DESIGN RULES FOR FILLET WELDS IN EUROCODE 3 AND AISC

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1 EUROSTEEL 2014, September 10-12, 2014, Nples, Itly DESIGN RULES FOR FILLET WELDS IN EUROCODE 3 AND AISC A.M. (Nol) Gresnigt Delt University o Technology, Fculty o Civil Engineering nd GeoSciences, The [email protected] INTRODUCTION The present design rules or illet welds in Eurocode 3 led to very thick illet welds or higher strength steels S420, S460 nd S690. Other design rules, e.g. the AISC speciiction [6] nd the Cndin stndrd CSA-S16-09 [8] give much smller illet weld sizes or these steel grdes. Another importnt dierence is tht the strength o the iller metl is directly tken into ccount in the design eqution, while in Eurocode 3 Prt 1-8 (EN , [3]) the strength o the iller metl is indirectly tken into ccount. EN prescribes mtching weld metl, while mny other codes lso llow undermtching weld metl or illet welds. Allowing undermtching weld metl or illet welds enbles the designer to choose n optiml combintion o prent metl, iller metl nd welding prmeters to comply with the requirements or ductility nd strength. A summry is given o previous nd present design rules nd the results o recent reserch. Proposls or modiiction o the Eurocode 3 design rules or illet welded joints re included, ocussing on high strength steel. 1 FROM IIW (1974) TO EUROCODE3 DESIGN RULES FOR FILLET WELDS In 1976, the Interntionl Institute o Welding (IIW) published its recommendtion "Design rules or rc welded connections in steel submitted to sttic lods" [1]. The rnge o ppliction o these IIW design rules is limited to stticlly loded welded connections in crbon nd low lloy steels hving minimum tensile strength less thn 600 N/mm 2, rtio o yield to ultimte strength 0,8 nd minimum elongtion 12%. The welds re ssumed to be mde by rc welding (i.e. covered electrode, gs shielded or submerged rc welding). During the drting o Eurocode 3, re-evlution ws crried out o the test results o the IIW interntionl test series, where more thn 700 tests were crried out in 11 countries. Also lter tests were considered in the dtbse. To extend the scope o Eurocode 3 to higher strength steel, reserch ws crried out on illet-welded connections in S460 [5], [13]. Design rules or S420 nd S460 were published in Annex D o ENV1993 [3]. In EN the scope o the min document ws extended to cover lso steels up to S460 nd the rules or welds in S460 were included in the min document. Becuse o the lrge sctter o the test results in S460 nd the limited number o tests, the resulting design rules give very thick illet welds [4], [13]. In recent yers severl reserch projects hve been perormed iming to develop design rules or structures in steel S690. This hs resulted in proposls or modiiction o EN nd EN The most importnt modiictions re the proposl to modiy the resistnce prt o the design eqution by including direct link to the weld metl strength, nd to llow undermtched weld metl. 2 DESIGN OF FILLET WELDS IN EUROCODE 3 Eurocode 3 gives two methods to clculte the design strength o illet welds: the directionl method nd the simpliied method.

2 2.1 Simpliied method ( in EN ) According to the simpliied method, beore clled the men stress method, the design resistnce per unit length is: Fw. Rd vw. d. with. vw d u / 3 (1) w where: vw.d = the design sher strength o the illet weld u = the nominl ultimte tensile strength o the weker prt joined M2 = prtil sety ctor or welded connections ( M2 = 1,25) w = correltion ctor (see Tble 1) M2 2.2 Directionl method ( in EN ) In the directionl method, beore clled the stress component method, the orces trnsmitted by weld re resolved into norml stresses nd sher stresses on the throt section o the weld. The norml stress // prllel to the weld xis does not need to be considered. The requirement is: u c 3 3 // nd 0,9 u (2) w M2 Mw ls weld t z x F end F kop ll b F side F zij l l t Fig. 1. End illet welds nd side illet welds. For end illet welds s in Fig weld weld u c weld nd // 0. With eqution (2) it ollows: 2 w M2 u weld w. u. end (4) w M2 2 For double end illet welds it ollows: Fend x t (5) 2 2 w. u. end w. u. end The gretest weld size is ound i in the connected plte x = y. In Tble 1 the required weld sizes re given or this cse. For side illet welds is 0 nd // weld. With eqution (2) it ollows: u weld w. u. side (6) w M2 3 Vlues or w.u.side nd w.u.end re given in Tble 1. (3)

3 Tble 1. Vlues o w, w.u.end, w.u.side or steels in Tble 3.1 o EN , Tbles 1 nd 3 o EN nd weld thickness or ull strength double end illet welds. Steel grde S235 / S235W S355 / S355W S355 N/NL S355 M/ML S420 N/NL/ M/ML S460 N/NL/ M/ML S690 Q/QL/ QL1 S690 Q/QL/ QL1 S690 Q/QL/ QL1 Plte thickness t (mm) y u eu *) w 0,80 0,90 0,90 0,90 1,00 1,00 1,00 1,00 1,00 w.u.end w.u.side For double end illet welds with x = y 0,46 t 0,55 t 0,58 t 0,60 t 0,75 t 0,75 t 0,79 t 0,95 t 1,39 t *) eu is the nominl weld metl strength. 2.3 Discussion - In cse o side illet welds, the simpliied method nd the directionl method give the sme throt thickness. In cse o end illet welds, there is dierence o ctor 3/ 2=1,22. This gives or the mount o weld metl to be deposited ctor 1,5. - From Tble 1 it ppers tht the design strength w.u.end nd w.u.side or higher strength steels thn S355 is lower thn or S355. This is not logicl. - For steel grdes up to S460, the strength o the iller metl shll t lest be mtching the strength o the prent metl. The dierence in the strength between the prent metl nd the weld metl is implicitly tken into ccount by the w -vlue. In prctice the mount o overmtching depends on the steel grde: in lower strength steel there is usully more overmtching thn in higher strength steel. - For steel grdes higher thn S460 up till S700, EN prescribes the use o the tensile strength o the pplied iller metl in the design equtions. Filler metl strengths grde 35, 42, 55, 62, nd 69 with nominl tensile strengths eu = 440, 500, 640, 700 nd 770 N/mm 2 re llowed. For high strength steels it is not esy to hve n overmtched weld metl tht lso ulils the requirements or economic welding, ductility etc. Undermtched weld metl cn then be good solution, i the lower strength o the weld metl cn be compensted by thicker welds. - Becuse o the high strin concentrtions in illet welds, see Fig. 2, the ductility o the weld metl is very importnt to llow locl yielding nd redistribution o stresses. The ductility my hve decisive inluence on the strength o the illet-welded connection [13]. Fctors governing the strength nd ductility o the deposited weld metl re:. the chemicl composition o the iller metl b. the chemicl composition o the prent metl c. the welding process nd welding prmeters (e.g. the prmeters tht govern the cooling rte, preheting, the thickness o the electrode, etc.) d. the weld thickness e. the geometry o the weld (weld discontinuities; stress nd strin concentrtions). - Another reson or the low design stresses o illet welds in S420 nd S460 ccording to the present design rules in Eurocode 3, ws the limited number o vilble test results nd the sctter in those results. - It is noted tht or severl steel types the design vlues or the ultimte strength o steel in EN re lower thn in the ENV This hs direct consequences or the design o illet welds s cn be seen in Tble 1. Whether this is justiied is not cler. It ws never beore.

4 elstic Elstisch-plstisch elstic Elstisch-plstisch plstic plstic Vol-plstisch Vol-plstisch Fig. 2. Stress nd strin concentrtions, locl yielding nd redistribution o stresses in illet welds. 3 DESIGN OF FILLET WELDS ACCORDING TO AISC In the AISC Speciiction or Structurl Steel Buildings [6] design rules or illet welds re given in Chpter J, Design o Connections. At severl plces reerence is mde to the Americn Welding Society Codes, e.g. to the AWS Structurl Welding Code Steel [7]. 3.1 Generl provisions The design strength R n o welded joints is deined s the lower vlue o the bse mteril strength F nbm A BM nd the weld metl strength F nw A we, where: F nbm = nominl stress o the bse metl, ksi ; or tensile orce F nbm = F u F nw = nominl stress o the weld metl, ksi A BM = cross-sectionl re o the bse metl, in. 2 (mm 2 ) A we = eective cross-sectionl re o the weld, in. 2 (mm 2 ) = resistnce ctor. The eective cross-sectionl re o the weld is the throt thickness times the eective length o the weld (the sme s in Eurocode 3). For the design o illet welds, iller metl with strength level equl to or less thn mtching iller metl is permitted to be used. In AWS D1.1/D1.1M tble 3.1, prequliied bse metl iller metl combintions or mtching strength re given. In Tble 2 summry is given or Shielded Metl Arc Welding (SMAW). Tble 2. Summry o AWS D1.1/D1.1M prequliied bse metl iller metl combintions or mtching strength or SMAW. Group Minimum yield strength rnge AWS electrode speciiction or SMAW Nominl electrode tensile strength in ksi Nominl electrode yield strength in ksi I 240 to 315 E60xx - E70xx ( ) ( ) II 315 to 415 E70xx 70 (483) 58 (400) III 415 to 450 E80xx 80 (552) 67 (462) IV 485 E90xx 90 (621) 77 (531) Mtching is on the bsis o yield strength. In the clcultion o the resistnce the introduction o overmtching weld metl is not llowed. The sections long the legs need not to be checked since in tests rupture ws never observed in this section, provided tht the weld metl is not overmtched. In order to chieve good structurl behviour, the qulity o the weld is importnt. The AWS codes contin mny provisions to ensure this. E.g. much emphsis is put on low hydrogen electrodes nd prequliied bse metl - iller metl combintions. The vlues o, F nbm nd F nw nd limittions thereon re given in Tble J2.5 o the AISC speciiction.

5 3.2 Design o end illet welds nd side illet welds in AISC The design ormul or the design strength o illet welds is: R F A with n nw we F F (7) nw 1,5 0,60 EXX (1,0 0,50sin ) where: = resistnce ctor = 0,75 F EXX = electrode clssiiction number, i.e. minimum speciied strength, ksi = ngle o loding mesured rom the weld longitudinl xis, degrees. For end illet welds is 90 degrees nd or side illet welds is 0 degrees A we = eective re o weld throt, in. 2 (mm 2 ) For end illet welds nd side illet welds the design strength becomes: Rn, endillet 0,675 FEXX Awe nd Rn, sideillet 0,450 FEXX Awe (8) For ull strength end illet welds s in Fig.1 this gives with mtching electrodes: Rn, plte t Fy Aplte 0,9 Fy t (9) Rn, endillet 0,675 FEXX Awe 0,675 FEXX 2 (10) With Rn, endillet Rn, plte 0,9Fyt 20,675 F it ollows: EXX F 0,667 F y EXX t Tble 3. Comprison o weld thickness or end illet welds ccording to AISC nd Eurocode 3 or mtching electrodes. (11) AISC SMAW mtching F y 235 F EXX (ksi N/mm 2 ) ,38t 0,33t ,49t Eurocode 3 (directionl method) Steel grde F w.u.end S ,46 t S355 S355 N/NL S355 M/ML ,55t 0,58t 0,60t ,51t S420 N/NL/M/ML 294 0,71t ,52t S460 N/NL/M/ML 305 0,75t S690 eu =770 S690 eu =640 S690 eu = ,79t 0,95t 1,39t For side illet welds the dierences between the AISC speciiction nd Eurocode 3 re smller. The min reson is the strength dierence between end illets nd side illets. For AISC it is ctor 1,5 while in EC3 it is 3 / 2 1,22. Nevertheless, or the higher strength steels S420 nd S460 the AISC speciiction still gives considerbly smller sizes (~20%). Side nd end illet welds hve dierent stiness nd deormtion cpcity limits, e.g. Butler, Pl nd Kulk [12]. This hs consequences or the structurl behviour o combintions o welds in dierent directions, e.g. in lp joints in gusset pltes. Reerence is mde to J2 o [6], section 4(c).

6 4 THE SWEDISH CODE BSK94 (1997) In the Swedish code BSK94 [11] the design strength or illet welds ollows rom: wd wuk with wuk ub euk (12) 1,2 n where: ub = nominl ultimte strength o the bse metl euk = nominl ultimte strength o the iller metl ϕ = reduction ctor depending on the weld type. For illet welds ϕ = 0.9 γ n = prtil coeicient with respect to sety clss γ n vries between 1,0 nd 1,2 For side illet welds the design strength is: F // 0,60 R wd For illet welds with dierent direction the design strength is: FR wd 2 cos 2 (14) For end illet welds this gives: FRend 0,71 wd (15) It is noted tht in BSK94, the design strength o illet welds depends on the strength o the bse metl nd on the strength o the plte metl. In ct, the ormultion is between the Eurocode nd the AISC speciiction. 5 PROPOSALS FOR MODIFICATIONS IN EUROCODE The design resistnce o illet welds In previous pper [16] the min directions or new design rules or illet welds were proposed: The resistnce unction or the illet weld strength should be depending on the electrode strength, either s it is in AWS D-1.1 nd AISC-LRFD or some "mixture", e.g. s in the Swedish code BSK94. Undermtching iller weld metls or illet welds should be llowed. Undermtching illet metls will give thicker welds, but on the other hnd, improved weldbility nd the better ductility will result in better overll structurl behviour. The design strength o illet welds should depend on the weld qulity, both in terms o ductility (possibilities or locl yielding nd redistribution o stresses) nd in terms o geometry (shpe nd bsence o weld deects). Whether this dependence cn be prcticlly introduced in the design rules needs urther considertion. In the mentime EN1993 Prt 1-12 on high strength steels [10] ws published which prescribes weld design bsed on the iller metl strength (iller metl grde). In 2012 Christin Rsche inlized her PhD on the Lod Bering Cpcity o Fillet Welded Connections o High Strength Steels [15]. She perormed more thn 100 tests on illet welded connections minly in S690 with vrition o iller metl strengths. In her nlysis she lso considered test results rom severl other sources. Numericl nlyses nd sttisticl evlutions ccording to EN 1990 Annex D were perormed. 5.2 Proposl or improvement [15] A proposl or improvement o the correltion ctor β w ccording to EN nd EN ws developed or high strength steels S460 nd S690: - For S460 N/NL/M/ML: β w = 0,85 insted o 1,00 s in the present EN For S690 N/NL/M/ML: β w = 1,10 insted o 1,00 s in the present EN (13)

7 This gives more logicl results or S460, resulting in smller dierence with the AISC speciiction. It is noted tht the test results in [15] show better ductility thn the test results in [5] tht were the bsis or the present correltion ctor β w or S460. As indicted beore: better ductility gives better redistribution o stresses in the throt o the weld nd thereore better strength. 5.3 Proposl or modiiction [15] The ollowing proposl or improvement (modiiction) o the present design resistnce or illet welds is presented or the directionl method [15]: , 25 u 0,75 eu c 3 3 // (16) w M2 where: u = the nominl ultimte tensile strength o the weker prt joined eu = the nominl weld metl strength M2 = prtil sety ctor or welded connections ( M2 = 1,25) w = correltion ctor depending on the nominl weld metl strength: - or weld metl G42/E42 w = 0,89 - or weld metl G46/E46/T46 w = 0,85 - or weld metl G69/T69 w = 1,09 - or weld metl G89 w = 1,19. With this modiied model or the design resistnce se nd more economic design o illet welded joints prticulrly o high-strength steels nd dierent iller metls is possible. 5.4 Connections to unstiened lnges In 4.10 (3) o EN , it is stted tht or n unstiened lnge o nd I- or H section the ollowing criterion should be stisied. b ( / ) b (17) e y, p u, p p Otherwise the joint should be stiened. The bckground o this requirement is o course the ssurnce or deormtion cpcity. At incresing lod it gurntees tht the connected plte will yield beore rupture in the connection. But deormtion cpcity is not lwys requirement. The rule tht i (17) is not ulilled, the connection should be stiened, is new. It ws not in the IIW recommendtions o 1974 or ny lter ECCS recommendtion nd not in the ENV Minimum weld strength or ductility In 4.9 o EN , it is stted tht in welded joints where deormtion cpcity is required the welds require suicient strength not to rupture beore generl yielding in the djcent prent mteril. For unstiened lnges rule is given, see previous section. But or e.g. joints like the ones in Fig. 1, no rules re given. In the IIW recommendtions o 1974 nd lter ECCS recommendtions nd ntionl stndrds the rule ws tht in such cses the welds should t lest be designed or 70% o the ull lod. In the ENV this vlue ws rised to 80%. The min reson ws tht or modern steels nd or higher strength steels thn S355 the reltion u / y is lower thn in the steels covered in previous recommendtions nd stndrds (S235 S355). A simple nd eective solution is similr to the requirement or unstiened lnges. The welded joint should be designed or t lest: / (18) x y, p u, p Where α is clibrtion ctor, e.g This pproch is similr to the one or net section deormtion cpcity veriiction o bolted lp joints. 6 CONCLUSIONS - Present design rules or illet welds in Eurocode 3 my give unnecessry thick welds or higher strength steels.

8 - Design rules where the iller metl is tken directly into ccount, llow more reedom or the designer to choose more optiml combintion o bse metl, iller metl nd welding prmeters to comply with requirements or ductility nd strength. - Undermtching iller metl my hve dvntges regrding weldbility nd better qulity o welded joints, possibly leding to better ductility nd redistribution o stresses nd thereore leding to higher strength. - Ductility especilly in the root o the weld is essentil or redistribution o stresses by locl yielding end thereby chieving n optimum strength. - Proposls or modiiction o Eurocode 3 design rules hve been ormulted. REFERENCES [1] Interntionl Institute o Welding (IIW), "Design rules or rc welded connections in steel submitted to sttic lods". Welding in the Word. Vol. 14. No. 5/6, [2] Gresnigt, A.M., Clcultion o illet welds in Eurocode No. 3. Rivist Itlin dell Sldtur, no 6, pp [3] ENV , Eurocode 3, Design o steel structures, Prt 1.1- Generl rules nd rules or buildings nd Annex D o Eurocode 3: The use o steel grde Fe E 460, CEN, Brussels. [4] Bckground Documenttion D.03, 1990: Evlutions o test results on welded connections mde rom Fe E 460 in order to obtin strength unctions nd suitble model ctors, Eurocode 3 Editoril Group, April [5] Snijder, H.H., De Koning, C.H.M., Gresnigt, A.M., Bijlrd, F.S.K., Tests on welded connections in Fe E 460, TNO-IBBC report B [6] ANSI/AISC , Speciiction or Structurl Steel Buildings. Americn Institute o Steel Construction, Chicgo, USA, Second printing: Februry [7] AWS D1.1/D1.1M-2010, Structurl Welding Code Steel, Americn Welding Society, Mimi, USA. [8] CAN/CSA-S16-09, Design o Steel Structures, Cndin Stndrds Assocition, Mississug, Ontrio, Cnd. [9] EN , Eurocode 3, Design o steel structures, Prt 1-8: Design o joints, CEN. [10] EN , Eurocode 3: Design o steel structures, Prt 1-12: Additionl rules or the extension o EN 1993 up to steel grdes S 700, CEN. [11] Boverkets hndbok om stlkonstruktioner, BSK 94, Boverket, Byggvdelningen, Krlskron, Februri 1997, ISBN: , ISSN: [12] Butler, L.J., Pl, S., nd Kulk, G.L., Eccentriclly Loded Welded Connections, Journl o the Structurl Division, ASCE, Vol. 98, No. ST5, My 1972, pp [13] Gresnigt, A.M., Strength nd Deormtion Cpcity o Fillet Welds in Fe E 460. Connections in Steel Structures II: Behvior, strength nd design. Proceedings o the Second Interntionl Workshop, Pittsburgh, pp Americn Institute o Steel Construction, ISBN [14] Treiberg, T., Inluence o bse nd weld metl strength on the strength o welds, Connections in Steel Structures II: Behvior, strength nd design. Proceedings o the Second Interntionl Workshop, Pittsburgh, pp Americn Institute o Steel Construction, ISBN [15] Christin Rsche, (2012). Zur Bestimmung der Trgähigkeit von Kehlnhtverbindungen höherester Bustähle (Determintion o the Lod Bering Cpcity o Fillet Welded Connections o High Strength Steels). PhD thesis in Germn lnguge. Germn Institut ür Konstruktion und Entwur, Sthl- Holz- und Verbundbu, Universität Stuttgrt. Mitteilungen des Instituts ür Konstruktion und Entwur; Nr [16] Gresnigt, A.M. (2002). Updte on design rules or illet welds. 3rd Europen Conerence on Steel Structures, September 19-20, 2002, Coimbr, Portugl.