10 HOLLOW SECTION CONNECTIONS

Transcription

1 0 HOLLOW SECTION CONNECTIONS 0. Introducton Connectng technology plays an mportant role n the perormance o hollow secton structures. A dstncton has to be made between CHS and RHS connected members, because the behavour o jonts, e.g. local behavour o members s derent. A partcular case s represented by beam-to-column jonts n buldng rames wth Concrete Flled Hollow Secton (CFHS) columns. Both welded and/or bolted connectons can be used n such a case. For beam-to-column jonts o hollow secton rames (e.g. RHS columns and beams or hollow secton columns and I or H secton beams), blnd boltng technology s avalable. Ths secton summarses the man aspects concernng the behavour and desgn o hollow secton connectons loaded predomnantly statcally. Ths means they can also be used or sesmc resstant buldngs, snce sesmc motons are not consdered as generatng atgue phenomena. European standard [pren : 2003], Chapter 7 gves detaled applcaton rules to determne the statc resstances o un-planar and mult-planar jonts n lattce structures composed o crcular, square or rectangular hollow sectons, and o un-planar jonts n lattce structures composed o combnatons o hollow sectons wth open sectons. K jont KT jont N jont T jont X jont Y jont DK jont KK jont X jont TT jont DV jont XX jont Fgure 0. Types o jonts n hollow secton lattce grders 90

2 The statc resstances o the jonts are expressed n terms o maxmum desgn axal and/or moment resstances or the brace members. The applcaton rules are vald both or hot nshed hollow sectons to EN 020 and or cold ormed hollow sectons to EN 029, the dmensons o the structural hollow sectons ull the necessary requrements. The nomnal wall thckness o hollow sectons should be lmted to a mnmum o 2,5 mm and should not be greater than 25 mm unless specal measures have been taken to ensure that the through thckness propertes o the materal s adequate. The types o jonts covered by standard pren are ndcated n Fgure 0.. The applcaton rules gven n paragraph 7..2 o pren may be used only where all the gven condtons are satsed. 0.2 Welded connectons Even bolted connectons to hollow sectons are utlsed to assemble preabrcated elements or space structures, the most used method to assemble CHS members s weldng, especally or trusses. Accordng to pren , the desgn jont resstances o connectons between hollow sectons and o connectons o hollow sectons to open sectons, should be based on the ollowng alure modes as applcable: Chord ace alure (plastc alure o the chord ace) or chord plastcaton (plastc alure o the chord cross-secton); Chord sde wall alure (or chord web alure) by yeldng, crushng or nstablty (crpplng or bucklng o the chord sde wall or chord web) under the compresson brace member; Chord shear alure; Punchng shear alure o a hollow secton chord wall (crack ntaton leadng to rupture o the brace members rom the chord member); Brace alure wth reduced eectve wdth (crackng n the welds or n the brace members); Local bucklng alure o a brace member or o a hollow secton chord member at the jont locaton. 0.3 Bolted connectons Connectng two hollow secton members or a hollow secton and an open prole or a plate drectly to each other by boltng can be dcult unless the jont s located close to the open end o a hollow secton member. Otherwse t s necessary to take measures, such as cuttng a hand access hole n the structural hollow secton member to enable the bolt to be tghtened rom the nsde or usng through or blnd bolts. The reason or ths specal stuaton s evdent, as the hollow secton allows ree access only to the outsde; any access to the nsde s restrcted. Bolted connectons reman nonetheless desrable n many cases n spte o the unque condton o non-accessblty to the nsde o a hollow secton. However, n these cases, the hollow sectons can be joned ndrectly usng lange or cappng plates, whch makes t possble to eect such bolted connectons n a smple and economcal manner. The man methods o assembly by boltng are descrbed bellow. Bolted connectons are mostly detachable. They are selected or the on ste assembly n order to avod ste weldng, whch may cause weldng errors due to envronmental dcultes. Ste weldng s also more costly than ste boltng. Ths paragraph s presented n accordance wth Desgn gude or structural hollow sectons n mechancal applcatons o J. Wardener et al, see [CIDECT, 995] and Gude on the use o bolts: sngle sded blnd boltng systems o N. F. Yeomans [Yeomans, 2002]. The man types o bolted connectons or hollow secton structures are: Bolted knee jonts, Flange connectons, Splce jonts, Jonts wth ork ends, Screwed tensoner, Through boltng, Bolted connectons wth lattened ends, Hnged support, Column bases, Fsh plate connectons, Bolted subassembles, and Fxng bolts through hand access holes [CIDECT, 995; Elremaly, Azznamn, 200b; Elremaly, Azznamn, 200a]. These connectons are realsed usng ntermedate connectng steel devces, whch are welded on the hollow secton members, the bolted connectons themselves beng desgned as normal connectons accordng to pren , Chapter 3 [pren : 2003]. For ths reason, desgn o hollow secton connectons does not mply specc requrements. 9

3 0.4 Desgn consderatons A structure made o hollow sectons and loaded by predomnantly statc loadng should be desgned n such a way that t has a ductle behavour. Ths means that the crtcal members or jonts should provde sucent rotaton capacty. In ths case, secondary bendng moments due to mposed deormatons or due to the jont stness may be neglected n desgn. Where the crtcal member or connecton do not provde sucent rotaton capacty, as n case o thn walled sectons, a second order elastc analyss should be used. Desgn o hollow secton connectons should be done n accordance wth pren [pren : 2003], Chapter 7: Hollow secton jonts and CIDECT Desgn gude or structural hollow sectons n mechancal applcatons, Chapter 5: Desgn consderatons or connectons [CIDECT, 995], [Kato, 988]. Connecton detalng or both welded and bolted connectons s gven n CIDECT Desgn gude or structural hollow sectons n mechancal applcatons, Chapter 6: Connecton detalng [CIDECT, 995]. Q&A 0. Crcular Hollow Secton Jonts What analytcal models are used or calculatng the resstance o CHS jonts? The ollowng three models are currently used: Tube model or chord ace alure Model or punchng shear alure Model or chord shear alure Tube model or chord ace alure The jont s modelled by a tube o eectve length B e, havng a geometry and mechancal characterstcs dentcal wth the CHS (Crcular Hollow Secton Jonts) chord as presented n Fgure 0.2. Fgure 0.2 Tube model or a X jont o CHS Neglectng axal and shear orces, and takng nto account the eectve length B e, whch s determned by tests, the eort correspondng to the plastcaton o the tube should be: N C t 2 0 o y =, (0.) C β snθ 92

4 where C 0, C are constants, θ s the angle between the dagonal and the chord and β = d /d 0 s the rato o dameters o the tubes [CIDECT, 99]. Ths model gves good results or T, Y and X jonts. For more complex jonts such as K and N, other parameters lke dstance between dagonals and axal orces should be taken nto account. Model or punchng shear alure Ths model s presented n Fgure 0.3 or a Y jont subject to tenson. The eort n the dagonal s obtaned by the ormula: N + θ 2 2 = π d2 to (0.2) sn θ2 Ths crteron s generally true only or reduced values o β, where β = d 2 /d 0, because β ncreases, the load wll be transerred to the chord by crcular stresses. Desgn rules based on the model or punchng shear alure are largely used n oshore desgn. Fgure 0.3 Model or punchng shear alure o the chord n a CHS jont Model or chord shear alure As seen n Fgure 0.4, or K and N jonts wth a gap between dagonals, the chord crosssecton may collapse n the gap secton, because o the combnaton o axal orce, shear orce and bendng moment. gap V Fgure 0.4 Model or chord shear alure n a CHS jont 93

5 I the chord s a compact secton, plastc desgn gves the ollowng relatons: N snθ 2 ( d o t o ) t o, (0.3) 3 N π d t t, (0.4) M o,gap o,gap ( o o ) o 2 ( do to ) to. (0.5) Generally, the bendng moments are qute small and only the nteracton between the axal orce and the shear orce has to be consdered 2 2 N o,gap N snθ +,0. (0.6) ( d t ) t 2 o o o π ( do to ) to 3 I the gap s small, the dagonals sten the chord, whch consderably ncreases the shear resstance. Q&A 0.2 Rectangular Hollow Secton Jonts What analytcal models are used or calculatng the resstance o RHS jonts? Analytcal models are used to descrbe the behavour o connectons and to study the eect o the prncpal parameters. Takng nto account all the parameters s dcult and too complcated. For these reasons, derent smpled models are used. In combnaton wth test results made n laboratores, these models have been used to establsh the desgn equatons. Y jont Model Fgure 0.5 Plastc lnes model or jonts o type T, Y or X (chord ace alure) 94

6 Plastc lne model The general prncple o the model, llustrated n Fgure 0.5, or an Y RHS jont, conssts o equalsng the work o the orce N wth a δ dsplacement, wth the nternal work n the plastc hnges (length l and rotaton angle ψ. I θ, s the angle between the brace and the chord, the capacty o the members [APK, 996] obtaned orm the ollowng expresson to 2 h N 4 β β bo snθ = +. (0.7) snθ Model or brace punchng shear alure Ths model s presented n Fgure 0.6 or a Y jont subject to tenson. The shear resstance or T, Y and X jonts s obtaned by the ormula 2 h N to 2 bep 3 snθ = +. (0.8) snθ a) longtudnal secton b) cross secton c) plane secton Fgure 0.6 Plastc lnes model or jonts o type T, Y or X (chord ace alure) Model o the brace eectve wdth The resstance s calculated as a uncton o the brace dmensons, whch or T, Y and X jonts s N = t 2 h 4 t + 2. (0.9) ( ) y be Model o chord shear alure The shear resstance o the chord may be calculated analytcally usng the plastc desgn o the secton: Av V pl =, (0.0) 3 where Av = (2 h 0 +α b 0 ) t 0 and α s a uncton o g/t 0. The rest o the chord secton s takng the axal eort. Usng the von Mses crteron o plastcty, the ollowng nteracton ormula s obtaned: 95

7 V Sd No,gap,Sd ( Ao Av ) + Av, (0.) V pl,rd where V Sd s the desgn shear eort and V pl,rd s the plastc resstance to the shear eort o a secton calculated wth relaton (0.0), dvded by the saety partal actor γ M0. 2 Fgure 0.7 Model or RHS chord shear n the gap o K or N jont Model or the plastcaton or the local bucklng o the lateral chord sde walls T, Y and X jonts, wth a hgh level o β, may reach alure by the plastcaton or the local bucklng o the lateral chord sde walls. For RHS jonts o the same wdth, the model presented n Fgure 0.8 gves N h = 2 to + 5 to. (0.2) snθ snθ Fgure 0.8 Model or the plastcaton or the local bucklng o the lateral chord sde walls Q&A 0.3 Jonts between Hollow and Open Secton Members What analytcal models are used or connectons between CHS or RHS members and I or H secton chords? The desgn resstance s calculated usng smpled models whch have been vered expermentally. 96

8 Model o the brace eectve wdth For jonts made by hollow sectons welded on I or H sectons, there s an unequal dstrbuton o the stresses and deormatons at the end o a hollow member, due to the web and the derent rgdty between the ends and the central part o the lange, see Fgure 0.9. I the load s ncreasng, ths phenomena s more mportant and may reach a premature alure o the jont, by the collapse between the member n tenson and the chord lange, or by local bucklng at the edge o the member n tenson. To cover ths rsk o collapse, the term o transversal crackng s used commonly and the majorty o authors have ntroduced the eectve wdth. Ths term s representatve or the permeter o the hollow secton, able to transmt the eort at the stage o the collapse, see Fgure 0.0 [APK, 996]. Fgure 0.9 Dstrbuton o the stresses and deormatons at the end o a RHS member Fgure 0.0 Noton o eectve permeter The ultmate resstance o the member or T, Y, X, K and N (wth gap) jonts may be computed as ollow: N = 2 t b, (0.3) Rd y e where b e beng equal wth the hal length o the eectve part o the permeter o the hollow member. The ollowng ormula may be used to compute b e : b + e = tw + 2 r 7 t (0.4) y Model o chord shear alure The rsk o alure by shear o the chord s the most probably alure mode or K and N connectons wth gap Fgure 0.. The shear resstance o the chord could be computed usng the ollowng ormula, where A v represents the eectve shear area: 97

9 Av Vpl = (0.5) 3 Fgure 0. Shear o the chord n a K jont wth gap Model o the local plastcaton o the chord web The approach o calculatng the local plastcaton o the chord web n tenson and the local crushng o the chord web s smlar to that used to determne the local tenson capacty o the web o the I and H secton. The method s shown n Fgure 0.2 and the resstance N,Rd s gven by the ollowng expresson tw bw NRd =. (0.6) snθ Fgure 0.2 Local plastcaton o the chord web Q&A 0.4 Desgn Charts What s the background to the desgn charts? The desgn charts shown n Fgure 0.3 are based on the ecency coecent C e o the brace [APK, 996]. The ecency coecent s obtaned by the ollowng ormula 98

10 N A C t k.rd o p = e. (0.7) y y t snθ Ecency parameter C e (C T or T and Y jonts, C X or X jonts and C K or K and N jonts) means the ecency o a jont wth k p =,0 and t = 90, wth dentcal thckness and steel grade n braces and chord. C e,0 0,9 0,8 0,7 θ to k. Rd p = CT y y t sn θ N A d 0 0 t 0 0,6 0,5 0,4 0,3 0, , 0,0 0,0 0, 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9,0 β Fgure 0.3 Desgn chart or T and Y jonts o CHS sectons In practse, t s mportant to evaluate quckly the jont resstance o RHS sectons, whch may be made by desgn charts gven or K, N, T, Y and X jonts. These charts are based on the EN recommendatons. The jont resstance s gven by an ecency coecent C e. Generally, the ecency s gven by the ollowng ormula [APK, 996]: N t Rd o kn = Ce (0.8) A t snθ y y For nstance, Fgure 0.4 presents the ecency o a K gap jont, o RHS. For jonts wth no gap, the total ecency s gven n the chart rom Fgure 0.5. The reerence [CIDECT, 995] contans desgn charts related to all the types o RHS jonts wth gap or overlapped. 99

11 C Kg,0 0,9 0,8 0,7 0,6 N A.Rd y0 0 kn = CKg y y t sn θ t b + b 2 2 b 0,5 0,4 0,3 0,2 0, 0, Fgure 0.4 Chart o ecency o braces or welded K and N RHS jonts wth gap ,6,4,2,0 0,8 0,6 b 0 t 0 N A y,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 2,0,75,5,25,0 yj t j y t 0, b 0,0 j t j Fgure 0.5 Chart o ecency o braces or welded K and N RHS jonts wth no gap Q&A 0.5 Blnd Boltng What derent types o blnd bolts are avalable? Flowdrll Drll System The Flowdrll system s a method or the extruson o holes usng a our lobed tungstencarbde rcton drll [Yeomans, 2002]. The process s shown schematcally n Fgure 0.6. A typcal beam end plate connecton s descrbed n Fgure 0.7. The results o a seres o tests on ndvdual low-drlled holes and on connectons made usng the Flowdrll system have shown that they are sutable or structural applcatons [Korol et al, 993] and [Ballern et al, 996]. These tests have shown that: holes can be produced n sectons rom 5,0 to 2,5 mm thck, [Barnett et al, 200], 00

12 threaded roll tapped holes wth M6, M20 and M24 ISO course thread proles can be made. I the threads are made usng a standard cuttng tap the pull-out capactes wll be lower than those shown n Table 0., the ull tenson capacty o grade 8.8 bolts can be carred by low-drlled and roll tapped holes, provded that the materal thckness s equal to or greater than the mnmum thckness shown n Table 0. and the materal has a nomnal yeld strength n the range 275 to 355 MPa, the shear and bearng capactes o the hole and bolt can be calculated n the normal manner. Fgure 0.6 Schematc o the Flowdrll process Fgure 0.7 Beam/column connecton Table 0. Mnmum materal thckness or ull grade 8.8 bolt tenson capacty Bolt sze and grade Mnmum materal thckness, mm M6 grade M20 grade M24 grade Lndapter HolloBolt Insert The HolloBolt s a three part pre-assembled unt consstng o a man body, a threaded truncated cone and a standard grade 8.8 bolt and s shown n Fgure 0.8a, a ve-parts system s also avalable [Yeomans, 2002]. Fgure 0.8 a) Insert (as suppled), b) beam/column connecton Huck Ultra-Twst Bolt The Ultra-Twst bolt s a pre-assembled unt [Yeomans, 2002]. An exploded vew o the bolt s shown n Fgure 0.9a. The Ultra-Twst bolt s nstalled usng an electrc boltng wrench n holes 2 mm larger than the outsde dameter o the bolts, whch provdes conventonal clearances or t-up, Fgure 0.9b. 0

13 Fgure 0.9 Huck Ultra-Twst bolt, a) exploded vew o the bolt, b) nstallaton procedure Stud Weldng Threaded studs welded to steel columns can also be used to produce connectons. A typcal beam end plate connecton, wthout counter sunk holes s shown n Fgure 0.20 [Maquo et al, 984]. 02

14 Welded nuts There are several other methods avalable or makng bolted connectons, whch can be xed rom one sde only [Sadr, 994]. Two o these are brely descrbed below. The rst method s smply drllng and tappng the steel secton, but ths generally needs a wall thckness o 6 mm or more to generate enough pull out capacty. Another method [Kato, 988], s to drll holes n the steel secton large enough or a nut o the requred sze to be nserted and then to weld the nut to the steel secton lush wth the outsde surace, see Fgure 0.2. Fgure 0.20 Welded threaded stud connecton Fgure 0.2 Nuts welded nto hollow secton wall Q&A 0.6 Hollow Secton Jonts usng Hgh Strength Steel Can the rules gven n pren be appled to hgh strength steels? The rules gven n pren may be appled to hgh strength steels. Clause 7.. (4) speces that nomnal yeld strength o hot nshed hollow sectons and the nomnal yeld strength o the basc materal o cold ormed hollow sectons should not exceed 460 MPa. For grades S 420 and S 460 the statc resstances gven n ths secton should be reduced by a actor 0,9. Accordng to EN 020 and EN 029 the requrements or materal s determned based on the end product, not on the base materal. CIDECT Desgn Gude [CIDECT, 995] stpulates that t s possble or structural hollow sectons n specal steels to be produced, e.g. very hgh strength steels wth yeld strengths up to 640 MPa or hgher, weatherng steels and steels wth mproved or specal chemcal compostons, etc.; oten large quanttes would requre to be ordered [Oyj, Vano, 2000]. Q&A 0.7 Oshore Constructon Can the recommendatons n pren be appled to the large sectons used n oshore constructon? The eld o applcaton o hollow sectons n engneerng s large and ther specc domans o applcaton are lsted n [CIDECT, 995] ncludng oshore structure. The desgn rules o pren : 2003, Chapter 7 could be used n the case o oshore structures, provded the general condtons are ullled. In the case o oshore structures these lmts o applcatons are mostly not ullled and specal requrements must be used. In addton atgue desgn condtons need to be consdered. 03