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IntroductionAccording to the recent Indianstandard code on earthquakeresistant design of structures, morethan 60-65% of the area of ourcountry falls under seismic zone IIIor above. This underlines theimportance of seismic detailing. Inany structure, the joints assumemore importance and have to bedetailed carefully so that they areable to withstand the inelastic jointrotations (in the order of 0.04radians) and drift that may resultduring an earthquake. The detailingof reinforced concrete structureshave been covered adequately inthe Indian codes. However, untilrecently such detailing of joints insteel structures was not covered inthe Indian code on steel structures.Though the recent version of thecode, IS 800:2007, containsprovisions for design and detailingfor seismic loads, it does notsuggest the type of connections

which are suitable for high orintermediate seismic zones.

Comparison of the damages inrecent earthquakes in Haiti andChile has shown that strictadherence to codal provisions andquality constructions could preventexcessive damages to constructionsand human loss. The Northridgeearthquake in California duringJanuary 1994 and the Kobeearthquake in Japan during 1995have shown that even well designedconnections are susceptible tosevere damages. Based on theresearch that followed theseearthquakes, The American Instituteof Steel Construction (AISC) hasstipulated that connections inspecial moment frames (SMF) orIntermediate moment frames (IMF)should be qualified for use bytesting. However, as testing ofconnections is time-consuming andexpensive, it has also specified afew pre-qualified connections, which

have been tested and found to besatisfactory (FEMA 355D, 2000),Gross et al, 1999). Theseconnections may be adopted inIndia also for better performance instrong or intermediate earthquakes.

Damages to Beam-column ConnectionsDuring EathquakesSubsequent to the Northridge andKobe earthquakes, it wasdetermined that some damage tomoment – resisting framesoccurred at the beam-columnconnections. These connectionsexperienced rotation levels wellbelow the plastic moment capacityof framing members. Failuresincluded non-ductile fractures ofbottom beam flange-to-columnflange complete-joint-penetration(CJP) groove welds, whichpropagated into the adjacent columnflange and web and into the beam

Though the recent version of the code, IS 800:2007, contains provisionsfor design and detailing for seismic loads, it does not suggest the type ofconnections which are suitable for high or intermediate seismic zones.Connections play an important role in seismic resistance. The Northridgeearthquake in California during January 1994 and the Kobe earthquakein Japan during 1995 have shown that even well designed connectionsare susceptible to severe damages. After an extensive testing initiated bythe Federal Emergency Management Agency, USA, the AISC has suggesteda few pre-qualified moment connections to be used in high or intermediateseismic zones. A brief description of these connections is provided for thebenefit of designers.

Dr. N. Subramanian, ConsultingEngineer, Gaithersburg, USA

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bottom flange. This failure wasaccompanied in some instances bysecondary cracking of the beamweb shear plate and failure of thebeam top flange weld. The factorsthat contributed to the damageinclude the following (FEMA, 2000):♦ Stress concentration at the

bottom flange weld, due to thenotch effect produced by backingstrips left in place,

♦ Poor welding practices, includingthe use of weld metal of lowtoughness

♦ uncontrolled deposition rates♦ The use of larger members than

those previously tested or the useof higher strength girders,

♦ Less system redundancy andhigher strain demands onconnections,

♦ Lack of control of basic materialproperties (large variation ofmember strength from theprescribed values)

♦ Inadequate quality control duringconstruction, and

♦ The tri-axial restraint existing atthe center of beam flanges andat the beam-column interface,which inhibits yielding.

In an attempt to ensuresatisfactory seismic performance,stringent specifications wereimposed on fully restrained momentconnections. Subsequent to theearthquake, a multi-billion dollarresearch was conducted for over10 years, to understand thebehaviour of such beam-to-columnconnections. This research resultedin the development of currentdesign provisions for momentresistant frames, prescribed in AISC341-05 (Seismic provisions forstructural steel buildings, AmericanInstitute of Steel Construction). Inaddition, AISC has developedanother American NationalStandards Institute approvedstandard, AISC 358-05, whichpresents materials, design,detailing, fabrication, and inspectionrequirements for a series of pre-qualif ied moment connections.AISC updates and reissues this

portions of the beam flanges areremoved in a pre-determinedfashion, adjacent to the beam-column connection, as shown inFig.1. In such a connection, yieldingand plastic hinges are forced toform away from the connection atthe reduced section of the beam.

The effect of dogbone is similarto that of cover plate connections.With cover plates the connection ismade stronger than the beam bystrengthening the connection. In thedogbone, the connection iseffectively made stronger than thebeam by weakening the beam.While producing the same effect ofcover plates, the dogboneconnection can be constructed withrelatively simpler details, resultingin a more reliable and economicsolution. Moreover, the strong-column and weak-beam design caneasily be achieved. The earliestapplication of dogbone connectionwas made in 1969 (Iwankiw andCarter, 1996, Engelhardt et al,1998).

However, the reduction in flangearea may reduce the stiffness ofbeam flange and may increase thesusceptibility of lateral torsionalbuckling in the reduced section.

Figure 1: Reduced beam section (RBS) moment connection (a) connection,(b) details of reduced beam flange (Ref.4,5)

standard from time to time, as andwhen additional research results areavailable. The draft AISC 358-2010contains a number of pre-qualifiedconnections and are discussedbriefly here.

Pre-qualified MomentConnectionsAISC 358-2010 gives the followingpre-qualified connections:1. Reduced–Beam section

connection2. Bolted un-stiffened and Stiffened

Extended End-Plate MomentConnections

3. Bolted Flange Plate (BFP)moment connection

4. Welded Un-reinforced Flange-Welded Web Moment Connection(WUF-W)

5. Kaiser Bolded Bracket (KBB)Moment connection

A brief discussion about thesejoints is given below. More detailsabout them and their methods ofdesign may be found from AISC358-2010.

Reduced–Beam SectionConnectionIn reduced beam section (RBS)moment connection (also known asthe ‘dog bone’ connection), some

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Hence additional lateral bracingsmay be provided in these locations.Note that various shaped flangessuch as straight cut, taper cut, arccut, and drilled flanges have beentested and the arc cut was found toprovide favorable results.

Bolted Un-stiffened andStiffened Extended End-Plate MomentConnectionsBolted end plate connections aremade by welding the beam sectionto an end plate which is in-turnbolted to the column flange. Threetypes of these connections are pre-qualified by AISC 358. It gives

equations to check the various limitstates of this type of connectionsuch as flexural yielding of thebeam section or end plate, yieldingof column panel zone, shear ortension failure of the end-platebolts, and failure of the variouswelded joints. These provisions areintended to ensure inelasticdeformation of the connection bybeam yielding.

Bolted Flange Plate (BFP)Moment ConnectionThese connections consist of plateswelded to column flanges andbolted to beam flanges as shownin Fig.3. Identical top and bottomplates are used. Flange plates are

connected to column flange byusing complete joint penetration(CJP) groove welds and beamflanges are connected to the platesby using high strength friction gripbolts. The web of the beam isconnected to the column flangeusing a bolted single-plate shearconnection, with bolts in short-slotted holes. In this connection,yielding and plastic hinge formationare designed to occur in the beamnear the end of the flange plates.The design procedure for this typeof connection is more complex thanother pre-qualified connections.

Welded Un-reinforcedFlange-Welded WebMoment Connection(WUF-W)Unlike other pre-qualif iedconnections, in the welded un-reinforced flange-welded web (WUF-W) moment connection, the plastichinge location is not moved awayfrom the column face. Rather, thedesign and detailing features areintended to allow it to achieveSpecial Moment Frame (SMF)performance without fracture. In thisconnection the beam flanges arewelded directly to the column flangeusing CJP groove welds. The beamweb is bolted to a single-plateshear connection for erection. Thisplate is used as a backing bar forwelding the beam web directly tocolumn flange using CJP grooveweld, which extends to the full depthof the web (that is, from weldaccess hole to weld access hole).A fillet weld is also used to connectthe shear plate to the beam web,as shown in Fig.4. A specialseismic weld access hole anddetailing, as shown in Fig. 4(b), arespecified for the WUF-W momentconnection, to reduce stress-concentration in the region aroundthe access hole,

Figure 2: Bolted un-stiffened extended end-plate (BUEEP) and bolted stiffenedextended end-plate (BSEEP) moment connections (Ref.4,5)

Figure 3: Bolted flange plate (BFP) moment connection (Ref.4,5)

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Kaiser BoltedBracket (KBB)Moment ConnectionIn Kaiser bolted bracket (KBB)moment connection, a cast steel(high-strength) bracket is fastenedto each beam flange and bolted tothe column flange as shown in Fig5. The bracket can be either boltedor welded to the beam. The bracketis proportioned to develop theprobable maximum momentstrength of the beam, such thatyielding and plastic hinge formationoccurs in the beam at the end of

Figure 4: Welded un-reinforced Flange – welded Web (WUF-W) moment connection(a) connection, (b) Detailing of connection (Ref.4,5)

bracket away from the columnflange. This connection is designedto eliminate field welding andfacilitate erection.

Several tests on this type boltedbracket connection were conductedat Lehigh University (Adan and Gibb,2009).Then it was patented with theUnited States Patent and TrademarkOffice by of Steel Cast ConnectionLLC. The advantage of usingcasting is that it will not have HAZissues or residual stresses thatwould be found in welds.

This bracket is available in USA,in various sizes and bolt patternsto match the demand required for

strength and ductility. The bracketis shop welded to the beam andfield bolted to the column. Thebrackets used in Wasatch propertyManagement CorporateHeadquarters building in Utah, USAare shown in Fig. 6. The use ofthese cast steel brackets resultedin a saving of $3,000 per joint, dueto the avoidance of completepenetration field welding, doublerplates, continuity plates and ultra-sonic testing and also from thereduced beam tonnage (Cartwright,2006).

The design procedure anddetailing requirements for these

Figure 5: Kaiser bolted bracket (KBB) moment connections (a) Beam welded tobracket, (b) beam bolted to bracket (Ref.4,5)

Figure 6: Use of Kaiser bolted bracketsin Wasatch property ManagementCorporate Headquarters building in

Utah, USA (Cartwright,2006)

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connections are given in AISC 358-2010.

A similar, but a bit complicated,field bolted cast modular connectoris under development and is shownin Fig. 7 (Sumer et al, 2007).

ConclusionThere seams to be an increasingearthquake activity throughout theworld. The recent earthquakes havedemonstrated that the damagesand loss of lives will be extensive ifthe buildings are not designed anddetailed properly. Though the recentversion of steel code containedprovisions for seismic design anddetailing, designers are not givenguidance to choose proper beam-to-column connections, in SMF andIMFs. After 10 years of extensiveresearch, initiated by FederalEmergency Management Agency,USA, the AISC has developed a fewpre-qualified connections, whichhave shown to provide the requiredamount of ductil ity. A briefdescription of such connections isgiven to aid the designers. Thedesign methods are discussed inAISC 385-05.

Figure 7: Typical configuration of cast modular connector (Sumer, et al 2007)

References:♦ FEMA 355D, State of the Art

Report on ConnectionPerformance, prepared by the SACJoint Venture for FederalEmergency Management Agency,Washington, D.C., 2000.

♦ http://www.sacsteel.org/♦ Gross, J.L., Engelhardt, m.D.,

Uang, C.-M., Kasai, K., andIwankiw, N.R., Modification ofExisting Welded Steel Momentframe Connections for SeismicResistance, STEEL Design Guideseries 12, American Institute ofSteel Construction, Inc., Chicago,IL, 1999.

♦ ANSI/AISC 358-05, Pre-qualifiedconnections for special andintermediate steel momentframes for seismic applicationsincluding Supplement No.1,American Institute of SteelConstruction, Inc., 2005. and DraftAISC 358-2010.

♦ Carter, C.J. and Grubb, K.A.,Prequalified moment connections(revisited), Modern SteelConstruction, Vol.50, No.1, Jan2010, pp. 54-57

♦ Iwankiw, N.R., and Carter, C.J.,The Dogbone: A new idea toChew on, Modern SteelConstruction, AISC, Vol. 36, No.4,April 1996

♦ Engelhardt, M.D., Winneberger,Zekany, A.J., and Potyraj, T.J.,Experimental Investigation ofDogbone Moment connections,Engineering Journal, AISC, vol. 35,No. 4, Fourth Quarter, 1998, pp.128-139

♦ Hamburger, R.O., Krawinkler, H.,Malley, J.O., and Adan, S.M.,Seismic design of steel specialmoment frames: A guide forpracticing Engineers, NEHRPSeismic design technical briefno.2, National Institute ofStandards and Technology,Gaithersburg, USA, June2009,33pp. (http://www.nehrp.gov/pdf/nistgcr9-917-3.pdf)

♦ www.steelcastconnections.com♦ Adan, S.M., and Gibb, W.,

Experimental Evaluation of Kaiserbolted Bracket Steel Moment-Resisting Connections,Engineering Journal, AISC, Vol. 46,No.3, Third Quarter 2009, pp. 181-196

♦ Cartwright,C., Wasatch PropertyManagement CorporateHeadquarters, The Newsletter ofthe Structural EngineersAssociation of Utah, Vol. XI, No.1,Sept 2006, pp.2-3

♦ Sumer, A., Fleischman, R.B., andHoskisson, B.E., Development ofa cast Modular Connector forSeismic-Resistant Steel MomentFrames, Part 1:Prototypedevelopment, Part 2: ExperimentalVerification, Engineering Journal,AISC, Vol. 44, No.3, Third Quarter,2007, pp. 195-231

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