Duri ng the final stage of testing thevisible damage was crushing of concrete andslight or serious buckling of thelongitudinal compression reinforcement.(See Fig . 16 ) . In the case of Uni t 2 the90 hoo ks in the cross ties wer e ob serv edto commence to open at a real displacementductility factor of about 9, and in thecase of Unit 3 the 90 bend s in the squa reperimeter hoops formed of lapped 'U' barscommenced to open at a real di splac ementductility factor of about 7, and in thenext cycle of loading the strength of themembers rapidly degraded due to bucklingof the longitudinal reinforcement andineffective confinement of the concrete.Note that the laps in the 1 U' bars inUni t 3 wer e in the faces of the colu mnparallel to the neutral axis.4.6.2 Concrete Compression Strains andBuckling of Longitudina l Reinf orcem ent

Initial crushing of the cover concretewas observed near or at peak load duringthe first cycle to a nominal dis pla ceme ntfacto r of =. 2 at concr ete surfacecompres sive strains which varied b etween0.004 and 0.0073. Subst antial spalli ngoff of the cover concrete occurred duringload ing to = 4 at concr ete surfa cecompressive strains of 0.011 to 0.017.These surface strains were calculated fromthe potentiometer readings for the 160 mm(6.3 in) gauge length adjacent to thecentral stub. The nominal displa cementductility factors when first crushing andspalling off of the cover concrete wasobserve d are marked on Figs. 12 to 15.The gradual nature of the observedspalling of the cover concrete mean t t hata sudden degradation of horizontal loadcarrying capacity did not occur. Hence itwould appear to be unnecessary to adopt alimiting co mpressive strain at which covercon cret e is assum ed to be suddenly l ostin analytica l moment -curvat ure stu dies, butrather the full range of the stress-straincurv e for unco nfin ed concre te for thecover concrete could be used.The calculated compressive strainson the surface of the core concrete atvariou s real displacem ent ductility factorsare shown in Fig . 18. The linear relati onship between these two quantities for thefour column units is apparent until a realdisplacement ductility factor of about 10is reached whe n the strains in Unit s 2 and3 increased more rapidly, evidently becauseof the less effective confinement from thecross ties with 90 and 180 end hooksand the perimeter hoop formed from 1 U' barswith tension splices.Buckling of longitudinal rei nforc ementwas observed in all column units andcomme nced at the stages marke d in F i g s .12 to 15 . In the case of Uni ts 1 and 4,only incipient buckling was visible in thefinal stages of testi ng. In the case of

Uni ts 2 and 3 the buckling in the finalstages of testing was more serious. InUnits 1 and 4 the buckling occurred betw eenh o o p s , whi le for Units 2 and 3 it oc curr edover a longe r length, as illu stra ted inFig . 19 . It can be conclu ded that crossties with 90 end hooks and peripheral


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E n *L j

Real Displacement Ductility Factor(top side - first cycle)2.0 3.5 5.5 83 12.0 15.9

Nominal Displacement Ductility FactorL 2 & 6 8 12

incipient buckling oflongitudinal reinforcement

Hu(Theoretical)

~8 -6 -L -2 J-&00Nominal Displacement Ductility FactorI I I I-12.7 -KU -6.1 -22Real Displacement Ductility Factor(top sid e - first cycle J

HORIZONTAL DISPLACEMENT (mm)

1st cycle2nd cycleo First yielding o f tension

reinforcementv First visible crushing of

cover concrete Onset of spalling off of

cover concrete

Fig. 12 Measure d Horizont al Load-Dis placemen t Loops for Unit 1.

Real Displacement Ductility Factor(bottom side-first cycfe )2J 5.5 93 133 18.1 22.2I I I I I I

Nominal Displacement Ductility Factor

Commence ment of opening outof 90 hook of supplementary tie

Commenc ement of visiblebuckling of longitudinal

reinforcement

(Theoretical)

(Theoretical)

-8 -6 - 4 -2Nominal Displacement Ductility Factor

15 -82 -5.1 Real Displacement Ductility(top sidefirst cycle)

2.2Factor


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Real Displacement Ductility Factor(bottom side - first cycle )2.0 4.9 9.1 U>.1Nominal Displacement Ductility Factor

2 6 8

Commencement of openingout of 90 turn ofsquare hoop

(Theoretical!-Large bucklingof long, reinf

-8 -6 -L -2Nominal Displacement Ductility FactorI I I I-M.5 -9.8 -5.7 -2.0Real Displacement Ductility Factor(bottom sidefirst cycle 1

HORIZONTAL DISPLACEMENT( mm) 1st cycle- - 2nd cycleo First yielding of

tension reinforcementFirst yielding ofcompression reinforcementFirst visible crushing ofcover concreteOnset of spoiling of offcover concrete

Fig. 14 Measur ed Horizontal Load-Displac ement Loops for Unit 3.

1 1

UNIT 4

Real Displacement Ductility Factor(bottom side - first cycle I

19 5.0 9.0 13.0 17.3 2U

Nominal Displacement Ductility Factor2 6 8 12

Commencement ofvisible buckling of

long, reinf.

H u(Theoretical)

-8 -6 -L -2Nominal Displacement Ductility F actor

I I I I-13.5 -9.9 -62 -22Real Displacement Ductility Factor

(bottom side - first cycle )

0 100HORIZONTAL DISPLACEMENT (mm)

2nd cycleFirst yielding oftension reinforcement

First visible crushingof cover concreteOnset of spoiling offof cover concrete

Fig. 15 Measu red Horizontal Load-Displac ement Loops for Unit 4.


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179

(a) Unit 1 at y = 12. The transverse (b) Unit 2 at y = 12. The 90 hooks atreinforcement is still effecti ve. one end of the interior cross tieshave opened.

Fig. 16 Visib le Damage to Column Units at Final Stages of Testi ng.


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Fig. 18 Extre me Fibre Core Concrete Compressive Strains With RealDisplacement Ductility Factor.


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1 8 1hoops with tension splices were lesseffectiv e for controlling the buckling oflongitudinal b a r s .4.6.3 Strai ns in Transverse Reinf orcem ent

Strains measured by electrica lresistance strain gauges on the tran sversereinfor cement of Unit 1 are shown plot tedagainst the real displacement ductilityfactors in F i g . 2 0 . The strains plottedare the average of pairs of strainsmeasured on opposite sides of the bar.The individual strain readings indicatedthat there was significant bending of thetransverse reinforc ement and hence averagingof the pairs of strain readings was necessaryto obt ain the axial tensil e str ain.For Unit 1, the average strainsmeasured seldom reached yield, even at thefinal stages of testing . The hig hes t

strains recorded were on the square hoops(gauges A, B and C) and these eventuallyreached yield strain on the hoop sides inthe dir ect ion of horiz ontal load and atright angles to it. The strai ns record edon the cross tie in the direction ofloading (gauge D) reached ab out 59 % ofyield, but the strains recorded on theother cro ss tie , which was very clos e tothe neutral axis, reached only about 38%of yield (gauge E ) .For Unit 2, the strains mea sur ed onthe cross tie with alter natin g 90 and 180end hooks in the direction of loadingindic ated a reduc tion in strain at a realdisplacement ductility factor of about 13,which coincided with the commencement ofvisibl e buckling of the longitud inalreinforcement.For Unit 3 the strains measu red onthe lapped 1 U ' bar hoops showed an abr uptdecrea se when a real displa cement ductil ityfactor of 9 was reached, which coincidedwith the serious opening of the 90 cornerbends of those bars and a degr ada tio n ofthe moment of resistance of the column.Correspo nding to this decrease in peri pheralhoop strain, the strain in the cross tienearest and parallel to the neutral axisbecame larger, indicating that the loss ofconfinemen t by the periphera l hoops had

resul ted in a deepe ning of the neut ral axisposit ion. The deepening of the neutralaxis positi on at this stage was confi rmedby the change in the calculated positionas obtain ed from the extreme fibre strainsmeasured on the column faces by the pairsof linear potentiometers.For Uni t 4, the strains meas ure d o nthe 1 J 1 bar cross ties at positions D andE in Fig . 20 we re very similar to th estrains measured at the same positi ons onthe cross ties with a 180 end hook ofUnit 1 at corresponding real displacementductili ty factor s. There fore, it can beconcluded that the anchorage by tensi on

spli ces of the 'J' bars was eff ect ive asfar as these columns were concerned.However, it should be noted that themaximum measured strain on the 1 J ! b arswas up to 60% of the yield s tra in, wh ichwas similar to the maximu m strainmea sur ed on cros s ties of Uni t 1. Thi smeans that the effectiveness of anchorage

by tension splices of 'J 1 bars was notfully investigated in these tests becausethe tension force induced in the 1 J 1 b arswas wel l bel ow the yield te nsio n force ofthose transverse b a r s . It should also benoted that at the peaks of applied momentthe measur ed neutra l axis depth increasedfrom about 0.3 to 0.6 of the sectiondepth, in propo rtion to the increase ofdisplacement ductility factor. That is,the neutral axis depth increased inproportion to the extent of damage to theconcrete in the compr essio n zone. Hencethere was always vertical compres sive stresspresent over part of the length of thetens ion splic es for Uni ts 3 and 4.4.6.4 Curvature Distribution, CurvatureDuctility and Equi valen t PlasticHinge Length

The curvature distribution measuredup the height of the columns showed thatyielding of the column generally wasspre ad over the end 400 mm (15.7 in) ofcolum n adj acent to the cent ral stub.

The curvature at first yield,


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182

I1Ictt o

1O

Unit 1 & U Unit 2 & 3Fig. 19 Obse rved Buckling of Longit udinal Reinfo rcemen t.

2000

1632 h

1000

P 0 5 10to

REAL DISPLACEMENT DUCTILITY FACTORFig. 20 Strains Measu red on Transvers e Reinforc ement of the Hoop Set Nearest theCentral Stub of Unit 1.

H

Fig. 21 Mom ent andIdealised CurvatureDistribut ion for Colu mns.

Cantilever Moment Idealized CurvatureDistribution Distribution


19/26

18 3measu red curve. The flexural strengths werewere calculated using two concretecompressive stress distributions, namely:1. The ACI recta ngula r stress block withthe meas ured f' valu e and an ext reme

fibre concrete compressive strain of0.003.2. The concrete compressive stressdistribution given by the modifiedKent and Park stress-strain relationship for concrete confined byrectangular arrangements of transversereinforc ement [ 12]. In the analysisthe extreme fibre concrete compressi vestrain was increased from zero byincrements of 0.00001 until thecalculated moment became a maximum.This extreme fibre concrete strain atthe maximum moment was found to be0.00456. The stress-str ain equations

are:For e < 0.002KcKf 'c

2e0.002K 0.002K

For e > 0.002Kcf c = K f c [ 1 - V e c - - 0 0 2 K )

where K 1 + P f u5 y hf 8c

(14)

(15)

(16)

where e concrete compressi ve strain,f c = concrete compressive stress, f 1concrete compressive cylinder cstrength, p ratio of volume oftransvers e reinforcement to volume ofconc rete c ore, f ^ = yiel d str ength oftransverse reinforcement, h" = widthof concrete core of section measuredto outside of perimeter hoop, ands^ = centre to centre spacing of hoopsets. For the concrete core of thecolumn units t\M P a , h"0.0255.

25.6 MPa, f h - 333= 320 mm, s^ = 80 mm Xnd p g =For the concrete coverP s = 0 was assumed (that is , thecover concrete was assumed to behave asunconfined c o n c r e t e ) . The concretestress-s train relations so define d areshown in Fig 23, whe re cur ve (a) is forthe core concrete and curve (d) is forthe cover concreteThe theoretical flexural strength forthe column units calculated using the ACIrectangular concrete compressive stressblock was 252 kNm (186 kip ft ) . Thetheoretical flexural strength calculatedusing the modified Kent and Park str ess-strain relations for the concrete was265 kNm (195 kip ft ) . The neutral ax isdepth at the theoretical flexural strengthobtained using the Kent and Park model

was 0.34 of the section depth.As shown in Tab le 1, the maxi mummoments measured in the column units inloading cycles to nominal displac ementduct ility fa ctors of = 2, 4 and 6

exceeded the theoretical value calculatedusing the ACI rectangular concretecompressive stress block by 11 to 15%,andexceeded the theoretical value calculatedusing the modif ied Ken t and Park str ess-strain relations for the concrete by 5 to9%.It is evident that using a concretecompressive stress distribution whichaccounted for the enhancement of concretecompressive strength due to confinementby transverse reinforcement gives a morerealistic calculated value for theflexural strength.The remaining difference between thetheoretical flexural strength and themaximu m measure d moments can be attrib utedto the additional confinement of the stiffcentral stub which would have strengthenedthe column section adjacent to the stubface. The visua l impres sion of thedamag ed zones of all colum n units confi rmedthat the cover concrete there was confinedand constrained from spalling off until anominal displacement ductility factor ofmore than 6 was reac hed. If the conf inement of the column at the face of the stubis assumed to be that provided by thetransverse reinforcement at a spacing of40 mm (that is one half of actual spa cin gin the potential plastic hinge r e g i o n ) ,the theoretic al flexural strength givenby the modifie d Kent and Park stress-stra inrelations for the concrete becomes 272 kNm(201 kip ft) which is 3% high er th an thefle xur al str eng th of 265 kNm (195 kip ft)calculated ignoring the additionalconfinement from the stub. This wouldrequ ire the cri tica l section for thecolum n at whi ch the flexural str ength is265 kNm to be 41 mm (1.6 in) from the stubface. It is likely that the additionalconfinement provided by the central stubwas higher than assumed above. To obtainthe measured column flexural strength atthe stub face would require the criticalsecti on at whi ch the flexural str ength is

265 kNm to be 75 to 133 mm (3.0 to 5.2 in )from the stub fac e. Pre vio us tests [13]hav e found this crit ical se ction to be atabou t 0.5 c away from the stub face ,whe re c = neut ral axi s depth. In thecurrent test series 0.5c ranged betweenab out 60 to 120 mm (2.4 to 4.7 in) w hi chgives a posit ion for the critical sectionin agreement with the previous tests.4.7.2 Theoretical Assessment of theEffect ivenes s of the Confinementfrom the Arra ngeme nts of Transv erseReinforcement

In order to asof the vario us typesreinforcement used imoment-curvature anaThe analyses s atisfistrain compatibilityThe stress-strain relongitudinal reinfor22 was as sume d, and

sess the effectivenessof transversen the test units,lyses were conducted.ed the requirements ofand equilibrium [11].lation for thecement shown in Fig.the stress-strain


20/26

184relation for concre te in compres sion usedwas the modifi ed Kent and Park relationsh ipgive n by Eq s. 14 to 17, wit h the co verconcrete assumed to act as if unconfinedand the core concrete as if confined. Themoment and curvature corresponding to arange of extreme fibre concrete compressionstrains were compu ted to trace the moment-curvature relations.

In Fig. 2 3, the stress-st rain curvemar ked (a) is for the core concre te and isobtained from Eqs. 14 to 17 assuming thatall three transverse bars in each directionare effective and hence that the volumetricratio of tran sve rse steel is p = 0.0255.Cur ve s (b) and (c) are for wh en p is twothirds of 0.0255 and one-third of S0 . 0 2 5 5respecti vely, keep ing the width of theconfined core and the spacing of transversereinforc ement the same. If the confinementfrom transverse reinforcement bars is lost(for example, if some tension splices orend anchorages become i n e f f e c t i v e ) , thestress-strain curve for the concrete willtend to curve (b) or in the wors t case tocurve (c).

Figs. 24 to 27 show the measuredmome nt curvatu re curve s for Units 1 to 4in the first cycle to nominal displace mentductility factors of y N = + 2, + 4, + 6 and+ 8, compared with theoretical monot onicmoment-curvature relations.For Unit 1 in Fig. 2 4 the theoreticalmomen t-cur vatur e relat ion marke d (A) wasobtained using st ress-s train curve (a) ofFig. 23 and hen ce assu ming t hat all theconfi ning steel is fully effe cti ve. It isapparent that the envelope curve ofmeasured moment-curvature response agreeswell with the theoretical monotonicmome nt-cu rvatu re relation and hence thatthe confining steel was effec tive.For Unit 2 in Fig. 25 the theor etica lmoment-curvature relation marked (A),obtained using stress-strain curve (a) inFig. 2 3, agrees quite well with theenvelope of the measured curves up to areal displ acemen t ducti lity factor u ofabout 13. It is notic eabl e that tnemeasured curvatures associated with they val ues are abo ut the same as for Unit 1at least unti l y = 9. Thi s mea ns that theanchorage of the 90 end hooks of thecross ties wa s still as com pete nt as thatof the 180 end hooks of Unit 1 at y R = 9.For Uni t 3 in Fig . 26 the th eoret icalmome nt-c urvat ure relation s marked (A) and(C) were obtained using stress-s traincurve s (a) and (c) of Fig . 23, respe ctiv ely.It can be seen that eventually themeasured moment-curvatu re curves approachedtheoreti cal relati on marke d (C) , which wasbased on confi nement from only one-thir dof the original volumetric ratio oftransverse reinforcement. This resultconfirms that the confinement provided bythe lapped 1 U 1 bars making up the perimeterhoops of Unit 3 beca me inef fective in thefinal stages of loading.For Unit 4 in Fig . 27 the theor etic almoment-curvature relation o btained usingstr ess -st rai n curv e (a) of Fig . 23 agrees

well with the envelope of the measuredc u r v e s . Henc e it was apparent that theconf inin g steel was fully effe ctiv e as forUnit 1.The divergence between the two

theoretical moment-curvature relations whichare ma rk ed (A) and (C) in Figs. 24 to 27would have been greater if the columns hadbe en subj ected to a highe r axial load thanthe 0.2f'A used in thes e tes ts. Thi s isbecau se the moment- curvatu re relatio n is mor edependent on the stress-strain relation ofthe concrete when the axial load level ishigh . Thi s is illustra ted in Fig . 28by the theoretical moment-curvaturerelations calculated for the column unitsusing the three stress-strain relations forthe concrete and for axial load levels of0.2f^A and 0.4f'A . It is evid ent thattests on columns w 2 t h a highe r axi alcompressive load than the 0.2f'A used inthese tests may have ind icat ed Ca ^ g r e a t e rdependence of column behaviour on cross tieeffectiveness.

5. CONCLUSIONSFour reinforced concrete columns, with4 00 mm (15.7 in) squa re cross secti ons wer etested under axial compressive load andcyclic flexure to simulate severe seismicloading. The longitudinal reinforceme ntconsisted of eight bars. The transversereinforcement consisted of square perimeterhoops surrounding all longitudinal barsand cross ties between the intermediatelongitudinal bars. The major variab le ofthe study was the type of anchorage usedfor the hoops and cross ties. The quantityof transverse reinforcemen t in the columnsatisfied both New Zealand and United Statesseismic code provis ions. The anchora gedetails used for that reinforcement did notalways satisfy those codes. The columns we resubjected to reversed horizontal loadingconsisting of two cycles to nominaldis pla cem ent ducti lity factors of 2, 4, 6, 8 and someti mes highe r. The axialload level was 0.2f1A , whic h resu lted ina neu tr al axi s deptfi f 0.3 to 0.6 thesection depth at the peaks of appliedmomen t. The transverse reinforc ement was

from deform ed bar with a yield stre ngth of333 MPa (48,300 ps i) .The following conclusions werereached for these columns:

1. With regard to the anchorage of the endsof the transverse reinforcement with bardiamete r d^ in potential plastic hingeregions by bending around lon gitudin alb a r s :(a) Satisfactory behaviour was observedfor:Perimeter hoops with a 135 end turnand an 8d^ exten sion into the c oreconcrete.Inter ior cross ties with a 180 endturn and a 5d^ extension into thecore concrete.1 J 1 bar interi or cross ties wit h endanchora ge as above and with 2 4d^tension splices in the core concrete.However it should be noted that the


21/26

18 5

D

to

to

721

500474

0

-- 721-247 (1.091 -9.091S S) 3' 45

I III

fy = 474MPaI III f u l t - . 721 MPa111

E s --200 000MPa

! i i i i 10 0.01 0.02 0.04 0.06 0.08

STRAIN, E s0.10 0.12

Fig. 22 Stress-Strain Model for 20 mm (0.79 in) Diameter Longitudin al Re inforce ment.

Fig. 23 Modifi ed Kent and Park Stress- Strain Mod el.


22/26

186

0 100 200 300CURVA TURE (x JO ' 61/mm)

Fig. 24 Comp aris on of Theoret ical Monotoni c and Measure d Cyclic Moment -Curva tureRelati ons for Unit 1.

CURVATURE (x W' 6 1/mm)

Fig. 25 Comp aris on of Theoret ical Monoton ic and Measure d Cyclic Mome nt-Cur vatureRelati ons for Unit 2.


23/26

187

Ecc=2.89%

Theory (monotonic hading) P s - 2.55%, s = 80mm p s = 0.85 % , s =80 mmExperimentUnit J , bottom sideFirst cycle of eachdisplacement ductility factor-

cc. Extreme compression fiberstrain of core concrete100 200 300

CURVATURE (* 10' 6l/mm)WO

fig. 2 6 Compari son of Theor etica l Monot onic and Measured Cyclic M oment -Curv atureRela tion s for Uni t 3.

CURVATURE (* 10 6 l/mm I

Fig. 27 Comparis on of Theor etical Monoton ic and Measur ed Cyclic Mo ment- Curvat ureRelations for Unit 4.


24/26

188

Yielding of compression andtension reinforcement

Yielding of compressionreinforcement

0 0

Yielding oftension reinforcement _ m i Pe = 0.2A g

--O.Cf^Ag p s =2.55% , s =80mm p s =1.70% , s =80mm p s - 0 . 8 5 % , s =80mm

cc= Concrete compressive strainat extreme fiber of core concretei I i _

100 200 300CURVATU RE (x 10~ 61/mm)

WO

Fig. 28 Compa rison of Theoret ical Moment- Curvat ure Relations for Columns WithTwo Different Axial Load Levels.


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18 9maximum measured strain i n the 'J'bars was up to 60% of the yield strai n.This means that the effectiveness ofanchorage by the tension splices wasnot fully investigated in these tests.

(b) The behavi our of inter ior cros s tieswith a 9 0 turn and 6d^ extension atone end and a 18 0 tur n and 5d^extension at the other end, placedso that consecutive cross ties havetheir 90 end hook on oppositesides of the column, behavedsatisfactorily up to a realdisplacement ductility factor ofabout 9. Beyond that displa cementlevel the 90 end hooks commenced toopen and the effectiveness of thoseend hooks was reduced.(c) The behaviour of perimeter hooks

formed from 'U 1 bars lapped in thecover concrete with a 17d^ tensionsplice was satisfactory up to a realdisplacement ductility factor ofabout 7. Beyond that displa cemen tlevel there was a rapid degr adat ionof strength due to the splice in thecover concrete becomi ng ineffectiv eas the cover concrete was lost andthe 90 corner turns of the 'U 1 b aropened. This transverse reinfo rcement detail is to be discouraged.2. With regard to the flexura l strengt hreached by the columns:(a) The maxi mum measur ed flexur alstrength exceeded the theoreticalflexural strength calculated usingthe ACI concrete compress iverectangular stress block and themeasured material strengths by 11to 15%.

(b) A mor e accurate theor etic al esti mateof the maximum measured flexuralstrength was obtained using themodifie d Kent and Park stress-str aincurve for compressed concrete whichtakes into account the enhancement ofconcrete strength and ductility dueto the confinement by transversereinfor cement. Als o, it was evidentthat additional confinement of thecritical section of the column wascaused by the presence of theadjacent stub which simulated anadjoining beam or other member.

ACKNOWLEDGEMENTSThis investigation was carried outin the Department of Civil Engineering ofthe Univers ity of Canter bury by Profe ssorB. McNamee while on leave from DrexelUniversity, U.S.A., and by AssociateProfessor H. Tanaka while on leave fromAkashi Technological College, Japan,

together with Professor R. Park of theUniversity of Canterbury.Thanks are due to techniciansMessrs A.M. Bell and G.E. Hill forassist ance with the const ructio n and testingof the column units.

The financial assistance provided bythe Nationa l Roads Board and theUniversity of Canterbury to carry out thisinvestigation is gratef ully acknowledged.

REFERENCES(1) "Code of Practice for the Design ofConcrete S tructure s (NZS 3101 , Part 1:1982) " and "Comm entar y on the Desi gnof Concret e Structur es (NZS 3101, Part2 : 1 9 8 2 ) " , Standards Association of NewZealand, Wellington.(2) "Building Code Requirements forReinforced Concret e (ACI 3 1 8 - 8 3 ) " and"Commentary on Building Code Requirements for Reinfor ced Concret e (ACI318-8 3 ) " , American Concrete Institute,

Detroit, 1983.(3) "Recommended Lateral Force Requirementsand Commentary", Seismology Committee,Structural Engineers Association ofCalifornia, San Francisco, 1975.(4) "Seismic Design of Concrete Structures.Second Dra ft of an App end ix to theCEB-FIP Model Cod e", Bulle tin d'Informati on No. 149, Comite Eu ro-International du Beton, Paris, March1982.(5) "Reinforced Concrete Structures.Desig n Code and Inter pretatio n(Commentary)", Architectural Instituteof Japan, Tokyo, 1982.(6) Pri estl ey, M.J .N, and Park, R.,"Strength and Ductility of BridgeSu bstructures", Research Report No.84-20, Depart ment of Civil Enginee ring,Universit y of Cante rbur y, December 1984 ,p . 1 2 0 .(7) Mo ehl e, J.P. and Cava nag h, T.,"Confinement Effectiveness of CrossTies in Reinfo rced Conc rete ",Structural Engineering, Proceedings ofthe Ameri can Society of Civil Enginee rs(to be p u b l i s h e d ) .(8) Oest erle , R.G., Fio rat o, A. E. andCorley , W.G. , "Reinforcement Detailsfor Earthq uake Resistant Stru cturalW a l l s " , Concrete International:Designand Construction, V.2 , No . 12 ,Decem ber 1980, pp.55-66 .(9) Ziro Suzuk i, Chief Edi tor , "GeneralRepor t on the Tokach i-oki Earthqua keof 1968", Keigaku Publishing Co. Ltd,Tokyo, 1971, p.754.

(10) Jenn ings , P.C., Edi tor, "EngineeringFeatures of the San FernandoEarthquake February 9, 1971",Earthquake Engineering ResearchLaboratory, California Institute ofTechnology, Pasadena, 1971, p.512.

(11) Park, R. and Paulay, T., "ReinforcedConcre te Struct ures ", John Wiley andS o n s , New York, 1975, p.769.


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190(12) Park, R., Pri est ley , M.J .N, and G i l l ,W.D., "Ductility of Square ConfinedConcrete Columns" , Journal ofStructural Division, Proceedings ofAmerican Society of Civil Engineers,Vol. 108, No. ST4, April 1982,pp.929-950.(13) Priest ley, M.J .N. , Park , R. andPotangaroa, R.T., "Ductility ofSpirally-Confined Concrete Columns",Journal of Structural Division,Procee dings of Amer ica n Society ofCivil Engineer s, Vol. 107, No. ST1,January 1981, pp.181 -202.

TABLE 1 ; TEST RESULTS FROM COLUMN UNITS

ColumnU n i t FirstYieldDisplacementAy

m m

FirstYieldCurvaturey

xlO ^1/miti

MaximumMomentMeasuredinLoadingCycles toy N = 2, 4and 6MmaxkNm

At NominalDisplacementDuctility FactorVi = 6 inSecond Cycle

At FinalStage ofTest

Comparisonof MaximumMeasuredMoments andTheoreticalFlexuralStrengths


m m


xlO ^1/miti


^N ^R M *y

My

M Mmax maxM MAC I MKP


m m


xlO ^1/miti


^N ^R Mm ax*y Mmax y

M Mmax maxM MAC I MKP

1 10.9 22.6 279 + 6-6 6.210. 6 0.940.91 9.2 +1211. 4i

15 . 9 0.94 20.0 1.11 1.052 10.9 21. 6 284 +6-6 9.68.4 0.940.88 10.8

!+1212. 4 ! 22 . 3 0.85 32.8 1.13 1.07

3 9.6 | 18. 4i

290 + 6-6 9.410.0 0. 910.88 11.8. -814.6 -14. 5 0.66 25 . 0 1.15 1.09

4 10.0 ! 19.8 28 5 + 6-6 9.210. 0 0.960.94 11. 91 +1213.o; 21 . 4 0.94 25. 5 1.13 1.08

N o t e :1 mm = 0.039 in. 1 kNm = 0.7375 kip ft.M = mea sur ed mom ent in colu mn at face of centr al stub .M = max imu m mea sur ed mom ent in column at face of centra l stubm a xM A C I = t n e o r e t i c a l flexural strength calculated using ACI concrete compressive stress block.M M K P = t n e o r e t i c a l flexural strength calculated using the modified Ke nt and Parkconcrete compressive stress distribution.

Anclaje Del Refuerzo de Confinamiento en Columnas

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