Welding Issues for Seismic Design Requirements

1 Technical BulletinWinter 20081 Welding Issues for Seismic Design RequirementsMetal Building SystemsbyW. Lee Shoemaker, , of Research & EngineeringBackgroundWelding Requirements for buildings designed to resist seismically-induced energy have undergone a variety of changes in thepast dozen years. With the publication of several specifications in 2005, some stability has now emerged. This bulletin isintended to outline and clarify the Welding Issues for Seismic Design that are specific to metal building systems. Applicable Codesand StandardsGeneral WeldingRequirementsWelding Requirements arestipulated in a number ofdocuments, including:(1) AISC Specification for SteelBuildings (AISC 360-05)(2) AISC Seismic Provisions forStructural Steel Buildings(AISC 341-05)(3) AISC Prequalified Connectionsfor Special and IntermediateMoment Frames for SeismicApplications (AISC 358-05)(4) AWS Structural Welding Code Steel (AWS ) (asreferenced by AISC 341-05)(5) AWS Structural Welding Code Seismic Supplement (.)

2 2005)Also, FEMA 353 RecommendedWelding Requirements for theseismic load resisting system (SLRS)depend on whether the weld iscategorized as a demand critical weld,or is just part of the SLRS. The seismicload resisting system is defined as theassembly of structural elements in abuilding that resists Seismic loads,including struts, collectors, chords,diaphragms and trusses. Demandcritical welds are those that have beendetermined both to be subject to yield-level stresses and to have the potentialto cause catastrophic results if theyfailed.

3 Demand critical welds caninclude complete-joint-penetrations(CJP) groove welds for column splicesin ductile frames1, CJP groove weldsfor beam flange to column flangeSpecifications and Quality AssuranceGuidelines for Steel Moment FrameConstruction for Seismic Applications,June 2000, was often referenced asa resource before the above codesand standards incorporated the appro-priate information. This is still some-times referenced separately, but shouldnot supersede these newer 341 and AISC 358 aresupplemental to the requirementsof AISC 360, while AWS issupplemental to AWS Therequirements of the supplemental doc-uments add to and are not in lieu of,the Requirements of the documents theysupplement.

4 AWS contains provi-sions that correspond to similar provi-sions in FEMA 353 and AISC 341 Appendix W. AWS may offeradvantages over FEMA 353 and AISC341 but it may be used only if referredto in a project specification or withapproval of an engineer. Continued on Page 21 For ductile moment frames (IMF and SMF) the strong column is necessary to force flexural yielding in the weak beam , which is ensured by the AISC341 beam and column proportioning Requirements . For the OMF, which is intended for minimal inelastic deformations only, the weak column strongbeam concept is permitted.

5 Hence, the demand critical weld does not guarantee better performance as long as the OMF column strength can be gov-erned by limit states other than flexural connections, and beam webto column flange CJP groove welds inmoment connections (See AISC 341 Section ).Whether a weld is demand criticalor not depends in part on the type ofseismic system involved, such as anordinary moment frame (OMF),intermediate moment frame (IMF) orspecial moment frame (SMF). In addition to the other applicableseismic Design criteria listed in of AISC 341, the classification of aweld is to be specified by the Engineerin the Contract Documents.

6 This doesnot give the Engineer the authority todisregard the code Requirements , butit is intended to makes it clear to thefabricator which welds are demandcritical. Therefore, from this specifiedcriteria, welds can be categorized asone of the following:(1) Welds that are not part of theSLRS (AWS applies)(2) Welds that are part of theSLRS, that are notdemandcritical (AISC 341, Section in addition to )(3) Welds that are part of the SLRS that aredemand critical (AISC341, Section applies inaddition to the Requirements ofAISC 341 Section andAWS )The typical metal building systemshould have only a few select weldsthat are demand critical.

7 Guidanceis provided below regarding theclassification of common Critical Weldsin Metal BuildingsIn typical metal building gableframes, the only welds that wouldbe considered demand critical in anOMF are CJP groove welds of theend-plates at the haunch IMF s, a prequalified connectionwould have to be used for this con-nection, but the bolted end-plateconnections in AISC 358 do not haveany additional demand critical weldrequirements other than what AISC341 requires for OMF s. Other weldrequirements of the web to flange inan IMF given in AISC 358 are dis-cussed separately in this paper.

8 Notethat fillet welds used to weld the end-plate to the beam end would notbeconsidered demand critical , end-plate connections elsewherein the frame, the ridge, are notdefined as demand Filler MetalRequirementsWeld filler metal Requirements canbe categorized as follows:(1) Welds that are part of theseismic load resisting systemthat are notdemand critical(AISC 341, Section ) shallhave a minimum CVN tough-ness of 20 ft-lb at 0 F as deter-mined by AWS A5 classificationor manufacturer MBMA members use gasmetal arc Welding (GMAW) forsemi-automatic Welding , andautomatic submerged arc weld-ing (SAW) for web-to-flangebuilt-up sections.

9 Other weldingprocesses are possible andacceptable, but these are themost common GMAW, a typical fillermetal is ER70S-3. For SAW,Lincoln 781/L50 or 781/L61 iscommonly used. These fillermetal combinations satisfy shielded flux cored arcwelding (FCAW-S) is not com-monly used for metal buildingfabrication today. However, inwelded joints where FCAW-Sis mixed with other processesincluding FCAW-G, the fillermetal combination must betested in accordance with (2) Welds that are part of theseismic load resisting systemthat are demand critical (AISC341, Section )

10 Shall havea minimum CVN toughness of20 ft-lb at -20 F as determinedby AWS A5 classification ormanufacturer certification, and40 ft-lb at 70 F as determinedby AISC 341 Appendix X, Annex A, or otherapproved previously discussed, theonly applicable welds that aredemand critical in an OMF orIMF are the CJP groove welds ofthe end-plates at the haunchconnection. In the metal build-ing industry, this weld wouldtypically be made with GMAW,and the typical filler metal usedis classified as ER70S-3. TheAWS classification forER70S-3 requires CVN tough-ness of 20 ft-lb at 0 F, not the-20 F required by AISC , AISC 341 also permits manufacturer certification asan acceptable alternative todemonstrate that the fillermetal is capable of achieving20 ft-lb at -20 F in a filler metalclassification test.