Design of Reduced Beam Section (RBS) Moment Frame Connection

STRUCTURALSTEELEDUCATIONAL COUNCIL

TECHNICAL INFORMATION & PRODUCT SERVICE

AUGUST 1999

Design of Reduced Beam Section (RBS) Moment Frame Connections

by

Kevin S. Moore, James O. Malley, Michael D. Engelhardt

DESIGN OF REDUCED BEAM SECTION (RBS) MOMENT FRAME CONNECTIONS

ABOUT THE AUTHORS

KEVIN S. MOORE is a Design Engineer with Degenkolb Engineers in San Francisco, Califor- nia. He ea rned his M.S. degree at The University of Texas at Aust in working u n d e r the direction of Dr. J. A. Yura and Dr. M. D. Engelhardt . While conduc t ing research , Kevin ass is ted Dr. Engelhard t with mater ia l test ing for the '~UT Tests," some of the first m o m e n t connect ion tests following the 1994 Northridge ea r thquake . He was the lead engineer for a 5-stolry SMF building utilizing RBS connec t ions cons t ruc ted in San Francisco and is a regis tered Profes- sional Engineer in California.

J A M E S 0. MALLEY is a Senior Principal at Degenkolb Engineers in San Francisco, Califor- nia. He is the Project Director for Topical Invest igations of the SAC Jo in t Venture Par tnership . The SAC Jo in t Venture was created to develop guideline d o c u m e n t s for the design, evaluation, and repair of steel m o m e n t frame bui ldings in response to the damage caused by the North- ridge ear thquake . J i m has been involved with m a n y steel design and peer review projects, inc luding the 5-story SMF bui lding listed above. He is a m e m b e r of the AISC Commit tee on Specifications and Chair of the Seismic Subcommi t t ee and has a u t h o r e d n u m e r o u s papers on steel design and cons t ruc t ion t h roughou t his career. He is also a regis tered S t ruc tu ra l Engi- neer in California.

MICHAEL D. ENGELHARDT is an associate professor of Civil Engineer ing at The University of Texas at Austin. Mike teaches courses on s t ruc tu ra l steel design at The University of Texas and conduc t s r esea rch on seismic res i s tan t steel framing. His previous work inc ludes major cont r ibut ions to the development and val idat ion of eccentr ical ly braced f rames (EBFs). Mike h a s been an active par t ic ipant in m o m e n t connec t ion resea rch since the 1994 Northridge ea r thquake and has worked extensively on RBS rela ted research . Mike is a m e m b e r of AISC Task Commit tee Number 113 on Seismic Design and is a registered Professional Engineer in California.


C O N T E N T S

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I N T R O D U C T I O N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 D E S C R I P T I O N O F S M F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1 .2 B A C K G R O U N D O F R B S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

H I S T O R Y O F T H E D E V E L O P M E N T O F R B S S M F C O N N E C T I O N S . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1 INITIAL R E S E A R C H . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

S U M M A R Y O F T E S T R E S U L T S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1 O V E R V I E W O F T E S T R E S U L T S F O R R A D I U S C U T R B S S P E C I M E N S . . . . . . . . . . . 4

R B S D E S I G N P R O C E D U R E F O R S M F S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4 .1 R B S D E S I G N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4 . 2 R B S S I Z I N G . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4 . 3 S T E P - B Y - S T E P P R O C E D U R E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 . 4 A D D I T I O N A L D E S I G N C O N S I D E R A T I O N S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 R B S D E S I G N E X A M P L E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 P R O C E D U R E S F O R A C C E P T A N C E O F D E S I G N B Y B U I L D I N G A U T H O R I T I E S . . . 2 1 6. I C O M M U N I C A T I O N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

6 . 2 M E T H O D O L O G Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 6 . 3 C O N S T R U C T I O N D O C U M E N T S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2

F A B R I C A T I O N A N D I N S P E C T I O N I S S U E S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 7 .1 C U T T I N G A N D G R I N D I N G . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2

7 . 2 W E L D I N G . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3

R E F E R E N C E S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 5

A P P E N D I X A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A i

LIST O F F I G U R E S

1.1 1.2

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2 . 2

4 .1

4 . 2 4 . 3

4 . 4

4 . 5

4 . 6

4 . 7

4 . 8 5 .1

5 .2 5 . 3

P R E - N O R T H R I D G E M O M E N T C O N N E C T I O N D E T A I L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

R A D I U S C U T R B S M O M E N T C O N N E C T I O N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

T A P E R E D C U T R B S M O M E N T C O N N E C T I O N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

E X A M P L E O F L A B O R A T O R Y B E H A V I O R O F R A D I U S C U T R B S T E S T S P E C I M E N ..... 4

(A) D E T A I L O F T E S T S P E C I M E N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

(B) R E S P O N S E O F T E S T S P E C I M E N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 M O M E N T D I A G R A M A N D B E A M G E O M E T R Y F O R R B S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

G E O M E T R Y O F R A D I U S C U T R B S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 T Y P I C A L M O M E N T F R A M E B E A M W I T H R B S C O N N E C T I O N S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

B E A M AT M I N I M U M S E C T I O N O F R B S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

F R E E B O D Y D I A G R A M B E T W E E N C E N T E R S O F R B S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

F R E E B O D Y D I A G R A M B E T W E E N C E N T E R O F R B S

A N D F A C E O F C O L U M N F L A N G E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

F R E E B O D Y D I A G R A M F O R C A L C U L A T I O N O F C O L U M N M O M E N T S . . . . . . . . . . . . . . . . . . . . . . 13

C O M P A R I S O N O F T E S T R E S U L T S F O R C O V E R P L A T E D A N D R B S C O N N E C T I O N S 17

R B S D I M E N S I O N S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 P O R T I O N O F E X A M P L E B E A M B E T W E E N R B S C E N T E R S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

C O N N E C T I O N D E T A I L F O R D E S I G N E X A M P L E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21


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

When subjected to a major ear thquake, buildings designed to meet the design require- ments of typical building codes, such as the UniI 'orm B u i l d i n g ~ C o d e (1997), are expected to have damage to both s t ruc tura l and non- s t ruc tura l elements. The s t ructura l design for large seismic events m u s t therefore explicitly consider the effects of response beyond the elastic range. The "Special Moment Frame" (SMF) steel building system is designed such tha t the connec t ions be tween the f rame b e a m s a n d c o l u m n s absorb s u b s t a n t i a l energy and provide major contr ibut ions to the d isplacement ductil i ty demand.

1.1 D e s c r i p t i o n o f SMF

Recent s tudies by Lee (1997) a n d others have demons t r a t ed tha t this a s s u m p t i o n is far different from the ac tual behavior.

l ~ ~ - - ~ C.P. ~70T-4

I I : I . 7/8" A325-X BOLTS 1

A SMF lateral force resist ing system is often preferred by bui lding owners and archi tec ts because this type of sys tem provides large u n o b s t r u c t e d spaces t h roughou t the building plan. This "open" layout offers the mos t flexibility for p rogramming the spaces as well as a rch i tec tura l appointments . For these reasons, steel buildings with SMF sys tems are quite c o m m o n in major commercia l and inst i tut ional s t ruc tures . Fur thermore , the SMF system is considered by m a n y to be one of the most ducti le steel building sys tems available to the engineer. For this reason, SMF sys tems have been widely used in a reas of high seismicity.

SMFs are typically comprised of connec- t ions be tween wide flange b e a m s a n d columns where beam flanges are welded to co lumn flanges utilizing complete joint pene- tration welds. Figure 1.1 shows a typical unreinforced design detail for a beam-to-col- u m n connect ion used in SMF sys tems prior to the 1994 Northridge ear thquake . Common practice prior to the Northridge ea r thquake was to ei ther bolt or weld the web to the col- u m n shear plate, and to weld the beam flanges to the co lumn flange us ing a complete joint penet ra t ion groove weld. Histori- cally, designers have a s s u m e d tha t beam shear is t ransferred to the co lumn by the beam web connect ion and the m o m e n t is t r ans fe r r ed t h r o u g h the b e a m flanges.

Figure 1. I Pre-Northridge M o m e n t C o n n e c t i o n Detai l

In the design of SMF connect ions , the engineer m u s t set objectives for both load and deformat ion capacit ies. Usually, the load capaci ty r equ i rement is based on the plastic m o m e n t of the beam. The connec t ion m u s t be s t rong enough to develop the s t rength of the beam, thus reduc ing the r isk of brittle failure in the connect ion. Inelast ic deforma- tion capaci ty is required to a s su re ducti l i ty in p rede te rmined locations when subjec ted to large deformat ion demands .

After some of the problems observed in SMF connec t ions after the Northridge ear thquake, a c o m m o n phi losophy has been to design the connec t ion to r ema in nominal ly elastic at the co lumn face, and force the inelastic deformat ion of the frame to occur in a portion of the beam, away from the connection. This phi losophy is executed by us ing a "capacity design" approach. The plastic m o m e n t and associa ted shea r of the beam is based on probable s t rengths of mater ials . These m a x i m u m s then become t h e design loads for the connect ion. The connec t ion of the beam to the co lumn flange is t hen des igned us ing nomina l mater ia l propert ies.

Most post-Northridge connec t ion designs locate the plast ic h inge (where inelast ic

DESIGN OF REDUCED BEAM SECTION (RBS} MOMENT FRAME CONNECTIONS

de fo rma t ions a re c o n c e n t r a t e d in the SMF beam) a w a y f rom the c o l u m n flange t h r o u g h re inforc ing a s h o r t por t ion of the b e a m n e a r the co lum n . By i n c r e a s i n g the s t r e n g t h of the b e a m in th is region, a p las t ic h inge will t end to form j u s t a d j a c e n t to the re inforced por- t ion of the beam. The i n h e r e n t difficulty wi th ut i l izing a re in forced b e a m - c o l u m n connec - t ion is the i n c r e a s e d ma te r i a l a n d labor cos ts a s soc i a t ed wi th th i s c o n n e c t i o n a n d the SMF sys t em as well as r equ i r ing welds t ha t are difficult a n d cost ly to m a k e a n d inspect .

1.2 B a c k g r o u n d o f RBS

Anothe r type of c o n n e c t i o n deve loped to force the ine las t ic d e f o r m a t i o n a w a y f rom the b e a m - c o l u m n in te r face is re fer red to as a "Reduced B e a m Sect ion" c o n n e c t i o n (RBS) or "dogbone". This c o n n e c t i o n relies on the selective remova l of b e a m flange ma te r i a l a d j a c e n t to the b e a m - t o - c o l u m n connec t ion , typical ly f rom bo th top a n d bo t tom flanges, to r e d u c e the c ross sec t iona l a r e a of the beam. This r e d u c t i o n in c r o s s sec t iona l a r e a will r e d u c e the m o m e n t capac i ty at a d i sc re te locat ion in the beam. Var ious s h a p e s of c u t o u t s a re poss ible , i nc lud ing c o n s t a n t cut , t ape red cut , r a d i u s cu t a n d o thers . Figure 1.2 i l lus t ra tes a r a d i u s c u t RBS connec t ion .

The L u x e m b o u r g - b a s e d steel m a n u f a c - t u r i ng c o m p a n y , ARBED, he ld a 1992 US p a t e n t on the r e d u c e d b e a m sec t ion (RBS).

' A

~L ~ - - . .

F i g u r e 1.2 R a d i u s C u t R B S M o m e n t C o n n e c t i o n

Fol lowing the Nor thr idge e a r t h q u a k e , t h e y wa ived all p a t e n t a n d c la im r igh ts a s s o c i a t e d wi th t he RBS for the benef i t of the profess ion . This g rac ious ges tu re a l lowed f u r t h e r devel- o p m e n t of the concep t for u s e in pos t -Nor th - r idge SMF bui ld ings .

The shape , size a n d locat ion of t he RBS all have a n effect on the c o n n e c t i o n d e m a n d s a n d pe r fo rmance . Var ious s h a p e s h a v e b e e n t e s t ed a n d u s e d in n e w c o n s t r u c t i o n d u r i n g the p a s t severa l years . Test p r o g r a m s have b e e n p e r f o r m e d to invest igate s t r a igh t c u t (Plumier , 1997), t ape r cu t (Chen, et .al . 1996) a n d r a d i u s cu t (Enge lhard t 1997; T remblay , et .al . 1997; Popov, et.al . 1998) r e d u c e d b e a m sec t ions .

The RBS forces yie lding a n d h inge fo rma- t ion to o c c u r wi th in the r e d u c e d sec t ion of the b e a m a n d l imits the m o m e n t t h a t c a n be deve loped at the face of the c o l u m n . By r e d u c i n g d e m a n d s on the b e a m flange groove w e l d s a n d the s u r r o u n d i n g b a s e m e t a l reg ions , the RBS r e d u c e s the poss ib i l i ty of f r a c t u r e s o c c u r r i n g in this v u l n e r a b l e region. A l t h o u g h the RBS essen t ia l ly w e a k e n s the b e a m , i ts i m p a c t on the overa l l l a t e r a l s t r e n g t h a n d st iffness of a s teel m o m e n t f r a m e is genera l ly qui te small .

The ine las t ic de fo rma t ion focused in a n RBS c o n n e c t i o n r e m a i n s in t he r e d u c e d b e a m sec t ion , w h i c h c a n be d e s i g n e d a n d loca t ed s u c h t ha t m i n i m a l pro tec t ive m e a s - u r e s n e e d to be t a k e n at the c o n n e c t i o n of b e a m to c o l u m n . The smal le r m o m e n t gener - a t ed a t the face of the c o l u m n for a n RBS c o n n e c t i o n , in add i t ion to r e d u c i n g s t r e s s levels on the welds , also offers s o m e a d v a n - t ages in sa t is fying s t rong c o l u m n - w e a k b e a m r e q u i r e m e n t s a n d in m i n i m i z i n g c o l u m n d o u b l e r p la te r e q u i r e m e n t s .

Fab r i ca t i on a n d e rec t ion of the RBS con- n e c t i o n avoids the add i t ion of s t r e n g t h e n i n g p l a t e s a n d spec i a l w e l d m e n t s t h a t a r e r e q u i r e d of m a n y pos t -Nor th r idge m o m e n t c o n n e c t i o n s . Consequen t ly , the RBS c o n n e c - t ion is very compet i t ive f rom a cos t p e r s p e c - tive. B e c a u s e of the compet i t ive cos t a n d e s t a b l i s h e d p e r f o r m a n c e b a s e d on ex tens ive t e s t ing a n d ana lys i s , the RBS c o n n e c t i o n a p p e a r s to be a cos t effective, c o n s i s t e n t l y pe r fo rming c o n n e c t i o n for u s e in t he se i smic des ign of SMF bu i ld ing s t r u c t u r e s .

2

DESIGN OFF REDUCED BEAM SECTION (RBS) MOMENT FRAME CONNECTIONS

. H i s t o r y o f t h e D e v e l o p m e n t o f R B S S M F C o n n e c t i o n s

A n u m b e r of significant events led to the cur- rent envi ronment su r round ing SMF design and cons t ruc t ion methodologies. Concerns over mater ia l properties, connect ion geome- try, design pa ramete r s and weld quali ty are j u s t a few issues which became a concern after brittle failures were observed in SMF m o m e n t connect ions after the 1994 North- ridge ear thquake .

SMF s t ruc tu res were still being des igned and reques ted by owners for all the reasons descr ibed earlier. The pre-Northridge connect ion detail had become a driving eco- nomic factor for the viability of the SMF system. To redesign m o m e n t connect ions in a SMF system utilizing expensive connect ion re inforcement t echn iques made this building system less competitive.

2 .1 I n i t i a l R e s e a r c h

A significant a m o u n t of research and t e s t i n g on RBS m o m e n t connect ions has a l ready been completed, a n d addi t ional work is underway. Appendix A provides a listing of tests on RBS connect ions . The list inc ludes key features of each test, inc luding m e m b e r sizes and s t rengths , connect ion details, RBS size and shape, and the plastic rotat ion achieved by each test assemblage. As indi- cated by the data- in Appendix A, successful tests have been conduc ted on cons tan t cut, tapered cut and rad ius cut RBS specimens.

The tapered cut, shown in Figure 2.1, is in tended to allow the section m o d u l u s of the beam to ma tch the seismic m o m e n t gradient in the reduced region, thereby promot ing more uniform yielding within the r educed section. This is in tended to create a reliable, uni form hinging location. However, s t ress concent ra t ions at the re -en t ran t corners of the flange cut may lead to f racture at these locations. After significant p las t i c rotation, both the cons tan t cut and tapered cut RBS connec t ions , have exper i enced f r ac tu re s within the RBS in some laboratory tests. These fractures have occurred at changes in section within the RBS, for example at the

m i n i m u m section of the tapered RBS. These changes of cross-sect ion in t roduce s t ress concen t ra t ions tha t can lead to f rac ture within the highly s t ressed r educed section of the beam.

I ~ ~ = ~

Figure 2. I T a p e r e d C u t RBS M o m e n t C o n n e c t i o n

The rad ius cut RBS appears to minimize s tress concent ra t ions , thereby reduc ing the chances of a f racture occurr ing within the reduced section (Engelhardt , et.al. 1996). F u r t h e r m o r e , tes t r e su l t s i nd i ca t e t h a t inelast ic deformat ions dis t r ibute over tl~e length of the r educed section. The rad ius cu t is also relatively simple to fabricate.

Figure 2.2 shows an example of a laboratory test of a r ad ius cut RBS specimen. The connect ion detail is shown in Figure 2.2(a) and the m o m e n t ve r sus plast ic ro ta t ion response is shown in Figure 2.2(b). As is typical of mos t r ad ius cu t RBS tests, this speci- m e n showed excellent performance.

As shown in Figure 2.2(a),.it is impor tan t to note tha t mos t RBS test spec imens , in addi t ion to incorpora t ing the RBS, also incorpora ted significant improvements in welding and in other detai l ing features as compared to the pre-Northr idge connect ion. All speci- m e n s were cons t ruc t ed us ing welding elec- t rodes tha t exhibit improved no tch toughness as compared to the E70T-4 electrode c o m m o n l y u s e d pr ior to the Nor thr idge ear thquake .

The major i ty of spec imens also incorpora ted improved pract ices with respec t to backing bars a n d weld tabs. In mos t cases, bot tom flange back ing bars were removed, backgouged a n d sealed with a fillet weld, and top flange back ing bars were seal welded to the column. Weld run-off tabs were removed in mos t cases. In addi t ion to welding re la ted improvements , mos t spec imens also incorpo-

3


~ ~ / B.U, bar to remain ---.--~"~J / ~ Remove weld tabs

• ~ "~'>~ . . . . . . . i~'i"" Note: i ~ ~ 45 ° ~ All field welds: E71T-8

r ~ ")(S~if~ed CVN = 20.-~

~ ~ \-~W~,lg4 ~ ~ltS: 1" A3 ~ 25 9" C-C ~

' ~Holes: 1-1/16" DIA. J • E ~8" x 6" x 2'-6" / ~ Z ................. ~

. . . .

~ ~ I k cleaned and ins~cted

~ R e a v e B.U. bar IN k Remove ~ l d tabs ~8 ~o o

" 3'-4" Radius

~ ~ /G r i nd Smi th 5 /1~ ~ / ~ / Grind Parallel to Beam Flange

/

~ ~ ~ 2.31"

~ ,~ ~ 9" 27"

(a) Detai l of ~ e s t S p e c i m e n

d CVN = 20 ft-lbs at -20 deg F)

40000 . '

$1:~¢. ~B4 I

i .,0000

i -20000

• .30000 I ~0000

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~ Moment ~nd RotafJon Computed v, lth Rs~pe¢~ to Faca o~ Col,~nn

I I I ,-0.03 -0.02 .0.01 0 0.01 0.02 0.03 0.04 0.05

Total Plastic Rotation (radian)

(b) Response of Test S p e c i m e n

Figure 2.2 Example of Laboratory Behavior of Radius Cut RBS Test S p e c i m e n

r a t e d a d d i t i o n a l d e t a i l i n g i m p r o v e m e n t s . C o n s e q u e n t l y , a l t h o u g h t h e b e a m f l ange c u t o u t s a re t h e m o s t d i s t i n g u i s h i n g f e a t u r e of t h e RBS c o n n e c t i o n , t h e s u c c e s s of t h i s c o n n e c t i o n in l a b o r a t o r y t e s t s is a l so l ikely r e l a t e d to t h e m a n y o t h e r w e l d i n g a n d deta i l - ing i m p r o v e m e n t s i m p l e m e n t e d in t h e t e s t s p e c i m e n s , i.e. t h e u s e of we ld m e t a l w i t h i m p r o v e d n o t c h t o u g h n e s s , i m p r o v e d p rac - t i ces w i th r e s p e c t to b a c k i n g b a r s a n d we ld t ab s , u s e of c o n t i n u i t y p l a t e s , etc.

3. S u m m a r y o f T e s t R e s u l t s

The t ab l e in A p p e n d i x A p r o v i d e s a l i s t ing of RBS t e s t da ta . Whi le t h i s l ist m a y n o t be e x h a u s t i v e or c o n t a i n every t e s t p e r f o r m e d o n RBS b e a m - c o l u m n s u b a s s e m b l i e s or anc i l l a ry t e s t i n g to s u p p o r t p e r f o r m a n c e , t h e l is t d o e s p rov ide t h e r e a d e r w i t h a s u b s t a n - t ial a m o u n t of d o c u m e n t e d p e r f o r m a n c e con- d i t i on s for t h i s c o n n e c t i o n . The t ab l e a l so i n c l u d e s RBS t e s t s c o m p l e t e d u n d e r t h e SAC P h a s e 2 r e s e a r c h p r o g r a m as of m i d - 1 9 9 9 . T h e s e t e s t r e s u l t s h a v e n o t b e e n fo rmal ly p u b l i s h e d , b u t a re i n c l u d e d b a s e d on avail- ab le t e s t r epo r t s .

T h e AISC Seismic Provisions for Structural Steel Buildings (1997) r e q u i r e qua l i f i ca t ion t e s t i n g for SMF c o n n e c t i o n d e s i g n s . The t e s t r e s u l t s r e p o r t e d in A p p e n d i x A m a y be u s e f u l

in sa t i s fy ing th~se qua l i f i c a t i on t e s t r equ i r e - m e n t s . A p p e n d i x S of t h e Seismic Provisions for Structural Steel Buildings p r o v i d e s gu ide - l ines o n e x t r a p o l a t i n g t e s t r e s u l t s b e y o n d t h e t e s t e d m e m b e r sizes.

A p p e n d i x A i n c l u d e s l i s t ings for 43 RBS tes t s . Th i s n u m b e r does n o t i n c l u d e t e s t s by P l u m i e r (1997), or s h a k i n g t ab le t e s t s by C h e n , Yeh a n d C h u (1996). Add i t i ona l t e s t s h a v e a lso b e e n c o n d u c t e d o n s p e c i m e n s in w h i c h t h e RBS w a s p r o v i d e d in t h e b o t t o m f lange on ly for u s e a s a re t rof i t m e a s u r e for ex i s t i ng m o m e n t f r a m e c o n n e c t i o n s . T h e s e RBS re t rof i t t e s t s a re n o t r e p o r t e d in A p p e n - dix A. I n f o r m a t i o n o n t h e t e s t s is ava i lab le in t h e AISC Steel Design Guide Series Twelve (Gross, et .al . 1999).

3.1 O v e r v i e w o f T e s t R e s u l t s f o r R a d i u s Cut RBS S p e c i m e n s

Thi s s ec t i on p r o v i d e s a n overv iew of t h e t e s t d a t a l i s t ed in A p p e n d i x A for r a d i u s c u t RBS t e s t s p e c i m e n s . T h e r e a re 27 r a d i u s c u t RBS t e s t s l i s ted in t h e table . E x a m i n a t i o n of t h i s d a t a i n d i c a t e s t h a t t h e s e c o n n e c t i o n s devel- o p e d p la s t i c r o t a t i o n s r a n g i n g f rom 0 .029 r a d to b e y o n d 0 .05 rad . T h e s e r e s u l t s s u g g e s t t h a t t h e r a d i u s c u t RBS c o n n e c t i o n c a n deve lop la rge p l a s t i c r o t a t i o n s o n a cons i s - t e n t bas i s . Also n o t a b l e is t h e fact t h a t a

4


large n u m b e r of rad ius cut RBS connect ions have been tested u n d e r a variety of conditions by a n u m b e r of different investigators, and there has not been a single test with poor performance. This suggests the connection is quite robus t and reliable.

The da ta in Appendix A demons t r a t e s the possible u l t imate failure modes for the rad ius cut RBS connect ion. In m a n y tests, spec imen s t rength gradual ly deter iorated due to local and lateral torsional buckl ing, and test ing was te rmina ted due to l imitat ions of the testing equ ipment or test setup. However, a n u m b e r of connec t ions have been loaded well past the occur rence of local flange buckl ing within the RBS, and ul t imately failed by low cycle fatigue f racture of the RBS. Only one of the 27 rad ius cut RBS spec imens experi- enced a f racture at the beam- to -co lumn connection. This specimen, des ignated "DBBW- C - Beam 2" in Appendix A, f rac tured in the beam bot tom flange base metal ad jacent to the groove weld, with the f racture init iat ing at the weld access hole. However, even this connect ion developed 0.038 rad. of plastic rotat ion prior to fracture.

Most of the rad ius cut RBS spec imens h a v e been tested pseudo statically, us ing a loading protocol in which applied displace- men t s are progressively increased. However, one spec imen ("S-l") was tested monotonically to failure. Two spec imens ("LS-2" and "LS-3") were tested us ing a loading protocol in tended to represen t nea r source g round motions tha t conta in a large pulse. Finally, two spec imens ("S-4" and "SC-2") were tes ted dynamically. The rad ius cu t RBS spec imens have performed well u n d e r all of these loading condit ions.

A wide range of beam sizes have been tested with the rad ius cu t RBS. The smal les t beam listed in Appendix A is a W530x82 (Canadian designation) which is rough ly equivalent to a W2 lx50. The heaviest beam tested is a W36x300. All co lumns for r ad ius cut RBS tests have been W14 sections. Most of the co lumns have been sized to provide for a very s trong panel zone, a l though a small n u m b e r of tests have inc luded modera te panel zone yielding. No tests have been conducted on spec imens with very weak panel

zones. However, such tests will be completed dur ing 1999.

Of the 27 rad ius cu t RBS spec imens listed in Appendix A, there are no repor ted cases of weld fracture. Beam flange groove welds for all r ad ius cut RBS spec imens have been made by the self shielded flux cored arc welding process (SS-FCAW) us ing electrodes with a m i n i m u m specified CVN toughness of 20 ft.-lbs, a t - 2 0 ° F. Three different electrode des ignat ions have been u sed in these tests: E71T-8, E70TG-K2, and E70T-6. For one of the rad ius cut RBS specimens , details of the backing bars were not reported. However, for the remain ing 26 spec imens in which backing bar details were reported, the bot tom flange backing was removed and the top flange backing was left in place. For the majori ty of these specimens , the top flange backing was seal welded to the face of the column, a l though these seal welds were not provided in four spec imens (WG-1 to WG-4). Note tha t only one of the 27 rad ius cu t RBS spec imens u sed cover plates at the beam-to- co lumn connec t ion as a supp lemen t to the RBS.. The remain ing 26 spec imens u sed no supp lementa l re inforcing m e a s u r e s (cover plates, ribs, etc.) at the connect ion.

Dimens ions of the RBS cuts for the 27 rad ius cu t spec imens vary over a fairly small range. The d is tance from the face of the col- u m n to the s tar t of the RBS cut (designated as L 1 in Appendix A) r anged from 50 to 75% of the beam flange width. The lengths of the cuts (designated as LRB S in Appendix A) have varied from 74 to 82% of the beam depth. The a m o u n t of flange width removed at the m i n i m u m sect ion of the RBS (desig- na t ed as FR in Appendix A) has var ied from 38 to 55%.

Two types of web connect ion details have been used for r ad ius cut RBS test specimens: a welded and a bolted detail. In the welded detail, the beam web is welded directly to the co lumn flange us ing a complete jo int pene- trat ion groove weld. For the bolted detail, fully tens ioned high s t rength bolts are used . Approximately half the spec imens have u s e d the bolted detail, a n d half the welded detail. The da ta indicates no significant difference in per formance for r ad ius cu t specimens .

5


Beam lateral bracing details have also varied among the rad ius cut RBS specimens. Of the 27 specimens, seven are repor ted to have provided a brace at the RBS. For the remain ing 20 specimens, the lateral brace was typically fur ther away from the RBS placed near the point of load application.

Finally, of the 27 rad ius cut spec imens listed in Appendix A, six were tested with a composite concrete floor slab. For Spec imens "SC-1" and "SC-2," a one- inch gap was inten- tionally left be tween the face of the co lumn and the slab, in an a t tempt to minimize com- posi te act ion. For S p e c i m e n s "DBBW-C Beams 1 & 2" and "DBWW-C Beams 1 & 2," no such gap was provided. No de t r imenta l effects of the slab were observed in any of these tests. In some tests, the investigators noted tha t the slab e n h a n c e d overall energy dissipat ion by delaying beam instability. Note tha t for all composite specimens, no shear s tuds were placed in the region of the RBS or between the face of the co lumn and the s tar t of the RBS.

As descr ibed above, a ra ther wide range of condi t ions has been investigated in RBS testing completed to-date. Testing of RBS connect ions is cont inuing u n d e r the SAC pro- gram and for specific building cons t ruc t ion projects. The reader is encouraged to r emain abreas t of this data, as it becomes available.

Even though m a n y variables have a l ready been invest igated in RBS testing, there are a n u m b e r of condit ions tha t have received less at tention. These condit ions, when they arise in design, should be approached with cau- tion since da ta is lacking in these areas. In such cases, addit ional test ing may be war- ranted. For example, no rad ius cut RBS connect ions to the weak axis of a wide flange col- u m n have been tested, a l though da ta for some other RBS connect ions to the co lumn weak axis are available (see Spec imens "COH-3" and "COH-4" in Appendix A). No spec imens with deep co lumns have yet been considered. Fur ther , no tests on spec imens with very weak panel zones have been conducted. Fu tu re resea rch is u n d e r w a y to address these and other issues.

4. RBS D e s i g n Procedure for SMFs

The following sect ions contain r ecommenda- tions for the design of new radius cut RBS m o m e n t connect ions. Based on the suc- cesses out l ined above, and the preference of engineers designing new SMF s t ruc tures , the design methodology presen ted herein focuses on the r ad ius cu t RBS shape. Globally impor tan t design pa ramete r s such as panel zone part icipation, beam shear and overall frame drift are addressed as par t of the rec- o m m e n d e d p rocedu re . Many i m p o r t a n t aspects of m o m e n t connect ion design are applicable and m u s t be considered when designing SMF RBS connect ions. The RBS design methodology should be performed in conjunct ion with available test resul ts as par t of the just i f icat ion of the design procedure.

The initial par t of the SMF/RBS design is to de termine the configuration of the m o m e n t frames, the typical bay sizes, p lan dimen- sions and frame locations. Many of these r e q u i r e m e n t s are d e t e r m i n e d by o thers , (architects , owners , developers), bu t the engineer should influence these decis ions based on sound design practices. One example would be to consider the bay size if a SMF/RBS system is to be utilized. Because of the high m o m e n t gradient ratio associa ted with short bays, more beam flange removal in RBS connect ions will be required for shor t bay f rames t han long bay frames. In addition, beam sizes may be affected. With proper guidance, the engineer can supply information tha t will help the archi tect develop a ra t ional , efficient bu i ld ing design. Upon de terminat ion of the basic s t ruc tura l pa ram- eters, the engineer can begin the m e m b e r and connect ion design process.

4 .1 R B S D e s i g n

The engineer will begin the design of the s t ruc ture by de termining the force level and drift limits to be incorpora ted as par t of the design. These pa ramete r s are typically set by a model building code such as the Uniform Building Code (1997) or, in the future , the

6


International Building Code. Once the force level is d e t e r m i n e d b a s e d on site cond t ions , s t r u c t u r a l sys tem, se i smic i ty of t he region a n d t a rge t drift l imits, t he eng inee r c a n begin the des ign of t he se i smic s y s t e m u s i n g the AISC Seismic Provisions for Structural Steel Buildings { 1997).

B a s e d on the r e q u i r e d des ign p a r a m e t e r s , the e n g i n e e r will d e t e r m i n e t he b e a m a n d c o l u m n sizes r e q u i r e d to m e e t drift l imits , etc. It is i m p o r t a n t t h a t t h e e n g i n e e r r e m e m - be r t h a t the f r a m e is less stiff d u e to t he RBS des ign , t h a n a "typical" n o n - R B S SMF.

After p r o p e r b e a m - c o l u m n sizes h a v e b e e n d e t e r m i n e d for t he f rame, t he RBS d e s i g n p r o c e d u r e s h o u l d be fo l lowed to develop the p rope r f lange r e d u c t i o n to pro- d u c e t he des i r ed p e r f o r m a n c e . M a n y of t he des ign s t eps a n d r e c o m m e n d a t i o n s para l le l i n fo rma t ion p rov ided in r epo r t s r e f e r e n c e d a t t he e n d of th i s d o c u m e n t .

The s t r e n g t h of t he b e a m at t he m i n i m u m sec t ion of t h e RBS m u s t sa t is fy code requ i re - m e n t s u n d e r all appl icab le load c o m b i n a - t ions i n c l u d i n g gravity, wind , a n d o the r loads a p p r o p r i a t e for t he s t r u c t u r e u n d e r cons ide r - a t ion. B e a m sizes in typica l SMFs are nor - ma l ly governed by code specif ied drift l imits. C o n s e q u e n t l y , even wi th a r e d u c t i o n in b e a m m o m e n t d u e to t h e add i t i on of t h e RBS, t he s t r e n g t h of t h e modi f i ed f r a m e will of ten be sa t i s fac to ry for all load c o m b i n a t i o n s . In some cases , a m i n o r i n c r e a s e in b e a m size m a y be n e e d e d .

The add i t i on of RBS c u t o u t s will r e d u c e t he s t i f fness of a s teel m o m e n t f rame. This r e d u c t i o n in st i ffness, a l t h o u g h genera l ly qui te smal l , m a y affect t h e abil i ty of t he f r ame to sa t is fy code specif ied drif t l imits . A r e c e n t s t u d y by G r u b b s (1997) e v a l u a t e d the r e d u c t i o n in e las t ic la te ra l s t i f fness of s teel m o m e n t f r a m e s d u e to the add i t i on of r a d i u s cu t RBS c o n n e c t i o n s . This s t u d y s h o w e d t h a t over a wide r a n g e of f r a m e h e i g h t s a n d conf igura t ions , t he ave rage r e d u c t i o n in stiff- n e s s for a 50 p e r c e n t f lange r e d u c t i o n w a s on the o rde r of 6 to 7 pe rcen t . For a 40 p e r c e n t f lange r e d u c t i o n , the r e d u c t i o n in e las t ic f r ame st i f fness w a s on the o rde r of 4 to 5 percent . If th i s r e d u c t i o n in s t i f fness is a concern , drift c a n be c o m p u t e d in t he u s u a l m a n n e r u s i n g a m o d e l t h a t does no t explic-

itly a c c o u n t for t he RBS, a n d t h e n i n c r e a s e d by the a m o u n t s n o t e d above to a c c o u n t for t he RBS c o n n e c t i o n s . Al ternat ively , a re f ined s t r u c t u r a l mode l , i n c l u d i n g the r e d u c e d stiff- n e s s a t e a c h c o n n e c t i o n d u e to t h e RBS, c a n be deve loped to c h e c k the s t i f fness of t h e f rame.

4 . 2 RBS S i z i n g

The loca t ion a n d size of t h e RBS will d ic ta te the level of s t r e s s a t t h e b e a m f l a n g e - c o l u m n f lange c o n n e c t i o n . The RBS se i smic m o m e n t d i a g r a m is p r e s e n t e d in F igure 4.1 a n d indi- ca t e s t he Nomina l Capac i ty , t he Probab le D e m a n d , a n d the Nomina l D e m a n d for t h e RBS b e a m . Note t h a t M ' p RBS is t h e maxi - m u m m o m e n t e x p e c t e d a t l~he face of t he col- u m n f lange w h e n t h e RBS h a s y ie lded a n d s t r a in h a r d e n e d u n d e r c o m b i n e d e a r t h q u a k e a n d gravi ty loads . M' p RBS is d i rec t ly influ- e n c e d by the P robab le i J e m a n d , a n d t h e loca- t ion of t h e RBS. M' P ,RBS is l a te r r e fe r red to as Mf in th i s d o c u m e n t .

r - - ~ r . . . . . . , ~ ; ~ , ~ - ~ , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i , \ I ,

~ ~,~as i

~--~,-,,~ o ~ Moment Diegrem

L ~

~am ¢ , ¢ ~ y

Figure 4. I M o m e n t D i a g r a m a n d B e a m G e o m e t r y for RBS

The overal l goal in s izing t h e RBS c u t is to limit t he m a x i m u m b e a m m o m e n t t h a t c a n develop a t t h e face of t h e c o l u m n to v a l u e s in the r a n g e of a b o u t 85 to 100 p e r c e n t of t h e b e a m ' s a c t u a l p l a s t i c m o m e n t . Th i s a p p r o a c h , in effect, l imi ts t he ave rage maxi - m u m s t r e s s a t t h e b e a m f lange groove we lds to v a l u e s on t h e o rde r of t h e a c t u a l y ie ld s t r e s s of t h e b e a m . E x p e r i m e n t s h a v e s h o w n t h a t c o n n e c t i o n s de t a i l ed in a c c o r d a n c e wi th

7


t h e r e c o m m e n d a t i o n s p r o v i d e d be low are c a p a b l e of s a f e ly r e s i s t i n g t h i s level of m o m e n t . As a p o i n t of c o m p a r i s o n , t e s t s o n p r e - N o r t h r i d g e m o m e n t c o n n e c t i o n s w i t h o u t RBS c u t o u t s o f ten s h o w m a x i m u m m o m e n t s a t t h e face of t h e c o l u m n of a b o u t 125 per - c e n t of M~ or g r ea t e r (Popov, S t e p h e n 1972; Tsai , PopoPv 1988; E n g e l h a r d t , H u s a i n 1993). C o n s e q u e n t l y , t h e a d d i t i o n of t h e RBS c u t o u t s in t h e b e a m r e s u l t s in a s u b s t a n t i a l r e d u c t i o n in m o m e n t a t t h e face of t h e col- u m n .

M u c h of t h e d e s i g n p r o c e d u r e p r e s e n t e d b e l o w fo l lows r e c o m m e n d a t i o n s of t h e Interim Guidelines: Evaluation, Repair, Modi- fication and Design o f Welded Steel Moment Frame Structures (FEMA 267) (1995) a n d t h e Interim Guidelines Advisory No. 1, Supple- ment to FEMA 267 (FEMA 267A) (1997), w i th severa l excep t ions . Mos t s ign i f i can t of t h e s e e x c e p t i o n s is t h a t FEMA 267A p l a c e s a l imi t on t h e m a x i m u m s t r e s s p e r m i t t e d a t t h e face of t h e c o l u m n e q u a l to n i n e t y p e r c e n t of t h e m i n i m u m spec i f ied y ie ld s t r e s s of t h e col- u m n . For t h e c a s e of a n A992 (A572 Gr. 50) c o l u m n , t h i s r e s u l t s in a l imi t of 45 ksi . Th i s l imi t w a s e s t a b l i s h e d to a d d r e s s c o n c e r n s r e g a r d i n g t h e p o t e n t i a l for t h r o u g h - t h i c k n e s s f a i lu res in c o l u m n f langes . The d e s i g n p roce - d u r e l imi t s t h e m a x i m u m s t r e s s a t t h e face of t h e c o l u m n to a v a l u e o n t h e o r d e r of t h e a c t u a l y ie ld s t r e s s of t h e b e a m . Th i s excep- t ion to t h e r e q u i r e m e n t s of FEMA 267A h a s b e e n a d o p t e d for severa l r e a s o n s . Fi rs t , spec- i m e n s d e s i g n e d a c c o r d i n g to t h e p r o c e d u r e s d e s c r i b e d h e r e i n h a v e p e r f o r m e d well in lab- o r a to ry t es t s . S e c o n d , s a t i s fy ing t h e 45 ks i s t r e s s l imit , w o u l d r e s u l t in la rge f lange c u t o u t s in m a n y cases , or w o u l d r e q u i r e s u p - p l e m e n t a l f lange r e i n f o r c e m e n t s u c h as cover p l a t e s or r ibs. F u r t h e r , r e c e n t l y c o m p l e t e d r e s e a r c h c o n d u c t e d u n d e r t h e SAC P h a s e 2 p r o g r a m s u g g e s t s t h a t t h e p o t e n t i a l for t h r o u g h - t h i c k n e s s f a i lu res is c o n s i d e r a b l y less t h a n p r e v i o u s l y t h o u g h t , a n d t h a t t he c u r r e n t l imit of 45 ks i c a n m o s t l ikely be i n c r e a s e d w i t h o u t p o s i n g a n i n c r e a s e in r i sk of f r a c t u r e in i t ia t ion .

The d e s i g n p r o c e d u r e a s s u m e s t h a t a r a d i u s c u t RBS is p r o v i d e d in b o t h t h e top a n d b o t t o m f l anges a t t h e m o m e n t c o n n e c - t i on a t e a c h e n d of a m o m e n t f r a m e b e a m .

T h e p r o c e d u r e a l so a s s u m e s t h e m i n i m u m spec i f ied y ie ld s t r e s s of t h e b e a m is 50 ks i or l e ss (Gr. 50 b e a m s ) , a n d t h a t t h e m i n i m u m spec i f i ed y ie ld s t r e s s of t h e c o l u m n is 50 ks i or g r ea t e r (Gr. 50 or Gr. 65 c o l u m n s ) .

F igu re 4 .2 s h o w s t h e g e o m e t r y of a r a d i u s c u t RBS, a n d F igu re 4 .3 s h o w s t h e en t i r e m o m e n t f r a m e b e a m . The key d i m e n s i o n s

I ~ ~ 1 ~ a

4 c ~ + d R = rad ius of cut 8c

C

~1 - - 1

b

Figure 4 . 2 G e o m e t r y o f R a d i u s C u t RBS

t h a t m u s t be c h o s e n by t h e d e s i g n e r a re a, t h e d i s t a n c e f rom t h e face of t h e c o l u m n to t h e s t a r t of t h e RBS cu t , b, t h e l e n g t h of t h e RBS cu t , a n d c, t h e d e p t h of t h e RBS c u t a t i t s m i n i m u m sec t ion . The r a d i u s of t h e c u t R c a n be r e l a t e d to d i m e n s i o n s b a n d c b a s e d o n t h e g e o m e t r y of a c i r cu l a r arc , u s i n g t h e e q u a t i o n in Fig. 4 .2 . T h e a m o u n t of f lange m a t e r i a l t h a t is r e m o v e d a t t h e m i n i m u m s e c t i o n of t h e RBS is s o m e t i m e s r e f e r r ed to the percent f lange removal w h i c h is c o m - p u t e d a s (2c/bf.) x 100, w h e r e bf i s the u n r e - d u c e d f l ange v~idth of t h e beam~

In p a s t r e s e a r c h t e s t s , t h e d i m e n s i o n s a a n d b h a v e gene ra l l y b e e n c h o s e n b a s e d o n t h e j u d g m e n t of t h e r e s e a r c h e r s . In genera l , t h e s e d i m e n s i o n s s h o u l d be k e p t a s sma l l a s

• w = uniform beam gravity load ~ II II RBS RBS

_ _ ~ ~.~_.£1l.~r.! ~ ~ 1 I } I I t ~ ~ 1 t I } ~ l ~ l ~ ~ . ! ? . . t . ~ . ! . | ~ [ ~ ]

' &4 i i ~ ,- ,n -~ ,n - ~

•

,, l l a + ~ " L' = distance be~een ~nters of RBS ~ts ~a+ ~ ~

I ~ L : distance between column ¢entedines

Figure 4 . 3 T y p i c a l M o m e n t F r a m e B e a m w i t h

RBS C o n n e c t i o n s

8


possible in order to minimize the increase of m o m e n t between the plastic hinge located in the RBS and the face of t_he column.

The d imens ion a should be large enough, however, to permit s t ress in the reduced section of the beam to spread uniformly across the flange width at the face of the column. Similarly, the d imens ion b should be large enough to avoid excessive inelast ic s t ra ins within the RBS. Based on an evaluat ion of successful pas t tests, the following sugges- t ions are made for selecting these d imen- sions:

(o.s to o.Ts) bf tl)

b ~ (65 to 0 .85 )d (2)

where by and d are the beam flange width and delSth. Examina t ion of RBS test da ta indicates tha t successfu l connect ion performance has been obta ined for a wide range of values for a and b. Consequent ly , a great deal of precis ion in choosing these values does not appear just if ied and Equa t ions 1 and 2 should be cons idered an approximate guide.

The remain ing d imens ion tha t m u s t be chosen w h e n sizing the RBS is c, the depth of the cut. The value of c will control the maxi- m u m m o m e n t developed within the RBS, and therefore will control the m a x i m u m m o m e n t genera ted at the face of the column. As noted above, the final d imens ions should be chosen so tha t the m a x i m u m m o m e n t at the face of the co lumn is in the range of abou t 85 to 100 percent of the beam's ac tua l plastic moment . At present , it is sugges ted to avoid utilizing flange reduc t ions greater t han about 50 percent. Thus, the value of c should be chosen to be less t han or equal to 0.25bf.

The basic approach t aken in "this procedure is to choose pre l iminary values for a, b, and c, then compute the m o m e n t at the face of the co lumn, and check this m o m e n t against the limit no ted above. Some iteration in the RBS d imens ions may be needed to arrive upon a sat isfactory design. Fur ther design checks are comple ted upon satisfac- tory sizing of the RBS.

The beam size will typically be chosen for drift requi rements , followed by some a m o u n t of flange reduct ion. The designer m u s t exam- ine the effect of all applied loads at the RBS

location. It is possible tha t beam size may need to be adjus ted , and different RBS sizing and location m u s t be de termined , to meet all design criteria.

This RBS sizing de te rmina t ion is also applicable when retrofit t ing existing SMF s t ruc tures . Access is l imited or impossible at the uppe r flange of the beam, due to the p resence of a floor slab, so RBS modificat ions typically occur at the bot tom flange of the m o m e n t beam only. If access is available to the top flange of the beam, it is r e c o m m e n d e d to apply the RBS design methodology to both flanges. There h a s been a great deal of effort and resea rch spent on the use of RBS modifications to existing SMFs. The AISC Design Guide Series Twelve (1999) tha t summar izes this work, conta ins a significant a m o u n t of informat ion regarding retrofit of SMFs utilizing RBS connect ion modifications. It is rec- o m m e n d e d tha t des igners us ing an RBS approach to retrofit an existing SMF refer to the AISC d o c u m e n t prior to utilizing the design methodology con ta ined herein.

Upon selection of the b e a m - c o l u m n combinat ion to be util ized in the SMF design and the location, shape a n d size of the RBS, fur- ther connect ion design checks are requi red to ensu re the design will perform in a ducti le manne r .

The first check shou ld be the "Strong Col- u m n - W e a k Beam" confirmation. This check is i n t ended to limit inelast ic deformat ions of co lumns outs ide of thei r pane l zone regions. It is generally recognized tha t co lumn yielding is an undes i r ab le mode because of the possible effect on the co lumn, and in turn , the global stability of the s t ruc tu ra l frame. The AISC Seismic Design Provisions (1997) out l ine an acceptable design level for the b e a m / c o l u m n relat ionship. As a m i n i m u m , this AISC proviso shou ld be met.

RBS connec t ion des ign m u s t also address the panel zone. The pane l zone is subjec ted to large shear forces as the b e a m s reach their full capacity. Based on FEMA 267A (1997), the panel zone m u s t be s t rong enough to develop at least 80% of the shea r s associa ted with Mfl The pane l zone r e q u i r e m e n t s can be met in one of two ways. One way is to provide a co lumn with a th ick e n o u g h web to resis t the requi red shear in acco rdance with the

9


d e s i g n r e q u i r e m e n t s . The o t h e r way to s u p - ply suf f ic ien t p a n e l zone s h e a r r e s i s t a n c e is to a d d d o u b l e r p l a t e s to t h e s e l ec t ed sec t ion . D o u b l e r p l a t e s s h o u l d c o n s i s t of t h e r e q u i r e d a d d i t i o n a l t h i c k n e s s of steel , a d d e d to o n e or b o t h s ides of t h e c o l u m n web. F a b r i c a t o r s i n d i c a t e t h a t t h e u s e of a heav ie r c o l u m n sec- t ion, i n s t e a d of d o u b l e r p l a t e s a n d o t h e r l ab o r i n t e n s i v e r e in fo r c ing de ta i l s , m a y r e s u l t in a m o r e e c o n o m i c a l s t r u c t u r a l f r ame .

The f inal d e s i g n c h e c k to be p e r f o r m e d on t h e s e l ec t ed b e a m - c o l u m n c o m b i n a t i o n is t h e b e a m s h e a r . T h e m a x i m u m b e a m s h e a r is d e v e l o p e d in t h e s ec t i on of t h e b e a m b e t w e e n t h e RBS a n d t h e c o l u m n f lange face, w h e r e gravi ty s h e a r a n d s e i s m i c s h e a r co inc ide . At t h i s loca t ion , s h e a r c a p a c i t y of t h e b e a m sec- t ion n e e d s to be c h e c k e d to e n s u r e t h a t t h e b e a m will h a v e a d e q u a t e s h e a r c a p a c i t y af ter t h e p l a s t i c h i n g e in t h e b e a m deve lops d u e to a p p l i e d l a te ra l loads .

The fol lowing s t e p - b y - s t e p p r e s e n t a t i o n o u t l i n e s t h e RBS d e s i g n p r o c e d u r e r e l a t i ng to t h e r e m o v a l of t h e b e a m f lange a n d t h e c h e c k s r e q u i r e d to e n s u r e p r o p e r b e h a v i o r a n d c o r r e l a t i o n w i t h t e s t a n d r e s e a r c h r e s u l t s .

4 . 3 S t e p - b y - s t e p P r o c e d u r e

STEP 2 C o m p u t e t h e p las t i c s ec t i on m o d u - lu s a t t h e m i n i m u m sec t ion of t h e RBS.

F igu re 4 .4 s h o w s a c r o s s - s e c t i o n of t h e b e a m at t h e m i n i m u m sec t ion of t h e RBS.

b~

"~'~"""''~P~ions cut from flange d/2 ~ ~ t w

Plastic Neutral Axis

d/2 /./.~Portions cut from flange

/ _ _ _ ~ ,~ , '~ t

~ ~.~ c c

F i g u r e 4 . 4 B e a m at M i n i m u m S e c t i o n o f R B S

B a s e d o n t h e d i m e n s i o n s s h o w n in t h i s fig- u re , Z R B S c a n be c o m p u t e d a s follows:

STEP 1 C h o o s e t r ia l v a l u e s for RBS d i m e n - s i o n s a, b, a n d c.

The t r ia l v a l u e s for a a n d b s h o u l d be c h o s e n w i t h i n t h e l imi t s of E q u a t i o n s 1 a n d 2. To e s t a b l i s h a t r ia l v a l u e of c, a f l ange r e d u c t i o n of a b o u t 40 p e r c e n t is s u g g e s t e d for t h e in i t ia l d e s i g n i t e ra t ion . T h u s , c h o o s e c ~ 0 .20 b f As n o t e d earl ier , v a l u e s for c in e x c e s s o f a p p r o x i m a t e l y 0 . 2 5 b f a re n o t rec- o m m e n d e d .

a (O.S to 0.75) bf

b ~ (0. 6 5 to O. 85) d

10

Z ~ s = Z b - 2 c t.f (d - t.f ) (3)

Where :

Z R B S = p la s t i c s e c t i o n m o d u l u s a t m i n - i m u m s e c t i o n of RBS

(1)

= p la s t i c s ec t i on m o d u l u s for full b e a m c r o s s - s e c t i o n

(i.e. w i t h o u t f l ange c u t o u t s )

o t h e r va r i ab l e s a s s h o w n in F igure 4.4.


STEP 3 E s t a b l i s h t h e e x p e c t e d y ie ld s t r e s s of t h e b e a m .

T he e x p e c t e d yie ld s t r e s s for t h e b e a m c a n be d e t e r m i n e d f rom S e c t i o n 6 .2 of t h e AISC Seismic Provisions for Structural Steel Buildings (1997). A c c o r d i n g to t h e s e provi- s ions :

Fy e = Ry Fy (4)

whe re :

Fy e = e x p e c t e d y ie ld s t r e s s

= m i n i m u m spec i f i ed y ie ld s t r e s s

= ra t io of e x p e c t e d to m i n i m u m spec i f i ed y ie ld s t r e s s

= 1.5 for A36 s tee l

T h e fac to r of 1.15 in E q u a t i o n 5 a c c o u n t s for s t r a i n h a r d e n i n g , a n d is b a s e d o n s t r a i n h a r d e n i n g v a i a e s m e a s u r e d in RBS tes t s .

STEP 5 C o m p u t e t h e s h e a r force a t t h e c e n t e r of t h e RBS c u t s a t e a c h e n d of t h e b e a m .

T h e s h e a r a t t h e c e n t e r of t h e RBS c a n be c o m p u t e d f r o m a free b o d y d i a g r a m of t h e m o m e n t f r a m e b e a m t a k e n b e t w e e n RBS c e n t e r s . S u c h a f ree b o d y d i a g r a m is i l lus- t r a t e d in F i g u r e 4 .5 for t h e c a s e of a u n i - fo rmly d i s t r i b u t e d grav i ty l oad w.

f R~BS RBS I w = uniform beam gravity ~oad • l!.~.,~ ~ ~ t ~ I t t t t I t t I t ~ ~ I t I I I t t ~ t . ! . . ! , {

. . . . . .

RBS RBS! i RBS RBS

i L' = distance between centers of RBS ' -I

Figure 4 .5 Free Body Diagram B e t w e e n

C e n t e r s o f RBS

= 1.1 for A572 Gr. 50 a n d A992 s tee l

T h e v a l u e of Fve r e c o g n i z e s t h a t t h e a c t u a l y ie ld s t r e n g t l ~ o f s t r u c t u r a l s tee l c a n s ign i f i can t ly e x c e e d t h e m i n i m u m spec i f i ed va lue .

S u m m i n g m o m e n t s a b o u t e a c h e n d of t h i s f ree b o d y d i a g r a m r e s u l t s i n t h e following:

2MRBs wL' V~S - L ' + - ~ - (6a)

S T E P 4 C o m p u t e t h e m a x i m u m m o m e n t e x p e c t e d a t t h e c e n t e r of t h e RBS.

MRB S = 1.15 ZRB S Fy e (5) 2 MRB s wL'

V~O~S - L ' 2 (6b)

w h e r e : w h e r e :

MRB S =

ZRB S =

m a x i m u m m o m e n t e x p e c t e d a t t h e c e n t e r of t h e RBS

p l a s t i c s e c t i o n m o d u l u s a t m i n - i m u m s e c t i o n of t h e RBS

e x p e c t e d y ie ld s t r e s s of b e a m

VRBS V' BS = s h e a r force a t t h e c e n t e r of t h e RBS a t e a c h e n d of b e a m

L ' = d i s t a n c e b e t w e e n c e n t e r s of RBS

W = u n i f o r m l y d i s t r i b u t e d g r av i t y l o a d o n b e a m

11

D E S I G N OF RE DUC E D B E A M S E C T I O N (RBS} MOMENT FRAME CONNECTIONS

For gravi ty load c o n d i t i o n s o t h e r t h a n a u n i f o r m load, t h e a p p r o p r i a t e a d j u s t m e n t c a n eas i ly be m a d e to t h e free b o d y d i a g r a m a n d to E q u a t i o n s 6 a a n d 6b.

E q u a t i o n s 6 a a n d 6b a s s u m e t h a t p l a s t i c h i n g e s will fo rm a t t h e RBS a t e a c h e n d of t h e b e a m . If t h e gravi ty load o n t h e b e a m is ve ry large, t h e p l a s t i c h i n g e a t one e n d of t h e b e a m m a y m o v e t o w a r d t h e in te r io r p o r t i o n of t h e b e a m s p a n . If t h i s is t h e case , t h e free b o d y d i a g r a m in F igu re 4 .5 s h o u l d be mod i - fied to e x t e n d b e t w e e n t h e a c t u a l p las t i c h i n g e loca t ions . To c h e c k if E q u a t i o n s 6 a a n d 6b a re valid, d r a w t h e m o m e n t d i a g r a m for t h e s e g m e n t of t h e b e a m s h o w n in F igu re 4 .5 , i.e., for t h e s e g m e n t of t h e b e a m b e t w e e n t h e c e n t e r s of t h e RBS cu t s . If t h e m a x i m u m m o m e n t o c c u r s a t t h e e n d s of t h e s p a n s , t h e n E q u a t i o n s 6 a a n d 6b a re valid. If t h e m a x i m u m m o m e n t o c c u r s w i t h i n t h e s p a n , a n d e x c e e d s Mp. e of t h e b e a m (see E q u a t i o n 8), t h e n t h e m o d i f i c a t i o n d e s c r i b e d above will be n e e d e d .

STEP 6 C o m p u t e t h e m a x i m u m m o m e n t e x p e c t e d a t t h e face of t h e c o l u m n .

M f = Mp, B s + VRB s a +

where :

(7)

= m a x i m u m m o m e n t e x p e c t e d a t t h e face of t h e c o l u m n

a l l o t h e r va r i ab l e s a s p r e v i o u s ~ de f i ned

E q u a t i o n 7 n e g l e c t s t h e gravi ty load on t h e p o r t i o n of t h e b e a m b e t w e e n t h e c e n t e r of t h e RBS a n d t h e face of t h e c o l u m n . Th i s s impl i f ies t h e e q u a t i o n a n d i n t r o d u c e s little error . If d e s i r e d , t h e grav i ty l oad on th i s sma l l p o r t i o n of t h e b e a m c a n be i n c l u d e d in t h e free b o d y d i a g r a m a n d in E q u a t i o n 7.

STEP 7 C o m p u t e t h e p l a s t i c m o m e n t of t h e b e a m b a s e d on t h e e x p e c t e d y ie ld s t r e s s .

Mpe = Z b Fy e (8)

T h e m o m e n t a t t h e face of t h e c o l u m n c a n be c o m p u t e d f rom a free b o d y d i a g r a m of t h e s e g m e n t of t h e b e a m b e t w e e n t h e c e n t e r of t h e RBS a n d t h e face of t h e c o l u m n f lange. S u c h a free b o d y d i a g r a m is i l l u s t r a t e d in F i g u r e 4 .6 .

RBS

- - M f ..... "~". VRB s MRB s

~ ,

I- b - - - N a +.-Z-

Figure 4 . 6 Free B o d y D iagra m B e t w e e n C e n t e r o f

RBS a n d Face o f C o l u m n F lang e

S u m m i n g m o m e n t s a b o u t t h e left e n d of t h i s f ree b o d y d i a g r a m r e s u l t s in t h e following:

whe re :

Mpe = p la s t i c m o m e n t of b e a m b a s e d on e x p e c t e d y ie ld s t r e s s .

STEP 8 C h e c k t h a t M f i s in t h e r a n g e of 85 to 100 p e r c e n t of Mpe.

M.f ~0 .85 to 1.0 (9) m pe

If E q u a t i o n 9 is n o t sa t i s f ied , m o d i f y t h e v a l u e s of c a n d / o r a a n d b a s n e e d e d , a n d r e p e a t S t e p s 2 t h r o u g h 8. Note t h a t t h i s c h e c k on m o m e n t a t t h e face of t h e c o l u m n is s impl i f i ed for d e s i g n p u r p o s e s , b a s e d on m o r e d e t a i l e d a n a l y s e s a n d p a s t t e s t r e su l t s . The a c t u a l force t r a n s f e r m e c h a n i s m a n d s t a t e of s t r e s s a n d s t r a i n a t t h i s l oca t ion is qu i t e c o m p l e x d u e to t h e c o n s t r a i n t gene r - a t e d by t h e c o n n e c t i o n to t h e c o l u m n f lange. For m o r e d e t a i l e d i n f o r m a t i o n on t h e i s sue , t h e r e a d e r is r e f e r r ed to (Lee, et .al . 1997).

12


STEP 9 Strong Column-Weak Beam Check

To c h e c k s t rong c o l u m n - w e a k b e a m Z Mc requi rements , the p rocedure presen ted in FEMA 267A (1997) will be used , with minor Where: modifications. The equat ion to be used to c h e c k th is r e q u i r e m e n t (from E q u a t i o n Vc = 7.5.2.5-1 of FEMA 267A (1997)) is as follows:

= M c t + Me b (14)

s h e a r force in the c o l u m n s above and below the connect ion

~ Z¢(F~c - J~) > 1.0 (10) Mct ZMc

= c o l u m n m o m e n t above connec t ion

immedia te ly

where: Mcb = c o l u m n m o m e n t immedia te ly below connec t ion

plast ic sect ion m o d u l u s of the c o l u m n sec t ion above a n d below the connect ion

ht dis tance from top of b e a m to point of inflection in the col- u m n above the connec t ion

YMc = m i n i m u m specified yield s t ress of the co lumn

= axial s t r e s s in the c o l u m n above and below the connect ion

~VM c s u m of the co lumn m o m e n t s at the top a n d bot tom of the panel zone c o r r e s p o n d i n g to the development of M R B S at the c e n t e r of the RBS in the a t t ached beams

Figure 4.7 shows a free body d iagram tha t can be u sed to es t imate co lumn m o m e n t s w h e n checking Equat ion 10. This free body cuts the beams at the RBS centers and cuts the co lumns at a s s u m e d points of inflection (often t aken as mid-height of the ad jacent stories for design purposes).

Based on Figure 4.7, £'M c can be esti- ma ted from the following equat ions:

, ,(de _,~ Z M R~s + (VR~s + V ~ s ) ~ - + a +

2J V~ : (11)

h t + d b + h b

Mct = Vch t (12)

Mcb = Vch b (13)

d c = depth of co lumn

hb dis tance from bot tom of b e a m to point of inflection in the col- u m n below the connec t ion

d b = depth of beam

All o ther variables as previously defined.

Mct

~ -,,~-.-.~ V ~i C

i Mcb

I

l I I

a+(b/2) d c a+(b/2)

Figure 4 . 7

~ MRBS V RBS

Free B o d y D i a g r a m for C a l c u l a t i o n o f C o l u m n M o m e n t s

ht

d b

hb

13

D E S I G N OF REDUCED B E A M S E C T I O N (RBS) MOMENT FRAME CONNECTIONS

T he a p p r o a c h p r e s e n t e d in FEMA 267A (1997) a c c o u n t s for t h e d i f fe rence in c o l u m n s h e a r forces a bove a n d be low t h e c o n n e c t i o n , w h e r e a s t h e s i m p l i f i e d a p p r o a c h a b o v e a s s u m e s t h e s a m e s h e a r force is p r e s e n t in t h e c o l u m n s above a n d be low t h e c o n n e c - t ion. A l t h o u g h t h e a p p r o a c h in FEMA 267A (1997) m a y be s o m e w h a t m o r e a c c u r a t e , t h e c o m p u t a t i o n of V c p r e s e n t e d in E q u a t i o n 11 above is s i m p l e r to i m p l e m e n t , a n d is still r e a s o n a b l y a c c u r a t e for in i t ia l d e s i g n p u r - p o s e s c o n s i d e r i n g t h e n u m e r o u s u n c e r t a i n - t ies invo lved in t h e s t r o n g c o l u m n - w e a k b e a m d e s i g n p h i l o s o p h y . T h e r e a d e r is r e f e r r ed to Sec t i on 7 .5 .2 .5 of FEMA 267A (1997) to i m p l e m e n t a m o r e a c c u r a t e ca lcu- l a t ion for V c to be u s e d in t h e f inal d e s i g n check .

S T E P 10 C h e c k Pane l Z o n e

To c h e c k t h e c o l u m n p a n e l zone , t h e p ro- c e d u r e u s e d in Sec t i on 6 . 6 . 6 . 3 . 7 of FEMA 2 6 7 A (1997) will be u s e d . T h i s s e c t i o n r e q u i r e s t h a t t h e p a n e l zone h a v e suf f ic ien t s t r e n g t h to deve lop t h e s h e a r force d e v e l o p e d by 0 .8 £'M/: B a s e d on t h i s a p p r o a c h , t h e p a n e l z o n e ' s h e a r force c a n be c o m p u t e d as follows:

M? = m a x i m u m m o m e n t e x p e c t e d a t o p p o s i t e c o l u m n face

All o t h e r va r i ab l e s as p r e v i o u s l y def ined .

The v a l u e of My c o m p u t e d a c c o r d i n g to E q u a t i o n 7 c o m b i n e s the , s e i s m i c m o m e n t d u e to (2XMRBs)/L' w i t h t h e m o m e n t d u e to grav i ty load. O n t h e s ide of t h e c o l u m n oppo- s i te to t h a t w h e r e My is deve loped , t h e m o m e n t a t t h e face of" t h e c o l u m n will be s o m e w h a t s m a l l e r s ince t h e gravi ty load m o m e n t will o p p o s e t h e s e i s m i c m o m e n t . T h i s s o m e w h a t s m a l l e r m o m e n t is c a l c u l a t e d u s i n g E q u a t i o n 17.

The s t r e n g t h of t h e p a n e l zone c a n be cal- c u l a t e d a s follows:

3b c t~ V = 0.55Fycdct 1 + dbdc--~ ~ (18)

where :

V = p a n e l zone s h e a r s t r e n g t h

M'f = M ~ S + V~S a + (15)

•Mf= Mf+ M~r (16)

o.8Z Vpz - 0.8V c (17)

0.95 d b

Where :

b c = w i d t h of c o l u m n f lange

tc f = t h i c k n e s s of c o l u m n f lange

= to ta l t h i c k n e s s of p a n e l zone i n c l u d i n g d o u b l e r p l a t e s

All o t h e r va r i ab l e s as p r e v i o u s l y def ined .

S T E P 11 C h e c k B e a m S h e a r

Vpz p a n e l zone s h e a r force corre- s p o n d i n g to t h e d e v e l o p m e n t of 80 p e r c e n t of t h e m a x i m u m e x p e c t e d c o l u m n face m o m e n t s

m a x i m u m m o m e n t e x p e c t e d a t t h e face of t h e c o l u m n , ca lcu- l a t ed a c c o r d i n g to E q u a t i o n 7

The f inal d e s i g n c h e c k s h o u l d be m a d e to e n s u r e t h a t t h e b e a m h a s a d e q u a t e c a p a c i t y for s h e a r a s s s o c i a t e d w i th l a te ra l a n d grav i ty loads . Th i s c h e c k c o m b i n e s t h e b e a m s h e a r a s s o c i a t e d w i t h t h e p l a s t i c m o m e n t w i t h i n t h e RBS u s i n g E q u a t i o n 6a, c o m b i n e d w i th t h e p o r t i o n of gravi ty load a d d i n g s h e a r to t h e b e a m w i t h i n t h e s e c t i o n b e t w e e n t h e RBS

14


center and the co lumn flange. This can be calculated us ing Equat ion 19:

VRB s q

( /- / , ) W - -

2 (19)

2

4 . 4 A d d i t i o n a l D e s i g n C o n s i d e r a - t i o n s

In addi t ion to es tabl ishing the d imens ions of the RBS cut, there are a n u m b e r of addi- t ional design and detail ing features tha t may significantly affect connect ion per formance and economy of this system. These i tems are d i scussed below.

The p rocedure p resen ted above for sizing the RBS cut permits a range of acceptable values for the d imens ions a, b and c. Fabri- cation can likely be simplified by s tandardiz- ing these d imens ions over a large n u m b e r of beams on a project. Making small changes on the RBS d imens ions from beam to beam is not likely to improve connect ion performance and may unnecessa r i ly increase fabricat ion costs. The designer may wish to con- sult with a fabricator before finalizing the RBS d imens ions to identify ways of r educ ing fabrication costs. For example, if the fabricator is m a k i n g RBS cu t s u s i n g a to rch m o u n t e d on a guide with a fixed radius , the economy of the connec t ion may be improved by main ta in ing a cons tan t rad ius of cu t R over a large n u m b e r of connect ions .

The RBS cut is normal ly made by the rmal cut t ing in the fabrication shop. The cut should be made to avoid nicks, gouges, a n d other discont inui t ies . After the cut is made , the surface should be ground, to aid in reduc ing the potent ial for f ractures occurr ing in the RBS at h igh plastic rotat ions and low cycle fatigue. The grinding should be done to avoid p roduc ing grind marks perpendicu la r to the beam flange, since they are perpendicular to the direction of principal stress. These m a r k s can act as s t ress risers. Varia- t ions on grinding me thods may be possible to reduce fabrication effort.

Another cons idera t ion for design of RBS m o m e n t connec t ions is welding. Research

conduc ted since the Northridge ea r thquake has demons t r a t ed the impor tance of weld metal t oughness in the groove welds of seismic res i s tan t m o m e n t connect ions (Kauf- m a n n , et.al. 1996; Tide 1998 I. The AISC Seis- mic Provis ions (1997) r e c o m m e n d s the use of a filler metal with a m i n i m u m specified ten- sile s t rength of 70 ksi, (assuming a 50 ksi base mater ia l specified yield) and a m i n i m u m specified CVN value of 20 ft.-lb, at -20 ° F. Previous resea rch tes ts on RBS connect ions have generally employed the self-shielded flux cored arc welding process (FCAW), us ing E70TG-K2, E71T-8 or E70T-6 electrodes. All of these electrodes provide a m i n i m u m specified CVN of 20 ft.-lb, at -20 ° F. A n u m b e r of other FCAW electrodes are available tha t provide this m i n i m u m CVN value. In addition, successful tes ts on other types of connec- t ions have employed the shielded metal arc welding {SMAW) process us ing an E7018 electrode. The final choice of welding process and electrode is bes t left to the fabricator. Other factors, s u c h as the mixing of different filler meta ls in the same weld jo in t may resul t in lower CVN values for the combinat ion, t han for one of the filler meta ls alone. A paper wri t ten on this subject , "The Effects of Intermixed Weld Metal on Mechanical Prop- erties" (Johnson, Q u i n t a n a 1998), may be useful to the engineer w h e n consider ing the inter-mixing of weld filler metals .

At the beam flange complete joint pene- t rat ion welds, it is r e c o m m e n d e d tha t the weld run-off tabs be removed at both the top and bot tom flanges, a n d tha t the edges of the groove welds be g round smooth. The preferred final profile of the weld tab g round surface is r ad iused , to fur ther reduce the possibility of f rac ture at these locations. This will minimize any potent ia l no tches intro- duced by the p re sence of the weld tabs, or by discont inui t ies con ta ined in the weld meta l within the run-off regions. In addit ion, it is r e c o m m e n d e d tha t the bot tom flange steel backing be removed a n d a reinforcing fillet be placed at the base of the weld after the jo in t is backgouged to s o u n d metal . This require- men t is in tended both to e l iminate the no tch effect p roduced by the steel backing, and to permit bet ter inspec t ion and u l t rasonic testing of the weld. At the top flange groove weld,

15


it is r e c o m m e n d e d t h a t t h e s teel b a c k i n g be sea l w e l d e d to t h e face of t he c o l u m n u s i n g a m i n i m u m size fillet weld, typica l ly a 5 / 1 6 " fillet. Ana lys i s h a s i n d i c a t e d t h a t t h e n o t c h effect of t h e s tee l b a c k i n g is n o t as severe a t t h e top f lange, a n d t h a t w e l d i n g t h e s tee l b a c k i n g to t h e c o l u m n f u r t h e r r e d u c e s t h e n o t c h effect. F u r t h e r , de fec t s a re l ess l ikely a t t h e t op f lange we ld s ince t h e groove weld is n o t i n t e r r u p t e d by t h e b e a m web, as it is a t t h e b o t t o m f lange.

M a n y r e s e a r c h e r s a n d d e s i g n e r s bel ieve t h a t t h e we ld a c c e s s ho le h a s a n i m p o r t a n t effect o n c o n n e c t i o n p e r f o r m a n c e . A l t h o u g h c u r r e n t r e s e a r c h is a d d r e s s i n g i s s u e s r e l a t e d to t h e we ld a c c e s s hole , t h e r e a p p e a r s to be n o c o n s e n s u s a s of ye t on t h e o p t i m u m size a n d s h a p e . C o n s e q u e n t l y , p e n d i n g f u r t h e r r e s e a r c h , a c c e s s ho le g e o m e t r y s h o u l d con- fo rm to t h e r e q u i r e m e n t s s h o w n in F igu re 5 .2 of AWS D 1 . 1 - 9 8 (AWS 1998). T h e r e is n o i n d i c a t i o n t h a t we ld a c c e s s ho le size, w i t h i n t h e AWS l imi ts , will adve r se ly affect t h e pe r - f o r m a n c e of RBS m o m e n t c o n n e c t i o n s . There fore , size a n d s h a p e of t h e a c c e s s ho le s h o u l d be left to t h e f ab r i ca to r to c o n f o r m to AWS r e c o m m e n d a t i o n s .

A n o t h e r i m p o r t a n t a s p e c t of w e l l - b e h a v e d m o m e n t c o n n e c t i o n s a r e t h e c o n t i n u i t y p l a t e s b e t w e e n t h e c o l u m n f langes . All of t h e s u c c e s s f u l t e s t s o n RBS c o n n e c t i o n s for n e w c o n s t r u c t i o n (Appendix A) h a v e e m p l o y e d c o n t i n u i t y p la tes . However , n o RBS t e s t s to d a t e h a v e o m i t t e d c o n t i n u i t y p la tes , so it is u n c l e a r u n d e r w h a t c o n d i t i o n s c o n t i n u i t y p l a t e s a re a c t u a l l y r equ i r ed . P e n d i n g t h e ou t - c o m e of f u r t h e r r e s e a r c h , it is r e c o m m e n d e d t h a t c o n t i n u i t y p l a t e s be p r o v i d e d for all RBS c o n n e c t i o n s , w i t h a c o n t i n u i t y p l a t e t h i ck - n e s s s imi l a r to t h e b e a m f lange t h i c k n e s s . Welds t h a t a t t a c h a c o n t i n u i t y p l a t e to t h e c o l u m n f lange or web, s h o u l d be m a d e wi th a n e l ec t rode w i t h a r a t e d CVN of a t l ea s t 20 ft.-lb, a t -20 ° F. B a s e d on e x p e r i m e n t a l r e s u l t s , r e m o v a l of b a c k i n g b a r s f rom cont i - n u i t y p l a t e we lds , however , d o e s n o t a p p e a r to be n e c e s s a r y . W h e n w e l d i n g t h e c o n t i n u i t y p l a t e s to t h e c o l u m n , w e l d i n g in t h e "k-area" of t h e c o l u m n s h o u l d be a v o i d e d (AISC 1997}.

All w e l d i n g s h o u l d be spec i f ied to be in c o n f o r m a n c e w i t h t h e l a t e s t ed i t i on of AWS

D 1.1. A c c e p t a n c e c r i te r ia for u l t r a s o n i c tes t - i ng of groove we lds is r e c o m m e n d e d to be in c o n f o r m a n c e w i t h Table 5.2 of AWS D 1.1-98. Add i t i ona l u s e f u l i n f o r m a t i o n on w e l d i n g m o m e n t c o n n e c t i o n s c a n be f o u n d in a n u m - be r of r e f e r e n c e s l i s ted at t h e e n d of t h i s doc- u m e n t .

R e c e n t t e s t s h a v e s h o w n t h a t RBS con- n e c t i o n s w i t h b o l t e d web de ta i l s c a n m e e t t h e r e c o m m e n d e d p la s t i c r o t a t i o n d e m a n d s of FEMA 267 (1995). However , it s h o u l d be n o t e d t h a t a t la rge r o t a t i o n d e m a n d s , t h e bo l t ed de ta i l a p p e a r s to be m o r e s u s c e p t i b l e to f r a c t u r e i n i t i a t i ng n e a r t h e weld a c c e s s hole . Th i s i s s u e is t h e s u b j e c t of f u r t h e r SAC s p o n s o r e d r e s e a r c h . Unt i l m o r e def ini t ive g u i d a n c e is p r o v i d e d in t h e u p c o m i n g SAC Guidelines, t h e e n g i n e e r s h o u l d ca re fu l ly c o n s i d e r r e q u i r e d c o n n e c t i o n a n d SMF per- f o r m a n c e w h e n c h o o s i n g a b e a m web connec t ion .

The m a j o r i t y of t h e w e l d e d web c o n n e c - t ion t e s t s h a v e u t i l i zed a c o m p l e t e j o i n t p e n - e t r a t i on (CJP) groove weld b e t w e e n t h e b e a m web a n d c o l u m n f lange over t h e full d e p t h of t he web. The s h e a r tab , w h i c h is w e l d e d to t h e c o l u m n a n d bo l t ed to t h e b e a m web, is still p rov ided . T h i s s h e a r t ab se rves severa l p u r p o s e s . Fi rs t , i t a c t s a s b a c k i n g for t h e C J P groove weld. S e c o n d , it ca r r i e s e r e c t i o n l oads a n d h e l p s m a i n t a i n t h e f r a m e in a p l u m b p o s i t i o n u n t i l w e l d i n g a t t h e c o n n e c - t ion is c o m p l e t e d . S ince t h e s h e a r t ab is p ro- v ided for e r e c t i o n p u r p o s e s only, it is r e c o m - m e n d e d t h a t t h e d e s i g n of t h e s h e a r t ab be left to t h e fabr ica to r . However , to e n s u r e t h a t t h e s h e a r t ab d o e s n o t r e s i s t l oads in t h e even t t h a t excess ive p l a s t i c r o t a t i o n s c a u s e t h e web c o n n e c t i o n to f r ac tu re , t h e d e s i g n e r c o u l d c o n s i d e r i n d i c a t i n g t h a t t h e s h e a r t ab be f a b r i c a t e d w i t h s h o r t h o r i z o n t a l s lo t t ed holes .

T r a d i t i o n a l l y t h e s h e a r t a b w o u l d be w e l d e d on b o t h s ides . However , w h e n uti l iz- i ng a web C J P weld, t h e "~backside" fillet we ld m a y p o s e p o t e n t i a l filler m e t a l m i x i n g a n d fit u p p r o b l e m s . The e n g i n e e r s h o u l d w o r k w i t h t h e f a b r i c a t o r to g e n e r a t e a n a c c e p t a b l e we ld ing s e q u e n c e . As a n a l t e rna t ive to a C J P groove weld, t h e b e a m web c o n n e c t i o n c a n a lso be m a d e u s i n g a h e a v y fillet w e l d e d s h e a r tab . The s h e a r t ab is typ ica l ly w e l d e d

16


to the co lumn us ing either fillet welds or a CJP groove weld. The shear tab, in turn, is then welded to the beam web with fillet welds. An example of such a connect ion can be found in "Moment Frame Connection Development and Testing for the City of Hope National Medical Center" (Zekioglu, et.al. 1997).

If the engineer chooses to use a bolted web connection, all aspects of the connect ion should be designed to resist the full shear applied to the beam due to gravity and earthquake loads. Short slotted holes may be utilized to futher protect the shear tab and beam web from possz'bie excesive deflections when the connect ion in subjected to large rotat ions as the system undergoes inelastic action dur ing an ear thquake. It should be noted tha t s t ructural steel erectors prefer s tandard holes to slotted holes to aid in erection.

One of the most d iscussed aspects of RBS design, and one of the most important , is the supplementa l lateral bracing required for this system. FEMA 267A (1997) r ecommends tha t a lateral brace be provided near the RBS. The following discussion presents an analysis of test resul ts that did not have lateral bracing provided near the RBS.

Virtually all m o m e n t connect ions that dissipate energy by yielding of the beam are subject to varying degrees of beam instability at large levels of inelastic rotation. This is true both for reinforced connect ions (cover plates, ribs, haunches , etc.) and for RBS connections. This instabili ty generally involves a combinat ion of flange buckling, web buckl ing and lateral torsional buckling and typically resul ts in deteriorat ion of the beam flexural strength, with increasing inelastic rotations. In the experience of some researchers , the degree of instabili ty and associated s t rength deterioration for RBS connect ions tested in the laboratory have been no more severe, and perhaps somewhat less severe than for many types of reinforced connect ions . This is demons t ra ted by the connect ion test results shown in Figure 4.8.

This figure shows a plot of beam tip load versus beam tip d isp lacement for two different test specimens. These two spec imens were virtually identical, except for the con-

nect ion detail. Both specimens were con- s t ruc t ed wi th the same m e m b e r sizes (W36xlS0 beam and W14x426 column) and hea ts of steel, and tested in the same test setup with identical member lengths, identical member end support conditions, and identical lateral bracing. Both spec imens were subjected to the same loading history. The only difference was that one specimen was cons t ruc ted with a cover plated connection and the other with an RBS connection. Both spec imens were provided with a single beam lateral support near the point of load application.

250

200

150

100 .

~ 5 0 .

~ o .

.~ -~0. - 1 0 0 ,

- 1 5 0 .

- 2 0 0 ,

-250

-6

Cover'Pla~ed Connectlon ~.______,~_ -~ - -~ - - ,~

RBS Connection ] * ~

\ ' ~ ~ -

_ _ _

- - - -

~ '~"'~'~'({~:;e ~ • I I

.~ -2

~ . ~ . ~ :~-~ :°~* °" ~ ~ '°~ ~

, , Displacement (inches)

Figure 4 .8 Compar i son o f Tes t Resu l t s for

Cover Plated and RBS C o n n e c t i o n s

As can be seen from Figure 4.8, the peak s t rength of the RBS connect ion is less t han that of the cover-plated connection. This, of course, is expected and is in fact a potential advantage of the RBS in that it reduces the m o m e n t genera ted at the connect ion and the m o m e n t delivered to the column. After reaching their peak s trength, both connect ions exhibited some s t rength deteriorat ion due to combined flange, web and lateral torsional buckl ing in the beam. Note however tha t the rate of deteriorat ion is less for the RBS specimen. In fact, at large inelastic deformations, the RBS exhibits the same s t rength as the cover-plated connect ion. This compar i son demons t ra tes the observation m a d e above, i.e., RBS c o n n e c t i o n s exhibi t no more s t rength deterioration, and pe rhaps somewhat less deteriorat ion than reinforced connections.

17


The t e s t d a t a s u m m a r i z e d in A p p e n d i x A i n d i c a t e s t h a t m a n y RBS c o n n e c t i o n t e s t s h a v e b e e n c o n d u c t e d w i t h o u t a n a d d i t i o n a l l a t e ra l b r a c e a t t h e RBS. T h e r e is n o i n s t a n c e w h e r e a n i n v e s t i g a t o r r e p o r t e d u n u s u a l l y severe or u n a c c e p t a b l e s t r e n g t h d e t e r i o r a t i o n d u e to t h e a b s e n c e of a l a te ra l s u p p o r t n e a r t h e RBS. F u t h e r , a s d i s c u s s e d above , s t r e n g t h d e g r a d a t i o n in t h e RBS is c o m p a r a - ble to t h a t s e e n in m a n y o t h e r c o n n e c t i o n t y p e s for w h i c h n o a d d i t i o n a l l a t e ra l b r a c i n g is p r e s e s n t l y r equ i r ed . C o n s e q u e n t l y , b a s e d on c u r r e n t l y ava i lab le da t a , a n a d d i t i o n a l lateral b r a c e at t h e RBS d o e s n o t a p p e a r n e c e s - s a r y in o r de r to ach i eve a c c e p t a b l e p e r f o r m - ance . However , t h e d e s i g n e r s h o u l d still a d h e r e to t h e n o r m a l code p r o v i s i o n s for b e a m la te ra l s u p p o r t a n d for b e a m f lange a n d web s l e n d e r n e s s l imits . La te ra l b r a c i n g for b e a m s in Spec ia l M o m e n t F r a m e s s h o u l d be p r o v i d e d a t a m a x i m u m s p a c i n g of 2 5 0 0

/FY, a s r e q u i r e d by Sec t ion 9 .8 of t h e AISC is~nic Provisions ( 1997}. As d e s c r i b e d ear l ier , m o s t m o m e n t con-

n e c t i o n s s h o w g r a d u a l s t r e n g t h d e g r a d a t i o n a t la rge levels of p l a s t i c r o a t a t i o n d u e to com- b i n e d local a n d la te ra l t o r s i o n a l b u c k l i n g of t h e b e a m . Th i s o c c u r s for t h e RBS as well a s for m o s t o t h e r c o n n e c t i o n types , a s i l lus- t r a t e d in F igu re 4.9. R e d u c i n g t h e la te ra l s u p p o r t s p a c i n g in t h e reg ion of t h e p l a s t i c h i n g e f r o m t h a t r e q u i r e d in Sec t ion 9 .8 of t h e AISC Seismic Provisions m a y t h e r e f o r e r e d u c e t h e r a t e of s t r e n g t h d e g r a d a t i o n for m o s t t y p e s of m o m e n t c o n n e c t i o n s . F u r t h e r def in i t ive r e c o m m e n d a t i o n s a n d r e s e a r c h r e s u l t s will be p r o v i d e d in t h e u p c o m i n g SAC Guidelines.

If a d e s i g n e r s h o u l d c h o o s e to p rov ide a l a te ra l b r a c e a t t h e RBS, t h e b r a c e s h o u l d n o t be l oca t ed w i t h i n t h e r e d u c e d s ec t i on of t h e b e a m . Welded or bo l t ed b r ace a t t a c h e - m e n t s in t h i s h i g h l y s t r a i n e d r eg ion of t he b e a m m a y se rve as f r a c t u r e in i t i a t ion si tes. C o n s e q u e n t l y , if a l a te ra l b r ace is p rov ided , it s h o u l d be l oc a t ed a t or b e y o n d t h e e n d of t he RBS t h a t is f a r t h e s t f rom t h e face of t h e col- u m n . If b r a c i n g is to be p r o v i d e d as p a r t of t h e des ign , r e q u i r e m e n t s a n d r e c o m m e n d a - t ions c a n be g a t h e r e d f rom d o c u m e n t s s u c h as FEMA 267A (1997) a n d " F u n d a m e n t a l s of B e a m Brac ing" (Yura 1993).

5 RBS Design Example

Description of Design Example Project

• C o m m e r c i a l Office B u i l d i n g / M e d i c a l Office B u i l d i n g

• L o c a t e d in S a n F ranc i sco , Ca l i forn ia • D i s t a n c e f r o m N e a r e s t E a r t h q u a k e

Fau l t : ~ 9 k i l o m e t e r s (San Andreas ) • High Se i smic i ty Z o n e w i th Near F a u l t

C h a r a c t e r i s t i c s

Description of Design Example Frame

P e r i m e t e r M o m e n t F r a m e s F r a m e c e n t e r l i n e d i m e n s i o n s :

s to ry h e i g h t = 13' - 0" b a y w i d t h = 22 ' - 8"

B e a m : W 2 4 x 1 1 7 A572 Gr. 50 (A992) Fy b = 50 ks i

C o l u m n : W14x311 A572 Gr. 50 (A992) Fy c = 50 ks i

Gravi ty load o n b e a m : (1.2D + .5L p e r Sect . 9 .2c of AISC Seismic Provisions):

2 k i p s / f t (0.17 k i p s / i n )

Gravi ty l oads are d u e to floor t r i b u t a r y l o a d s a s well a s ex te r io r wal l loads .

D e s i g n typ ica l in te r io r m o m e n t c o n n e c t i o n of p e r i m e t e r f rame.

I ~ V l ~ a

R = radius of cut = 4c~+ b ~

8c

_1 - - I b

Figure 5 .1 RBS D i m e n s i o n s

18


S e c t i o n Proper t i e s : From Equat ion 5:

W 2 4 x 1 1 7 :

d b = 2 4 . 2 6 in. b f = 12 .80 in.

fw = 0 .85 in. = 0 . 5 5 in.

Zxb = 3 2 7 in. 3 W 1 4 x 3 1 1 :

d c = 17 .12 in. bc f = 16 .23 in. t c f = 2 . 2 6 in. t cw = 1.41 in. Zxc = 6 0 3 in. 3

STEP 1 C h o o s e t r i a l v a l u e s for RBS d i m e n - s i o n s a, b a n d c

MRB S = 1.15 ZRBS_Fy e = 1 1 5 x 2 1 8 x 5 5 = 13789 i n - k i p

STEP 5 C o m p u t e t h e s h e a r fo rce a t t h e c e n t e r s of t h e RBS a t e a c h e n d of t h e b e a m

L ' = L - d c - 2 a+ =272-17 .12-2 7+ =222in.

F r o m E q u a t i o n s 6 a a n d 6b:

2Me~ s wL' 2×13789 0.17x222 Vm~ s - - - + - ~ =143kips

L' 2 222 2

a -~'(0.5 to 0.75) b f ~ 6 in. to 10 in. Try: a = 7 in.

b ~ ( 0 . 6 5 to 0.85) d b ~ 16 in. to 21 in. Try: b = 19 in.

c ~ 0 . 2 b f ~ 2 . 6 in. Try: c = 2 . 7 5 in.

STEP 2 C o m p u t e t h e p l a s t i c s e c t i o n m o d u - l u s a t t h e m i n i m u m s e c t i o n of t h e RBS

F r o m E q u a t i o n 3:

ZRB S = Zxb- 2 c t f ( d b - t ~ = 327 - 2 x 2.75 x 0.85 x (24.26 - 0.85) = 218 in.3

STEP 3 E s t a b l i s h t h e e x p e c t e d y ie ld s t r e s s of t h e b e a m

For A572 Gr. 50 s tee l , Ry = 1.1.


V~ s _ 2M~s wL'_ 2×13789 0.17×222 =105kips L' 2 222 2

F i g u r e 5 .2 s h o w s t h e s h e a r fo rce d i a g r a m , t h e b e n d i n g m o m e n t d i a g r a m , a n d t h e f ree b o d y d i a g r a m t h e for t h e p o r t i o n o f t h e b e a m b e t w e e n R B S c e n t e r s . O b s e r v e t h a t t h e m a x - i m u m m o m e n t o c c u r s a t t h e e n d s , i .e. , a t t h e c e n t e r s of t h e RBS. If t h e g r a v i t y l o a d w e r e e x t r e m e l y la rge , c o m p a r e d to t h e m o m e n t

143 105

V (k i p )

M ( k i p - i n )

13789

-13789

Fy e = RyFy b = 1 . 1 x 5 0 = 5 5 k s i

STEP 4 C o m p u t e t h e m a x i m u m m o m e n t e x p e c t e d a t t h e c e n t e r o f t h e RBS

~ REDS w = 0.17 kips/in. ~ RIBS

Ii . , . l . . i ~ I i ~ I i I I I I ~ t I t i i I I I t I ~ I i i . l . . ! j

. . . . . . . . t J 143 ' "~05k ~

, L' ~ 222 in.

F i g u r e 5 . 2 P o r t i o n o f E x a m p l e B e a m

b e t w e e n R B S C e n t e r s

19

D E S I G N O F R E D U C E D B E A M S E C T I O N (RBS) M O M E N T FRAME CONNECTIONS ,

d e v e l o p e d d u e to a p p l i e d l a te ra l l oads , t h e c u r v e d p o r t i o n of t h e m o m e n t d i a g r a m c o u l d dr ive t h e p las t i c h i n g e t o w a r d t h e c o l u m n , a w a y f rom t h e RBS. Th i s e x a m p l e i n d i c a t e s t h a t t h e gravi ty l oad is n o t la rge e n o u g h to fo rm a p l a s t i c h i n g e w i t h i n t h e s p a n , a w a y f rom t h e RBS. C o n s e q u e n t l y , t h e ca lcu la - t i on s above for t h e m o m e n t a n d s h e a r forces , a t t h e RBS cu t s , a r e valid.

S T E P 6 C o m p u t e t he m a x i m u m m o m e n t e x p e c t e d a t t h e face of t h e c o l u m n

M s


=Mees + Veas(a + 2b-/= 13789 + 143(7 + ~ ) = 16149in - kip

S T E P 7 C o m p u t e t h e p l a s t i c m o m e n t of t h e b e a m b a s e d on t h e e x p e c t e d yie ld s t r e s s


Mpe = Zxb Fy e = 327 x 55 = 17985 in -k ip

S T E P 8 C h e c k t h a t Mfis in t h e r a n g e of 85 to 100 p e r c e n t of Mpe


ZMc > 1.0 (Equa t i on 10)

R e t u r n i n g to t h e e x a m p l e , a s s u m i n g t h a t p o i n t s of in f lec t ion in t h e c o l u m n s o c c u r a t t he i r m i d - h e i g h t s , a n d a s s u m i n g a n axial s t r e s s (fa) of 15 ks i in t h e c o l u m n s u n d e r c o m b i n e d e a r t h q u a k e a n d gravi ty loading , t h e fol lowing c a l c u l a t i o n s resu l t .

F r o m E q u a t i o n s 11, 12, 13 a n d 14:

h~ + d b + h b

2 x 13789+ (143 + 105(17;12 + 7 + ~ )

156 = 217kips

Met

Mcb

= V c h t = 2 1 7 x (156 - 2 4 . 2 6 ) / 2 = 14294 in -k ip

14294 in -k ip

= 2x14294 = 28588 in - k i p

M f 16149 - -

Mpe 17985 - - - 0.90 OK

T h u s , t h e p r e l i m i n a r y d i m e n s i o n s a re OK.

Use: a = 7 i n . b = 1 9 i n . c = 2 .75 in.

S T E P 9 S t r o n g C o l u m n - W e a k B e a m C h e c k

To c h e c k s t r o n g c o l u m n - w e a k b e a m r e q u i r e m e n t s , t h e p r o c e d u r e p r e s e n t e d in FEMA 267A (1997) will be u s e d , w i th t h e m i n o r m o d i f i c a t i o n s n o t e d in Sec t ion 4. The f inal e q u a t i o n to be u s e d to c h e c k t h i s r e q u i r e m e n t ( f rom E q u a t i o n 7 . 5 . 2 . 5 - 1 of FEMA 267A) is as follows:

~Zc(Fyc-.f~) 2×603(50-15) - = 1.5 > 1 OK

~ M ~ 28588

S T E P 10 C h e c k C o l u m n P a n e l Z o n e

To c h e c k t h e c o l u m n p a n e l zone , t h e p ro- c e d u r e d i s c u s s e d in Sec t i on 4 will be u s e d .

B a s e d on t h e e x a m p l e , t h e c o l u m n p a n e l zone s h e a r is c o m p u t e d a s follows:

Mf = 16149 in -k ip (Equa t i on 7)

F r o m E q u a t i o n s 15, 16 a n d 17:

27Mf = Mf+ M:f = 16149 + 15522 = 3 1 6 7 4 in -k ip

, Mf=M~Bs+V~Bs a+ =13789+105 7+ =15522 in - kip

i | 1

2 0


Vez - 0.8z..,~'Mr 0.8Vc Vc - 0.8x31671 0.8x217 = 926kips 0.95dt) 0.95 × 24.26

Panel zone s t rength is computed as follows:

From Equation 18:

= 0.55F~,~d~tIlL + 3b~ft~d+d~t 1

I 3 x 16"23 x (2"26)~ ] = 0.55xSOx17.12x1.41 1+ 24.26xlT.12xl.41J = 946 kips

946 > 926 .'.No doubler plates required

STEP 11 Check Beam Shear

From Equat ion 19:

w ( l - l ' ) /272~222/ V~ 4 2 0.17 -

' 2 143 ÷ 2

= 145kips

V, = A,,,Fy = (0.55)(24.26)(5 O) = 667 k ips > 145 k ips

RBS flange reduct ion is approximately 43 percent. Consequently, it is expected that the inclusion of tlae RBS the beams will increase interstory drift by about 5 percent.

S ~ e ~ c Abut

,~ ~ . B.U. bar to remain

I / ~ ~ Remove weld tabs IE 718" x 6" ~,.~,,.T-~-~'~"r-.~ / ~ IP {B.S.) ~ I ! [ I / _1 16 ~ Weld B.U. bar Io coiutnn

• ~ l ~ . l I~ . ~ _ 5 . ° - - N ....

~ l / * ~

I.t' i I w2,.,,7 ~i I.I I'\i I g;-~'~-------------~,~,:,,d~,,,~,,~,,oo,~d,

~, tose~v a s b a c i~g C~, ~ - - ~ I Z . . . . ooo,0,.to , . ] ~ , ~ , _ ~ ~ ~ \ , ~ , ~ . ~ : ~ column and beam byfabdcato~.

I I 5/16 \ cleaned and inspected

.

Configure plate comes to \ ~ 17 75" Radius =.o,o0, . . . . . . / . of column Grind Smooth

~ ~ J ~ 1 ~ 2.75" 7.3" 2.75"

5 / ' I ' ~ NI welds: ET0 ~lI groove welds: electrodes must be rat~;I for

'° CVN of at teast 20 It-fos at -20 deg. F. All welding shall conform to AWS D1.1

Figure 5 .3 C o n n e c t i o n Detai l for Des ign E ~ m p l e

6 P r o c e d u r e s f o r A c c e p t a n c e o f D e s i g n b y B u i l d i n g A u t h o r i t i e s

Continuity Plates

Use cont inui ty plates with a th ickness approximately equal to the beam flange thickness . The beam flange th ickness is 0.85 inches. Therefore, use 7 /8" thick cont inui ty plates (0.875"). Connect cont inui ty plates to co lumn flanges us ing CJP groove welds, and the web us ing double fillet welds. The corners of cont inui ty plates should be config- ured to avoid welding into the k-area of the column.

Beam Web Connection

Connec t b e a m web to co lumn flange us ing CJP groove weld over full depth of web (between weld access holes).

A drawing of a generic final connect ion detail is shown in Figure 5.3. The resul t ing frame should be checked for all code specified s t rength and drift limits. Note tha t the

The design of SMF building systems require that the design account for inelastic deformat ion d e m a n d s on the connection. The AISC Seismic Provisions for Structural Steel Buildings (1997), Section 9.2, p resen ts the requ i rements for SMF structures . The RBS connec t ion is an opt ion tha t can m e e t requ i rements set by bui lding codes and con- s ensus documents . The following c o m m e n t s are in tended to describe actions that can be followed to help facilitate the permi t t ing process for a SMF building system.

6.1 C o m m u n i c a t i o n

It is r e c o m m e n d e d that early in the process, the Structural Engineer of Record communi - cate with the bui lding official regarding the proposed use and per t inent aspects of the RBS m o m e n t connect ion. The engineer may need to provide background documen ta t ion to the bui lding official if he or she is unfamil- iar with the design and terminology relating

21


to the design. The use of this d o c u m e n t may aid the bui lding official in u n d e r s t a n d i n g the design intent.

6 . 2 M e t h o d o l o g y

Once the bui lding official u n d e r s t a n d s the design in tent and sys tem behavior, it is impor tan t to clearly state the design methodology to be used early so tha t any misunder - s tandings can be avoided. This d o c u m e n t presen ts a general design methodology, utilizing some simplifying a s s u m p t i o n s and some of the bet ter aspects of m a n y different design methods . There are other ways to design an RBS m o m e n t connect ion and SMF system than tha t represen ted in this docu- ment . If other me thods are utilized, the engineer should be sure to clearly indicate the me thod u sed and the impor tan t aspects tha t show design compliance with the governing building code.

Any design methodology utilized should correlate well with other publ i shed methods , test resul ts and research papers. Section 9.2 of the AISC Seismic Provisions require tha t the design be based on qualifying cyclic tests. The table in Appendix A will help to satisfy this r equ i rement for the RBS connect ion. Any significant deviation from es tabl ished methodologies or tests should be justified. It is impor tan t to u n d e r s t a n d tha t m a n y rec- ommenda t ions conta ined in this d o c u m e n t are based on exper imenta l research. Design equat ions and RBS sizing values are based on successful research , both analytically and experimentally. Therefore, any new design equat ions should be comparable to estab- l ished equat ions.

6 .3 C o n s t r u c t i o n D o c u m e n t s

After a design is complete, it is imperative to convey the information accurate ly on con- s t ruct ion documents . While calculat ions are impor tan t and describe the final cons t ruc ted connect ion, cons t ruc t ion documen t s provide direction to the fabricator and erector. The e lements expressed on the drawings will be more impor tan t to the final quality of the design than any calculation.

The documenta t ion related to the RBS connect ion should be clear and concise, yet provide enough detail for the fabricator to properly incorporate all the difficult and impor tan t aspects of the connection. The information should be such tha t any fabricator or erector can utilize the information provided, and cons t ruc t the final connect ion in such a m a n n e r tha t the performance will directly correlate with the design intent.

Impor tan t aspects of the design to be inc luded in the drawing details are welding detai ls , RBS shape a n d locat ion, no tes regarding grinding of the RBS after cutting, shear tab detail information and beam web to co lumn flange connect ion details. It is rec- o m m e n d e d to provide a set of notes specific to the RBS connect ions , relat ing to welding pract ices and connect ion cons t ruc t ion proce- dures to help the contractor u n d e r s t a n d the connect ion and the impor tance it has on the building sys tem performance. Reference to applicable port ions of AWS D I.1 and other AWS or AISC documen t s should be inc luded in these notes to clearly state a level of expected quality. This level of informat ion will also facilitate obtaining the appropria te level of inspect ion required for this type of connection.

7 Fabricat ion and I n s p e c t i o n I s sues

A n u m b e r of fabr icat ion and inspec t ion i ssues are impor tan t to ensu re a well-con- s t ruc ted RBS connect ion. As d i scussed earlier proper fabrication and erect ion of this connect ion is a critical port ion of the sys- t em 's pe r fo rmance . If welds are poorly placed, the s t ress at which f racture init iates and propagates is m u c h lower than the stress a tough weld metal , placed with care, can resist. Cut t ing and grinding are critical aspects of fabrication which m u s t be well executed to p roduce a high quality final connection.

7.1 C u t t i n g a n d G r i n d i n g

The cut portion of both the curved RBS section, as well as the prepara t ion of the end of

22

DESIGN OF REDUCED BEAM SECTION (RBS} MOMENT FRAME CONNECTIONS •

the beam, needs to be smooth and free of notches . This smoothness is impor tan t for reasons d i scussed earlier. Many fabrication shops have the ability to make virtually no tch free the rmal cuts. While this is a ben- efit to reduce the n u m b e r of pe rpendicu la r notches , which may presen t s t ress risers, small imperfect ions exist tha t may affect connect ion performance.

Therefore, it is impor tan t to clearly identify wha t is the adequa te a m o u n t of mater ia l to remove (by grinding) from the cut surface. FEMA 267A (1997) d i scusses a level of acceptable surface roughnes s value less t han or equal to 1000 as defined in ANSI/ASME B46.1. This level is difficult to de termine wi thout a significant a m o u n t of equ ipment and expertise. Therefore, this d o c u m e n t rec- o m m e n d s tha t the the rmal cuts be ground smooth in the following manne r : "It is impor- t an t tha t the pa t te rn of any cuts m a d e in the flange be propor t ioned so as to avoid sharp cut corners. All c o m e r s should be rounded to minimize no tch effects and in addit ion, cut edges should be cut or g round to have a surface roughness meet ing the requ i rements of AWS C4.1-77 class 4, or smoother."

The designer should d i scuss the in tent with the fabricator and develop criteria for an acceptable mock-up to be made for reference du r ing fabr icat ion inspec t ions . The final grinding tha t the engineer and fabricator have agreed upon , shou ld be inspec ted by the fabricator 's representa t ive as well as the owner 's test ing agency, to ensu re compl iance with the accepted mock-up .

Many beams u s e d for SMF sys tems are large with th ick flanges and webs. Shear p u n c h i n g holes in these th ick port ions of the member could lead to localized de laminat ion or tearing. In s i tua t ions where hole diame- ters are smaller t h a n the base mater ia l th ickness , the des igner m a y consider tha t holes required for fabricat ion of e lements and port ions of the RBS beam be drilled ra ther t han punched . No resea rch resul ts indicate tha t a reduc t ion in connect ion per formance is a t t r ibutable to p u n c h i n g holes in RBS beams.

7.2 W e l d i n g

Welding is a very critical par t of the proper fabrication of this connect ion. A significant a m o u n t of effort ha s been made to produce a b e a m wi th a r e d u c e d sec t ion m o d u l u s , de s igned to yield pr ior to developing m o m e n t s which deliver very high s t resses to beam flange - co lumn flange welds. However, if the welding requi red for this connect ion is done poorly, the s t ress at wh ich brittle behavior m a y occur is m u c h lower than the engineer expects. Good welds, us ing tough filler metal , will resis t h igher loads than sur- round ing base metal . Therefore, it is imperative tha t the welding for this type of connec- t ion be of h igh qual i ty , to p r o d u c e a connec t ion tha t will perform as designed.

Any specific i s sues re la ted to welds, such as weld profiles, s equence , submi t t a l of mater ia ls or cert if ications tha t are considered impor tan t for compl iance of the fabricator's work to mee t the design intent , should be clearly s ta ted in the cons t ruc t ion documents . I tems s u c h as p rehea t shou ld be addres sed in the project specifications and cons t ruc t ion drawings. Typically, AWS will adequate ly addres s mos t i ssues , and for new design will provide the fabr icator ample direction to complete the cons t ruc t ion in a safe and high quali ty m a n n e r .

The engineer shou ld be clear in the project specifications a n d cons t ruc t ion drawings tha t filler meta l s shal l no t be mixed in s u c h a way as to p roduce a CVN value below tha t specified a n d requ i red for a single filler metal. Most fabricat ion shops present ly use gas shielded FCAW m e t h o d s for welds to co lumns a n d beams . The erect ion crews, especially w h e n welding complete joint pene- t ra t ion groove welds , typical ly u s e self shielded FCAW. Also, the re are different filler metals u sed for the flat posit ion as well as other positions. Some combina t ions of filler meta ls in the same jo in t m a y p roduce a combined CVN value, wh ich could p resen t "brit- fie behavior". The engineer should carefully review the in format ion provided in "The Effects of In termixed Weld Metal on Mechan- ical Properties" (1998) and the submi t t ed WPS prior to fabr icat ion to ensu re tha t the fabricator and erector are not creat ing a

23

DESIGN O F REDUCED BEAM SECTION (RBS} MOMENT FRAME CONNECTIONS

potential problem by inappropriately mixing filler metals.

Parameters should be set for quali ty control of shop welding and fabrication. The fabricator m u s t have an acceptable Quali ty Con- trol (QC) procedure in place th roughout the fabrication of the project. In addition, Quality Assurance measures should be t aken to help ensure tha t the QC procedure is being imple- mented and followed. Typically QA or Verifi- cat ion Inspect ion is provided by special inspectors , hired by the owner. It is the responsibi l i ty of the engineer to es tabl ish inspect ion protocol, request a pre-fabrication and pre-erection meeting, and impress upon the fabricator and erector the impor tan t

i s sues s u r r o u n d i n g the RBS connec t ion details and construction. Complete joint pen- etrat ion groove welds should be inspected by a Level II qualified NDT inspector as defined in the AWS D 1.1. Each joint should be ultra- sonically tested and all welds associated with the connect ion should receive cont inuous special inspection. Field inspect ion should be sensitive to such i ssues as weld preparat ion and fi t-up, weld profile and weld p a s s sequence, back-up bar removal and grinding of run-of f tabs . The inspec to r s shou ld develop an acceptable protocol for inspect ion and reports in regards to welding and connect ion completion.

24


References

"AISC Initiates Research Into k Area Crack- ing," Modern Steel Construction, Vol. 37, No. 9, September 1997, pp.23-24.

Grubbs, K.V., "The Effect of the Dogbone Connection on the Elastic Stiffness of Steel Moment Frames," M.S. Thesis, Department of Civil Engineering, the Uni- versity of Texas at Austin, Austin, Texas, August 1997.

Blodgett, O., Funderburk, S., and Miller, D., "Fabricators ' and Erectors ' Guide to Welded Steel Construction," The Lincoln Electric Company, Cleveland, 1997.

International Conference of Building Officials (ICBO), The Uniform Building Code (UBSC), April 1997.

Chen, S.J., Yeh, C.H. and Chu, J.M, "Ductile Steel Beam-to-Column Connections for Seismic Resistance," Journal of Structural Engineering, Vol. 122, No. 11, November 1996, pp. 1292-1299.

Iwankiw, N., "Ultimate Strength Considera- tions of Seismic Design of the Reduced Beam Section (Internal Plastic Hinge)," Engineering Journal , American Institute of Steel Construction, Inc., Vol. 34, No. 1, First Quarter 1997.

Engelhardt, M.D. and Husain, A.S., "Cyclic Loading Performance Of Welded Flange - Bolted Web Connections," Journal o f Structural Engineering, ASCE, Vol. 119, No. 12, December 1993.

Engelhardt, M.D., Winneberger, T., Zekany, A.J. ,and Potyraj, T., ~The Dogbone Con- nection: Part II." Modern Steel Construc- tion, August 1996.

Engelhardt, M.D., Winneberger, T., Zekany, A.J. ,and Potyraj, T., "Experimental Inves- tigation of Dogbone Moment Connec- tions," Proceedings: 1997 National Steel Construction Conference, American Insti- tute of Steel Construction, Chicago, May 1997.

Johnson, M., Quintana, M., '~The Effects of Intermixed Weld Metal on Mechanical Properties, Part III," Proceedings, Interna- tional Conference on Welded Construc- tions in Seismic Areas, AWS, October 1998.

Kaufmann, E., Xue, M., Lu, L., and Fisher, J. , "Achieving Ductile Behavior of Moment Connections," Modern Steel Con- struction, Vol. 36, No. 1, American Insti- tute of Steel Construction, J anua ry 1996.

Lee, K., Goel, S.C., Stojadinovic, B., "Bound- ary Effects in Welded Steel Moment Con- nections," Research Report No. UMCEE 97-20, December 1997.

Engelhardt, M.D. and Sabol, T.A., "Reinforc- ing of Steel Moment Connections with Cover Plates: Benefits and Limitations," Engineering Structures, Vol. 20, No. 6, pp. 510-520, 1998.

Noel, S. N., "Reduced Beam Section Design for Seismic Retrofit of Steel Moment Frame Connections," M.S. Thesis, Divi- sion of Structural Engineering, University of California, San Diego, 1997.

Gross, J., Engelhardt, M., Uang, C., Kasai, K., and Iwankiw, N., "Modification of Existing Steel Welded Moment Frame Connect ions for Seismic Resistance," Steel Design Guide Series Twelve, Ameri- can Institute of Steel Construction, Inc., Chicago, 1999.

Plumier, A., "The Dogbone: Back to the Future," Engineering Journal, American Institute of Steel Construction, Inc. 2nd Quarter 1997.

25

DESIGN O F REDUCED BEAM SECTION (RBS} MOMENT FRAME CONNECTIONS

Popov, E. and Stephen, R., "Cyclic Loading of Full Size Steel Connections," Bulletin No. 21, American Iron and Steel Institute, 1972.

SAC Joint Venture, Background Reports on Metallurgy, Fracture Mechanics, Welding, Moment Connections and Frame Systems Behavior, Published by the Federal Emer- gency Management Agency, Report FEMA 288, 1996.

SAC Joint Venture, Interim Guidelines: Eval- uation, Repair, Modification and Design of Welded Steel Moment Frame Structures, Published by the Federal Emergency Management Agency, Report FEMA 267, August 1995.

SAC Joint Venture, Interim Guidelines Advi- sory No. 1 - Supplement to FEMA 267, Published by the Federal Emergency Management Agency, Report FEMA 267A, March 1997.

Seismic Provisions for Structural Steel Build- ings, American Institute of Steel Con- struction, Inc., Chicago, April 15, 1997.

"Structural Welding Code - Steel," AWS D 1.1- 98, American Welding Society, Miami, 1998.

Tide, R., "Stability of Weld Metal Subjected to Cyclic Static and Seismic Loading," Engi- neering Structures, Vol. 20, Nos. 4-6, April-June 1998.

Tsal, K.C. and Popov, E.P., "Steel Beam-Col- umn Joints In Seismic Moment Resisting Frames", Report No. UCB/EERC - 88/19, Earthquake Engineering Research Cen- ter, University of California at Berkeley, 1988.

Yura, J.A., "Fundamentals of Beam Bracing," Proceedings, Structural Stability Research Council Conference, "Is Your Structure Suitably Braced?," 1993.

Zekioglu, A., Mozaffarian, H. and Uang, C., "Moment Frame Connection Development and Testing for the City of Hope National Medical Center," Proceedings; Structures Congress XV, Portland, April 13-16, 1997, American Society of Civil Engineers, 1997.

26

APPENDIX A

Summary of Experiments on Reduced Beam Section Moment Connections for New Construction

Ref

[1]

[1]

[1]

[1]

[1]

Spec.

YC-1

YC-2

PC-1

PC-2

PC-3

Beam

Built-up W shape d=23.6", b~=l 1.8", tf=0.79", tw=0.47"

Lb=73" A36 steel Fy_f =40 ksi Fo.~ =66 ksi Fy.w =40 ksi Fu.w =65 ksi

Column

Built-up Box: 19.7"xl 9.7"x.79"

Lc = 87" A572 Gr. 50

Fy =56 ksi Fu =82 ksi

Flange Welds

SS-FCAW E70T-7

No weld tabs used

Web Connection

Bolted: 7-7/8" A325

RBS Details and Other

Flange Modifications

Tapered cut L1=2"

LRBS=I 3.8" FR=20%

Tapered cut L~=2"

LRBS=17.7" FR=25%

Tapered cut L1=4.7"

LRBS=I 5.7" FR=34%

Tapered cut L1=4.7"

LRSS = 17.7" FR=42%

Tapered cut L1=4.7"

LRss=I 7.7" FR=42%

Op (%)

2.4

2.9

4.1

4.8

3.8

Comments

Fracture of beam flange initiating at weld access hole





I m~

Ref

[2]

[2]

[2]

[2]

[3,4]

[3,4]

Spec.

DBT- 1A-99-

176

Beam

W30x99 A572 Gr. 50

L~= 138"

Column

W14x176 A572 Gr. 50

Lc=168"

Flange Welds

SS-FCAW E70TG-K2;

backing bar removed

Web Connection

Bolted: 7-1" A325



Tapered cut L1=7.5"

LRBS=20.25 ''

DBT- 1 B-99-

176

DBT- 2A-150-

257

DBT- 2B-150-

257

ARUP- 1

Fy.w = 61.6 ksi Fu.w = 82.8 ksi

W30x99 A572 Gr. 50

Lb=138" Fy. w = 51.5 ksi Fu.w = 72.1 ksi

W36x150 A572 Gr. 50

Lb=138" F~.w = 60.2 ksi Fu.w = 72.3 ksi

W36x150 A572 Gr. 50

Lb=138" Fy.w = 62.9 ksi Fu.w = 83.1 ksi

W36x150 A572 Gr. 50

Lb=132"

Fy.w =55.6 ksi Fu.w =70.7 ksi

W14x176 A572 Gr. 50

Lc=168" Fy.w =55.5 ksi Fu.w =71.8 ksi

W14x257 A572 Gr. 50

Lc=168" Fy.w =59.6 ksi Fu.w =75.2 ksi

W 14x257 A572 Gr. 50

Lc=168" Fy.w =64.5 ksi Fu.w =83.2 ksi

W 14x426 A572 Gr. 50

Lc=136"

at bottom flange

SS-FCAW E70TG-K2

backing bar left in

Bolted: 9-1" A325

welded (heavy shear tab groove

FR=45%

Tapered cut L1=7.5"

LRBS=20.25 " FR=45%

Tapered cut L1=9"

LaBs=24" FR=45%

Tapered cut L1=9"

LRBS=24 '' FR=45%

Tapered cut L1 =9"

LABS=24"

COH-1

Fy.f =55.5 ksi Fu4 =73 ksi

Fy.w =62.5 ksi Fu-w =77 ksi

W27x178 A572 Gr. 50

Lb= 132" Fy.f =44 ksi Fu.f =62 ksi Fy.w =46 ksi Fu-w =62 ksi

W 14x455 A572 Gr. 50

Lc=136" Fy.f =55 ksi Fu4=84 ksi Fy.w =54 ksi Fu-w =86 ksi

place w/seal weld at top flange;

backing bar removed at bottom flange

welded to column and fillet welded

to beam web)

FR=44% top & bottom

flanges reinforced with

vertical ribs Tapered cut

L~=7" LABS=20" FR=38%

top & bottom flanges

reinforced with vertical ribs

0p (%)

2.8

4.0

Comments

no failure; test stopped due to limitations in test setup

no failure; test stopped due to limitations in test setup

3.5 ' Fracture of beam top flange near groove we d

1.7 Fracture of beam top flange we d; propagated to divot- type fracture of column flange

3.5 Flange fracture at minimum section of RBS

3.5

A-2

[3,4]

[3,4]

[3,4]

[3,4]

COH-4 ~¢ =~

COH-5 |~

[5,6]

[5,6]

Spec. Beam Column Flange Welds Web Connection



COH-2 (~ =¢ ~

COH-3 Wl 4x455 A572 Gr. 50

Lc=136" Fy.f =55 ksi Fu.f =84 ksi Fyow =54 ksi Fu-w =86 ks i

Beam connected to column web

W33x152 A572 Gr. 50

Lb=132" Fy.f =57.6 ksi Fu.f =78.5 ksi Fy.w =62 ksi

Fu-w =84.5 ksi

Tapered cut L1=9"

LRBS=26" FR=43%

top & bottom flanges

reinforced with vertical side

plates

Ref

DB1 Wl 4x426 A572 Gr. 50

Lc=136"

W 14x426 A572 Gr. 50

Lc=136" Fy.f =50 ksi

Fu4 =74.5 ksi Fy.w =50 ksi Fu.w =75 ksi

W33x152 A572 Gr. 50

Lb=132" F~4 =62.8 ksi Fu.f =86 ksi

F~.w =69.1 ksi Fu.w =93.7 ksi

SS-FCAW E71T-8

backing bar left in place w/seal weld at

top flange; backing bar removed

at bottom flange

W36x160 L~=134"

Fy.f =54.7 ksi Fu4 =75.6 ksi Fy.w =53.5 ksi Fu-w =79.2 ksi

welded (beam web

W36x150 Lb=134"

Fy.f =41.4 ksi Fu4=58.7 ksi Fy.w =47.1 ksi Fu-w =61.8 ksi

DB2

Constant cut L1=9"

groove welded to column)

LRBS=I 9.5" FR=40%

Radius cut L1=9"

L~Bs=27" FR=40%

Gp Comments (O/o)

3.8

3.2

4.0

1.8

2.0 Flange fracture at RBS

3.0 Testing stopped due" to limitations of test setup

A-3

Ref

[5,6]

[5,6]

[5,6]

[7]

Spec.

DB3

DB4

DB5

DB1

Beam

W36x170 L~=134"

Fy.f =58 ksi Fu.f =73 ksi

Fy,w =58.5 ksi Fu.w =76.7 ksi

W36x194 Lb=134"

Fy.f =38.5 ksi Fu4 =58.6 ksi Fy,w =43.6 ksi Fu.w =59.8 ksi

W30x148 Lb=134"

Fy.f =46.6 ksi Fu.f =64.5 ksi Fy.w =48.5 ksi Fu.w =65.4 ksi

W36x135 A36 Steel Lb=134.5"

Column

W 14x426 A572 Gr. 50

Lc=136"

W 14x426 A572 Gr. 50

Lc=136" Fy4=50 ksi

Fu4 =74.5 ksi Fy,w =50 ksi Fu.w =75 ksi W 14x257

A572 Gr. 50 Lc=136"

Fy.f =48.7 ksi Fu.f =69 ksi

Fy. w =49.4 ksi Fu.w =66.2 ksi

W 14x257 with 1-5/16" thk.

cover plates (cover plates welded

across flanges of W14x257 to form

box) A572 Gr, 50

L~=132"

Flange Welds

SS-FCAW E71T-8

(details of backing and weld tabs not

available)

Web Connection

Not Available



Radius cut L1=9"

LRBS=27 '' FR=40%

Radius cut L1=9"

LRBS=27 " FR=38%

Radius cut L1 =5"

LRas=25 " FR=38%

Radius cut L1=8"

LRBS=28 '' FR=40%

~p (%)

3.8

3.7

4.0

3.0

Comments

Testing stopped due to limitations of test setup; significant column panel zone yielding

Testing stopped due to limitations of test setup

A-4

Ref

[8]

[8]

[8]

[8]

[8]

Spec. Beam Column

S-1

S-2A

SC-1

S-3

S-4

W530x82 (Canadian Designation)

d=20.8", bf=8.2", tf=0.52", tw=0.37"

wt.=54 Ib/ft. Lb= 142"

CSA G40.41-350W steel

Fy.f =52.4 ksi Fo.f =76.6 ksi Fy.w =57.5 ksi

Fu.w =81 ksi (~

W 14x 120 A572 Gr. 50

Lc=120"

Flange Welds

SS-FCAW E71T-8



at bottom flange

Web Connection

Bolted: 5-1" A325



Radius cut L1=4.7"

LRss=l 5.7" FR=55%

0p (%)

9.0

3.6

3.4

note (8)

note (9)

Comments

Specimen loaded monotonically; testing stopped due to limitations of test setup

Testing stopped due to limitations of test setup Composite slab included (6); testing stopped due to limitations of test setup statically applied simulated earthquake loading (7); testing stopped due to reaching end of simulated earthquake loading; no connection failure dynamically applied simulated earthquake loading (7); testing stopped due to reaching end of simulated earthquake loading; no connection failure

A-5

Ref

[8]

[11]

[11]

[11]

[11]

[12]

[12]

Spec.

SC-2

LS-1

Beam Column

W30x99 A572 Gr. 50

W14x176 A572 Gr. 50

Flange Welds

SS-FCAW E70T-6

Web Connection

welded (Beam web



Radius cut L1 = 7"

LS-2

LS-3

LS-4

DBBW Beam 1

Lb = 141" Fy.f = 54.0 ksi Fu4= 71.9 ksi Fy.w = 58.0 ksi Fu.w = 74.8 ksi

W36x150 A572 Gr. 50

Lb = 141"

Lc = 150" Fy.f= 55.5 ksi Fu4 = 74.0 ksi Fy.w= 54.0 ksi Fu.w= 73.1 ksi

(~

W 14x398 A572 Gr. 50

Lc = 146"



at bottom flange ~

SS-FCAW E70T-6

backing bar left in


Bolted: 10 - 1" A490

LaB s = 20" FR = 50%

Radius cut L1 = 9"

LaBS = 27" FR = 50%

DBBW

Beam 2 m

Fy.f = 54.3 ksi Fo.f = 68.8 ksi Fy.w = 59.4 ksi Fu.w= 72.0 ksi

Fy = 53.0 ksi Fu = 73.0 ksi

(based on CMTR)

place w/seal weld at top flange;


.

0p (%)

Note (9)

Comments

Composite slab included (6); dynamically applied simulated earthquake loading (6); testing stopped due to reaching end of simulated earthquake loading; no connection failure

4.0 No connection failure

+1.0 note (12) /-5.0 -1.0/ note (12) +5.0

4.0 No connection failure; testing stopped due to limitations of test setup

4.0 No connection failure; test stopped due to limitations of test setup;

see note (13)

4.0

A-6

Ref

[12]

[12]

[13]

[13]

[13]

[13]

Spec.

DBBW- C

Beam 1 DBBW-

C

Beam 2

DBWW

Beam 1

DBWW

Beam 2 DBWW

-C

Beam 1 DBWW

-C

Beam 2

Beam Column Flange Welds Web Connection

W36x150 A572 Gr. 50

Lb= 141" Fy.f= 54.3 ksi Fu.f = 68.8 ksi Fy.w = 59.4 ksi Fu.w= 72.0 ksi

¢¢

W 14x398 A572 Gr. 50

Lc = 144" F v = 53.0 ksi Fu = 73.0 ksi

(based on CMTR)

SS-FCAW E70T-6



at bottom flange ( (

welded (Beam web




Op (%)

5.0

3.8

3.5

Comments

Low cycle fatigue fracture in RBS;

see note (14) Fracture of bottom beam flange adjacent to groove weld; fracture initiated at weld access hole;

see note (14) No connection failure; test stopped due to limitations of test setup

see note (13)

3.5

5.0 Low cycle fatigue

5.0

fracture in RBS

see note (14) Low cycle fatigue fracture in RBS

A-7

Ref Spec.

[14] WG-1

[14] WG-2

[14] WG-3

[14j

Notes:

Beam

W33x201 A572 Gr. 50 Lb = 160.5"

F~.f = 52.0 ksi Fu-f = 72.8 ksi Fy.w = 51.5 ksi Fu-w = 68.0 ksi

W36x300 A572 Gr. 50

Lb = 159" F~.f = 56.0 ksi Fu4 = 72.9 ksi Fy.w = 56.7 ksi Fu.w = 74.5 ksi

WG-4 "

Column

W14x311 A913 Gr. 65

Lc = 152" Fy.f = 69.0 ksi Fu4 = 88.3 ksi Fy-w = 68.0 ksi F..w= 86.5 ksi

5/8" doubler plates (A572 Gr. 50)

provided on each side of column web

W14x550 A913 Gr. 65

Lc = 152" Fy.f = 67.0 ksi Fu4= 86.8 ksi Fy.w= 68.1 ksi Fu.w = 87.6 ksi

Flange Welds

SS-FCAW E70TG-K2;


Web Connection

Bolted: 13-1" A490

Bolted: 20 - 1" A490 (2 rows of 10 bolts each)



Radius cut L1 = 9.3"

LRBS = 25" FR = 54%

Radius cut L1 = 10"

Lass = 27" FR = 51%

~p (%)

2.9

2.9

3.5

Comments

fracture of RBS at local buckle in RBS

see note (15)

No connection failure; test stopped due to limitations of test setup

1~

" 4.5 "

1. All specimens are single cantilever type, except DBBW, DBBW-C, DBWW, and DBWW-C 2. All specimens are bare steel, except SC-1, SC-2, DBBW-C and DBWW-C 3. All specimens subject to quasi static cyclic loading, with ATC-24, SAC or similar loading protocol, except S-1, S-3, So4, SC-2, LS-2 and LS-3 4. All specimens provided with continuity plates at beam-to-column connection, except Popov Specimen DB1 (Popov Specimen DB1 was provided with

external flange plates welded to column). 5. Specimens ARUP-1, COH-1 to COH-5, S-1, S-2A, S-3, S-4, SC-1, SC-2 and LS-4 provided with lateral brace near loading point and an additional

lateral brace near RBS; all other specimens provided with lateral brace at loading point only. 6. Composite slab details for Specimens SC-2 and SC-2:118" wide floor slab; 3" ribbed deck (ribs perpendicular to beam) with 2.5" ~oncrete cover;

normal wt. concrete; welded wire mesh reinforcement; 3.4" dia. shear studs spaced at 24" (one stud in every other rib); first stud located at 29" from face of column; 1" gap left between face of column and slab to minimize composite action.

A-8

7. Specimens S-3, S-4 and SC-2 were subjected to simulated earthquake loading based on N10E horizontal component of the Llolleo record from the 1985 Chile Earthquake. For Specimen S-3, simulated loading was applied statically. For Specimen S-4 and SC-2; simulated loading was applied dynamically, and repeated three times.

8. Specimen S-3: Connection sustained static simulated earthquake loading without failure. Maximum plastic rotation demand on specimen was approximately 2%.

9. Specimens S-4 and SC-2: Connection sustained dynamic simulated earthquake loading without failure. Maximum plastic rotation demand on specimen was approximately 2%.

10. Tests conducted by Plumier not included in Table. Specimens consisted of HE 260A beams (equivalent to W10x49) and HE 300B columns (equivalent to W12x79). All specimens were provided with constant cut RBS. Beams attached to columns using fillet welds on beam flanges and web, or using a bolted end plate. Details available in Refs. 9 and 10.

11. Shaking table tests were conducted by Chen, Yeh and Chu [1] on a 0.4 scale single story moment frame with RBS connections. Frame sustained numerous earthquake records without fracture at beam-to-column connections.

12. Specimens LS-2 and LS-3 were tested using near field loading protocol. The specimen was subjected to peak pulses corresponding to 6% story drift ratio. Loading was repeated six times for LS-2 and four times for LS-3. The specimens eventually failed due to low cycle fatigue fracture at the narrowest section in the RBS.

13. Specimens DBBW and DBWW were cruciform t~,pe specimens with beams attached to each column flange. 14. Specimens DBBW-C and DBWW-C were cruciform type specimens with composite floor slab. Composite slab details:

96" wide slab; 2" ribbed metal deck (ribs parallel to beam) with 3.5" topping of normal weight concrete; concrete compressive strength at time of testing = 3600 psi for DBBW-C and 6800 psi for DBWW-C; slab reinforced with #4 Gr. 60 bars in each direction; 3.4" dia. shear studs spaced at 12"; first stud located at 36" from face of column (at end of RBS).

15. Specimens WG-1 to WG-4: Test report provided slightly conflicting data on location along length of beam where displacement was measured. Values of plastic rotation reported above are based on an estimated location for displacement measurements.

A-9

Notation: Fy.f = flange yield stress from coupon tests Fu_f = flange ultimate stress from coupon tests Fy_w = web yield stress from coupon tests Fu-w = web ultimate stress from coupon tests Lb = Length of beam, measured from load application point to face of column Lo = Length of column L~ = distance from face of column to start of RBS cut EBBS = length of RBS cut FR = Flange Reduction = (area of flange removed/original flange area) xl00

(Flange Reduction reported at narrowest section of RBS) ep = Maximum plastic rotation developed for at least one full cycle of loading, measured with respect to the face of the column (based on occurrence

of fracture or based on end of loading)

References: [1] Chen, S.J., Yeh, C.H. and Chu, J.M, "Ductile Steel Beam-to-Column Connections for Seismic Resistance," Journalof Structural Engineering, Vol.

122, No. 11, November 1996, pp. 1292-1299. [2] Iwankiw, N.R., and Carter, C., "The Dogbone: A New Idea to Chew On," Modern Steel Construction, April 1996. [3] Zekioglu, A., Mozaffarian, H., and Uang, C.M., "Moment Frame Connection Development and Testing for the City of Hope National Medical Center,"

Building to Last- Proceedings of Structures Congress XV, ASCE, Portland, April 1997. [4] Zekioglu, A., Mozaffarian, H., Chang, K.L., Uang, C.M. and Noel, S., "Designing After Northridge," Modem Steel Construction, March 1997. [5] Engelhardt, M.D., Winneberger, T., Zekany, A.J. and Potyraj, T.J., "Experimental Investigation of Dogbone Moment Connections," Proceedings; 1997

National Steel Construction Conference, American Institute of Steel Construction, May 7-9, 1997, Chicago. [6] Engelhardt, M.D., Winneberger, T., Zekany, A.J. and Potyraj, T.J., "The Dogbone Connection, Part II, Modem Steel Construction, August 1996. [7] Popov, E.P., Yang, T.S. and Chang, S.P., "Design of Steel MRF Connections Before and After 1994 Northridge Earthquake," International

Conference on Advances in Steel Structures, Hong Kong, December 11-14, 1996. Also in: Engineering Structures, 20(12), 1030-1038, 1998. [8] Tremblay, R., Tchebotarev, N. and Filiatrault, A., "Seismic Performance of RBS Connections for Steel Moment Resisting Frames: Influence of

Loading Rate and Floor Slab," Proceedings, Stessa '97, August 4-7, 1997, Kyoto, Japan. [9] Plumier, A., "New Idea for Safe Structures in Seismic Zones," IABSE Symposium - Mixed Structures Including New Materials, Brussels, 1990. [10] Plumier, A., "The Dogbone: Back to the Future," Engineering Journal, American Institute of Steel Construction, Inc. 2nd Quarter 1997. [11] Uang, C.M., Unpublished preliminary test reports for SAC Phase 2 RBS tests, University of California at San Diego, December 1998 and February

1999. [12] Engelhardt, M.D. and Venti, M., Unpublished preliminary test reports for SAC Phase 2 tests, University of Texas at Austin, 1999. " [13] Fry, G., Unpublished preliminary test reports for SAC Phase 2 tests, Texas A & M University, 1999. [14] Unpublished report of connection proof tests for building construction project in southern California; project title withheld at request of building owner,

January, 1999.

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Documents

Design of Reduced Beam Section (RBS) Moment Frame Connection