BOLTED CONNECTIONS Dr S R Satish Kumar IIT

BOLTED CONNECTIONS Dr S R Satish Kumar, IIT Madras 1

CONTENTS • Introduction • Bolted Connections • Bolts and Bolting • Force Transfer Mechanism • Failure of Connections In shear In tension Combined shear and tension Block shear Dr S R Satish Kumar, IIT Madras 2

INTRODUCTION • 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 Dr S R Satish Kumar, IIT Madras 3

TYPES OF CONNECTIONS -! Classification based on type of force in the bolts Single shear Double shear a) Lap Connection b) Butt Connection Shear Connections support (a) (b) Tension Connection and Tension plus Shear Connection Dr S R Satish Kumar, IIT Madras 4

BOLTS AND BOLTING Bolt Grade: Grade 4. 6 : - fu = 40 kgf/mm 2 and fy = 0. 6*40 = 24 kgf/mm 2 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 Dr S R Satish Kumar, IIT Madras 5

FORCE TRANSFER MECHANISM (a) Bearing Connection T Bearing stresses T (b) Friction Connection T Clamping Force, PO Tension in bolt Frictional Force T T Clamping Force, PO Bolt Shear Transfer – Free Body Diagram Dr S R Satish Kumar, IIT Madras 6

TIGHTENING OF HSFG BOLTS 1) Turn-of-nut Tightening 2) Calibrated Wrench Tightening 3) Alternate Design Bolt Installation 4) Direct Tension Indicator Method ¾ turn position snug-tight position Tightening of HSFG bolts (a) Standard (b) Oversized (c )Short Slot (d) Long slot Feeler gauge Hole types for HSFG bolts Dr S R Satish Kumar, IIT Madras 7

FAILURE OF CONNECTIONS Shear Connections with Bearing Bolts Fig. 9 (a) Shearing of Bolts Ps = ps As where As = 0. 8 A (b) Bearing on Bolts Pbb = pbb d t (c) Bearing on Plates Zone of plastification Pbs = pbs d t ½ e t pbs Dr S R Satish Kumar, IIT Madras 8

10. 3 Bearing Type Bolts 10. 3. 2 Shear capacity of bolt 10. 3. 1. 1 Reduction factor in shear for Long Joints 10. 3. 1. 2 Reduction factor in shear for Large Grip Lengths lg = 8 d /(3 d+lg) 10. 3. 2. 3 Reduction factor for Packing Plates pk = (1 - 0. 0125 tpk) Dr S R Satish Kumar, IIT Madras 9

10. 3 Bearing Type Bolts 10. 3. 3 Bearing Capacity of bolt on any ply Vsb = (2. 5 d t fu )/ γmb 10. 3. 4 Tension Capacity Tb =(0. 90 fub An)/ γmb 10. 3. 5 < (fyb Asb (γm 1 / γm 0))/ γmb Bolt subjected to combined shear and tension Dr S R Satish Kumar, IIT Madras 10

FAILURE OF CONNECTIONS-1 Shear Connections with HSFG Bolts (a) Slip Resistance Vsf = (µf ne Kh Fo)/ γmf Kh =1. 0 (clearance hole) = 0. 45 (untreated surfaces) Fo= proof load (b) Bearing on Plates Vbf = (2. 2 d t fup ) / γmf < (3 d t fyp)/ / γmf Dr S R Satish Kumar, IIT Madras 11

10. 4. 1 Slip resistance Friction Grip Type Bolting Vsf = (µf ne Kh Fo)/ γmf Where, µf = coeff. of friction (slip factor) as in Table 10. 2 (µf < 0. 55) ne = number of effective interfaces offering frictional resistance to slip Kh = 1. 0 for fasteners in clearance holes = 0. 85 for fasteners in oversized and short slotted holes = 0. 7 for fasteners in long slotted holes loaded parallel to the slot. γmf = 1. 10 (if slip resistance is designed at service load) γmf = 1. 25 (if slip resistance is designed at ultimate load) Fo = minimum bolt tension (proof load) at installation ( 0. 8 Asb fo) Asb = shank area of the bolt fo = proof stress (= 0. 70 fub) Note: Vns may be evaluated at a service load or ultimate load using appropriate partial safety factors, depending upon whether slip resistance is required at service load or ultimate load. Dr S R Satish Kumar, IIT Madras 12

TABLE 10. 2 TYPICAL AVERAGE VALUES FOR COEFFICIENT OF FRICTION (µf) Treatment of surface Coefficient of friction (µf) Clean mill scale 0. 33 Sand blasted surface 0. 48 Red lead painted surface 0. 1 Dr S R Satish Kumar, IIT Madras 13

10. 4 Friction Grip Type Bolting 10. 4. 2 Bearing capacity Vbf = (2. 2 d t fup ) / γmf < (3 d t fyp)/ / γmf 10. 4. 3 Tension capacity Tf = (0. 9 fu A)/ / γmf 10. 4. 4 Combined Shear and Tension Reduction factor in shear for Long Joints will apply here Dr S R Satish Kumar, IIT Madras 14

BOLTS UNDER TENSION AND PRYING EFFECT 2 T Bearing type connection T 2 T (b) HSFG Connection To To+ T 2 T Bolt force B k. N HSFG B Bearing type Proof Load Po Applied load 2 T (k. N) ( c) External Tension versus bolt force Dr S R Satish Kumar, IIT Madras b n A Q Q T+Q (d) Prying Effect 15

10. 4 Friction Grip Type Bolting 10. 4. 5 Prying Force = 2 for non-pretensioned and 1 for pretensioned = 1. 5 for LSM be = effective width of flange per pair of bolts Dr S R Satish Kumar, IIT Madras (Conti…. ) 16

DESIGN STRENGTHS FOR BOLTED CONNECTIONS Table 1 Bolt Strengths in Clearance Holes in MPa Bolt strengths Shear strength ps Bearing strength pbb Tension strength pt Bolt grade 4. 6 8. 8 160 375 435 970 195 450 Table 2 Bearing Strengths of Connected Parts in MPa Steel grade ST 42 S Gr. 43 Gr. 50 Bearing bolts pbs 418 460 550 HSFG bolts pbg 650 825 1065 Dr S R Satish Kumar, IIT Madras 17

10. 5. 9 Stresses due to Individual forces 10. 5. 10 Combination of stresses 10. 5. 10. 1 Fillet welds Combined bearing, bending and shear Dr S R Satish Kumar, IIT Madras (Conti…. ) 18

10. 2 Fasteners spacing and edge distance 10. 2. 1 Minimum Spacing - 2. 5 times the nominal diameter 10. 2. 2 Maximum Spacing - shall not exceed 32 t or 300 mm, whichever is less, where t is thickness of the thinner plate 10. 2. 2. 2 pitch shall not exceed 16 t or 200 mm, in tension members and 12 t or 200 mm, whichever is less, in compression members 10. 2. 3 Edge and End Distances minimum edge shall be not less than that given in Table 10. 1. maximum edge distance should not exceed 12 t , where = (250/fy)1/2 10. 2. 4 Tacking Fasteners spacing in line not exceeding 32 t or 300 mm If exposed to the weather, 16 t or 200 mm max. spacing in tension members 1000 mm max. spacing in compression members 600 mm Dr S R Satish Kumar, IIT Madras 19

GENERAL ISSUES IN CONNECTION DESIGN Assumptions in traditional analysis • 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 Dr S R Satish Kumar, IIT Madras T V d C V e M = Td (a) (b) Standard Connections (a) moment connection (b) simple connection 20

CONTENTS -1 • 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 Dr S R Satish Kumar, IIT Madras 21

TYPES OF CONNECTIONS Classification based on type of resultant force transferred (a) (b) Concentric Connections (a) (b) Moment Connections Dr S R Satish Kumar, IIT Madras 22

COMBINED SHEAR AND MOMENT IN PLANE • Bolt shear due to Px and Py Rxi = Px/n and Ryi = Py/n • M = Px y’ + Py x’ • Rmi = k ri Mi = k ri 2 MR = k ri 2 = k ri 2 • Bolt shear due to M Rmi=M ri/ ri 2 Combined shear Dr S R Satish Kumar, IIT Madras Rmi x’ ri P y’ O Bolt group eccentrically loaded in shear 23

COMBINED SHEAR AND MOMENT OUT-OF-PLANE Ti d li d/6 (a) Li NA (b) Li C (c) Bolt group resisting out-of-plane moment Ti = kli where k = constant M = T i L i = k li L i Ti = Mli/ li Li Shear assumed to be shared equally and bolts checked for combined tension+(prying)+shear Dr S R Satish Kumar, IIT Madras 24

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 Dr S R Satish Kumar, IIT Madras 25

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. Dr S R Satish Kumar, IIT Madras 26

BEAM-TO-COLUMN CONNECTIONS (a) e V (b) (c) Simple beam-to-column connections a) Clip and seating angle b) Web cleats c) Curtailed end plate (a) Economical when automatic saw and drill lines are available (b) Check end bearing and stiffness of seating angle (c) Clip angle used for torsional stability (b) If depth of cleats < 0. 6 d design bolts for shear only (c) Eliminates need to drill holes in the beam. Limit depth and thickness (d) t < /2 (Gr. 8. 8) and /3 (Gr. 4. 6) Dr S R Satish Kumar, IIT Madras 27

BEAM-TO-COLUMN CONNECTIONS column web stiffeners (a) diagonal stiffener (b) web plate (c) Rigid beam-to-column connections a) Short end plate b) Extended end plate c) Haunched Dr S R Satish Kumar, IIT Madras 28

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 Splice plate Gusset Plate (a) Apex Connection e support (b) Support connection Truss Connections Dr S R Satish Kumar, IIT Madras 29

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 www. steel-insdag. org Thank You Dr S R Satish Kumar, IIT Madras 30