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209SEI SM IC ST R EN GT H E N I N G OF E X I ST I N G

R EI N F OR C ED C ON C R ET E B U I L D I N GS I N JA PA NShunsuke Sugano*

2. DESIGN AND CONSTRUCTION1. INTRODUCTIONA number of reinforced concreteb u i l d i n g s , damaged by recent severe earthquakes , required extensive repair and alsos t r e n g t h e n i n g ^ " 4 ) . The Tokac hi-oki E arth quake of 1968 heavily da maged a large numberof low rise bui ldi ngs . Some of these werestrengthened by the addition of structuralw a l l s . These buildings are still in use.Because of the lack of guidelines, thedesig n as well as the cons tructi on forstrengthen ing was based on engineering judgement alone. The destructive 1978 Miyagike n-oki Earthquake was also followed by the

strengthening of a number of buil dings .However, in this case materials and techniques for construction were specificallyselected and the design was based on experimental or analytical investigations or onguidelines, where these were available.

2.1 General PrinciplesThe approach to the design and construction for the strengthening of hazardous buildings in Japan is summarized in theflow chart of Fig. 1. Detail ed disc uss ionsare held before the design commences andthe cons truct ion techniq ue is chos en. Theresults of the seismic evaluation and incertain cases also those of field investigations are required. Laboratory tests maybe necessary to provide additional information for design and construction.The aims of the strengthening are toprovide:

(1) Increased streng th with resp ect tolateral loading, orThese studies have also indicatedtha t ther e is a wid e sc atte r in the lev el ofseismic resistance of existing buildings.It was found that a cons iderab le numbe r oflow to medium rise building s, designed andconstructed in accordance with previousbuilding codes , may need strengthening( 16) .Cons eque ntly a number of public and privat eb u i l d i n g s , considered hazardous, werestrengthened or rebuilt.It is intended to provide not onlyincreased strength, so as to preventcollapse, but also increased stiffness togive increased protection against damage tonon-str uctural building components . Toestabl ish guidel ines for design and construction , several experimental studies,rele vant to the seismic s treng thenin g of _existi ng s tructures, have been conducted *7 , 9 , 1 7 ) m However, test data, accumulatedover some 8 year s, have not been sys tematically reviewed as yet.The necessity for strengthening ofhazardous buildings was recognised for some

t i m e , and as a conseq uence an advis orycommittee for the Japanese Ministry of Construction prepared design guidelines in1 9 7 7 ( 1 2 ) a These design guidelines wereinten ded to be used in conju nctio n wit h themethod of evaluation of seismic safety ofexist ing buil dings , proposed by the samecommitt ee. This method has been describedin some detail in New Zealand(13). j n anumber of cases these guidelines havealready been used in Japan.

This paper describes techniqueswhereby current knowledge developed in Japancan be used to give increased resistance toexist ing substandard buil ding s. First thegenera l design procedure is described andthen brief reviews of some of the relevantexperim ental studies are given. Some application of strengthening techniques to existing buildings will also be described.* " Chief Research Engineer , TakenakaTechnical Research Laboratory, Tokyo.

BULLETIN O F T H E N E W ZEALAND NATIONAL SOCIETY F O R

(2) Increased ductility or(3) A proper combina tion of these twofeatures.These concepts are illustrat ed in Fig. 2.The combination of strength and ductilityinvolve the proper balance betwee n s trengthand stiffness.

To provide increa sed stre ngth is considered as being the most promising approachin the strengthening of low to medium riseb u i l d i n g s . Even if ductility is provided,increase d strengt h is expect ed to reducethe magnitudes of inelastic dis plac ements .For ducti le structures it is conside red tobe particularl y important to reduce eccentriciti es result ing often from the irreg ulardistribution of stiffness within a storeyor throughout the entire structure.2.2 Construction Techniques

Examples of construction techniquesto meet both, the increased st rengt h andincreased ductility, criteria for strengthening are given in Fig. 3. Gener ally neweleme nts may be added to exis ting stru ctures to give increased strength, or existing framing elements may be reinforced withnew mater ials to improve their duct ili ty.Typical strengthening methods considered inJapan are assembled in the chart of Fig. 4.Infilled walls and wing walls, extensivelyused in Japan and shown in Figs. 3(a) and(b), are cast in situ or precast wallelements which are attached to frames or tocolumns, as appropriate.

For the systems shown in Fig. 3( a) ,(b) and (d) , cast in situ or prec ast concrete is commonly used with the variousconnection techniques that are listed inFig. 4. Typical detail s for such connections are given in Figs. 5 and 6 whereadditional information required in thedesign is also given. Careful attentionmust be paid to connections, because theyEARTHQUAKE ENGINEERING, V O L . 14, N O . 4, DECEMBER 1 9 8 1

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{START)

1PRELIMINARY

INVESTIGATIONIANALYTICALINVESTIGATION

TRENGTHENING

SUFFICIENTLY SAFEOR

STRENGTHENING IMPRACTICAL

EVALUATION OFSEISMIC SAFETY

1ESTABLISH AIMS

PRELIM/NARY INVESTIGATION* =SELECT CONSTRUC TION METHOD

DETAILING FORCONSTRUCTIONI\DESIGN CALCULATIONS]

ANALYTICAL EVALUATION(SEISMIC SAFETY)

NO

[DRAWINGS & SPECIF/CATIONS \ CONSTRUCTION

STRENGTHENINGDESIGN AND

CONSTRUCTION

FEASIBILITY STUDYON CONSTRUCTION1OWNER'S OR USER'S

REQUIREMENTSLABORATORY TEST AND/OR

FIELD INVESTIGATION

F I G . 1 - F L O W C H A R T O F D E S I G N A N D C O N S T R U C T I O N O F S E I S M I C S T R E N G T H E N I N G

S T R E N G T H R E S I S T A N T T Y P EX U L T I M A T ED I S P L A C E M E N ' I

R E S P O N S E ^ - S T R E N G T H E N E D

S T R E N G T H E N E D^ - X - ^ U N S T R E N G T H E N E D

U N S T R E N G T H E N E DR E Q U I R E D EL E X U R A LS T R E N G T H

S T O R Y D I S P L A C E M E N T(a ) TYPE OF EARTHQU AKE RESISTANCE

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

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a S T R E N G T H O F B R I T T L E M E M B E R S(b ) A IM OF SEISMIC STRENGTHENING

F I G . 2 - A I M O F S E I S M I C S T R E N G T H E N I N G

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will strongly affe ct the behav iour of thestrengt hened struct ure, as well as for theplac ing of concrete on the site. Highpressure pumping of the fresh concrete maybe neces sary to avoid the formati on ofcavit ies between the new and the existin geleme nts. Detailing of bracing elementsshould be such as to avoid stress concen tration s .In the process of increasing theducti lity of exist ing colum ns, such as shownin Fig. 3( e) , (f) and (g) , one of the aimsis to increase their shear streng th. Thisis achieved by the wrapp ing tech niques shownin these illu strat ions. A narrow gap at theends of the encase ment is provid ed to avoidthe undesired increase of the flexuralstrengt h of the member at that se ction .

2.3 Design Procedu reThe safety of the strengthen ed build ing is assessed with the recently intro duced Japanese evaluation procedure forexisting reinforced concrete buildings,report ed in detail by A o y a m a ( 1 3 ) . If theyare more detailed, other procedures are alsopermis sible. The guideline(12) gives specific calcul ation techniq ues for infilledwalls, wing walls and encased columns.These calcul ation proced ures are based ontests.

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But t resses

As an example , the desig n streng thof an infilled wall assembly of Fig. 5(d)is given as the smaller of either:(1) The total shear str engt h of the wal l

panel and both colu mns, treated asindependent elements or(2) The total shear str engt h of onecolumn and the connect ion to the wallalong the beam and the punch ing(sliding) shear strength of the othercolumn.

These procedures assume that failurewill occu r either in the wall pan el or alongthe connections where shear is bei ng int roduced to the infill wa ll. The streng th ofthe connec tions is evaluate d either fromtheoretical considerations or from empiricalequat ions applicable to the tech niqu es shownin Fig. 5(a) to (c) . The punc hing or sliding shear strength of the column is givenin terms of the princi pal t ensile stress inthe concrete.

The contribution to flexural andshear strength by a column stre ngthe ned bywing walls , as shown in Fig. 6 (a ), is estimated to be 80 % of that of a colum n withidentical properties but cast monolithic-ally with the wing wa ll s. In the case ofwing walls with dowel connections, anv // ;J> // // //

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d ) B Y B U T T R E S S E S B Y W E L D E D W I R E F A B R I C SFIG. 3 - C O N S T R U C T I O N T E C H N I Q U E S F O R S E IS M I C S T R E N G T H E N I N G

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A I M O FS T R E N G T H E N I N G

I S E I S M I CS T R E N G T H E N I N G

( a ) T O I N C R E A S ES T R E N G T H

(b) T O I N C R E A S EB O T H S T R E N G T HA N D D U C T I L I T Y

C ) T O I N C R E A S ED U C T I L I T Y

( a ) R e s i s t a n c e r e l y i n g o n s t r e n g th

( c ) R e s i s t a n c e r e l y i n go n d u c t i l i t y

\ ^ n s t r e n g t h e n e d_ mmD I S P L A C E M E N T

F I G . 4 - T Y P I C A L S T R E N G T H E N I N G M E T H O D S

T Y P E O FS T R E N G T H E N I N G

T E C H N I Q U ES T R E N G T H E N I N G

E L E M E N T C O N N E C T I O N

I N F I L L I N GW A L L S

B R A C I N G

C A S T - I N - P L A C EC O N C R E T E P A N E LP R E C A S T C O N C R E T E W A L LP A N E LR I B B E D S T E E L P A N E L

^ C O N C R E T E B L O C K S

' ' T E N S I O N A N DC O M P R E S S I O NC R O S S B R A C I N G( S T EE L O R C O N C R E T E )T E N S I O N C R O S SB R A C I N G ( S T E E L )K - B R A C E S( S T E E L , C O N C R E T E )

B U T T R E S S I N G O-BRACESX ( S T E E L )

A D D I N GW I N G W A L L S

C A S T - I N - P L A C EC O N C R E T E

R E I N F O R C I N GC O L U M N S

P R E C A S T C O N C R E T EP A N E L

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W E L D E D T R A N S V E R S ER E I N F O R C E M E N TE N C A S E M E N T O F S T E E LS E C T I O N ( C I R C U L A R ,R E C T A N G U L A R )S T E E L S T R A P S

\ W E L D E D W I R E F A B R I C S

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D O W E L W I T HW E D G E A N C H O RW E L D E D D O W E L W I T HE X I S T I N G R E I N F O R C E M E N TW E L D E D D O W E L W I T HM E C H A N I C A L L YA N C H O R E D P L A T EH O O K E D D O W E L O NE X I S T I N G R E I N F O R C E M E N TC H I P P E D S H E A R K E YA D H E S I V E S H E A R K E YB O L TW I T H O U T C O N N E C T I O N

idealiz ed truss syste m is used to model loadtransfer, as shown in Fig. 6( c) .In the case of encased columns of thetype shown in Fig. 3 (e ), (f) and (g) , theshear and flexural strength is evaluate d asfor ordinary monolithic reinforced concretec o l u m n s , using the increased dimensions aswell as the contribution of the added steele l e m e n t s .For other strengthening methods,evaluation by testing is recommended.

2.4 Selection of Construction MethodsThe selecti on of const ructi on metho dsshould be based on the overal l co nside rations of the work involved on the site,wei ght s and convey ance of eleme nts to behandled, cost and also on the structuralcharacteristi cs re levant to each alternati ve of struct ural solutio n? Table Iillus trate s these asp ect s. It is based onthe author * s exper iment al study of one-ba y,one- stor ey simple frames which were str engt he ne d by fi ve di ff er en t t ech ni que s (**) The quantiti es in brac kets in the tablerepresent normalised values in terms of theinfilled concrete wall construction whichwas taken as unity.For this type of structure it wasfound, as Table I indic ates , that rein forced concrete constructio n has merits in

terms of cost, strength and stiffness whileit is penali zed for site work and conve yanceof comp onen ts. In general steel brac ing wasfound to offer advantage s in conveya nce,weig ht and ductil ity but it was the mos texpensive solution.3. RESEARCH

The number of streng thened struct uresthat have been examined ex peri ment allyserved as a backg round in formul ating theguidelines for design^ 1

The earli est tests were aiming atthe improvement of ductility in columns(5,9)by the techni que s shown in Fig . 3, and atthe boost ing of the strength of frames bythe addition of precast and cast in situwalls(5,6,17) ^ Subsequently one storeyinfilled frames with various conne ctio ndetails(5,17) a nd bracing systems(7 , 8 ,14)were exam ined. Curren tly, three storeyframes, strengthened by infilling andbraci ng, were also tested. In the followinga brief r eview of the findings of some ofthese studies is given.3.1 Infilled Concret e Wall s

In a series of tests wit h threetypes of infilled walls it was found that:(a) These provid ed signi fica nt increa sein strength.

http://welded/http://welded/

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(c ) PRECAST SHEAR KEY CONNECTION

Q w u = Shear s t re ng th of wal l pane lQ s w = S tren gth of an inf i l led w al lQpc = Punch ing she ar s t r eng th a ttop f columnQj = To ta l s t r e ng t h o f connec t ion s .

along a beamQc = St re ng th o f a co lumn

(d) DESIGN OF INFILLED WALL

F I G . 5 - D E S I G N A N D C O N S T R U C T I O N O F I N F I L L E D W A L L S ( 1 2 )

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( a) M O N O L I T H I C A L L Y C A S T W I N G W A L L S

( b) W I N G W A L L S W I T H D O W E L C O N N E C T I O N S

( c ) DE SI GN OF W I N G W A L L SW I T H D O W E L C O N N E C T I O N

F I G . 6 - D E S I G N A N D C O N S T R U C T I O N O F W I N G W A L L S ( 1 2 )

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TABLE I - FEASI BILIB Y STUDY OF STRENGTHEN ED ONE-STO REY FRAMESConstruction Cost of Structural Capacity

Workability Conveyance Curing Weight Structures Stiffness Strength DuctilityInfilledConcreteWall

much worknecessarydifficultconveyanceof concrete necessary

heavy(1.00) cheap(1.00) high(1.00) high(1.00) low(1.00)InfilledConcreteBlock Wall

easy work,few workman easylocalcuringnecessary

heavy(1.00)slightlyexpensive(1.61)

low(0.30) low(0.30) low(1.13)InfilledSteelWall

easy work,but accuracyneeded easylocalcuringnecessary

light(0.39) expensive(2.41) moderate(0.49) m o d e r a t e(0.78) low(0.96)CompressionBraces

simpleconnection,easy work easy

localcuringnecessary

light(0.39)

slightlyexpensive(1-47)

low(0.27)

low(0.63)

high(1-7)

TensionBraceseasy workbut accuracyneeded easy

necessaryfor chippedslabslight(0.44) expensive(2.93) low(0.24) low(0.67) high(1.7)

(b) Dowels connec ting wall to framesfailed simultan eously at the threadedshank by shear.(c) It was eff ect ive to prov ide gapsalong column s to allow walls tobeha ve in a ductile m anner .(d) Chip ped shear key s prov ided as good a

shear connection as monoli thic co nstruction .

3.2 Braced FramesAvailable data on the response ofbraced frames is as yet limi ted. X, K anddiamond shaped braci ng systems were studied.These indicated moder ate incre ases instrength but adequate ductil ity and abili tyto dissipate energy . The studi es indicatedthat connecti on details requi re care ful

attent ion as they might strong ly influenc ethe overall hysteretic respo nse.When adequate connection, placed continuo usly around all bounda ry membe rs, wereprov ided , infill frames exhibite d the samestren gth as monol ithic walls with id enticalboundary elemen ts. Recently tested multipleprec ast infill wall panels indicated goodd u c t i l i t y p r o p e r t i e s ( 5 , 7 , 1 1 ) f but, asexpected, significantly less strength wasattained. The predominance of bendingbeh avi our in three storey infilled frame swas observ ed in contrast to shear domina ncethat contro lled the response of one storeyinfilled structures.

(8)Using this data , Sugano propos edthe following guidelines with respect tothe strength of infill panels with dow elc o n n e c t i o n s :(1) Whe n the req uir ed stren gth of astren gthen ed structu re is more than60% of that obtainable with an identical monoli thic w all , Q , or more than0.6 / fj MPa , in terms or co ncreteshear stre ss, dowels should be desig ned to posse ss a shear stress equiva lent to 1.0 MPa.(2) Wall s with out any connecti on to aboun dary frame may develop a strength

corr espo ndin g to 0.4 Q w or 0.3 /f ^M Pa .(3) Wa lls con nec ted to beam s but not tocolumns could be expected to develop0trength up to 0.6 Q w orin terms of shear stress. 6 /fj" MPa

3.3 Wing Wall Constr uctionThe effect of small wal ls place dadjacen t to existin g columns or placedseparate ly in the frames was also studied '6 ) . Cast in place wall add ition s pr ovidedalmost as much strength as identica l mono lithic construction. The addition of precast units resulted in less strength but itproduced more ductility.

3.4 Reinforce d Column sThe types of stre ngth enin g techni quesshown in Fig. 3 were examined(5,9 ) exper imentally . Both the strength and ductilityof poor columns was signifi cantl y increasedby these strengthening met hods . Whilewelde d wire fabric wrap ping res ulted in considerab le increase in duct ilit y, the use ofsteel straps preven ted shear failu res anddelayed the crushing of the concrete.

3. 5 General Effects of Strengtheni ngTypical load-displacement relation ships for the structures studied are presented in Fig. 7. These are only qu alit ative indications of the order of strengthand ductilit y that might be attained using

different strengthening techn iques . Thelateral (shear) force, required to develo pthe full strength of a mono lith ic rein forced concrete wall with boundary elements, isdenoted as Q w , while that of the originalframe or a colu mn is given as Q F and Q crespe ctive ly. It is seen that, whe n ade quate shear connect ors are pro vide d, f rames

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strengthened by infilled wall panels candevel op st rength of the order of 0.6 Q w or3.5 Q F respectively. Steel braces andmultiple precast concrete panels withoutshear connectors gave less strength increase but resulted in improved d ucti lity .Win g wal ls attached to columns have appro ximately doubled the strength of the originalcolumn while giving considerably increasedd u c t i l i t y .

Fig. 8 shows the dramati c improvem entattain ed by streng thening of a column u singwelded wire fabric wrapping and mortar .Fig. 8(a) shows the brittl e failure of thistype of short column that has been ext ensively used in Japanes e construc tion. D i s placement ductilities larger than 6 couldbe attained with the strengthening technique employed.Fig. 9 compares the observe d r esponseof strengthened three storey and one storeyfram es. As stated earlier becaus e of thedominance of bending effects, the threestorey frames exhibit ed larger duct ility ,while the one storey frames suffered brittleshear failures.

4. STRENGTHENING OF EX ISTING BUILDINGSMore than ten reinforced concret ebui ldin gs suffered very severe damageduring the 1978, June 12th, Miyagiken-o kiEar thq uak e (M = 7.4) in and arou nd the cityof Sendai. An earlier earthquake on Februa ry 2 0 of the same year (M = 6.5) a lso

damaged several buildings in Northern Japan.Although some of these buildings weredemolished and subsequently rebuilt, many

were repaired and also s treng thene d.Different techniques were used inthese undertaki ngs. Unfortunate ly no

detailed evidence is available, however, toenable a reliable survey to be mad e. Inmost cases cast in place rei nforc ed c oncrete walls with dowel or chipped shear keyconnections to existing structures wereused for strengthening*2) in some casesdetails similar to those given in Fig. 6and Fig. 3(g) were us ed, but exact detailsare not available.Some example s of stren gthen ing ofboth, damaged and undamaged buildings, arebriefly described in the foll owin g.

4.1 Damaged School Building "A "( 1 8 )A five storey college building, shownin Fig . 10, and Fig. 14( a) to (d) suffere dsevere damage during the June 1978 earthquake . Particularly "captive columns",such as seen in Fig. 1 4 ( b ) , were affected.Severely damaged columns were replace d withnew concrete and additi onal reinf orcem enthas also been prov ided . Overal l strengthwith respect to lateral loadings wasincreased by the addition of infilled wallsand also by the thickening of existingwal ls. In the long dire ction of the building, cross bracing was added to the framing,as seen in Fig. 14(c) and (d ). Bracingmembers were connected at every floor toexisting exterior beams by means of steelp l a t e s .An exper iment al study was also undertaken to investi gate the beha viou r of these

Q w

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