CONNECTION DESIGN REQUIREMENTS

CONNECTION DESIGN REQUIREMENTS OUTLINE INTRODUCTION COMPLEXITIES OF CONNECTIONS TYPES OF CONNECTIONS CONNECTION DESIGN PHILOSOPHY COST OF CONNECTIONS SUMMARY

INTRODUCTION Necessity for Connections Importance of Connection Limited Length of Members Rolling & Transportation Constraints Larger Size of Structures Importance of Connection Structure is only as strong as the weakest link Connection failure to be avoided before member failure The full strength of members is to be utilised Connection failure is usually not ductile

INTRODUCTION Requirements of Connection Designs Strength, Stiffness, and Ductility Deflection control & stability under service load Large Defection & Ductility at Ultimate load (over load) Connections are Complex Because They are more complex to design than members They have greater potential for variability in behaviour & strength Most vulnerable component of a structure Failure of a connection often leads to failure of the structure

COMPLEXITY OF CONNECTIONS Relaibility or Safety of a design depends on Variability of loads Variability of the member strength Variability of Connection Strength Larger Uncertainty of Connections is Due to Complexity of Connection Geometry Highly Indeterminate Stress concentration Non-Linearity due to slip, & local yielding Geometric Imperfections Residual Stresses & Strains

COMPLEXITY OF GEOMETRY Beam Tee Bolted Connection Welded Connection Bolts Column Bracket Angle Flange Plate Stiffener

COMPLEXITY OF CONNECTIONS

COMPLEXITY OF CONNECTIONS GEOMETRIC IMPERFECTIONS Bow in members as rolled Lack of fit in Black Bolts in clearance holes Fabrication Errors Member deflections Welding distortions Gaps & tolerances in fabrication & erection RESIDUAL STRESSES & STRAINS Differential cooling after hot rolling, gas cutting & welding Premature yielding under loading Lack of fit in bolted fabrication (Distortions) Member strength ?

Failure of pipe connection Partial safety factor for connection = 1.25 1.5 (field fabrication)

TYPES OF CONNECTIONS WELDED CONNECTIONS BOLTED CONNECTIONS Fillet welding Butt welding BOLTED CONNECTIONS Bearing type Carbon steel / High strength Friction type HSFG RIVETED CONNECTIONS Mild steel High strength steel

BOLTED CONNECTIONS Bearing Type: X Bearing Stress Notice slip in bearing type of connection Clamping Force, P0 T Frictional Force T Contact Pressure, P0

Bolt Shear Transfer – Free Body Diagram (a) Bearing Connection (b) Friction Connection T Frictional Force T Clamping Force, PO Bearing stresses Tension in bolt FORCE TRANSFER MECHANISM Dr S R Satish Kumar, IIT Madras

BOLTED CONNECTIONS Bearing Type: Friction Type: Notice no slip is observed in-between plates in HSFG Connection T Bearing Stress X T Clamping Force, P0 T Frictional Force T Contact Pressure, P0 T

Merits Welded Connections Transfer of forces between elements more direct Requires little additional elements like gussets Shorter length of joints No reduction in member strength due to bolt holes etc. Rigid connections easy to achieve

Demerits Requires skilled manpower Requires special equipment not easy to achieve at difficult locations less ductile prone to defects & fatigue cracks under cyclic loading

Merits & Demerits Bolted Connections Bearing Type Easy to install even at difficult locations Economical Does not require highly skilled manpower Slip causes flexible joint Joint size larger

Merits & Demerits Bolted Connections Friction Type Rigidity of connection Better fatigue performance Expensive due to material & installation labour Requires skilled manpower Requires better inspection

RIGIDITY OF MOMENT CONNECTIONS Type of connections Rigid Hinged Semi-Rigid >90 h Mr

RIGIDITY OF MOMENT CONNECTIONS h Moment Mr Rotation Rigid joint Hinged joint Semi-rigid joint

CONNECTION DESIGN PHILOSOPHY Connections are complex The strength is variable Large numbers have to be designed ‘Exact’ analysis: Complex but possible Accuracy depends on assumptions Not practically feasible Practical, simplified Methods are Appropriate Should satisfy equilibrium Ductility requirements (static loading) Fatigue strength requirements (cyclic loading)

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 transferred Replace the forces/moments transferred by member Equivalent system of forces on interface elements in the joint Consider the mechanism of force transfer in the member Transfer to elements in consistency with their relative stiffness The system of forces be in equilibrium with the force to be transferred

TRANSFER OF MEMBER FORCES (a) Connection (b) Freebody Diagram Critical section for block shear V V

FORCE FLOW IN A JOINT Force flow is complex High degree of indeterminacy Local stress concentration and yielding Effect of Residual stress, which is unknown Connection force flow Analysis Making simplifying Assumptions – sharing of forces Satisfying Equilibrium Ensuring adequacy of strength Ensuring adequacy of stiffness Ensuring adequacy of ductility

SIMPLIFIED ANALYSIS OF JOINTS Assume sharing of forces among alternate elements Stiffer elements attract larger proportion of the imposed force Plate elements are stiff in resisting forces imposed in their plane Plate elements are flexible in resisting forces imposed out-of-plane The assumed forces may be at variance from the elastic results Equilibrium & Compatibility are to be satisfied in elastic analysis Only equilibrium is assumed in the simplified analysis Ensure adequacy of ductility to redistribute forces as assumed Redistribution is necessary since assumed sharing may at variance Ensure adequacy of strength of elements in the load path

SIMPLIFIED ANALYSIS OF JOINTS C T S1 S2

COST OF CONNECTIONS Design cost Fabrication / Erection cost Consumes a major portion of efforts Simplified design methods reduce the cost Standardised designs and details are desirable Design handbook, aids and software Fabrication / Erection cost Repetitive use of standard details Good access, easy support, ease of joining at location Mix of automatic and manual fabrication Choice of connection method Other factors Simple detail, simple techniques appropriate to requirement

relative stiffness of elements, and ductility of elements Sound Connection Design Requires Understanding of the requirement force flow relative stiffness of elements, and ductility of elements Good design can considerably reduce cost of steel structure

BOLTED CONNECTIONS

Analysis of Bolt Groups Combined Shear and Moment in-Plane Combined Shear and Moment out-of-plane Beam and Column Splices Beam to Column Connections Beam to Beam Connections Truss Connections Fatigue Behaviour

Designed more conservatively than members because they are more complex to analyse and discrepancy between analysis and design is large In case of overloading, failure in member is preferred to failure in connection Connections account for more than half the cost of structural steel work Connection design has influence over member design Similar to members, connections are also classified as idealised types Effected through rivets, bolts or weld Codal Provisions

Concentric Connections TYPES OF CONNECTIONS Classification based on type of resultant force transferred (a) (b) Concentric Connections (a) (b) Moment Connections

Tension Connection and Tension plus Shear Connection TYPES OF CONNECTIONS -! Classification based on type of force in the bolts Single shear a) Lap Connection b) Butt Connection Double shear Shear Connections support (a) (b) Tension Connection and Tension plus Shear Connection

BOLTS AND BOLTING Bolt Grade: Grade 4.6 :- fu = 40 kgf/mm2 and fy = 0.6*40 = 24 kgf/mm2 Bolt Types: Black, Turned & Fitted, High Strength Friction Grip Black Bolts: usually Gr.4.6, made snug tight, ductile and cheap, only static loads Turned & Fitted; Gr.4.6 to 8.8, Close tolerance drilled holes, 0.2% proof stress HSFG Bolts: Gr.8.8 to 10.9, less ductile, excellent under dynamic/fatigue loads

TIGHTENING OF HSFG BOLTS snug-tight position ¾ turn Tightening of HSFG bolts 1) Turn-of-nut Tightening 2) Calibrated Wrench Tightening 3) Alternate Design Bolt Installation 4) Direct Tension Indicator Method (a) Standard (b) Oversized (c )Short Slot (d) Long slot Feeler gauge Hole types for HSFG bolts

Bolt Shear Transfer – Free Body Diagram (a) Bearing Connection (b) Friction Connection T Frictional Force T Clamping Force, PO Bearing stresses Tension in bolt Clamping Force, PO FORCE TRANSFER MECHANISM

BOLTS UNDER TENSION AND PRYING EFFECT (b) HSFG Connection Bearing type connection 2T T To To+T (d) Prying Effect Q B A b n T+Q 2T Proof Load Po Bolt force B kN Applied load 2T (kN) HSFG Bearing type ( c) External Tension versus bolt force

Bolted Steel Connections Bolts in tension 6 x 200 =1200 kN Bolts in shear

Failure modes of bolts in shear Hole bearing Hole tearout Bolt shear

PRYING EFFECT AND END PLATE DESIGN Minimum prying force Q is given by = 2 (non-preloaded) = 1.5 for limit state design w = width/pair of bolts Po= proof load in consistent units n is the minimum of end distance or the minimum thickness of the plate is obtained as follows The 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.

Tnb = 0.90 fub An < fyb Asb (γmb / γm0) FAILURE OF CONNECTIONS Shear Connections with Bearing Bolts (a) Shearing of Bolts (b) Bearing on Bolts Vnpb = 2.5 kb d t f’u Zone of plastification (c) Tension capacity Tnb = 0.90 fub An < fyb Asb (γmb / γm0)

BOLTS UNDER TENSION AND PRYING EFFECT (b) HSFG Connection Bearing type connection 2T T To To+T (d) Prying Effect Q B A b n T+Q 2T Proof Load Po Bolt force B kN Applied load 2T (kN) HSFG Bearing type ( c) External Tension versus bolt force

Vnsf = µf. ne. Kh. Fo FAILURE OF CONNECTIONS Shear Connections with HSFG Bolts (a) Slip Resistance Vnsf = µf. ne. Kh. Fo Kh =1.0 (clearance hole) = 0.45 (untreated surfaces) ne = no of effective interfaces Fo= proof load (b) Bearing on Plates Pbg = pbgd t  1/3 e t pbg

DESIGN STRENGTHS FOR BOLTED CONNECTIONS Table 1 Bolt Strengths in Clearance Holes in MPa Bolt strengths Bolt grade 4.6 8.8 Shear strength ps 160 375 Bearing strength pbb 435 970 Tension strength pt 195 450 Table 2 Bearing Strengths of Connected Parts in MPa Steel grade ST42S Gr.43 Gr.50 Bearing bolts pbs 418 460 550 HSFG bolts pbg 650 825 1065

COMBINED SHEAR AND TENSION (a) Bearing Bolts (a) HSFG Bolts

BLOCK SHEAR FAILURE Capacity=Shear Capacity of AB + Tension Capacity of BC T A B C Tdb = ( Avg fy /( m0) + 0.9Atn fu /m1 ) Block Shear or Tdb = (0.9Avn fu /( m1) + Atg fy /m0 )

GENERAL ISSUES IN CONNECTION DESIGN Assumptions in traditional analysis M = Td Standard Connections (a) moment connection (b) simple connection e V T C d (a) (b) Connection elements are assumed to be rigid compared to the connectors Connector behaviour is assumed to be linearly elastic Distribution of forces arrived at by assuming idealized load paths Provide stiffness according to the assumed behaviour ensure adequate ductility and rotation capacity provide adequate margin of safety

COMBINED SHEAR AND MOMENT IN PLANE Bolt shear due to Px and Py Rxi = Px/n and Ryi = Py/n  P ri Rmi O x’ y’ M = Px y’ + Py x’ Rmi = k ri Mi = k ri2 MR =  k ri2 = k  ri2 Bolt shear due to M Rmi=M ri/ ri2 Bolt group eccentrically loaded in shear Combined shear

COMBINED SHEAR AND MOMENT OUT-OF-PLANE Ti li Li NA d/6 (a) (b) (c) C Bolt group resisting out-of-plane moment Ti = kli where k = constant M =  Ti Li = k  li Li Ti = Mli/ li Li Shear assumed to be shared equally and bolts checked for combined tension+(prying)+shear

Strength, stiffness and ease in erection BEAM AND COLUMN SPLICE Strength, stiffness and ease in erection Assumptions in Rolled-section & Plate Girders (a)Conventional Splice (b) End-Plate Splice Bolted Beam Splice Column Splices – bearing type or HSFG moment splices

BEAM-TO-COLUMN CONNECTIONS (a) Simple – transfer only shear at nominal eccentricity Used in non-sway frames with bracings etc. Used in frames upto 5 storeys (b) Semi-rigid – model actual behaviour but make analysis difficult (linear springs or Adv.Analysis). However lead to economy in member designs. (c) Rigid – transfer significant end-moments undergoing negligible deformations. Used in sway frames for stability and contribute in resisting lateral loads and help control sway.

BEAM-TO-COLUMN CONNECTIONS V Simple beam-to-column connections a) Clip and seating angle b) Web cleats c) Curtailed end plate Economical when automatic saw and drill lines are available Check end bearing and stiffness of seating angle Clip angle used for torsional stability If depth of cleats < 0.6d design bolts for shear only Eliminates need to drill holes in the beam. Limit depth and thickness t < /2 (Gr.8.8) and /3 (Gr.4.6)

Rigid beam-to-column connections column web stiffeners web plate diagonal stiffener (a) (b) (c) Rigid beam-to-column connections a) Short end plate b) Extended end plate c) Haunched

BEAM-TO-BEAM AND TRUSS CONNECTIONS Beam-beam connections similar to beam-column connections Moment continuity may be obtained between secondary beams Check for torsion in primary beams Gusset Plate Splice plate GussetPlate e support (a) Apex Connection (b) Support connection Truss Connections

FATIGUE BEHAVIOUR Fatigue leads to initiation and growth of cracks under fluctuating stresses even below the yield stress of the material (High-cycle fatigue) Fatigue cracks grow from points of stress concentrations To avoid stress concentrations in bolted connections Use gusset plates of proper shape Use match drilling Use HSFG bolts Fatigue also depends on range of stress fluctuations and reversal of stress pre-tensioned HSFG avoid reversals but lead to fretting corrosion Fatigue design carried out by means of an S-N curve on a log-log scale Components are designed below the endurance limit

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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’=200+250/2=325 mm M = Pyx’ = 100x325 = 32500 kN-mm

Try the arrangement shown in Fig Try the arrangement shown in Fig. E1 Note: minimum pitch = 60 mm and minimum edge dist. = 60 mm

2) Bolt capacity Try M20 HSFG bolts Bolt capacity in single shear = μf n Kh Fo = 0.48 × 1.0 × 177 = 85 kN ISHB 400 flange is thicker than the bracket plate and so bearing on the bracket plate will govern. Bolt capacity in bearing = d t pbg = 20 × 8 × 650 × 10-3 = 104 kN ∴ Bolt value = 85 kN > 81.79 safe.

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.

Solution: 1) Flange Splices : Flange force =BM/(D-tf) = 150 × 103/(450-17.4) = 346.7 kN

Slip resistance per bolt = 0.33 × 183 = 60.4 kN Bearing resistance on flange per bolt = 20 × 17.4 × 650 × 10-3 = 226.2 kN Bolt value = 60.4 kN Use 3 rows of 2 bolts at a pitch of 60 mm Flange capacity = (250/1.10) × 1844 × 10-3 = 400.9 kN > flange force OK Try 150 mm wide splice plate Thickness of splice plate required = 346.7 × 103/1.0 × 250(150-2 × 22)/1.10 = 15.8 mm Use 16 mm Use flange splice plate of size 400×150 × 16

2) Web Splice For M20 HSFG bolts of Gr.8.8 in double shear Slip resistance per bolt = 2 ×60.4 = 120.8 kN Try 8 mm thick web splice plates on both sides of the web. Bearing Resistance per bolt = 20 × 9.4 × 650 × 10-3 =122.2 kN Bolt value = 120.8 kN Try 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 kN Vertical Shear force per bolt = 100/3 = 33.3 kN Resultant shear force = √(22.52+33.32) = 40.2 kN < 120.8 (bolt cap) OK Use web splice plate of size 270×160×8 - 2 nos.

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.

The support reaction acts as a UDL over length (b+ 2.5h2) on the web 1) Seating Angle The support reaction acts as a UDL over length (b+ 2.5h2) on the web Length 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 mm Length of bearing on cleat = b = 100-2.5 h2 =25 mm end clearance of beam from the face of the column c= 5mm allow tolerance d = 5 mm minimum length of angle leg required for seating = b+c+d = 35 mm Try ISA 110×110×15 angle of length w = bf = 140 mm

Distance from end of bearing on cleat to root of angle (A to B) = b + c + d - (t+r) of angle; = 25 + 5 + 5 – 25 = 10 mm assuming the load to be uniformly distributed over the bearing length b moment at the root of angle =(200/10)× 102/2 = 1.0 kN-m Moment capacity = (250/1.1)×(140×152/4) ×10-6 = 1.79 kN-m > 1.0 kN-m OK Shear Capacity of outstanding leg of cleat = [(250/1.10)/1.732]× 140×15×10-3 = 275.5 kN >200 kN OK

2) Connection of seating angle to column flange Bolts required to resist only shear Try 4 bolts of 22 mm dia and grade 4.6, capacity = 52.7kN/bolt Total shear capacity = 4×52.7=210.8 kN > 200 kN OK 3) Provide nominal clip angle of ISA 50 × 50 × 8 at the top

Design Example 4: Design a bolted web cleat beam-to-column connection between an ISMB 400 beam and an ISHB 200 @ 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.

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 = 50+50+8.9=108.9 mm (g for ISHB200 is 100 mm )OK

2) Connection to web of beam- Bolt capacity shear capacity of bolt in double shear = 2×160×245×10-3=78.4 kN bearing capacity of bolt on the beam web = 418×20×9.0×10-3= 75.24 kN bolt value = 75.24 kN Try 4 bolts as shown in the Figure with vertical pitch of 75 mm Assuming the shear to be acting on the face of the column, its eccentricity with the centre of the bolt group will produce horizontal shear forces in the bolts in addition to the vertical shear.

horizontal shear force on top bolt due to moment due to eccentricity e = Px e ri/Σ ri2 = 150×50×112.5/2(37.52+112.52) = 30.0 kN vertical shear force per bolt = 150/4 = 37.5 kN resultant shear = √(30.02+37.52) = 48.0 kN < bolt value Safe !

3) Connection to column flange: Bolt capacity shear capacity of bolt in single shear = 160×245×10-3 = 39.2 kN bearing capacity of bolt on column flange = 418×20×9.0×10-3= 75.24 kN bolt value = 39.2 kN Try 6 bolts as shown in the Fig.E5 with vertical pitch of 75 mm 4) Check bolt force Similar 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.

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/(502+1252+2002) =12.9 kN vertical shear force per bolt = 150/6 = 25.0 kN resultant shear = √(12.92+25.02) = 28.13 kN < bolt value OK Use 2 Nos ISA 90x90x8 of length 375 mm as angle cleats ISA 90x90x8 Length 375mm

Design Example 5: Design a bolted end plate connection between an ISMB 400 beam and an ISHB 200 @ 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.

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 F 4F× 384 = 150× 103 F = 97.6 kN tension capacity of M20 bolt = 0.9Fo = 159.3 kN allowable prying force Q = 159.3-97.6 = 61.7 kN

2) design for prying action try 30 mm thick end plate of width be = 180 mm distance from the centre line of bolt to prying force n is the minimum of edge distance or 1.1T√βPo/Py = 1.1× 30 √(2× 512/250) = 55.66 mm n = 40 mm assuming 10 mm fillet weld, distance from center line of bolt to toe of fillet weld b = 60-10 = 50 mm; moment at the toe of the weld = Fb-Qn = 97.6×50-61.7×40 = 2412 N-m effective width of end plate per bolt w = be/2 = 180/2 = 90 mm moment capacity =(fy/1.10)×(wT2/4) =(250/1.10)(90×302/4)=4402 N-m > 2412 N-m Safe !

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