Download - BOLTED CONNECTIONS
Dr S R Satish Kumar, IIT Madras 1
BOLTED CONNECTIONS
Dr S R Satish Kumar, IIT Madras 2
• Introduction• Bolted Connections• Bolts and Bolting• Force Transfer Mechanism• Failure of Connections
In shearIn tensionCombined shear and tensionBlock shear
CONTENTS
Dr S R Satish Kumar, IIT Madras 3
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 4
Shear Connections
a) Lap Connection b) Butt Connection
support(a)
(b)
Tension Connection and Tension plus Shear Connection
TYPES OF CONNECTIONS -!
Singleshear
Double shear
Classification based on type of force in the bolts
Dr S R Satish Kumar, IIT Madras 5
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 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
Dr S R Satish Kumar, IIT Madras 6
Bolt Shear Transfer – Free Body Diagram
(a) Bearing Connection
(b) Friction Connection
T
Frictional Force TClamping Force, PO
Bearing stresses
Tension in bolt
T
T
T
Clamping Force, PO
FORCE TRANSFER MECHANISM
Dr S R Satish Kumar, IIT Madras 7
snug-tightposition
¾ turnposition
Tightening of HSFG bolts
Feeler gauge
TIGHTENING OF HSFG BOLTS
1) Turn-of-nut Tightening2) Calibrated Wrench Tightening3) Alternate Design Bolt Installation4) Direct Tension Indicator Method
(a) Standard (b) Oversized
(c )Short Slot (d) Long slot
Hole types for HSFG bolts
Dr S R Satish Kumar, IIT Madras 8
FAILURE OF CONNECTIONS
(a) Shearing of Bolts
(b) Bearing on Bolts
(c) Bearing on PlatesZone of plastification
Fig. 9Shear Connections with Bearing Bolts
Ps = ps As where As = 0.8A
Pbb = pbb d t
Pbs = pbs d t ½ e t pbs
Dr S R Satish Kumar, IIT Madras 9
10.3.2 Shear capacity of bolt
mb/ sbAsnnbAnnufsbV
3
10.3.1.1 Reduction factor in shear for Long Joints
1.0ljβ0.75but
/200d)j(l1.075ljβ
-
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)
10.3 Bearing Type Bolts
Dr S R Satish Kumar, IIT Madras 10
10.3.3 Bearing Capacity of bolt on any ply
10.3.4 Tension Capacity
10.3.5 Bolt subjected to combined shear and tension
10.3 Bearing Type Bolts
Vsb = (2.5 d t fu )/ γmb
Tb =(0.90 fub An)/ γmb < (fyb Asb (γm1 / γm0))/ γmb
0.1
22
ndT
eT
sdV
V
Dr S R Satish Kumar, IIT Madras 11
FAILURE OF CONNECTIONS-1
Shear Connections with HSFG Bolts
(a) Slip Resistance
(b) Bearing on Plates
Kh =1.0 (clearance hole) = 0.45 (untreated surfaces)Fo= proof load
Vsf = (µf ne Kh Fo)/ γmf
Vbf = (2.2 d t fup ) / γmf < (3 d t fyp)/ / γmf
Dr S R Satish Kumar, IIT Madras 12
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.
10.4.1 Slip resistanceVsf = (µf ne Kh Fo)/ γmf
10.4 Friction Grip Type Bolting
Dr S R Satish Kumar, IIT Madras 13
TABLE 10.2 TYPICAL AVERAGE VALUES FOR COEFFICIENT OF FRICTION (µf)
Clean mill scale 0.33
Sand blasted surface 0.48
Red lead painted surface 0.1
Treatment of surfaceCoefficient of friction
(µf)
Dr S R Satish Kumar, IIT Madras 14
10.4 Friction Grip Type Bolting
10.4.2 Bearing capacity
10.4.3 Tension capacity
10.4.4 Combined Shear and Tension
Reduction factor in shear for Long Joints will apply here
Vbf = (2.2 d t fup ) / γmf < (3 d t fyp)/ / γmf
Tf = (0.9 fu A)/ / γmf
Dr S R Satish Kumar, IIT Madras 15
(b) HSFG Connection
Bearing type connection
2T
T T
2T
To To To+T To+T
Proof LoadPo
Bolt forceB kN
Applied load 2T (kN)
HSFG
Bearing type
( c) External Tension versus bolt force
BOLTS UNDER TENSION AND PRYING EFFECT
(d) Prying Effect
Q Q
B
A
bn
T+Q
2T
T+Q
Dr S R Satish Kumar, IIT Madras 16
10.4 Friction Grip Type Bolting
10.4.5 Prying Force
2v
le
l27
4te
bo
f
eT
el2v
lQ
= 2 for non-pretensioned and 1 for pretensioned = 1.5 for LSM
be = effective width of flange per pair of bolts
(Conti….)
Dr S R Satish Kumar, IIT Madras 17
Bolt strengths Bolt grade
4.6 8.8
Shear strength ps 160 375
Bearing strength pbb 435 970
Tension strength pt 195 450
Steel grade ST42S Gr.43 Gr.50
Bearing bolts pbs 418 460 550
HSFG bolts pbg 650 825 1065
Table 1 Bolt Strengths in Clearance Holes in MPa
Table 2 Bearing Strengths of Connected Parts in MPa
DESIGN STRENGTHS FOR BOLTED CONNECTIONS
Dr S R Satish Kumar, IIT Madras 18
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
wta lt
Pqorf =
2q3br
fb
f2
brf
2
bf
ef +++=
mw
ufqafef
3232
(Conti….)
Dr S R Satish Kumar, IIT Madras 19
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 32t or 300 mm, whichever is less, where t is thickness of the thinner plate
10.2.2.2 pitch shall not exceed 16t or 200 mm, in tension members and 12t 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 32t 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 20
GENERAL ISSUES IN CONNECTION DESIGN
M = Td
Standard Connections (a) moment connection (b) simple connection
eV
T
C
dV
(a) (b)
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 21
• 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
CONTENTS -1
Dr S R Satish Kumar, IIT Madras 22
Concentric Connections
(a) (b)
Moment Connections
(a) (b)
TYPES OF CONNECTIONS
Classification based on type of resultant force transferred
Dr S R Satish Kumar, IIT Madras 23
COMBINED SHEAR AND MOMENT IN PLANE
Bolt group eccentrically loaded in shear
Pri
Rmi
O
x’
y’
• Bolt shear due to Px and Py
Rxi = Px/n and Ryi = Py/n
• M = Px y’ + Py x’
• Rmi = k ri
Mi = k ri2
MR = k ri2 = k ri
2
• Bolt shear due to M Rmi=M ri/ ri
2
22 sincos imiyiimixii RRRRR
2
22
2
22 )()( ii
iy
ii
ixi yx
Mx
n
P
yx
My
n
PR
Combined shear
Dr S R Satish Kumar, IIT Madras 24
COMBINED SHEAR AND MOMENT OUT-OF-PLANE
Bolt group resisting out-of-plane moment
Ti
d li Li
NAd/6
Li
(a) (b) (c)
C
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
Dr S R Satish Kumar, IIT Madras 25
BEAM AND COLUMN SPLICE
Bolted Beam Splice
(a)Conventional Splice
(b) End-Plate Splice
Strength, stiffness and ease in erection
Assumptions in Rolled-section& Plate Girders
Column Splices – bearing type or HSFG moment splices
Dr S R Satish Kumar, IIT Madras 26
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 27
V
BEAM-TO-COLUMN CONNECTIONS
Simple beam-to-column connections a) Clip and seating angle b) Web cleats c) Curtailed end plate
e(a) (b) (c)
(a) Economical when automatic saw and drill lines are available Check end bearing and stiffness of seating angle Clip angle used for torsional stability (b) If depth of cleats < 0.6d design bolts for shear only(c) Eliminates need to drill holes in the beam. Limit depth and thickness t < /2 (Gr.8.8) and /3 (Gr.4.6)
Dr S R Satish Kumar, IIT Madras 28
BEAM-TO-COLUMN CONNECTIONS
Rigid beam-to-column connections a) Short end plateb) Extended end plate c) Haunched
column webstiffeners
diagonalstiffener
web plate
(a) (b) (c)
Dr S R Satish Kumar, IIT Madras 29
BEAM-TO-BEAM AND
TRUSS CONNECTIONS
(a) Apex Connection
Truss Connections(b) Support connection
GussetPlate
Spliceplate
GussetPlate
e
support
Beam-beam connections similar to beam-column connectionsMoment continuity may be obtained between secondary beamsCheck for torsion in primary beams
Dr S R Satish Kumar, IIT Madras 30
FATIGUE BEHAVIOUR
Fatigue 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 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 scaleComponents are designed below the endurance limit
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