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E L S E V I E RPIh S0141-0296 97)00022-9

Engineering StructuresVol . 20 , Nos 4 6 , pp . 336-346 , 1998© 1 9 9 7 E l s ev i e r S c i e n c e L t d

Al l r igh t s rese rved . Pr in ted in Grea t Br i t a in0 1 4 1 4 1 2 9 6 / 9 8 1 9 . 0 0 + 0 . 0 0

R e c e n t d e v e l o p m e n t s i n h y b r i d

s t ructures in Jap an researchdes ign and cons t ruc t ionS . M o r i n o

Department of Architecture Mie University Tsu 514 Japan

A var ie ty of uses for combined re inforced concrete and s tee l haverecent ly been appl ied in actual cons t ruct ion wh ich are ca l ledhybr id s t ructures . The appearance o f the hy br id con st ruct ion is theresul t of pursuing the econ om ical h igh-eff ic iency of the const ruc-t ion. This paper f i rs t in t roduce s the present s i tuat ion o f des ign con-s t ruct ion and research on hyb r id s t ructures and deta i ls of severalse lected hybr id s t ructura l sys tems are d iscussed: SRC s t ructuresCFT co lum n sys tems RC co lum n S beam sys tem and hybr id wa l lsys tem. The US-Japan Cooperat ive Ear thquake Research Programon Co mp osi te and Hybrid Structures i s br ief ly expla ined andf inal ly research needs are show n.

Keywords: s t e e l r e i n f o r c e d c o n c r e t e , c o n c r e t e - f i l l e d t u b e , R C c o l -u m n S beam sys tem hybr id wa l l

1 . I n t r o d u c t i o n

Concre te -encased s tee l cons t ruc t ion i s ca l led s tee l -r e i n f o r c ed c o n c r e t e S R C ) c o n s t r u c t io n i n J a p a n a n d i sof ten appl ied for mid- to h ighr i se bu i ld ings , in which a l lb e a m s a n d c o l u m n s a r e u s u a l l y m a d e o f S R C . R e c e n t l y,d i f fe r e n t t y p e s o f c o m b i n e d r e i n f o r c e d c o n c r e t e R C ) a n ds tee l S) s t ruc tura l sys tem s a re encoun tered , which a reca l led hybr id s t ruc tures. A w ide var ie ty of hybr id cons t ruc-t ion i s found in ac tua l bu i ld ings , f rom s imple compos i tem e m b e r s t o a s y s t e m i n w h i c h a r e i n f o rc e d c o n c r e t e c o r e i scombined wi th s tee l f rames . In order to c la r i fy the presen t

s i tua t ion o f the des ign and con s t ruc t ion o f hybr id s t ruc tures ,an inves t iga t ion was car r ied ou t in 1989 us ing ques t ion-na i res sen t to genera l cont rac tors , enginee r ing off ices , fabr i -ca tors and s tee l ma nufac ture rs ~. Ques t ion s c oncern ing thef o l lo w i n g w e r e a s k e d : a p p l i c a ti o n o f th e s y s t e m , n u m b e rof cons t ruc t ions , to ta l f loor a rea , number of f loors , typ ica lspan , charac te r i s t ics and expec ted mer i t s , and des ign con-s idera t ions . Ques t ionna i res were aga in d i s t r ibu ted in 1990to c la r i fy the de ta i l s o f the s t ruc tura l des ign and cons t ruc-t ion of th ree popula r hybr id s t ruc tures : concre te - f i l l ed tubu-la r CF T) co lumn sys tem , s t ruc tura l wal l wi th encas ed s tee lb r a c e o r p la t e , a n d c o m b i n a t i o n o f R C c o l u m n w i t h Sbea m 2 .

In th i s repor t , we f i r s t in t roduce the c lass i f ica t ion of

hybr id s t ruc tures wi th examples , the presen t s i tua t ion ofdes ign and cons t ruc t ion , des ign s tandards , and research onhybr id s t ruc tures , base d on the resu l t s o f l i te ra ture survey sand the ques t ionna i re d i s t r ibu ted to s t ruc tura l engineers .

Deta i l s o f the presen t des ign and co ns t ruc t ion o f the th reepopular hybr id s t ruc tura l sys tems a re d i scussed , in addi t iont o t h e S R C s t r u c t u r a l s y s t e m . T h e U S - J a p a n C o o p e r a t i v eE a r t h q u a k e R e s e a r c h P r o g r a m o n C o m p o s i t e a n d H y b r i dSt ruc tures i s b r ie f ly expla ined and , f ina l ly, research needsare shown.

2 . P r e s e n t s t a t u s o f h y b r i d s t r u c t u r e s

2.1 . Des ign and cons t ruc t ionAcco rd ing to a 1989 inves t iga t ion ~, hybr id cons t ruc t ions

a r e c l a ss i fi e d i n to fi v e g r o u p s : l ) h y b r i d m e m b e r s ;2 ) h y b r i d a s se m b l a g e s ; 3 ) h y b r i d s y s t e m s ; 4 ) h a l f -p r e -c a s t P C a ) c o n s t ru c t io n s ; a n d 5 ) o t h e r c o n s t r u c t io n s , i nwhich the ord inary SRC s t ruc ture i s no t inc luded . A re la -t i v e l y l a rg e n u m b e r o f a p p l i c a t i o n s o f t h e f o l l o w i n g i t e m si n e a c h c a t e g o r y w e r e r e p o rt e d : 1 ) C F T c o l u m n a n d c o n -cre te -encased s tee l tubular co lumn wi th or wi thout f i l l ed-c o n c r e te ; 2 ) c o m b i n a t i o n o f S R C c o l u m n w i t h R C o r S Cb e a m , w h e r e S C m e a n s s t e e l e n c a s e d b y c o n c r e t e w i t h o u tmain re inforcements o ther than th in longi tud ina l bars sup-p o r t in g s t i rr u p s , a n d a c o m b i n a t i o n o f R C c o l u m n w i t h So r S R C b e am ; 3 ) c o m b i n a t i o n o f S R C o r R C c o re w it hs tee l per im eter f rame; 4) ha l f -preca s t s lab ; and 5) spec ia luse of s t ruc tura l cab les and s t rands for p res t ressed concre te

s t ruc ture . D eta i l s o f the appl ica t ion o f hybr id s t ruc tureswere repor ted in Ref . [3 ] andF i g u r e 1 s h o w s a n e x a m p l eof ha l f -precas t cons t ruc t ion .

S t ruc tura l engineers expec t to ob ta in the fo l lowing mer i t s

6



Recent deve lopm ents in hybr id s t ruc tures in Japan: S Me r ino 3 3 7

Figure 1 Half-precast const ruct ion

b y e m p l o y i n g t h e h y b r i d s t ru c t u ra l s y s t e m s : t h e i n c r e a s e o fs t i ffness , s t rength , d uc t i l ity an d f i re res i s tance capac i ty ; ther e d u c t i o n o f a m o u n t o f s t e e l ; t h e c o m p a c t c r o s s - s e c t i o n o fh i g h r i s e a n d l o n g - s p a n f r a m i n g s y s t e m s ; a n d t h e r e d u c t i o no f c o n s t r u c ti o n c o s t a n d r i m e . I t w a s n o t e d t h a t th e m o t i v -a t i o n b e h i n d e m p l o y i n g t h e h y b r i d s y s t e m w a s t o m a k e u pf o r m u t u a l d e f i c ie n c i e s o f R C a n d S s y s t e m s . D e s i g n c o n -s idera t ions were main ly g iven to the eva lua t ion of s t i ffness ,s t rength , duc t i l i ty, and s t ruc tura l charac te r i s t ic coeff ic ien tD s; m o d e l i n g o f r e s t o r i n g - fo r c e c h a r a c t e r is t ic s f o r d y n a m i cana lys i s ; conf i rmat ion of smooth s t ress t ransfer ; and prob-lems assoc ia ted wi th the cons t ruc t ion a t the s i te , such asconcre te c om pac tness in 1 :he s tee l tube . M any of these w eres o l v e d e x p e r i m e n t a l l y.

I t seems tha t the des ign and the cons t ruc t ion techniquehave a t ta ined a suff ic ien t l eve l o f appl ica t ion for the ac tua lb u i l d in g i n th e c a s e o f t h e C F T c o l u m n s y s t e m , th e f r a m i n gs y s t e m w i t h R C c o l u m n s a n d S b e a m s , t h e s t ru c t u ra l w a l lw i t h e n c a s e d s t e e l b r a c e o r p l at e , th e f r a m e s y s t e m c o m b i n -i n g S R C o r R C c o r e s w i t h a S fr a m e , a n d h a l f - p r e c a s t m e m -b e r s , a m o n g w h i c h t h e f o r m e r t h r ee s y s t e m s w e r e a l s oinves t iga ted us ing ques t ionna i res in 19902 . The resu l t s a red i s c u s s e d b e l o w.

2 2 Structural design standardsStruc tura l s tandards of ten used in the des ign of hybr ids t ruc tures a re the s tanda rds for SR C s t ruc tures 4 and rec-o m m e n d a t i o n s f o r c o m p e s i t e c o n s t r u c t i o n s 5 , b o t h p u b l i s h e dby A rchi tec tura l Ins t i tu te of Japan (A IJ) , and the s t ruc tura lr e q u i r e m e n t s p u b l i s h e d b y t h e B u i l d in g C e n t e r o f J a p a n(BC J) 6 . I t i s o f ten the c~ .se tha t new sys tem s a re des igne du s i n g f o r m u l a s d e r i v e d f r o m e x p e r i m e n t a l a n d a n a l y t i c a li n v e s ti g a t io n s . T h e p r e s e n t e d i ti o n o f S R C s t a n d a rd s o fA I J~ does no t t ake in to a ccou nt the conf in ing e ffec t o f as tee l tube on concre te s t rength in the CFT column. Theconf in ing e ffec t s on concre te s t rength were taken in to

account in the eva lua t ion in the CFT repor t7 , which was ane x p e r i m e n t a l i n v e s ti g at ie , n p e r f o r m e d j o i n t l y b y a n u m b e ro f c o n s t r u c ti o n c o m p a n i e s a n d s t e e l m a n u f a c t u r e r s.

2 3 Recent research trendsFigure 2 s h o w s t h e n u m b e r o f p a p e r s o n h y b r i d c o n s t ru c -t ion repor ted a t the Ann ual M eet ings o f AIJ . A s ign i f ican ti n c r e a s e i n th e n u m b e r o f p a p e r s o c c u r r e d t w i c e , i n 1 98 7and 1990. A bou t ha l f the ; pape rs presen ted each year wereon CFT s t ruc tures un t i l 1989 , bu t decreased grea t ly a f te r1990. Th e percen tages of papers rep or ted in the las t 7 yearsf o r e a c h g r o u p o f h y b r i d c o n s t r u c t i o n a r e s h o w n i nFigure3 . U p t o 1 98 9, m o s t p a p e r s m a i n l y c o n c e r n e d t h e g r o u p s o fh y b r i d m e m b e r s a n d h y b r i d a s s e m b l a g e s , w i t h t h e f o r m e rs h o w i n g a g r a d u a l d e c r e a s e . H o w e v e r, t h e g r a p h f o r 1 9 9 3shows tha t the subjec ts o f inves t iga t ion a re sca t te red ra there v e n l y a m o n g s t t h e h y b ~ f d c o n s t r u c t i o n g r o u p s .Figure 4ind ica tes the k inds of hybr id s t ruc tures tha t hav e been

1-- ] Tota lpapers193 1991 P a p e r son C FT structures 44)

Number of papers in paren theses ) 1 2 7 ~ ~122 (29)1 ILj

N u m b e rof Papers 2 )~ ~ [,JIq

5 0 I- 4 2 ~ n f l l l

40 ~1)

30 17

2 8 to t i le

10 7 7 ~ ~ 7) 8X3)7 4 6

7 0 7 2 7 4 7 6 7 8 8 0 8 2 8 4 8 6 8 8 9 0 9 2Ye a r

Figure2 Number of papers on hybrid construct ions

Hybrid members

~ ] Hybr id systems

Year

I [ H y b r i d assemblages

[EElH a l f - P C aConstructions

E 1

81

8,

0 10 20 30 40 50 60 70 80 90 100

Figure3 Percen tage o f pape r s i n each g roup o f h ybr id con-s t ruct ions

inves t iga ted dur ing the per iod f ro m 1987 to 1993 . About6 6 o f t h e p a p e r s w e r e o n C F T c o l u m n s in t h e g r o u p o fh y b r i d m e m b e r s , a n d 6 4 o n t h e c o m b i n a t i o n o f R C c o l -u m n s w i t h S o r S R C b e a m s i n th e g r o u p o f h y b r i d a s s e m -blages .

The objec t ives of research a re c lass i f ied in to four ca teg-



8 Recent developm ents in hy br id s t ruc tures in Japan: S Me r ino

Figure

7-1993

i c v r

D Concrete-encased-and-filleds tee l tubular co lum ns

~7~ Concrete-encased steelbeams (SC)

I ~ Prestressed concrete (PC)

beams wi th encased s t ee l1 RC structural walls with

encased-steel braceor plate[] Others

(a) Hybrid members

3/168

~ ~10 8 /1 68 )

87-1993

1 Combination of SRCco lumns w i th RC o rSCbeams

~ ] Combina t ion o f RCco lumnswith S or SRCbeams

~ ] Combination of hybridmembers with convent ionalor hybrid members

[~ Others

(b) Hybrid assemblages

Percentage of papers in the groups of hybrid members and hybrid assemblages

1 Structural ~ ] Construction m e t h o dperformance and ma nagem ent

~ ~ O u m b i l i ~ a n d ~ Structuralfire resistance t=zaplanningYear

92

90

89

88

87

0 10 20 30 40 50

Figure5 Percentage of papersgroup

60 70 80 90 100

in each research objective

ories based on the kind of improvement that could be achi-eved by employing the hybrid system: structural perform-ance, construction methods and management, durability andfire resistance, and structural planning. Figure 5 shows thepercentage of papers classified according to the objectives,and the items of improvement in the papers published in

1990 and 1993 are shown in Figure 6. A large percentageof the improvements was concerned with the structural per-

formance, but that concerning construction methods andmanagement has recently increased, which may be con-sidered to be the result of countermeasures against theshortage of manpower and skilled workers. The improve-ments of strength and ductility in the structural performancewere the main concerns of the engineers. The hysteresisloop and stress transfer were also considered important. Forthe construction methods and management, it was aimed tosimplify the form work for concrete casting, make precastmembers, increase construction efficiency, save manpower,achieve low-cost construction and simplify the arrangementof reinforcing bars. The development of long-span, light-weight and highrise framing systems and a free choice ofstructural systems were the main concerns in structuralplanning.

3 . S t e e l -r e i n f o rc e d c o n c r e te ( S R C ) st r u c t u r e s

3 1 Design and constructionComposite structures composed of steel members encasedin reinforced concrete are called steel-reinforced concrete(SRC) structures, and they have become one of the mostpopular structural systems in Japan. They originated in Eur-ope and came to Japan in the 1910s. The good seismicperformance of the SRC structures was recognized after theGreat Kanto earthquake in 1923 and, since then, most ofthe building structures of 6-9-stories have been constructedwith SRC systems. Figure 7 shows the change in the cross-section of SRC columns and beams. In the very early timesof SRC construction, large rolled steel sections wereimported, but open-web steel composed of angles and flat



Recent developments in hybr id s t ruc tures in Japan: S Me r ino

5/126

~0

(a ) S t r uc tu ra l p e r fo rman ce

9 3

0 1 7 3

~0 93

( b ) Co ns t ruc t i on me tho d an d managem en t

C r a c k

[ ~ ] Strength

[7~ Stiffness

~ ] Duct il ity

Hysteresisl o o p

[ ] Dy n amic cha ra c t e rs t i cs

[ ff ] S t ress t ransfer

F o r m w o r k

D P a

~ '~ Cons t ruc t i on e f f i c i ency

M a n p o w e r

In spec t i on

~ L o w c o s t

~-~ Re bararrangement

[ ] L o n g s p an

9

[ -- -J L ight weig ht

[ ] ighrise

Figure

PO(c ) S t ruc tura l p lanning

Improve men t s p roposed in pape r s pub l i shed in 199 and in 1993

9 3 [ ] F r e e d o m

column

1 Fb e a m

(a ) 19 10 -1920c o l u m n c o l u m n b e a m

C o 1 9 3 0 - 1 9 7 0

l lz lv J i l [ z o l u m n b e a m c o l u m n c o l u m n

(c ) 196 0 - 197 0 (d ) a f t e r 1970

Figure 7 Change of cross-.sections

] I Fb e a m

bars , as shown inFigure 8 were main ly used unt i l 1970,s ince fu l l -web s teel was expens ive . H owe ver, fu l l -web s tee lm ad e o f H- shaped s e c t i on s ha s b een u s ed s i nce 1 970 fo rsevera l reasons : in i t ia t ion of the manufac ture of H-shapedrol led sec t ions , popular iza t ion of welded cons t ruc t ion , andr i se in the cos t of fabr ica t ion of bol ted or welded op en-

web steel . Figure 9 shows a t yp i ca l SR C b eam- to - co lumnconnect ion cur rent ly used .

The cos t of an SRC s t ruc ture per uni t a rea i s the h ighes tamong a l l s t ruc tura l sys tems. This i s re la ted to the amountof s tee l used , w hich i s approxim ate ly equal to 120 kg/ m 2 ,40 o f wh ic h co r re spon ds t o t he r e in fo r ceme n t s . O the r



3 4

a) Batten plate Co) Latt ice

Figure 8 Types of steel web

(c) Full web

Figure 9 SRC beam to column connection

charac te r i s t ics o f SRC co ns t ruc t ion a re as fo l lows: i t is veryof ten used fo r bu i ld ings ta l le r than 10-s tor ies ; the f loor a reap e r b u i l d in g c a n b e m u c h l a rg e r t h a n t h o s e o f o t h e r s t ru c -t u r al s y s t e m s ; a n d m o s t o f th e S R C b u i l d in g s a r e a p a r t m e n thouses and off ices .

T h e s t a n d a rd d e s i g n s t r e n g th o f c o n c r e t e F c fo r S R Cs t ru c t u re s i n J a p a n i s u s u a l l y l e s s t h a n 3 6 0 k g f / c m 2,

because of the l imi t spec i f ied in JASS58, and Fc of2 4 0 k g f / c m2 a n d 2 7 0 k g f / c m 2 a r e p o p u l a r l y u s e d . G r a d e sof re inforcem ents a re SD295 (s tandard des ign s t rength F= 3 0 0 0 k g f / c m 2 ) f o r D 1 6 a n d s m a l l e r b a rs , S D 3 4 5 ( F =3 5 0 0 k g f / c m 2 ) f o r D 1 9 t o D 3 2 , a n d S D 3 9 5 ( F =4 0 0 0 k g f / c m2 ) f o r D 3 5 a n d l a rg e r b a r s . T h e m a x i m u m s t ee lg r a d e u s e d f o r S R C s t r u c t u r e s h a s b e e n a s h i g h a s S M 5 2 0( F = 3 6 0 0 k g f / c m2) in accordance wi th the SRC s tandardso f A l L H o w e v e r, i n t e n s i v e r e s e a r c h h a s b e e n d o n e f o r t h eappl ica t ion of h igh-s t rengths t e e l 9a n d d e s i g n f o r m u l a s f o rS R C m e m b e r s u s i n g s t e e l w h o s e u l t i m a t e s t r e n g t h i s a sh i g h a s 8 0 0 0 k g f / c m 2 h a s b e e n p r e p a r e d ~°.

3.2 . Adva ntages and d isadvantagesT h e a d v a n t a g e s o f S R C c o n s t r u c t i o n o v e r R C c o n s t r u c t i o nare : g rea te r duc t i l i ty, more compact c ross -sec t ion , smal le rc r e e p d e f o r m a t io n , a n d u s e o f to p - d o w n m e t h o d o f c o n c r e t ecas t ing . Those over S cons t ruc t ion a re : mul t ip le ro les of

R e c e n t d e v e l o p m e n t s i n h y b r i d s t r u c t u r e s i n J a p a n : S M o r i n o

concre te as s t ruc tura l , f i reproof ing and buckl ing- res t ra in inge lements , h igher s t i ffness , and grea te r damping . Disadvan-t a g e s a r e h i g h e r c o n s t r u c t i o n c o s t s c o m p a r e d t o R C c o n -s t ruc t ion , and heavier weight compared wi th S cons t ruc t ion .In addi t ion , the grea tes t d i sadvantage i s the very compl i -ca ted ,a r rangement of s tee l and re inforceme nts , espec ia l lyi n t h e b e a m - t o - c o l u m n c o n n e c t i o n , a s s h o w n i nFigure 9.I n o r d e r t o o v e r c o m e t h i s c o m p l e x i t y, a s i m p l e r a r r a n g e -m e n t o f r e in f o r c e m e n t s h a s b e e n r e c o m m e n d e d ~, a n d t h eu s e o f p r e c a s t S R C b e a m s h a s b e e n a t t e m p t e d .

3 .3 . SRC s t andards o f AMThe f i r s t ed i t ion of SRC s tandards of AIJ wa s publ i shed in1 95 8 , b a s e d o n i n t e n s i v e r e s e a r c h s u p e r v i s e d b y t h e C o m -mi t tee for SRC s t ruc tures in AIJ . This f i r s t ed i t ion wascharac te r ized by the in troduc t ion of the conc ept o f super-p o s e d s t r e n g t h t o t h e d e s i g n f o r m u l a s f o r S R C m e m b e r ssubjec ted to ax ia l fo rce and bending moment , tha t i s , thec o n c e p t t h a t t h e s t r e n g t h o f t h e S R C m e m b e r i s g i v e n b yt h e v e c t o r s u m o f th e s t r e n g th s p r o v i d e d b y S a n d R C p o r-

t ions . The sup erpose d u l t imate s t rength i s i l lus tra ted inFig-ure 10 w h e r e t h e s t re n g t h v e c t o r O C o f a n S R C c o l u m nis g iven by the s t rength vec tors OA o f the S por t ion andO B o f t h e R C p o r t i o n . T h e t h e o r y o f s u p e r p o s e d s t r e n g thwas es tab l i shed for the ca lcu la t ion of the u l t imate s t rengtho f a c o m p o s i t e m e m b e r b a s e d o n t h e m a t h e m a t i c a l t h e o r yof p las t ic i ty 2 .~3, bu t i t has b een appl ied to the ca lcu la t iono f a l l o w a b l e s t r e n g t h f o r c o n v e n i e n c e . T h e s e c o n d e d i t i o nwas publ i shed in 1963 wi th a smal l change: the provis ionsf o r s h e a r d e s i g n , w h i c h w e r e s h o w n i n t h e C o m m e n t a r y i nthe f i r s t ed i t ion , were inc luded in the main body.

In the th i rd ed i t ion pu bl i shed in 1975, s ign i f ican t changesin the concept o f shear des ign were in t roduced , in which

e m p h a s i s w a s p l a c e d o n t h e d e f o r m a t i o n c a p a c i t y o f th emember : the member should fa i l in f lexure than in shear ;S and RC por t ions should ind iv idua l ly res i s t the shearforces ac t ing on them, wi thout cons ider ing the bondb e t w e e n t h e t w o p o r t i o n s ; a n d t h e a x i a l f o rc e o f t h e c o l u m nshould be l imi ted to a cer ta in va lue . These were based onthe resu l t s f rom the Tokachi -oki ear thquake in 1968 , dur ingwhich br i t t l e fa i lu re of RC columns occur red .

The four th , tha t i s , the presen t ed i t ion , was publ i shed in1987, fo l lowing the rev is ion of the Enforcement Order ofthe Bui ld ing S tandard Law of Japan e nforce d in 1981 ~4, and

Figure 10

~

Superposed strength of column section



Recent developments in hybrid structures in Japan: S Morino 34

inc luded research f ind ings s ince 1975 . The main fea tures ofthe rev is ion a re as fo l lows:

( 1 ) A s m a n y d e s i g n f o r m u l a s a s p o s s ib l e a r e in c l u d e d i nthe main bo dy o f the s tandards . The y a re for the f lex-u r a l, s h e a r a n d b o n d d e s i g n s o f n o t o n l y th e b e a m a n dt h e c o l u m n , b u t a l so o f t h e b e a m - t o - c o l u m n c o n n e c -t ion , the co lum n base , the s tee l jo in t and the s t ruc tura lwal l . Al l fo rm ulas a re sys tema t ica l ly wr i t t en , based onthe concept o f supeqgosed s t rength .

(2) Formulas used to eva lua te the u l t imate s t rength andi tems necess ary for the eva lua t ion of the u l t imate hor i -z o n t a l l o a d - c a r r y i n g c a p a c i t y o f a f r a m e a r e p r e s e n t e d .T h e c a l c u l a t i o n o f t h e u l t i m a t e l o a d - c a r r y i n g c a p a c i t yi s requi red for the s l ruc ture s ize and he ight o f whiche x c e e d t h e l i m i t s p e c i f i e d b y t h e E n f o r c e m e n t O r d e rof the Bu i ld ing S tandard Law of Japan ~4.

( 3 ) D e s i g n p r o v i s i o n s f o r c o m p o s i t e s t ru c t u r es c o n s i s t in gof s tee l tubes and concre te a re inc luded .

T h e E n f o r c e m e n t O r d e r o f t h e B u i l d i n g S t a n d a r d L a wof Japan rev ised in 1 981 requi res two s teps of s t ruc tura ldesig n fo r building struc~Eures exc eed ing a certain l imit inte rms o f the s ize and he ight . F i r s t, the s t ruc ture shouldb e h a v e e l a s t i c a l l y u n d e r a m o d e r a t e e a r t h q u a k e , a n d m e m -b e r s s h o u l d b e p r o p o r t i o n e d b y t h e a l l o w a b l e s t r e ss d e s i g nm e t h o d ; t h e n i t m u s t b e p r o v e d t h a t t h e m a x i m u m h o r i z o n -ta l load-car ry ing capac i ty of the des igned s t ruc ture , de te rm-ined by ine las t ic ana lys i s , i s l a rger than the va lue requi redto res i s t a severe ear thq~aake . The requi red load-car ry ingcapac i ty i s spec i f ied in te rms of the s t ruc tura l charac te r i s t iccoeff ic ien t Ds (the ra t io of the requi red hor izonta l s t reng thof an ine las t ica l ly des igned s t ruc ture to tha t o f an e las t ica l lydes igned s t ruc ture , equiva len t to the coeff ic ien t Rw in theUS ) , wh ich i s de te rmin ed re la t ive to the duc t i l i ty o f thes t ru c t ur e . S R C s t a n d a rd s s h o w t h e m e t h o d f o r d e t e r m i n i n gthe u l t imate s t rength des ign , bu t no provis ions for D~. BCJrequi rem ents 6 a re for the s t ruc tura l des ign of th ree m ajors t ruc tures , i . e . S , RC and SRC, and ind ica te the va lues ofDs of SRC s t ruc tures : 0 .30 and 0 .55 for duc t i le and br i t t l es t ruc tures , respec t ive ly, t lybr id s t ruc tures a re no t coveredb y t h e S R C s t a n d a r d s o r t h e B C J r e q u i r e m e n t s . T h e s t r u c -t u r al d e s i g n e r u s u a l l y d e t e r m i n e s t h e v a l u e o f D s o f h y b r i ds t r u c t u r e s b y a p p l y i n g a p p r o p r i a t e p r o v i s i o n s a c c o r d i n g t othe hypoth e t ica l fa i lu re mode , tha t is , i f the RC c olum nf a i ls a t t h e u l ti m a t e s t a g e o f th e f r a m e c o m p o s e d o f R Cc o l u m n s a n d S b e a m s , p r o v i s i o n s f o r R C s t r u c t u r e s i n B C J

requi rem ents a re app l ied , and i f the s tee l beam fa i ls , p ro-v i s i o n s f o r S s t ru c t u r e s ~ e a p p l ie d . T h e c o n c e p t o f a l i m i ts t a t e d e s i g n h a s n o t y e t b e e n e m p l o y e d i n e i t h e r t h e A I Js tandards or the BCJ requi rements , except in the draf t o fS s tandards of AIJ 5 .

3.4. Da ma ge due to eaJ thquakesT h e r e w a s n o s e v e r e e a r t h q u a k e d a m a g e t o S R C b u i l d i n g sr e p o r t e d d u r i n g t h e 5 5 y e a r s b e t w e e n t h e G r e a t K a n t o e a r t h -q u a k e i n 1 9 23 a n d t h e M i y a g i k e n - o k i e a rt h q u a k e i n 1 9 7 8 ,s i n c e n o m a j o r S R C b u i l d i n g s h a d b e e n b u i l t i n t h e a r e a sa f f e c t e d b y t h e s e e a r t h q u a k e s , a s w e l l a s b y t h e N i i g a t ae a r t h q u a k e i n 1 9 6 4 a n d th e To k a c h i - o k i e a r th q u a k e i n1968, except for the shear fa i lu re of nons t ruc tura l wal l si n a p a r t m e n t h o u s e s d u r i n g t h e M i y a g i k e n - o k i e a r t h q u a k e .H o w e v e r , t he H y o g o k e n - N a n b u e a r th q u a k e in 1 9 95 c a u s e dv e r y s e v e r e d a m a g e t o a l l s t ru c t u ra l s y s t e m s i n c lu d i n g S R Cs t ru c t u re s . M o s t o f t h e S R C b u i l d i n g s d a m a g e d i n t h e

H y o g o k e n - N a n b u e a r th q u a k e w e r e b u i lt b e f o re 1 97 5, a n do p e n - w e b s t e e l o f b a t te n p l a t e t y p e a n d l a tt i ce t y p e c o m -p o s e d o f a n g l e s a n d f l a t b a r s h a d b e e n u s e d . T h e t h i rd e d i -t io n o f t h e S R C s t a n d a r d s o f A I J i n 1 9 75 r e c o m m e n d e d t h eu s e o f f u l l - w e b s t e e l , b a s e d o n e x p e r i m e n t a l i n v e s t i g a t i o nw h i c h v e r i fi e d t h at t h e e n e rg y d i s s ip a t i o n c a p a c i t y o f a S R Cc o l u m n w i t h o p e n - w e b s t e e l i s r a t h e r p o o r. A n o u t l i n e o ft h e o b s e r v e d d a m a g e t o S R C s t ru c t u re s c a u s e d b y t h isearth qua ke is g iven bel ow 16,17.

( a ) C o l u m n : b u c k l i n g a n d f r a c tu r e o f t h e a n g le u s e d a s th em a i n m e m b e r o f th e b u i l t -u p s t e e l c o l u m n , f r a c t u r e o fthe sp l ice p la te a t the co lumn jo in t , and f rac ture of thew e b m e m b e r a t t h e n e t s e c t i o n i n t h e r i v e t e d c o n n e c -t i o n w e r e o b s e r v e d .

( b ) U n e m b e d d e d c o l u m n b a s e : f ra c t u r e o f t h e m a i n m e m -b e r o f t h e s t ee l c o l u m n , b u c k l i n g a n d f r a c t u r e o f m a i nr e i n f o rc e m e n t s , c r u s h i n g o f c o n c r e t e u n d e r t h e b a s epla te , and expuls ion and f rac ture of anchor bo l t sw e r e o b s e r v e d .

( c ) B e a m - t o - c o l u m n c o n n e c t io n : o n e c a s e o f s h e a r f a i lu r eof a connec t ion pane l was repor ted , a l though de ta i l ss u c h a s t y p e o f s t e e l w e b , a m o u n t o f s h e a r r e i n f o r c e -men ts , and de te r iora t ion of concre te s t rength , werenot clarif ied.

( d ) Tr a n s i t i o n z o n e f r o m S R C t o R C c o n s t r u c t i o n : t h em o s t d i s t i n ct i v e c h a r a c te r i s ti c o f t h e d a m a g e c a u s e d b yt h is e a r t h q u a k e w a s t h e c o n c e n t r a t io n o f d a m a g e a t th et r a n s i t i o n z o n e , w h i c h c a u s e d a c o m p l e t e d i s a p p e a r-a n c e o f a c e r t a i n s t o r y i n s o m e c a s e s . S i m i l a r d a m a g ew a s o b s e r v e d i n t h e S R C c o l u m n a t th e t r a n si t io n z o n ef r o m t h e f u l l - w e b t o t h e o p e n - w e b s t e e l .

( e ) B u i l d i n g f r a m e i n w h i c h t h e f l o o r a r e a o f t h e u p p e r

s t o r y w a s m u c h s m a l l e r t h a n t h a t o f t h e l o w e r s t o r y :dam age conce nt ra t ion a t the t rans i tion zone , a t whicht h e f lo o r a re a b e c o m e s m u c h s m a l l e r t h a n t h e a r e a o n ef l o o r b e l o w w a s o b s e r v e d , s i n c e t h e v i b r a t io n c h a r a c -te r i s tics d i ffe r be tw een the upper and the lowe r par t sof the bu i ld ing .

( f ) Wal l : s truc tura l wai l s who se hor izonta l re inforce me ntsw e r e n o t s e c u r e l y a n c h o r e d t o c o l u m n s w e r e d a m a g e d ,a l t h o u g h t h e d a m a g e t o t h e s u r r o u n d i n g S R C f r a m ewas ra ther s l igh t . Many cases of shear fa i lu re of non-s t ruc tura l wal l s occur red , main ly because wal l s weren o t a b l e t o a c c o m m o d a t e t h e d u c t i l e d e f o r m a t i o n o fa d j a c e n t S R C c o l u m n s .

( g ) P o o r c o n s t r u c t io n a n d d e t e r i o ra t i o n o f c o n c re t e : d a m -a g e w h i c h m a y h a v e b e e n c a u s e d b y p o o r c o n s tr u c t io nwas repor ted , such as insuff ic ien t concre te cover th ick-ness , insuff ic ien t hook angle of shear re inforcements ,s h o r t a g e o f s t re n g t h a t m a i n r e i n f o r c e m e n t j o i n e d b ypressure weld ing , and de te r iora t ion of concre te wi th alow s tandard d es ign s t rength F t . Tes t da ta of concre tec o r e s p e c im e n s t a k e n f r o m c o l u m n s o f d a m a g e d S R Cb u i l d in g s s h o w e d t h a t th e s t r e n g th d e t e r i o r a te d t o 9 3 -1 87 k g f / c m 2 in t h e c a s e o f a b u i l d i n g d e s i g n e d w i t hF c e q u a l to 2 1 0 k g f / c m 2 c o n s t r u c te d i n 1 9 57 , a n d 1 2 3 -1 78 k g f / c m 2 i n t h e c a s e o f a b u i l d i n g d e s i g n e d w i t hFc equa l to 180 kg f /c m 2 cons t ruc ted in 1971.

For bu i ld ings cons t ruc ted a f te r rhe regula t ion concern-i n g t h e s e i s m i c d e s i g n w a s c h a n g e d b y t h e r e v i s i o n o f t h eE n f o r c e m e n t O r d e r o f th e B u i l d in g S t a n d a r d L a w o fapani n 1 9 8 1 , n o c a s e o f c o m p l e t e c o l l a p s e w a s r e p o r t e d , a n d



34 Recent develo pments in hybrid structures in Japan: S Mori no

thus it may be concluded that the new regulation was effec-tive. There was no report of damage to CFT structures.

4 . Concre te f i l led s t ee l tubu la r s t ruc tu res

4.1 . Chara c te r i s t ics and research t rends

The advantages of CFT structures are as follows: highstrength and ductility, resistance to local buckling of steeltube due to concrete filling, increase of concrete strengthdue to confining effect provided by steel tube, reduction ofconstruction costs, and elimination of form work. On theother hand, the following issues are usually raised: stresstransfer mechanism from a steel beam to a CFT columnrelated to low bond strength between the steel tube andconcrete, and compactness of cast in si tu concrete under-neath the diaphragm at the beam-to-column connection. Inaddition, engineers often complain that the same amount offireproof material as in the case of a pure steel structure isstill required for the CFT column by law. Recent researchinvestigations have focus ed on these issues, and the struc-tural behavior and performance of normal CFT columnsand beam-to-column connections, shown in Figure 11 havebeen experimentally investigated. Concerning stress trans-fer, the use of steel tubes with embosses on the inner sur-face and cast-steel stiffener has been considered. It has beenrevealed that in Japan, the steel tube had been expected toact as both a load-carrying element and a confining elementfor concrete simultaneously, while it is applied only as aconfining element in the United States. The latter conceptwas recently adopted also in Japan, and the behavior of areinforced concrete column strengthened by steel tube,which does not carry either axial force or bending momenthas been investigated. Experimental work on the fire resist-

ance characteristics of CFT members has been carried out.

4 .2 . Des ign and cons t ruc t ionThe results of the investigation using questionnaires in19902 were as follows. The use of concrete-encased-and-filled steel tubular colum ns was rare. The use o f CFT struc-tures was not limited, but they were mainly used for officeand hotel buildings. The structural walls were not oftenUsed in CFT structures. The merits o f CFT structures com-pared with RC and SRC structures were applicability tohighrise and long-span structures, improve ment o f stiffness,reduction of manpower and elimination of form work. Thestructural performance of CFT structures was on the basis

of S RC standards 4 in most cases. In some cases, the con-crete was taken into account only to evaluate the stiffness.

Square and circular tubes were used with comparable fre-quencies in CFT structures. Circular tubes (diameter o f450-1000mm, diameter-thickness ratio of 17-65) wereused for buildings with irregular plan grids, and square andrectangular tubes (width of 400-900 mm, width-thicknessratio of 10-54) were used in the case of regular plan grids.Most tubes were cold-formed, which were inexpensive andwidely available on the market. Box sections built-up bywelding were used when the plate became thick and/orlarge ductility was required. Cast-steel tubes were used tosimplify the beam-to-column connection. Annealing for theremoval of residual stresses was rarely performed in Japan.

The type of diaphragm of the beam-to-column connec-tion shown in Figure 11 was selected according to platethicknesses of the column and the beam. The through-typediaphragm was often employed when the beam plate wasthicker than the column plates; otherwise, an inner dia-phragm was employed. This tendency led to the use ofthrough-type diaphragms for cold-formed tubes, and innerdiaphragms for built-up box sections. The use of an outerdiaphragm was adopted as an easy way of assuring concretecompactness of CFT. Through-type and inner diaphragmsusually have an opening with a diameter of 200-300 mmfor concrete casting, with several small holes for the pas-sage of air. Beam-to-column connections were built-upusing welding flanges and welding or bolting webs.

The design standard strength of concrete was between210 and 270 kgf/cm 2 in most cases, and administrative per-mission must be obtained for any construction in whichthe concrete strength exceeds 360 kgf/cm 2. The maximumstrength reported in the questionnaires was 870 kgf /cm 2

concrete for which was compacted by the centrifugalmethod and cured in an autoclave. In the questionnaires,80 of respondents stated that special consideration wasgiven to the mixing proportion of concrete. Air-entraininghigh-range water-reducing agent was often used to obtaingood workability of concrete with low water content. Con-crete was cast by pumping or by the use of a tremie tubein most cases. The casting height was about 30 m in theformer case and about 10 m in the latter. In about 50of the responses, the efficiency of the casting method wasconfirmed experimentally. The construction technique iswell established on the basis of actual construction of high-rise buildings taller than 100 m.

Outer diaphragm Inner diaphragm Tlu~ugh type diaphragm

Figure 11 Types of di phr gm



Recent developments in hybrid structures in Japan: S Morino 4

T h e s t i f f n e ss a n d s t r e n g th o f C F T c o l u m n s a n d t h es t re n g t h o f b e a m - t o - c o l u m n c o n n e c t i o n s w e r e e v a l u a t e d b yt h e m e t h o d o f s u p e r p o s k i o n a c c o r d i n g t o S R C s t an d a r d s .T h e i n c r e a s e o f c o n c r e t e s t r e n g t h a n d d e f o r m a t i o n c a p a c i t yo f t h e C F T c o l u m n d u e t o t h e c o n f in i n g e f f e c t w e r e t a k e ni n to a c c o u n t a c c o r d i n g t o t h e C F T r e p o r t 7 i n s o m e c a s e s .

The s t ruc tura l charac te r i s tic coeff ic ien t Ds was de te rm -i n e d a c c o r d i n g t o t h e r a n k o f b e a m s , s i n c e t h e f r a m e w a susua l ly propor t ioned so tha t i t would fa i l wi th the p las t ich i n g e s f o r m i n g i n b e a m s . H o w e v e r, e n g i n e e r s t h o u g h t t h a tD~ should be de te rmined accord ing to the overa l l charac te r-i s ti c s o f th e f r a m e . T h e l o a d - d e f l e c t i o n c u r v e o f t h e f r a m ewas approximated as t r i i l inear for the dynamic ana lys i s ,w h i c h w a s o b t a i n e d f r o m t h e i n c r e m e n t a l a n a l y s is , b a s e do n t h e b i l i n e a r c h a r a c t e r i s t i c s a s s u m e d f o r C F T c o l u m n sa n d S b e a m s . Vi s c o u s d a m p i n g w a s o f t e n a s s u m e d t o b ep r o p o r t i o n a l t o t h e f r e q u e n c i e s , a n d t h e d a m p i n g c o n s t a n twas taken to be 2 . Des ign c r i te r ia were usua l ly se t asfo l lows: e las t ic response and s tory dr i f t angle no t g rea te rthan 1 /200 under the leve l 1 se i smic input which cor re -

s p o n d s t o a m o d e r a t e e a r t h q u a k e ; a n d p l a s t i c d e f o r m a t i o nra t io no t g rea te r than 2 and s tory dr i f t angle no t g rea te r than1/100 under the leve l 2 se i smic input which cor responds toa m a j o r e a r t h q u a k e .

5 . S t r u c t u r a l w a l l s w ith e n c a s e d s te e l b r a c e s o rp l a t e s

5 2 Des ig n and cons t ruc t ionThe resu l t s o f the inves t iga t ion us ing qu es t ionna i res in19902 were as fo l lows . S t ruc tura l wal l s conta in ing s tee le l e m e n t s w e r e e m p l o y e d i n t h e c o n s t r u c t i o n o f a p a r t m e n thouses , ho te l s and off ice bu i ld ings . Both bon ded andu n b o n d e d s t e el e l e m e n t s w e r e u s e d i n a p a r t m e n t h o u s e s a n dh o t e l s , b u t t h e b o n d e d e l e m e n t s w e r e u s e d i n a l m o s t a l lcases of o ff ice bu i ld ings . In cont ras t, the unbo nded e lem entw a s o f t e n u s e d i n t h e c o n s t r u c ti o n o f h i g h ri s e a p a r t m e n tbui ld ings and hote l s . Precas t wal l s wi th f la t bars were on lyu s e d f o r a p a r t m e n t h o u s e s a n d h o t e ls , a n d t h e c o n c r e t e p o r-t ion d id no t car ry the load . On the o ther hand , severa l typeso f r o l le d s h a p e s w e r e u s e d f o r o f f ic e b u il d i n g s, w h e r e c o n -cre te was cas tin s i tu The ra t io of the la te ra l load cardedby the wal l to the to ta l l a te ra l load was more than 50 ina l l responses .

S t i f f n e s s o f w a l l s i n a p a r t m e n t h o u s e s a n d h o t e l s w a seva lua ted main ly in te rms of the s tee l e lements , whi le tha tfor o ff ice bu i ld ings w as eva lua ted tak in g in to account theRC por t ion o f the wal l as wel l . This i s beca use the s t ruc-

tura l wal l conta in ing s tee l e lements i s used as par t i t ionsand i s ra ther th in in the case of the former, whi le tha t inthe la t te r is used to t ransfer the s t resses in the upp er S f ram et o t h e l o w e r R C s t ru c t u re s m o o t h l y. E n g i n e e r s s e e m t o f e e ltha t the va lue of Ds should be equiva len t to tha t for theu n b r a c e d f r a m e , a n d s h o u l d b e d e t e r m i n e d a c c o r d i n g t o th ed e f o r m a t i o n c a p a c i t y.

5 1 Chara c te r i s t ics and research t rendsThe s t ruc tura l wal l conta in ing s tee l b race i s used to cont ro lthe la te ra l s t i ffness and s t rength of a bu i ld ing f rame. Whenthe sway def lec t ion of the f rame i s smal l , the re inforcedconcre te wal l main ly res i s t s the la te ra l load , and the bracecar r ies the load a f te r shear c racks occur in the wal l . Thero le of the wal l i s to prevent the s t rength de te r iora t ion oft h e b r a c e d u e t o b u c k l i n g , s o t h a t t h e b r a c e c a n a c h i e v el a rg e e n e rg y d i s s i p a t i o n u n d e r r e p e a t e d l o a d i n g c a u s e d b year thquakes . The bond be tween the s tee l b race and sur-r o u n d i n g c o n c r e t e i s u s u a ll y c u t t o a v o i d e a r l y c r a c k in g o fconcre te a long the brace ., and to s im pl i fy the eva lua t ion ofthe overa l l behavior. When appl ied to the s tee l f rame, th i st y p e o f w a l l i s u s u a l l y m a d e o f p r e c a s t c o n c r e t e , a n d t h ew a l l a n d t h e b r a c e a r e c o n n e c t e d t o t h e f r a m e b y b o l t s .Some d i ff icu l t ies a r i se concern ing such de ta i l s as those ofbol t ing , s t resses genera ted in the brace under the grav i tyload condi t ion and f i reproof ing for s tee l e lements .

S o m e t i m e s , t h e c o n c r e t e w a l l i s o n l y c o n n e c t e d t o t h ef l o o r s l a b , b u t n o t t o t h e c o l u m n s o r t h e u p p e r b e a m s , a n di t ac t s o n l y t o p r e v e n t b u c k l i n g o f t h e b r a c e . T h e s a m e i d e aw a s r e a l i z e d i n t h e c o m p o s i t e b r a c e m a d e o f a s t e e l p l a t ec o r e m e m b e r e n c l o s e d i n a c o n c r e t e - f il l e d s te e l t u b e w h i c hw a s n o t c o n n e c t e d t o t h e f r a m e .F i g u r e 1 2 s h o w s a ne x a m p l e o f a n u n b o n d e d b r a c e e n c a s e d i n C F T 18. N o t e t h a tthe hys te res i s loops be tween ax ia l load P and ax ia l defor-m a t i o n A o f t h e u n b o n d b r a c e a r e s p i n d l e - s h a p e d a n ds tab le , ind ica t ing equa l s t rength in bo th tens ion and com-p r e s s i o n , w h i l e a n o r d i n a r y b r a c e s h o w s t h e b o o m e r a n g -shaped hys te res i s loops ind ica t ing s t rength de te r iora t iondue to buckl ing . For the s t ruc tura l wal l conta in ing s tee le lements , hys te re t ic behavior, de ta i l s o f the connec t ion ,de ta i l s to prevent the loca l buckl ing occur r ing a t the veryend of the p la te brace enc losed in the precas t concre te wal l ,d e t a il s f o r u n b o n d i n g , a n d r e i n f o r c e m e n t o f c o n c r e t e i n t h en e i g h b o r h o o d o f t h e b r a c e h a v e b e e n i n v e s t i g a t e d .

6 . F r a m e s c o m p o s e d o f R C c o l u m n s an d Sb e a m s

6 1 Chara c te r i s t ics and research t rends

T h e f r a m e c o m p o s e d o f R C c o l u m n s a n d S b e a m s i sc h a r a c t e r i z e d a s a g o o d c o m b i n a t i o n f o r i t s c a p a b i l i t y i nc a r r y i n g h i g h g r a v i t y l o a d o f th e R C c o l u m n w i t h t h e s p a n -n i n g a b i l i t y o f t h e S b e a m . T h e k e y p o i n t o f t h i s s y s t e mi s t h e m o m e n t t r a n s f e r m e c h a n i s m a t t h e b e a m - t o - c o l u m nc o n n e c t i o n ; a n u m b e r o f i n n o v a t io n s h a v e b e e n r e p o r t e dwi th the resu l t s o f the pro of tes ts inJapan 3 T h o s e i n n o -va t ions a re separa ted in to f ive grou ps acco rd ing to them e t h o d o f t r a n s fe r r in g t h e b e n d i n g m o m e n t f r o m t h e b e a mt o t h e c o lu m n , a s s h o w n i nFigure 13 T h e m o m e n t t r a n s f e rd e v i c e i s c o m p o s e d o f v e r t i c a l p l a t e s p r o j e c t e d f r o m t h ef lange of the be am or face bear ing p la tes , and re l ies on thebear ing s t ress genera ted in concre te by the p ly ing ac t ion .T h e s e m o m e n t t r a n s f e r m e c h a n i s m s w e r e a l l d e v e l o p e d b yl e a d in g c o n s t r u c ti o n c o m p a n i e s , a n d s o m e w e r e m a d e u s i n gthe precas t sys tem.

6 2 Des ign and cons t ruc t ionThe resu l t s o f the inves t iga t ion us ing ques t ionna i res in1 9 9 0 2 w e r e a s f o l l o w s . T h e s y s t e m c o m p o s e d o f R C c o l -u m n s a n d S b e a m s w a s o f t e n a p p l i e d t o f r a m e s w i t h l o wh e i g h t a n d l o n g s p a n . A b o u t h a l f th e b u i l d in g s m e n t i o n e di n r e s p o n s e s w e r e n o t c o m b i n e d w i t h s t ru c t u ra l w a l l s , e v e nw h e n t h is s y s t e m w a s a p p l i e d t o a s h o p b u i l d in g s u r r o u n d e dby b l ind wal l s which can be used as s t ruc tura l wal l s . Mer i t so f th i s s y s t e m c o m p a r e d w i t h S , R C a n d S R C s y s t e m s w e r ea p p l i c a b i l i t y t o l o n g - s p a n f r a m e s a n d c a p a b i l i t y t o r e d u c ethe cons t ruc t ion cos t and t ime. Rat iona l iza t ion o f cons t ruc-t ion was ta rge ted in the e ffor t s to prefabr ica te the beam-t o - c o l u m n c o n n e c t i o n i n c l u d i n g b e a m b r a c k e t s a n d p r e c a s tc o l u m n s , a n d t o u s e a s - r o l l e d S b e a m s .



4 4 R e c e n t d e v e l o p m e n t s i n h y b r i d s tr u c t u r e s in J a p a n : S M o r i n o

hJ .bond e St l S teelm a t e r i a l t u b e t u b e [

Compression Compression( a ) C o m p o s i t i o n o f a n u n b o n d e d b r a c e ( b ) B e h a v i o r o f a n u n b o n d e d b r a c e ( c ) B e h a v i o r o fan o r d i n a r y

b r a c e

Figure 12 U n b o n d e d b r a c e

igure 13 M o m e n t t r a n sf e r m e c h a n i s m o f R c o l u m n S b e a m s y s te m

B e a m - t o - c o l u m n c o n n e c t i o n s o f t hi s s y s t e m w e r ed e s i g n e d s o t h a t t h e f a i l u r e o f t h e c o n n e c t i o n s h o u l d n o to c c u r p r i o r t o t h e f a i lu r e o f c o l u m n s o r b e a m s . T h e s t re n g t ho f t h e c o n n e c t i o n w a s c o n f i r m e d b y t e st s i n m o s t c a s e s .T h e v a l u e o f D s w a s d e t e r m i n e d a c c o r d i n g t o t h e r a n k o fm e m b e r s i n w h i c h p l a s t i c h i n g e s p r e s u m a b l y f o r m e d , o r ava lue equiva len t to tha t o f the RC s t ruc ture was taken tobe a sa fe va lue . So me en gineers fe l t tha t the va lue of Dsshould be based on tes t resu l t s , i f p las t ic h inges form inb o t h R C c o l u m n s a n d S b e a m s i n th e s a m e s t o ry. T h i s s y s -tem has been appl ied in smal l s t ruc tures and , thus , thed y n a m i c a n a l y s i s w a s u s u a l l y n o t p e r f o r m e d . H o w e v e r,r e s p o n d e n t s s a i d t h a t i f d y n a m i c a n a l y s i s w a s n e e d e d , s i m i -la r types of res tor ing- force charac te r i s t ics and des ign c r i -t e r i a a s t h o s e e x p l a i n e d p r e v i o u s l y f o r C F T s t r u c t u r e s

should be adopted wi th some modi f ica t ions : t r i l inearr e s t o r in g - f o r c e c h a r ac t e r is t ic s a s s u m e d f o r R C c o l u m n , a n dd a m p i n g c o n s t a n t e q u a l t o 3 % .

7 . The US Ja pan coopera tive ea r thquakeresearch program

T h e U S - J a p a n C o o p e r a t iv e E a rt h q u a ke R e s e a r c h P r o g r a mb e g a n i n 1 9 7 9 f o l l o w i n g t h e r e c o m m e n d a t i o n s o u t l i n e d i nt h e fi n a l r e p o r t o f th e U S - J a p a n P l a n n i n g G r o u p f o r t h ep r o g r a m 19. T h e o v e r a l l o b j e c t i v e o f t h e p r o g r a m i s t oi m p r o v e s e i s m i c s a f e t y p r a c t ic e s i n b o t h c o u n t r i e s t h r o u g hc o o p e r a t i v e s t u d i e s t o d e t e r m i n e t h e r e l a t i o n s h i p a m o n gfu l l - sca le tes t s , smal l - sca le tes ts , com pon ent tes t s , andre la ted ana ly t ica l and des ign impl ica t ion s tud ies . The f i r s tf o u r p h a s e s o f t h e p r o g r a m c o n c e r n e d r e i n f o r c e d c o n c r e t es t ruc tures , s tee l s t ruc tures , masonry s t ruc tures , and precas tconcre te s t ruc tures . Research on mixed s tee l /RC s t ruc tures

has been ident i f ied in the p lanning group repor t as ani m p o r t a n t p h a s e o f th e p r o g r a m t o g e t h e r w i t h t h e o t h e rf o u r p h a s e s .

D u r i n g t h e p a s t 1 0 y e a r s t he u s e o f c o m p o s i t e a n d h y b r i ds t ruc tures has increased in the US and Japan . In sp i te o fs o m e r e s e a r c h a n d d e v e l o p m e n t w o r k , n o t e n o u g h i s k n o w na t p r e s e n t r e g a r d in g t h e i r s e is m i c b e h a v i o r o r p e r f o r m a n c e .D e s i g n p r o c e d u r e s a n d c o d e s f o r u s e i n t y p i c a l d e s i g noff ices a re cur ren t ly nonexis ten t . Cons ider ing the impor-t a n c e o f d e v e l o p i n g d e s i g n g u i d e li n e s ( a u n i f i e d c o d ed e v e l o p m e n t ) f o r t h es e t y p i c a l c o m p o s i t e a n d h y b r i d s t r u c -tures tha t a re cur ren t ly used in prac t ice , and a l so ofd e v e l o p i n g n e w a n d i n n o v a t i v e c o m p o s i t e s t r u c t u r a le l e m e n t s a n d h y b r i d s y s t e m s u s i n g a d v a n c e d n e w m a t e r i a l sa n d / o r d e v i c e s , a f i v e - y e a r r e s e a r c h p r o g r a m o n c o m p o s i t e

a n d h y b r i d s t r u c t u r e s w a s r e c o m m e n d e d a s t h e f i f t h p h a s eo f t he U S - J a p a n C o o p e r a t i v e E ar th q u a k e R e s e a r c h P r o -g r a m .

B e c a u s e o f t h e d i v e r s e s c o p e o f t h e s u b j e c t a r e a , t h er e s e a r c h p r o g r a m w a s o rg a n i z e d i n t o t h e f o l l o w i n g f o u rg r o up s : ( 1 ) C F T c o l u m n s y s te m s ; ( 2 ) R C c o l u m n a nd Sb e a m s y s t e m s ; ( 3 ) h y b r i d w a l l s y s t e m s ; a n d ( 4 ) n e wmater ia l s , e lements and sys tems . The de ta i led research rec-o m m e n d a t i o n s f o r e a c h g r o u p w e r e b a s e d o n a n u m b e r o ft e c h n ic a l m e e t i n g s o f t h e U S a n d J a p a n P l a n n i n g G r o u p sa n d a J o i n t P l a n n i n g G r o u p Wo r k s h o p h e l d i n B e r k e l e y i nSep tem ber 19922°,21 . Exc ept for g ro up (4) , the wo rk in ea chg r o u p i n v o l v e s e x p e r i m e n t a l i n v e s t i g a ti o n o f t h e s t ru c t u ra lb e h a v i o r, d e v e l o p m e n t o f d e s i g n r e c o m m e n d a t i o n s c o n t a i n -i n g d e s i g n f o r m u l a s , m a t h e m a t i c a l m o d e l i n g o f th e s y s t e mfor s ta t ic and dynamic ana lyses and cons t ruc t iona l de ta i l s ,d e v e l o p m e n t o f a d a t a b a s e i n c l u d in g d a t a o f s i m i la r w o r kdone in the pas t , t r i a l des ign of theme s t ruc tures to c la r i fy



Recent deve lopm ents in hyb r id s t ruc tures in Japan : S. M or in o 34 5

des ign charac ter i s t ics and mer i t s of the sys tem, andexchan ge of the des ign docum ents prepared in rea l prac t ice .The ex pe r imen t s do ne s o f a r by t he J apanese s i de i n e achcorrespo nding group are on the fo l lowing: 1 ) cent ra l ly andec cen t r i c a l l y l oaded comp res s i on me mber s a nd bea m-co l -umn s o f re c t angu l ar an d c ir cu l a r CF Ts ; 2 ) fo r c e - t r an s f e rmech an i sm and i n t e r i o r an d ex t e r i o r c on ne c ti ons o f t h roughcolumns wi th severa l d i ffe ren t force-bear ing mechanisms;and 3 )b oun da ry l ink beam, T- an d L - shape d co re wa l ls ,and a 12-s tory T-shap ed coupled wal l. In addi tion , in ten-s ive ana ly t ica l inves t iga t ion has been car r ied out on thebeha v io r o f CFT b eam-c o lu mn s , t he be ha v io r o f t he RCco lumn and S beam sys t em us i ng t he f in i te e l e me n t me thod ,and s t a t ic and d ynam ic r e sp ons e o f t h r ee - d imens ion a lf rames wi th core wal l s . Seve ra l tr ia l des igns o f theme s t ruc-t u r es u s ing the sy s tem s o f g r oup s 1 ) t o 3 ) have beenpresented by the Japan St ruc tura l Consul tan ts As socia t ionJSCA ) , a mem ber o f t h e p ro g r am . In g ro up 4 ) , f e a sib i li t y

s tudies have been cont inued on the appl ica t ion of f iber-re inforced p las t ics to RC panels , to re t rof i t t ing of the ex is t -

ing RC members , and to e lec t r ic fac i l i t i es having the prob-lems of e lec t r ic cor ros ion of re inforcements and e lec t r icno i s e, and on t he deve lo pm en t o f h i gh -pe r fo rma nc e con -cre te , tha t i s , u l t ra ligh t -weight , h igh-s t rength concre te , andhigh- tens i le s t rength and ]a igh-ducti l ity concre te . S om e ofthe resu l t s of inves t iga t ions ment ioned above werep r e s e n t ed a t t he 2nd Jo in t Tech n i ca l Coo rd ina ti ng Com mi t -tee Meet ing he ld in Hono lu lu in June 199522, and publ i shedin Ab s t r ac t s o f Ann ua l M ee t i ngs o f AI J . Ove r v i ews o f t h ework done by each g r oup have b een r epo r t e d annua l l y23.The work on the US s ide i s about 2 years behind tha t ofthe Japanese , and now a to ta l of 10 pro jec ts a re on-goingconc e rn ing CFT be am-co l um ns , conn ec t i ons i n CFT co l -umn s y s t ems , bond b e t ween conc re t e a nd s t e e l t ube , con -nec t i on s i n RC co lu mn and S be am sys t e ms , compos i t ebe am s , compos i t e c ou p l i ng be am s , a nd f i b e r- r e in fo r ce dconcre te-encased s tee l jo i s t s .

8 Conc lud ing r emarks

The appearance o f hybr id cons t ruc t ions i s a resu l t o f pursu-ing econo mica l , h igh-eff ic iency cons t ruc t ion . At present inJapan , i t i s requi red to obta in spec ia l permiss ion f rom theM in i s t e r o f Cons t ruc t i o n fo r t he cons t ru c ti o n o f m os t o f t hehyb r i d s t r uc tu r e s men t i o ne d ab ov e , becau s e a me thod o fassur ing the s t ruc tura l sa fe ty of these s t ruc tures has no t ye tbeen fu l ly es tab l i shed . However, the ques t ionnai re survey

on the present s i tua t ion of hybr id s t ruc tures revea led tha tthe des ign and the cons t ruc t ion technique of the fo l lowingthree hybr id sys tems have a t ta ined a suff ic ien t leve l ofappl ica t ion for the ac tua l bu i ld ing; CFT column sys tems,RC column and S beam sys tems, and s t ruc tura l wal l s wi thencased steel braces or p]iates. For these structures, inten-s ive research has been done , and des ign and cons t ruc t iontechniques have b een es tab l i shed as a resu l t o f ac tua l con-s t ruc t ion exper ience . Prepara t ion of the necessary docu-ments for the des ign and cons t ruc t ion i s now needed.

Sm ooth s t ress t ransfer i s a key poin t in successfu l ly con-s t ruc t ing a hybr id s t ruc ture , bu t in many cases i t i s con-f i rmed on ly by expe r imen t beca use o f t he l a ck o f an e s t ab -l i shed theory. S t ruc tura l engineers fee l tha t a ra t iona lmeth od o f eva lua t ing the s t ruc tura l charac ter is t ic coeff ic ien tDs for hybr id s t ruc tures based on i t s overa l l per formanceshould be es tab l i shed and , thus , research work on Ds i su rgen t l y needed . I n fo rma t i on ob t a ined f rom t he US- Ja pa n

Coope ra t i ve E a r thquake Resea r ch P ro g ram on c ompos i t eand hybr id s t ruc tures wi l l he lp to c la r i fy these poin ts .

The s t r uc tu r a l de s ign me thod i s now mov ing t owardsl imi t s ta te des ign , which i s a wor ldwide tendency. This i sg i v in g r is e t o a new p rob l em o f how to de t e rm ine t he l o a dfac tors , s ince i t depends not only on the s ta t i s t ica l charac-te r i s tics of loads , bu t a l so on the charac ter i s t ics of the s t ruc-ture and re l iab i l i ty index se t in the des ign . The research onthis topic has not yet s tarted.

Acknowledgemen t s

A p ar t of th i s repor t was wr i t ten based o n the resu l t s of thein ve s ti ga ti o n a nd d i s cus s ion ca rded ou t by t he Co mm i t t e efo r t h e I nvest i ga ti on o f Hybr id Cons t ruc t ion Cha i rman :P ro fe s so r Se i j i Nakano , Tokyo Denk i Un ive r s i t y ) andTechn i ca l Coo rd ina t ion Comm i t t e e o f t he J apanese s i de o ft h e US - Jap an C oop e ra t i v e E a r th quake Resea rc h P r og r a mCha i rman : P ro f e s so r H i royuk i Aoyama , N ihon

Univers i ty) , bo th a t tached to the Bui ld ing Center of Japan .

The au thor wishes to express h is s incere gra t i tude to thememb er s o f t he se c om mi t t e e s , pa r t i cu l a r l y Dr Ma sa h ikoTach ib an a , Tokyo De nk i Un iv e r s i t y, Dr I s ao N i sh iyama ,BRI , and Dr Noboru Sakaguchi , Shimizu Corpora t ion .

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Building Center of Japan (BCJ), 19902 Report of Structural Design and Construction of Hybrid Structures,

BCJ, 19913 Morino, S. Nishiyama, I. and Sakaguchi, N. Hybrid structures in

Japan-- research and practice , in Proc. Annual Tech. Session ofStructural Sta bility Res. Counc ilBethlehem, 1994, pp. 207-221

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Edition, BCJ, 198615 Standard for Limit State Design of Steel Structures (draft), English

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Earthquake in 1995, BRI, MOC, 199517 Interim Report of Investigation on Damage by Great Hanshin Awaji

Earthquake in 1995, BCJ, 199518 Ono, T., Nakamura, H. and Watanabe, A. Development of unbond

brace , Buildin g Letter B CJ 1992, 47-5519 Recommendations for a US-Jap an Cooperative Research Program

Utilizing Large-Scale Testing Facilities, Report no. UCB/EERC 79-26, 1979

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4 6 Recent deve lopm ents in hybr id s t ruc tures in Japan: S Mo r ino

22 Handouts at 2nd Joint Technical Coordinating Committee MeetingHonolulu 1995

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