Modern Steel Construction / November 2001

Hollow structural sec-tions (HSS) are beingused more and moreoften in building con-struction. A synergy

between this increased use and theadvances in connection design hasresulted in new HSS design specifica-tions and connection design aids.

There are two primary reasons touse HSS in building construction: stan-dard structural applications wheretheir use results in cost savings (typicalapplications are columns or beams thatare unbraced and/or bi-axially loaded),and architectural or aesthetic appear-ance. Both of these applications are pri-marily the result of the closed shape ofthe HSS. This closed shape is also whatmakes HSS connections unique andoften challenging to design and fabri-cate. When HSS are used for architec-tural applications and appearancebecomes a design constraint, it be-comes even more challenging to designa connection that is economical to fab-ricate and erect.

Almost all of the current HSS con-nection design methods and specifica-tions are based on research doneoutside of the United States. A group ofinternational HSS producers, CIDECT,gathered information and issuedMonograph No. 6 in 1986 giving a stateof the art approach to welded HSS con-nections. The International Institute ofWelding (IIW) has been working withCIDECT to develop HSS design knowl-

Overlapped K ConnectionFigure 1.

Gapped K Connection

Stepped ConnectionFigure 2.

Larry Kloiber, P.E.

edge into a specification format, andtheir design recommendations formthe basis of most specifications. NorthAmerican research on HSS connectionshas been lead by Professor J. A. Packerof the University of Toronto. The Cana-dian Institute of Steel Constructionpublication, “Hollow Structural Sec-tion Connections and Trusses—A De-sign Guide” by J. A. Packer and J. E.Henderson, is regarded as the primaryreference for design information onHSS connections.

Until very recently the only specifi-cation for design of HSS connections inthe United States was found in theAmerican Welding Society (AWS) D1.1Structural Welding Code—Steel. TheAWS provisions for design of tubularconnections are similar to IIW/CIDECTrecommendations.

The AISC Specification for The De-sign of Steel Hollow Structural Sec-tions published in 1997 along with theAISC “Hollow Structural SectionsConnection Manual” was a major steptowards bringing comprehensive HSSconnection design information to engi-neers in the United States. The AISCHSS Connections Manual provides de-sign procedures and a series of connec-tion design tables for a variety of HSSconnections. Included are simple shearconnections, moment connections, ten-sion and compression connections, capand base plate connections along withwelded truss connections. HSS cross-section properties in the manual are

based on the actual wall thicknessbeing 93% of the nominal thickness perthe AISC Specification Section 1.2.2.This reduction in thickness, which ispermitted by ASTM A 500, has becomestandard practice.

Connections to HSS almost alwaysrequire some type of welded connec-tion. Even where bolts are used tomake the field connection, it is usuallynecessary to weld some type of detailmaterial to the HSS. The strength of aconnection welded to the face of anHSS is primarily a function of the geo-metric parameters of the connectionand cannot be increased by simply in-creasing the weld strength. However,the weld strength must be adequate toaccommodate the highly non-uniformload distribution along the weld lengthdue to the varying stiffness across thewall of the HSS. Unlike open sections,such as wide flange shapes, it is notpossible to add a stiffener to the back-side of the connection to provide amore uniform load transfer. Instead,the weld must be designed to ensurethat yielding and redistribution willoccur in the base metal, thereby allow-ing the weld to develop the requireddesign load before reaching its ulti-mate breaking strength.

Direct Welded ConnectionsThe most common direct welded

connections are axially loaded trusstype members forming joints in a T, Yor K configuration. The connection

strength, as noted above depends onthe geometric parameters of the mem-bers, and therefore it is important thatthe designer choose member sizes withthe connection design as part of the se-lection criteria. The main member orchord should always be checked tomake sure the wall thickness is ade-quate for all of the connection forcesdelivered by branch member.

The joint configuration for K typetruss connections can be classed asgapped or overlapped and as steppedor matched as shown in Figures 1, 2 &3. The AISC HSS Specification cur-rently only provides design rules forgapped type connections. Overlappedconnections should only be consideredwhere branch loads are so large theyrequire some type of load transfer di-rectly between branches. Gapped con-nections are more economical tofabricate because they require only asingle bevel cut and the gap allows fitup tolerance. The gap also permits theentire truss to be assembled beforewelding, which helps to reduce cost.

Matched connections require aflare-bevel-groove weld that, depend-ing on the corner radius of the chord,may require some type of steel backingor a special backing weld to close theroot of the joint. Flare-bevel-groovewelds are relatively inefficient becauseof the limited effective throats allowedfor AWS pre-qualified joints. It is possi-ble to qualify these welds with a muchlarger effective throat by following the

Matched ConnectionFigure 3.

Single–Plate ConnectionFigure 4.

November 2001 / Modern Steel Construction

procedure qualification provisions ofD1.1. Procedure qualification should beconsidered if the effective throat of thepre-qualified joint is not adequate forthe design loads.

Stepped connections are more eco-nomical to fabricate than matched con-nections because they permit the use offillet welds. The chord and branchmember sizes should be selected toallow the fillet welds to be placed onthe flat of the chord. The chord widthshould be approximately the branchwidth plus four times the chord wallthickness plus two times fillet weldsize. If the leg of the fillet weld does ex-tend slightly onto the corner radius, itis still possible to get the required effec-tive throat by modifying the weld pro-file.

Direct welded HSS connectionsmust have enough strength to providefor the non-uniform distribution ofload along the weld length at the re-quired load. This can be accomplishedby either providing welds that will de-velop the yield strength of the branchbefore rupturing or by sizing weldsusing the “effective weld length” con-cept in the AISC HSS Specification sec-tion 9.2. The design procedure fordeveloping the branch strength is cov-ered in the AWS D1.1 provisions. Theeffective weld length procedure is pre-ferred because it is based on actualmember loads and results in more eco-nomical weld sizes, but it is applicableonly to rectangular HSS connections.

The effective weld length formulaswere developed by Packer and arebased on extensive physical tests ofHSS joints. More recent work by Packerindicates that the AISC provisions areconservative because they are based onthe rectangular dimensions of the HSSin calculating the effective length in-stead of the actual weld length. TheAISC HSS Specification also does notpermit the use of the fillet weld in-crease in strength due to the directionof the load that is allowed by AISCLRFD Specification Appendix J2.4,while Packer’s work again indicatesthis increase can be used.

The weld strength required is deter-mined by dividing the design load inthe member by the effective length cal-culated per the appropriate formula inSection 9.2. The same strength weld isrequired around the entire perimeter of

the member even though the effectivelength maybe substantially less. Filletwelds the preferred weld detail to usefor these joints. When detailing filletwelds on surfaces that do not intersectat 90 degrees the weld size or joint de-tail should be modified to provide therequired effective throat. Completejoint penetration (CJP) welds should beavoided in these truss type joints. Ex-cept for joints subject to cyclic loading,CJP details are not required. Truss typejoints because of the geometry do notpermit the use of steel backing. CJPwelds that are made without backingrequire special welder qualifications,special fit-up and require special in-spection procedures. It is possible todevelop the required connectionstrength for most HSS using filletswelds and/or partial joint penetrationwelds.

The preferred HSS truss joint isgapped, stepped and fillet welded withthe welds sized using the “effectiveweld length” procedure.

Welded / Bolted ConnectionsThe most important advance in HSS

connection economy has been the de-velopment of the HSS single plateframing connection by Dr. Donald RSherman of the University of Wisconsinat Milwaukee. The single plate connec-tion shown in Figure 4 is the most eco-nomical way to connect a wide flangebeam to HSS columns and beams. Thefabrication cost of a single plate con-nection is less than 1/4 the cost of the oldthrough plate connection and substan-tially less than a WT or a double angleconnection. Single plate connectionscan be used as long as the wall of theHSS is not classed as a slender element.For A500 Grade B material the b/t ratiomust be 35 of less.

When the reaction is larger thanwhat a single plate connection cancarry, the next choice is usually a dou-ble angle connection shop welded tothe HSS. Stiffened seats welded to thewall of the HSS can also be used.

The AISC HSS Connections Manualalso gives a procedure for sizing col-umn cap plates supporting beams andjoist girders. If the reaction is within theface of the HSS, the load is delivered indirect bearing to the wall of the HSS,and the controlling limit state is eitherthe local yielding or crippling of the

wall of the HSS. Increasing the platethickness will only increase the bearinglength slightly and result in only a min-imal increase in strength.

Through-bolting of HSS is not verypractical. Holes in the HSS have to bemanually drilled, the bolt lengths re-quired exceed normal stock lengthsand the bolt is difficult to install. Whendesigning through bolts, it is importantto remember that the snug tight re-quirements of the AISC Specificationrequires all plies in the connection to bein firm contact. This is not possible withan HSS unless some type of interior fillis used. The AISC HSS Specificationtreats through bolts as pin type connec-tions with reduced bearing values. It issuggested through bolts be used onlyfor erection aids with oversize holes orslots to allow for easier field fit-up.

Architecturally Exposed Connections

When HSS are used for aesthetic orarchitectural reasons, the connectionsare often specified to be welded con-nections. Connections that are fieldwelded usually require some type oftemporary bolted erection aid (connec-tion) to locate and support the memberuntil it can be welded. These connec-tions, including the one shown in Fig-ure 5, are the means and methods ofconstruction and not part of the perma-nent structure. Therefore, the structuralengineer does not normally designthese connections or may not even in-dicate they are required. This can leadto possible conflicts when the fabrica-tion and erection is bid separately. Thedesign drawings should at least indi-cate that erection aids are to be pro-vided as necessary to allow for theerection and field welding. If it is possi-ble to locate these connections so theycan remain and be part of the finishedstructure the structural engineer shouldshow this on the connection details.

Project specifications should refer-ence the Architecturally Exposed Struc-tural Steel (AESS) provisions in theAISC Code of Standard Practice. Oftenproject specifications have provisionsthat require “all welds to be ground.”Grinding welds, especially fillet welds,is expensive and usually does not im-prove the appearance. The AESS provi-sions require “reasonably smooth anduniform welds.” Where multi-pass fil-

let welds are used and a special ap-pearance is required, it is more eco-nomical to dress the surface with aplastic filler material like those used inauto repair. The use of a joint mockupis often helpful in establishing reason-able appearance standards for exposedwelds. The architect should be requiredto view the mockup from the same dis-tance he or she will see the finishedweld in the structure. This samemockup can be sectioned to verify theeffective throats of the various weld de-tails.

HSS Connection Design ReferencesThere have been significant ad-

vances in HSS connection design in re-cent years that make the design processmore user-friendly and the fabricationand erection more economical. TheHSS connection manuals published byCISC and AISC can be ordered fromtheir web sites: www.cisc-icca.ca andwww.aisc.org. In addition to these keyreferences, structural engineers design-ing truss type connections m ight find anew computer program by the Univer-sity of Toronto Tube Group helpful.This Windows-based program covers awide variety of truss connections inmetric or U.S. customary units. Formore information, e-mail Professor Jef-frey Packer at packer@ecf.utoronto.ca.

Larry Kloiber P.E. is Vice President ofEngineering for LeJeune Steel Company inMinneapolis, MN.

HSS to HSS Erection ConnectionFigure 5. Note: Erection connection is only for locating and supporting HSS member.All field welds must be completed before any other load is applied. After field weldingHSS, remove plate connection and grind flush.

HSS CONNECTION TIPS• The single plate connectionshown in Figure 4 is the most eco-nomical way to connect a wideflange beam to HSS columns andbeams.• The design drawings should atleast indicate that erection aidsare to be provided as necessary toallow for the erection and fieldwelding.• Grinding welds, especially filletwelds, is expensive and usuallydoes not improve the appearance.

November 2001 / Modern Steel Construction

HSS ConnectionsSHORT COURSEat NASCC 2002—Seattle

If you need to know more about the artand science of connection design for hol-low structural sections and steel pipe, the2002 NASCC will feature a short courseproviding the practical knowledge youneed. Whether you design connectionsor evaluate the connections shown onshop drawings, this course will help youunderstand the fundamentals—and pecu-liarities—of HSS connection design, in-cluding welding, bolting and connectingelements. You’ll also gain insight into thedetailed design of shear, moment, brac-ing, truss (including HSS-to-HSS) andmany other connections in tubular steelstructures.

When: Saturday, April 27, 8:00 - 1:00Where: NASCC 2002—Seattle, WASpeaker: Donald R. Sherman, Ph.D.,P.E., University of Wisconsin, MilwaukeeRegistration and Cost: Visitwww.aisc.org or see the NASCC advanceprogram in the December issue of Mod-ern Steel Construction.