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Teaching Resources © IIT Madras, SERC Madras, Anna Univ., INSDAG Calcutta 1
BOLTED CONNECTIONS
Teaching Resources © IIT Madras, SERC Madras, Anna Univ., INSDAG Calcutta 2
• Introduction• Bolted Connections• Bolts and Bolting• Force Transfer Mechanism• Failure of Connections
In shearIn tensionCombined shear and tensionBlock shear
CONTENTS
Teaching Resources © IIT Madras, SERC Madras, Anna Univ., INSDAG Calcutta 3
• 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
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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
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Concentric Connections
(a) (b)
Moment Connections
(a) (b)
TYPES OF CONNECTIONS
Classification based on type of resultant force transferred
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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
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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
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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
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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
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(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
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PRYING EFFECT AND END PLATE DESIGN
Minimum prying force Q is given by
2
4
272 nb
wtpT
n
bQ o
y
o
p
ptn
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.
QnTbMQnM BA ;pBA M
TbMM
2 415.1
2wtpM
yp
wp
Mt
y
p
415.1min
= 2 (non-preloaded) = 1.5 for limit state designw = width/pair of boltsPo= proof load in consistent units
n is the minimum of end distance or
the minimum thickness of the plate is obtained as follows
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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
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FAILURE OF CONNECTIONS-1
Shear Connections with HSFG Bolts
(a) Slip Resistance
(b) Bearing on Plates
Psl = 1.1 Ks po
Pbg = pbgd t 1/3 e t pbg
Ks =1.0 (clearance hole) = 0.45 (untreated surfaces)po= proof load
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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
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ft/Pt
10.4
Shear and Tension Interaction Curve
fs/Ps
1
0.4
COMBINED SHEAR AND TENSION
4.1t
t
s
s
P
f
P
f
0.18.0 t
t
sl
sl
P
f
P
f
(a) Bearing Bolts
(a) HSFG Bolts
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Block Shear
BLOCK SHEAR FAILURE
T
A
B C)()( 5.06.0 BCeyABey ApApT
Capacity=Shear Capacity of AB + Tension Capacity of BC
T = (0.62 Avg fy/M0 + Atn fu/M1)
orT= (0.62 Avn fu/M1 + Atg fy/M0)
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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
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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
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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
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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
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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.
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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 (c) If depth of cleats < 0.6d design bolts for shear only(d) Eliminates need to drill holes in the beam. Limit depth and thickness t < /2 (Gr.8.8) and /3 (Gr.4.6)
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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)
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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
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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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