2 CONNECTION DESIGN REQUIREMENTS OUTLINEINTRODUCTIONCOMPLEXITIES OF CONNECTIONSTYPES OF CONNECTIONSCONNECTION DESIGN PHILOSOPHYCOST OF CONNECTIONSSUMMARY
3 INTRODUCTION Necessity for Connections Importance of Connection Limited Length of MembersRolling & Transportation ConstraintsLarger Size of StructuresImportance of ConnectionStructure is only as strong as the weakest linkConnection failure to be avoided before member failureThe full strength of members is to be utilisedConnection failure is usually not ductile
4 INTRODUCTION Requirements of Connection Designs Strength, Stiffness, and DuctilityDeflection control & stability under service loadLarge Defection & Ductility at Ultimate load (over load)Connections are Complex BecauseThey are more complex to design than membersThey have greater potential for variability inbehaviour & strengthMost vulnerable component of a structureFailure of a connection often leads to failure of the structure
5 COMPLEXITY OF CONNECTIONS Relaibility or Safety of a design depends onVariability of loadsVariability of the member strengthVariability of Connection StrengthLarger Uncertainty of Connections is Due toComplexity of Connection GeometryHighly IndeterminateStress concentrationNon-Linearity due to slip, & local yieldingGeometric ImperfectionsResidual Stresses & Strains
6 COMPLEXITY OF GEOMETRY BeamTeeBolted ConnectionWelded ConnectionBoltsColumnBracketAngleFlange PlateStiffener
8 COMPLEXITY OF CONNECTIONS GEOMETRIC IMPERFECTIONSBow in members as rolledLack of fit in Black Bolts in clearance holesFabrication ErrorsMember deflectionsWelding distortionsGaps & tolerances in fabrication & erectionRESIDUAL STRESSES & STRAINSDifferential cooling after hot rolling, gas cutting & weldingPremature yielding under loadingLack of fit in bolted fabrication (Distortions)Member strength ?
9 Failure of pipe connection Partial safety factor for connection = 1.251.5 (field fabrication)
10 TYPES OF CONNECTIONS WELDED CONNECTIONS BOLTED CONNECTIONS Fillet weldingButt weldingBOLTED CONNECTIONSBearing type Carbon steel / High strengthFriction type HSFGRIVETED CONNECTIONSMild steelHigh strength steel
11 BOLTED CONNECTIONS Bearing Type: XBearingStressNotice slip in bearing type of connectionClampingForce, P0TFrictional Force TContactPressure, P0
12 Bolt Shear Transfer – Free Body Diagram (a) Bearing Connection (b) Friction ConnectionTFrictional Force TClamping Force, POBearing stressesTensionin boltFORCE TRANSFER MECHANISMDr S R Satish Kumar, IIT Madras
13 BOLTED CONNECTIONS Bearing Type: Friction Type: Notice no slip is observed in-between plates in HSFG ConnectionTBearingStressXTClampingForce, P0TFrictional Force TContactPressure, P0T
14 Merits Welded Connections Transfer of forces between elements more directRequires little additional elements like gussetsShorter length of jointsNo reduction in member strength due to bolt holes etc.Rigid connections easy to achieve
15 Demerits Requires skilled manpower Requires special equipment not easy to achieve at difficult locationsless ductileprone to defects & fatigue cracks under cyclic loading
16 Merits & Demerits Bolted Connections Bearing Type Easy to install even at difficult locationsEconomicalDoes not require highly skilled manpowerSlip causes flexible jointJoint size larger
17 Merits & Demerits Bolted Connections Friction Type Rigidity of connectionBetter fatigue performanceExpensive due to material & installation labourRequires skilled manpowerRequires better inspection
18 RIGIDITY OF MOMENT CONNECTIONS Type of connectionsRigid Hinged Semi-Rigid>90hMr
19 RIGIDITY OF MOMENT CONNECTIONS hMomentMrRotationRigid jointHinged jointSemi-rigid joint
20 CONNECTION DESIGN PHILOSOPHY Connections are complexThe strength is variableLarge numbers have to be designed‘Exact’ analysis:Complex but possibleAccuracy depends on assumptionsNot practically feasiblePractical, simplified Methods are AppropriateShould satisfy equilibriumDuctility requirements (static loading)Fatigue strength requirements (cyclic loading)
21 TRANSFER OF MEMBER FORCES Factors:Understand the expected connection behaviour.Model this appropriately in analysis.Determine the forces and moments transferred to the connection.Consider the joint size to reduce forces transferredReplace the forces/moments transferred by memberEquivalent system of forces on interface elements in the jointConsider the mechanism of force transfer in the memberTransfer to elements in consistency with their relative stiffnessThe system of forces be in equilibrium with the force to be transferred
22 TRANSFER OF MEMBER FORCES (a) Connection(b) Freebody DiagramCritical sectionfor block shearVV
23 FORCE FLOW IN A JOINT Force flow is complex High degree of indeterminacyLocal stress concentration and yieldingEffect of Residual stress, which is unknownConnection force flow AnalysisMaking simplifying Assumptions – sharing of forcesSatisfying EquilibriumEnsuring adequacy of strengthEnsuring adequacy of stiffnessEnsuring adequacy of ductility
24 SIMPLIFIED ANALYSIS OF JOINTS Assume sharing of forces among alternate elementsStiffer elements attract larger proportion of the imposed forcePlate elements are stiff in resisting forces imposed in their planePlate elements are flexible in resisting forces imposed out-of-planeThe assumed forces may be at variance from the elastic resultsEquilibrium & Compatibility are to be satisfied in elastic analysisOnly equilibrium is assumed in the simplified analysisEnsure adequacy of ductility to redistribute forces as assumedRedistribution is necessary since assumed sharing may at varianceEnsure adequacy of strength of elements in the load path
26 COST OF CONNECTIONS Design cost Fabrication / Erection cost Consumes a major portion of effortsSimplified design methods reduce the costStandardised designs and details are desirableDesign handbook, aids and softwareFabrication / Erection costRepetitive use of standard detailsGood access, easy support, ease of joining at locationMix of automatic and manual fabricationChoice of connection methodOther factorsSimple detail, simple techniques appropriate to requirement
27 relative stiffness of elements, and ductility of elements Sound Connection Design Requires Understanding ofthe requirementforce flowrelative stiffness of elements, andductility of elementsGood design can considerably reduce cost of steel structure
29 Analysis of Bolt Groups Combined Shear and Moment in-PlaneCombined Shear and Moment out-of-planeBeam and Column SplicesBeam to Column ConnectionsBeam to Beam ConnectionsTruss ConnectionsFatigue Behaviour
31 Designed more conservatively than members because they are more complex to analyse and discrepancy between analysis and design islargeIn case of overloading, failure in member is preferred to failure inconnectionConnections account for more than half the cost of structural steelworkConnection design has influence over member designSimilar to members, connections are also classified as idealised typesEffected through rivets, bolts or weldCodal Provisions
32 Concentric Connections TYPES OF CONNECTIONSClassification based on type of resultant force transferred(a)(b)Concentric Connections(a)(b)Moment Connections
33 Tension Connection and Tension plus Shear Connection TYPES OF CONNECTIONS -!Classification based on type of force in the boltsSinglesheara) Lap Connectionb) Butt ConnectionDouble shearShear Connectionssupport(a)(b)Tension Connection and Tension plus Shear Connection
34 BOLTS AND BOLTINGBolt Grade: Grade 4.6 :- fu = 40 kgf/mm2 and fy = 0.6*40 = 24 kgf/mm2Bolt Types: Black, Turned & Fitted, High Strength Friction GripBlack Bolts:usually Gr.4.6,made snug tight,ductile and cheap,only static loadsTurned & Fitted;Gr.4.6 to 8.8,Close tolerance drilled holes,0.2% proof stressHSFG Bolts:Gr.8.8 to 10.9,less ductile,excellent under dynamic/fatigue loads
35 TIGHTENING OF HSFG BOLTS snug-tightposition¾ turnTightening of HSFG bolts1) Turn-of-nut Tightening2) Calibrated Wrench Tightening3) Alternate Design Bolt Installation4) Direct Tension Indicator Method(a) Standard(b) Oversized(c )Short Slot(d) Long slotFeeler gaugeHole types for HSFG bolts
36 Bolt Shear Transfer – Free Body Diagram (a) Bearing Connection (b) Friction ConnectionTFrictional Force TClamping Force, POBearing stressesTensionin boltClamping Force, POFORCE TRANSFER MECHANISM
37 BOLTS UNDER TENSION AND PRYING EFFECT (b) HSFGConnectionBearing typeconnection2TTToTo+T(d) Prying EffectQBAbnT+Q2TProof LoadPoBolt forceB kNApplied load 2T (kN)HSFGBearing type( c) External Tensionversus bolt force
39 Bolted Steel Connections Bolts in tension6 x 200 =1200 kNBolts in shear
40 Failure modes of bolts in shear Hole bearingHole tearoutBolt shear
41 PRYING EFFECT AND END PLATE DESIGN Minimum prying force Q is given by= 2 (non-preloaded)= 1.5 for limit state designw = width/pair of boltsPo= proof load in consistent unitsn is the minimum of end distance orthe minimum thickness of the plate is obtained as followsThe corresponding prying force can then be obtained as Q = Mp/n.If the total force in the bolt (T+Q) exceeds the tensile capacity of the bolt,then the thickness of the end plate will have to be increased.
42 Tnb = 0.90 fub An < fyb Asb (γmb / γm0) FAILURE OF CONNECTIONSShear Connections with Bearing Bolts(a) Shearing of Bolts(b) Bearing on BoltsVnpb = 2.5 kb d t f’uZone ofplastification(c) Tension capacityTnb = 0.90 fub An < fyb Asb (γmb / γm0)
43 BOLTS UNDER TENSION AND PRYING EFFECT (b) HSFGConnectionBearing typeconnection2TTToTo+T(d) Prying EffectQBAbnT+Q2TProof LoadPoBolt forceB kNApplied load 2T (kN)HSFGBearing type( c) External Tensionversus bolt force
49 Vnsf = µf. ne. Kh. Fo FAILURE OF CONNECTIONS Shear Connections with HSFG Bolts(a) Slip ResistanceVnsf = µf. ne. Kh. FoKh =1.0 (clearance hole)= 0.45 (untreated surfaces)ne = no of effective interfacesFo= proof load(b) Bearing on PlatesPbg = pbgd t 1/3 e t pbg
50 DESIGN STRENGTHS FOR BOLTED CONNECTIONS Table 1 Bolt Strengths in Clearance Holes in MPaBolt strengthsBolt grade4.68.8Shear strength ps160375Bearing strength pbb435970Tension strength pt195450Table 2 Bearing Strengths of Connected Parts in MPaSteel gradeST42SGr.43Gr.50Bearing bolts pbs418460550HSFG bolts pbg6508251065
51 COMBINED SHEAR AND TENSION (a) Bearing Bolts(a) HSFG Bolts
52 BLOCK SHEAR FAILURECapacity=Shear Capacity of AB + Tension Capacity of BCTABCTdb = ( Avg fy /( m0) Atn fu /m1 )Block ShearorTdb = (0.9Avn fu /( m1) + Atg fy /m0 )
54 GENERAL ISSUES IN CONNECTION DESIGN Assumptions in traditional analysisM = TdStandard Connections(a) moment connection(b) simple connectioneVTCd(a)(b)Connection elements are assumed tobe rigid compared to the connectorsConnector behaviour is assumed tobe linearly elasticDistribution of forces arrived at byassuming idealized load pathsProvide stiffness according to theassumed behaviourensure adequate ductility and rotationcapacityprovide adequate margin of safety
55 COMBINED SHEAR AND MOMENT IN PLANE Bolt shear due to Px and PyRxi = Px/n and Ryi = Py/nPriRmiOx’y’M = Px y’ + Py x’Rmi = k riMi = k ri2MR = k ri2 = k ri2Bolt shear due to MRmi=M ri/ ri2Bolt group eccentricallyloaded in shearCombined shear
57 COMBINED SHEAR AND MOMENT OUT-OF-PLANE Ti liLiNAd/6(a)(b)(c)CBolt group resisting out-of-plane momentTi = kli where k = constantM = Ti Li = k li LiTi = Mli/ li LiShear assumed to be shared equally and bolts checked for combined tension+(prying)+shear
58 Strength, stiffness and ease in erection BEAM AND COLUMN SPLICEStrength, stiffness and ease in erectionAssumptions inRolled-section& Plate Girders(a)ConventionalSplice(b) End-PlateSpliceBolted Beam SpliceColumn Splices – bearing type or HSFG moment splices
59 BEAM-TO-COLUMN CONNECTIONS (a) Simple – transfer only shear at nominal eccentricityUsed in non-sway frames with bracings etc.Used in frames upto 5 storeys(b) Semi-rigid – model actual behaviour but make analysisdifficult (linear springs or Adv.Analysis). However leadto economy in member designs.(c) Rigid – transfer significant end-moments undergoingnegligible deformations. Used in sway frames forstability and contribute in resisting lateral loads andhelp control sway.
60 BEAM-TO-COLUMN CONNECTIONS VSimple beam-to-column connections a) Clip and seating angleb) Web cleats c) Curtailed end plateEconomical when automatic saw and drill lines are availableCheck end bearing and stiffness of seating angleClip angle used for torsional stabilityIf depth of cleats < 0.6d design bolts for shear onlyEliminates need to drill holes in the beam. Limit depth and thicknesst < /2 (Gr.8.8) and /3 (Gr.4.6)
61 Rigid beam-to-column connections column webstiffenersweb platediagonalstiffener(a)(b)(c)Rigid beam-to-column connectionsa) Short end plateb) Extended end platec) Haunched
62 BEAM-TO-BEAM ANDTRUSS CONNECTIONSBeam-beam connections similar to beam-column connectionsMoment continuity may be obtained between secondary beamsCheck for torsion in primary beamsGusset PlateSpliceplateGussetPlateesupport(a) Apex Connection(b) Support connectionTruss Connections
63 FATIGUE BEHAVIOURFatigue leads to initiation and growth of cracks under fluctuating stresseseven below the yield stress of the material (High-cycle fatigue)Fatigue cracks grow from points of stress concentrationsTo avoid stress concentrations in bolted connectionsUse gusset plates of proper shapeUse match drillingUse HSFG boltsFatigue also depends on range of stress fluctuations and reversal of stresspre-tensioned HSFG avoid reversals but lead to fretting corrosionFatigue design carried out by means of an S-N curve on a log-log scaleComponents are designed below the endurance limit
65 Design Example 1: Design a bolted connection between a bracket 8 mm thick and the flange of an ISHB 400 column using HSFG bolts, so as to carry a factored vertical load of 100 kN at a distance of 200 mm from the face of the column as shown in Fig. E1.Solution:1) Bolt force:Px = 0; Py = 100 kN;Total eccentricity x’= /2=325 mmM = Pyx’ = 100x325 = kN-mm
66 Try the arrangement shown in Fig Try the arrangement shown in Fig. E1 Note: minimum pitch = 60 mm and minimum edge dist. = 60 mm
67 2) Bolt capacityTry M20 HSFG boltsBolt capacity in single shear = μf n Kh Fo = 0.48 × 1.0 × 177 = 85 kNISHB 400 flange is thicker than the bracket plate and so bearing on thebracket plate will govern.Bolt capacity in bearing = d t pbg = 20 × 8 × 650 × 10-3 = 104 kN∴ Bolt value = 85 kN > safe.
68 Design Example 2: Design a bolted splice for an ISMB 450 section to transfer a factored bending moment of 150 kN-m and a factored shear of 100 kN. Assume that the flange splices carry all of the moment and that the web splice carries only the shear.
70 Slip resistance per bolt = 0.33 × 183 = 60.4 kN Bearing resistance on flange per bolt = 20 × 17.4 × 650 × 10-3 = kNBolt value = 60.4 kNUse 3 rows of 2 bolts at a pitch of 60 mmFlange capacity = (250/1.10) × 1844 × 10-3 = kN > flange force OKTry 150 mm wide splice plateThickness of splice plate required= × 103/1.0 × 250(150-2 × 22)/1.10 = 15.8 mm Use 16 mmUse flange splice plate of size 400×150 × 16
71 2) Web SpliceFor M20 HSFG bolts of Gr.8.8 in double shear Slip resistance per bolt = 2 ×60.4 = kNTry 8 mm thick web splice plates on both sides of the web.Bearing Resistance per bolt = 20 × 9.4 × 650 × 10-3 =122.2 kNBolt value = kNTry 3 bolts at 100 mm vertical pitch and 45 mm from the center of joint.Horizontal shear force on bolt due to moment due to eccentricity= 100 × 45 × 100/(2 × 1002) = 22.5 kNVertical Shear force per bolt = 100/3 = 33.3 kNResultant shear force = √( ) = 40.2 kN < (bolt cap) OKUse web splice plate of size 270×160×8 - 2 nos.
72 Design Example 3: Design a Seating angle connection for an ISMB 400 beam to an ISHB 200 column so as to transfer a shear of 200 kN.
73 The support reaction acts as a UDL over length (b+ 2.5h2) on the web 1) Seating AngleThe support reaction acts as a UDL over length (b+ 2.5h2) on the webLength of bearing required at root line of beam (b+2.5 h2)= V/(twpyw)= 200 × 103/(8.9 × 250/1.10) = 98.9 say 100 mmLength of bearing on cleat = b = h2 =25 mmend clearance of beam from the face of the column c= 5mmallow tolerance d = 5 mmminimum length of angle leg required for seating = b+c+d= 35 mmTry ISA 110×110×15 angle of length w = bf = 140 mm
74 Distance from end of bearing on cleat to root of angle (A to B) = b + c + d - (t+r) of angle;= – 25 = 10 mmassuming the load to be uniformly distributed over the bearing length bmoment at the root of angle =(200/10)× 102/2 = 1.0 kN-mMoment capacity = (250/1.1)×(140×152/4) ×10-6= 1.79 kN-m > 1.0 kN-m OKShear Capacity of outstanding leg of cleat= [(250/1.10)/1.732]× 140×15×10-3= kN >200 kN OK
75 2) Connection of seating angle to column flange Bolts required to resist only shearTry 4 bolts of 22 mm dia and grade 4.6, capacity = 52.7kN/boltTotal shear capacity = 4×52.7=210.8 kN > 200 kN OK3) Provide nominal clip angle of ISA 50 × 50 × 8 at the top
76 Design Example 4: Design a bolted web cleat beam-to-column connection between an ISMB 400 beam and an ISHB 40 kg/m column. The connection has to transfer a factored shear of 150 kN. Use bolts of diameter 20 mm and grade 4.6.
77 1) The recommended gauge distance for column flange is 100 mm 1) The recommended gauge distance for column flange is 100 mm. Therefore required angle back mark is 50 mm. Use web cleats of ISA 90x90x8 giving gauge g = =108.9 mm (g for ISHB200 is 100 mm )OK
78 2) Connection to web of beam- Bolt capacity shear capacity of bolt in double shear = 2×160×245×10-3=78.4 kNbearing capacity of bolt on the beam web = 418×20×9.0×10-3= kNbolt value = kNTry 4 bolts as shown in the Figure with vertical pitch of 75 mmAssuming the shear to be acting on the face of the column, its eccentricitywith the centre of the bolt group will produce horizontal shear forces inthe bolts in addition to the vertical shear.
79 horizontal shear force on top bolt due to moment due to eccentricity e = Px e ri/Σ ri2= 150×50×112.5/2( ) = 30.0 kNvertical shear force per bolt = 150/4 = 37.5 kNresultant shear = √( ) = 48.0 kN < bolt value Safe !
80 3) Connection to column flange: Bolt capacity shear capacity of bolt in single shear = 160×245×10-3 = 39.2 kNbearing capacity of bolt on column flange = 418×20×9.0×10-3= kNbolt value = 39.2 kNTry 6 bolts as shown in the Fig.E5 with vertical pitch of 75 mm4) Check bolt forceSimilar to the previous case, the shear transfer between the beam web and the angle cleats can be assumed to take place on the face of the beam web.
81 However, unlike the previous case, no relative rotation is possible between the angle and the beam web.Assuming centre of pressure 25 mm below top of cleat (point A),horizontal shear force on bolt due to moment due to eccentricity e=(V/2)exri/Σri2= (150×50/2)× 200/( ) =12.9 kNvertical shear force per bolt = 150/6 = 25.0 kNresultant shear = √( ) = kN < bolt value OKUse 2 Nos ISA 90x90x8 of length 375 mm as angle cleatsISA 90x90x8 Length 375mm
82 Design Example 5: Design a bolted end plate connection between an ISMB 400 beam and an ISHB 40 kg/m column so as to transfer a hogging factored bending moment of 150 kN-m and a vartical factored shear of 150 kN. Use HSFG bolts of diameter 20 mm and Grade 10.9.
83 1) bolt forces taking moment about the centre of the bottom flange and neglecting the contribution of bottom bolts and denoting the force in the top bolts by F4F× 384 = 150× 103F = 97.6 kNtension capacity of M20 bolt = 0.9Fo = kNallowable prying force Q = = 61.7 kN
84 2) design for prying action try 30 mm thick end plate of width be = 180 mmdistance from the centre line of bolt to prying force n isthe minimum of edge distance or 1.1T√βPo/Py = 1.1× 30 √(2× 512/250) = mmn = 40 mmassuming 10 mm fillet weld,distance from center line of bolt to toe of fillet weld b = = 50 mm;moment at the toe of the weld = Fb-Qn = 97.6× ×40 = 2412 N-meffective width of end plate per bolt w = be/2 = 180/2 = 90 mmmoment capacity =(fy/1.10)×(wT2/4) =(250/1.10)(90×302/4)=4402 N-m > 2412 N-m Safe !