The Challenge of Welding Jumbo Shapes - · PDF file3 Welding Innovation Vol. X, No. 1, 1993...

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The Challenge of Welding Jumbo Shapes Reprinted Articles from Welding Innovation Magazine The James F. Lincoln Arc Welding Foundation by Omer W. Blodgett, Sc.D., P.E. and Duane K. Miller, Sc.D., P.E.
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  • The Challenge of WeldingJumbo ShapesReprinted Articles from Welding Innovation Magazine

    The James F. Lincoln Arc Welding Foundation

    by Omer W. Blodgett, Sc.D., P.E. and Duane K. Miller, Sc.D., P.E.

  • 2Welding Innovation Vol. X, No. 1, 1993

    Australia and New ZealandRaymond K. RyanPhone: 61-29-772-7222Fax: 61-29-792-1387

    CroatiaProf. Dr. Slobodan KraljPhone: 385-1-6168306Fax: 385-1-6157124

    HungaryDr. Gza Gremsperger Phone: 361-156-3306

    IndiaDr. V.R. KrishnanPhone: 91-11-247-5139Fax: 91-124-321985

    JapanDr. Motoomi OgataPhone: 81-565-48-8121Fax: 81-565-48-0030

    Peoples Republic of ChinaDai Shu HuaPhone: 022-831-4170Fax: 022-831-4179

    RussiaDr. Vladimir P. YatsenkoPhone: 07-095-238-5543Fax: 07-095-238-6934

    United KingdomDr. Ralph B.G. YeoPhone & Fax:44-1709-379905

    3 Part I: The AISC Specifications

    6 Part II: Increasing Ductility of Connections

    10 Part III: Case Study

    12 Consider Welded Vs. Bolted Connections for Jumbo Sections

    The serviceability of a prod-uct or structure utilizing thetype of information presentedherein is, and must be, thesole responsibility of thebuilder/user. Many variablesbeyond the control of TheJames F. Lincoln Arc WeldingFoundation or The LincolnElectric Company affect theresults obtained in applyingthis type of information.These variables include, butare not limited to, weldingprocedure, plate chemistryand temperature, weldmentdesign, fabrication methods,and service requirements.

    INTERNATIONAL ASSISTANT SECRETARIES

    Volume XNumber 1, 1993

    EditorDuane K. Miller, Sc.D., P.E.

    Assistant EditorR. Scott Funderburk

    The James F. Lincoln Arc Welding Foundation

    Omer W. Blodgett, Sc.D., P.E.Design Consultant

    The Challenge of Welding Jumbo Shapes

    Visit Welding Innovation online at http://www.lincolnelectric.com/innovate.htm

  • 3 Welding Innovation Vol. X, No. 1, 1993

    The Challenge of Welding Jumbo Shapes

    Part I: The AISC SpecificationsBy Duane K. Miller, P.E.

    Welding Design EngineerThe Lincoln Electric Company, Cleveland Ohio

    IntroductionIn 1989, the American Institute of SteelConstruction developed specifications forwelding jumbo shapes. The next three arti-cles will review the AISC specifications, dis-cuss additional welding engineering principlesand structural details that will contribute to thesuccess of similar projects, and present acase study of a recent successful projectwhich utilized these principles extensively.

    Special Problems ofWelding Jumbo ShapesDuring fabrication and erection of weldedassemblies that utilized Group 4 and 5shapes, commonly called jumbo sections,cracking problems were experienced on anumber of projects. Particularly alarmingwere complete, through-section cracks thatoccurred in the tension cords of large truss-es. The failures were classic brittle-type frac-tures that occurred in the complete absenceof service loads. The typical crack wouldbegin in the region of the weld access holeand propagate through the web, or theflange, or both. The fracture was usually inthe base metal, with the weld unaffected.Fortunately, since these incidents occurredduring construction, their impact was mini-mized. Nevertheless, these experiencescaused many engineers to look to alternatematerials or revert to bolted connectionswhen jumbo shapes were used.

    Group 4 and 5 shapes are very heavy rolledsections that weigh up to 848 pounds per lin-ear foot (see Figure 1). During the initialsolidification of the ingot used to make theseshapes, it is possible for carbon and otheralloys to segregate, causing an enriched con-centration of elements in the center of theingot. Upon rolling, the outer surfaces of theshape being formed will be cooled by the millrolls and other external cooling methods. In

    addition, the surfaces of the shape receivesignificant mechanical working, improving thetoughness of these surfaces, but the centerregion receives minimum mechanical work-ing. As shown in Figure 2, a cast structureis formed in the middle of the section whichmay exhibit poor notch toughness. Wherefailures have been experienced in the past,poor toughness in this region has been char-acteristic.

    The greatest problems with welding on jumboshapes occurred when these materials, origi-nally contemplated for compression (or col-umn) applications, were used in tensionapplications. Since the failures occurredbefore the structures were subject to serviceloads, the difference was not inherent to theapplication, but rather to the type of welddetails used in the two types of applications.In primary tensile applications, CompleteJoint Penetration (CJP) groove welds typical-ly are required. For compression applica-tions, Partial Joint Penetration (PJP) groovewelds generally are sufficient. When CJPgroove welds are used, weld access holes(colloquially known as rat holes) arerequired. For PJP groove welds, they arenot. The presence or absence of weldaccess holes, and the difference in residualstresses experienced by CJP welds (whichhave greater weld metal volume than PJPgroove welds) explain the difference inbehavior between tension and compression

    applications. The PJP groove weld may notintersect the as-cast core structure in thecenter of the section, while the weld accesshole used with CJPs automatically intrudesinto this region.

    A common method of preparing the variousrolled sections for welding is to use the oxyfuel thermal cutting process, which generatesa change in microstructure. The surface maybe enriched in carbon content, and the hotsteel on the surface of the cut is rapidlycooled by the conduction of heat into the sur-rounding steel. As a result, a small, thinlayer of relatively hard, brittle microstructuremay form. Under some conditions, micro-cracks have resulted in this zone. Goodworkmanship is also required in this area. Insome cases, ragged edges resulted in crack-like notches in the weld access hole region.Further complicating the cutting process isthe semicircular nature of the weld accesshole, where good manual dexterity isrequired in order to create a uniform surface.Finally, as the cut approaches the region ofthe web-to-flange interface, the natural radiusthat occurs in rolled shapes makes precisecutting difficult. In many examples involvingfracture, poor workmanship resulted in inade-quate preparation of the weld access holes.

    When welds cool from elevated tempera-tures, they must shrink in size due to thethermal contraction that takes place.

    Figure 1. Group 4 and 5 shapes can weighup to 848 pounds per linear foot.

    Figure 2. A cast structure is found in themiddle of the section.

  • Welding Innovation Vol. X, No. 1, 1993 4

    As the hot, but already solidified, weld metalshrinks in size, it induces shrinkage strainson the surrounding materials. These strainsinduce stresses that cause localized yielding.As the weld metal cools to near-room tem-perature, the remaining strain may be insuffi-cient to cause yielding, but will result inresidual stresses that are present after weld-ing is complete. For example, a weld that isused to join flanges will establish a residualstress pattern that is transverse and longitu-dinal to the direction of welding. The webweld will similarly set up a longitudinal andtransverse shrinkage stress. Surrounding theregions of high residual tensile stress, therewill be a region of residual compressivestress. When weld access holes are small insize, the residual stresses from these twowelds can combine and develop into a triaxi-al state of stress. Under these conditions,the steel may be unable to exhibit its normalductility, although the same material underuniaxial conditions may behave in a veryductile fashion. The formation of this high tri-axial stress in the region of the web-to-flangeinterface is the driving force behind the initia-tion of these brittle cracks.

    Figure 3 illustrates the interaction of weldtype (CJP vs. PJP), weld access holes, andthe cast core region. In the top figures, theCJP preparation automatically intersects theregion of the shape with questionablemicrostructure. For the PJP, the absence ofaccess holes, plus the reduced depth ofbevel, minimizes or eliminates the amount ofcore region that is intersected by the cuttingprocess. As a result, weld metal, and theshrinkage stress it will induce, does notdirectly act on the core region.

    Fracture mechanics methods may be used todetermine the required notch toughness toprevent brittle fracture. In the case of thejumbo section failures, the cracks located inthe weld access holes were of sufficient size,and the residual stresses from welding of asufficient level, that they exceeded the resist-ing force, or the fracture toughness, of thebase material, particularly in the cast coreregion. To ensure reliable fabrication, theAISC Task Group addressed all three issues:notch toughness of the base material, qualityof the weld access holes, and a reduction inthe residual stresses developed by examin-ing the geometry of the weld access hole.

    The AISC ResponseThe AISC task group set up to study thismatter established, and the SpecificationCommittee adopted, a series of controls topermit problem-free welding of jumbo sec-tions. These were initially published in 1989as Supplements 1 and 2 to the AISC SteelConstruction Manual. They have now beenincorporated into the ninth edition of the ASDManual. The first was a requirement that thebase metal exhibit a minimum notch tough-

    ness of 20 foot-pounds at +70F. It isrequired that the Charpy specimens be takenfrom the web-to-flange interface, the regionexpected to have the poorest toughness. Avariety of methods may be employed toimprove the notch toughness in this area,including the use of semi-killed steel, fine-grain practice, and special cooling tech-niques.

    To provide additional resist