Bolted Connections[1]

Fasteners for Structural Steelwork

Bolts and Bolted ConnectionsFasteners for Structural SteelworkFasteners for Structural Steelwork

Bolts and Bolted ConnectionsBolts and Bolted Connections

Rupen GoswamiDepartment of Civil Engineering

Indian Institute of Technology Madras

FastenersFastenersFasteners

Faculty Development Programme on Design of Steel Structures – Anna University, Chennai – 2 December 20103

Necessity of Connections Larger size of structures

• Limited length of members Rolling & Transportation constraints

Join two or more members

IntroductionIntroductionIntroduction


Fasteners for ConnectionsFasteners for ConnectionsFasteners for Connections

Fasteners Rivets BoltsWelds


Fasteners Rivets Bolts

RivetRivet High StrengthHexagonal Head Bolt

High StrengthHexagonal Head Bolt

High StrengthInterference Body Bolt

High StrengthInterference Body Bolt

Early DaysEarly DaysEarly Days


Early Days…Early DaysEarly Days……

Rivets Better than common hand-tightened black bolts

• Initial Pre-stress Due to heating and cooling in riveting process No control on amount of axial tension

o Could not be accounted for in design

Difficult process• Hammering• Heating• Noise• Time consuming

Labour cost!!


Bolts Fast construction

• Good performance!

Early Days…Early DaysEarly Days……


1960s1960s1960s

WeldingNeat!

Groove WeldGroove Weld

Fillet WeldFillet Weld

Slot WeldSlot Weld

Plug WeldPlug Weld


JointsJointsJoints

5 types

Butt JointButt Joint Lap JointLap Joint

Edge JointEdge JointTee JointTee JointCorner JointCorner Joint


Unique Aspects of Steel ConstructionUnique Aspects of Steel ConstructionUnique Aspects of Steel Construction

On & Off Site Fabrication Fast field erection using bolts

• Field connections are typically bolted

Welding better suited to shop fabrication under controlled environment

• Or where bolting is either impractical or undesirable!


Pure boltingMay not be possible in large steel structures

Beam

High Strength Bolts

Full Penetration Groove Weld

End Plate

Column

Courtesy:: UW

Unique Aspects of Steel Construction…Unique Aspects of Steel ConstructionUnique Aspects of Steel Construction……

Basics of BoltingBasics of BoltingBasics of Bolting


BoltingBoltingBolting

Steps Two bolts per connection

• Basic stability

Remaining bolts to be installed and tightened after the member is aligned / plumbed

• Overall geometry

Systematic pattern of tightening• Avoid local stress• Uniform load distribution


Grade of BoltsGrade of BoltsGrade of Bolts

Mild Steel Bolts (Turned & Fitted) Class 4.6

• Medium grade steel• Ductile• Light low-rise structures

High Strength Bolts Class 8.8, 10.9

• Alloy steel• Heat-treated• Low ductility• Extensively used in construction

Good under dynamic / fatigue loads


fy

fu = 100Xσ fy/fu = .Y

ε

Grade of Bolts…Grade of BoltsGrade of Bolts……

Yield & Tensile Strength Class X.Y (e.g., 4.6, 8.8, 10.9…)

Tensile Strength = X × 100 MPa• Examples

Grade 4.6:: fu = 400 MPa Grade 8.8:: fu = 800 MPa …

Yield Strength (Proof Stress) = 0.Y × fu MPa• Examples

Grade 4.6:: fy = 0.6 × 400 = 240 MPa Grade 8.8:: fy = 0.8 × 800 = 640 MPa …


Force Transfer MechanismForce Transfer MechanismForce Transfer Mechanism

Tension

Shear

Bearing• Of bolt shank• Of plate

Friction

Combination!

Two sub-types of High Strength Bolts• Bearing Type• Friction Type (Slip Critical)

As early as in early 1930s in the UK


Bearing Type Snug-tight bolts

• Enough tension to ensure that surfaces are in contact and bear on each other

Usual application – gravity load

Joint may slip• Load resistance by bearing and shear

Snug-tightSnug-tight

Force Transfer Mechanism…Force Transfer MechanismForce Transfer Mechanism……


Bearing Type• Bolt shank in bearing• Bolt shank in shear

Bearing on Bolt

Bearing on Plate

Shear in Bolt

P

P

Force Transfer Mechanism…Force Transfer MechanismForce Transfer Mechanism……


Failure Modes of Bolted ConnectionsFailure Modes of Bolted ConnectionsFailure Modes of Bolted Connections


Shear Failure of Bolt Single shear Double shear

P

P

P

P/2

P/2

Bolt in Single ShearBolt in Single Shear Bolt in Double ShearBolt in Double Shear

Failure Modes of Bolted Connections…Failure Modes of Bolted ConnectionsFailure Modes of Bolted Connections……


Bearing Failure Bolt on plate Plate on bolt

Failure Modes of Bolted Connections…Failure Modes of Bolted ConnectionsFailure Modes of Bolted Connections……

Bearing failure of boltBearing failure of bolt Bearing failure of plateBearing failure of plate

General RequirementsGeneral RequirementsGeneral Requirements


SizeSizeSize

DiameterNominal diameter DGross diameter at thread dgross

Root diameter at thread droot

d gross

d rootD

Shank ThreadBoltHead

d gross

d rootD



Three types (Table 19, pp 73)

Standard clearance holeOver size hole Slotted hole

• Short• Long

Standard(STD)

Oversized(OVS)

Short-slotted(SSL)

Long-slotted(LSL)

18 20

18

22 56

Different holes for 16mm (nominal) diameter boltDifferent holes for 16mm (nominal) diameter bolt

Bolt HolesBolt HolesBolt Holes


SpacingSpacingSpacing

Minimum Spacing 2.5D

To reduce interference

Maximum SpacingMinimum of 32t or 300 mm

To minimize unconnected length

d

b

t

g

p

g

g

p p p


Maximum PitchMinimum of 16t or 200 mm

• Tension members

Minimum of 12t or 200 mm• Compression members

To ensure participation Reasonable connection

length d

b

t

g

p

g

g

p p p

Spacing…SpacingSpacing……


Minimum Edge and End Distance 1.7d

• Sheared or hand-flame cut holes

1.5d• Rolled, machine-flame cut holes, sawn and planed edges

To prevent end ruptureEnd Distance eEnd Distance e

d

b

t

g

p

g

g

p p pEdge DistanceEdge Distance



Maximum Edge and End Distance 12tε

To prevent local buckling / flapping of unstiffened part

End DistanceEnd Distance

d

b

t

g

p

g

g

p p pEdge DistanceEdge Distance


Design PhilosophyDesign PhilosophyDesign Philosophy



Design:: To ensure Safety

• Capacity > Demand

Economy• By how much?



Capacity > Demand Capacity

• Material• Geometry

Demand• Loads• Deformations



Capacity > Demand Factored Resistance > Factored Load

• Limit State Design

Freq

uenc

y

Capacity or Demand

Demand Capacity

Safety Margin

Freq

uenc

y

Capacity or Demand

Design Strength:: Bearing BoltsDesign Strength:: Design Strength:: Bearing BoltsBearing Bolts


Two TypesTwo TypesTwo Types

Bearing Type Bolts Section 10.3

Friction Grip Type Bolts Section 10.4


Bearing Type BoltsBearing Type BoltsBearing Type Bolts

Shank Area As

Net Tensile Stress Area An

Effective Area Ae

• Depending on location of shear plane

2

4DAs

2

4 rootn dA

sne AAA or

d gross

d rootD


d gross

d rootD



Design Strength in Tension Shear Combined Tension and Shear Bearing

Bolts in Tension

Bearing Type Bolts…Bearing Type BoltsBearing Type Bolts……


Design Strength in Tension Tdb

Tdb = Tnb / mb (mb = 1.25)

Tnb = Nominal Tensile Capacity of a bolt

Tnb = 0.9 × fub × An

< fyb × As × (mb / m0)Tensile Rupture of threaded areaTensile Rupture of threaded area

Yielding of shank areaYielding of shank area

Nominal tensile capacity equal to tensile rupture strength of threaded area subjected to maximum of yield strength of shank area



Design Strength in Shear Vdsb

Vdsb = Vnsb / mb (mb = 1.25)

Vnsb = Nominal Shear Capacity of a bolt

Vnsb = (fub / √3) × (nn × Anb + ns × As)

Note: Nominal shear capacity determined at fubP

P/2

P/2



Design Strength in Shear Vdsb × βlj × βlg × βpk

Connections often have multiple bolts• Contribution of each bolt• Overall contribution not equal to sum of the individual

capacities

Reductions due to• Long Joints (βlj)• Large Grip Lengths (βlg)• Packing Plates (βpk)





Reduction due to• Long Joints

Non-uniform distribution of shear Similar to shear lag effect Farthest bolt NOT as effective as the nearest one

175.0 lj

D

l jlj 200

075.1

P

P

Shear distribution in Long JointsShear distribution in Long Joints

lj




Reduction due to• Large Grip Length

Long bolts in bending Similar to slender versus deep beam behaviours

Dlg

lj

8lg

glg lD

D

3

8

73.0lg Dlg 8for

P/2

P

Bending of bolt in joint with Large Grip LengthBending of bolt in joint with Large Grip Length

lj

P/2

lg




Reduction due to• Packing Plates

Increase joint thickness and length of bolts Similar effect as for large grip length

801 pk

pk

t



Combined Tension and Shear• Based on von Mises yield criterion

222 3 yxyx f

222

1 3

y

xy

y

x

ff

13

22

ys

xys

yt

xt

fA

A

fAA

0.122

dsb

sb

db

b

VV

TT

T

V



Design Strength in Bearing Vdpb

Vdpb = Vnpb / mb (mb = 1.25)

Vnpb = Nominal Bearing Strength of a bolt

Vnpb = 2.5 × D × t × fup

Note: Nominal bearing strength determined at fubt = Summation of thicknesses of plates

experiencing bearing in the same direction

P

P/2

P/2



Reduction due to• End failure (bursting)

Insufficient end distance

• Bearing failure of thinner plate

Vnpb = 2.5 × D × t × fup × kb

kb is smaller of


0.1 , ,25.03

,3 holehole up

ub

ff

dp

de

End Distance eEnd Distance e

dhole

t

p p p p




Additional Reduction due to• Oversize / Slotted Holes

In direction normal to slot

Also check for Block Shear

Case Multiplying Factor

Oversize 0.7

Short-slotted 0.7

Long-slotted 0.5

Friction Grip BoltsFriction Grip BoltsFriction Grip Bolts


P,Δ

Slip in Common Bolts

Bearing & Shear in Bolt

P,Δ

Δ

P

Why Friction Grip BoltsWhy Friction Grip Bolts


Pinching Common in bearing type bolts

Why Friction Grip Bolts…Why Friction Grip Bolts…

Pinching


Pinching Bolt holes elongate and cause more slip

Pinching



Pinching Poor energy dissipation

• Need to avoid pinching under working loads

Δ

P



Friction Type (Slip Critical) Pre-tension

• Usually large tensile stress in bolts ~ 0.7-0.8 fu

Usual application – dynamic / fatigue loads• Good for seismic applications

Joint slip only when critical shear is exceeded• Load resistance by friction before slip

Commonly called High Strength Friction Grip (HSFG) Bolts Pre-torquePre-torque



Friction Type Shank in tension only

• Till slip occurs

Tension T in BoltCritical Shear Force = μT

P = μT

P



Friction Type Shank in tension only

• Till slip occurs

Shank in tension, bearing and shear

• Post slip

Tension T in Bolt

Critical Shear Force = μT

P = μT

POverall Elongation

Stage 1

Stage 2

Stage 3

Stage 4

FrictionLinear response

Bearing

Slip

Elastic DeformationLinear response

Yielding of plate / Fastener; tensile / shear fracture

Applie

d L

oad

PShear in Bolt



Pre-torquePre-torque

Advantages of HSFG boltingUp to pre-load

• Enlarging of bolt holes not an issue up to critical friction• Bolt not subjected to

shear and bearing



Slip Critical Design Strength • At Working Load

Tension Friction

Post Slip Design strength • Ultimate Load

Tension Shear Combined Tension and Shear Bearing

Same as Bearing Typeo Pinching like common bearing type bolts but only at

ultimate load levels


Design Strength:: Friction Grip BoltsDesign Strength:: Design Strength:: Friction Grip BoltsFriction Grip Bolts


Design Strength in Slip Resistance Vdsf

Vdsb = Vnsf / mf mf = 1.10 at service load= 1.25 at ultimate load

Vnsf = Nominal Shear Capacity of a bolt governedby slip

Vnsf = μf × ne × Kh × F0

Friction Grip BoltsFriction Grip Bolts



μf factor

(Table 20)

0.10Red lead painted surface

0.20Untreated surface

0.48Sand blasted surface

0.33Clean mill surface

μfSurface Treatment

Friction Grip Bolts…Friction Grip Bolts…



Kh factor

0.7LSL – parallel loading0.85LSL – perpendicular loading0.85Oversize, SSL1.0ClearanceKhCase

Standard(STD)

Oversized(OVS)

Short-slotted(SSL)

Long-slotted(LSL)

18 20

18

22 56

Different holes for 16mm (nominal) diameter boltDifferent holes for 16mm (nominal) diameter bolt




F0 = minimum bolt load= Anb × f0

= Anb × 0.7 × fub

Pre-torquePre-torque



P

P

Shear distribution in Long JointsShear distribution in Long Joints

lj175.0 lj

D

l jlj 200

075.1


Design Strength in Slip Resistance Vdsf Reduction due to

• Long Joints Non-uniform distribution of friction force Similar to shear lag effect


Design Strength After Slipping Shear & Bearing Capacity at Ultimate Load

• Same procedure as for Bearing Type Bolt

Vdsf = {(fub / √3) × (nn × An + ns × As)} / mf

Vdpf = (2.5 × D × t × fup) / mf

Also check for Block Shear

P

P/2

P/2



Design Strength in Tension Tdf

Tdf = Tnf / mf mf = 1.10 at service load= 1.25 at ultimate load

Tnf = Nominal Tensile Capacity of a friction bolt

Tnf = 0.9 × fub × An

< fyb × As × (m1 / mf)

Tensile Rupture of threaded areaTensile Rupture of threaded area

Yielding of shank areaYielding of shank area

Nominal tensile capacity equal to tensile rupture strength of threaded area subjected to maximum of yield strength of shank area



Combined Tension and Shear• Based on von Mises yield criterion

222 3 yxyx f

222

1 3

y

xy

y

x

ff

13

22

ys

xys

yt

xt

fA

A

fAA

T

V

0.122

dsb

sb

db

b

VV

TT



Tension in HSFG Bolt Due to pre-torque ( 0.7fu) Due to external load Due to prying force


PryingPryingPrying


Bolts under Prying EffectBolts under Prying Effect

Tension due to Prying Force

End plane beam-column connectionEnd plane beam-column connection

Bolts in Tension


2T

Fbt=0

Applied Load (2T)

Fbt=T Fbt=TFbt=0

Ten

sion in B

olt (

F bt)

2T

Fbt=F0 Fbt=F0+ΔT Fbt=F0+ΔTFbt=F0

Proof Load F0

HSFG BoltingHSFG Bolting

Bearing Type BoltingBearing Type Bolting

Bolts under Prying Effect…Bolts under Prying Effect…


Prying Force Q

β = 1 for pre-tensioned bolt= 2 for ordinary bolt

η = 1.5 (LSM)

2

40

272 ve

ee

e

v

lltbf

Tll

Q

yde f

ftel 01.1 , Min

lv le

2Te

Te + Q Te + Q

QQ

t

ed



End Plate Thickness

eA QlM

eveevveC QllTllQlQTM

req plate,2 pve

e

CA

MlT

Ql

MM

lv le

2Te

Te + Q Te + Q

QQ

t

ed

B AC



End Plate Thickness

Minimum Thickness

Prying Force

10.144

2

0

2

plate,ye

m

yep

ftbftbM

ey

p

bf

Mt req plate,min

1.14

e

p

l

MQ plate,

lv le

2Te

Te + Q Te + Q

QQ

t

ed

B AC



Finally check

And

mfnubdfe AfTQT /9.0

25.1/9.0 nube AfQT

0.122

dsf

sf

df

e

VV

TQT

lv le

2Te

Te + Q Te + Q

QQ

t

ed

B AC



IS 800: 2007IS 800: 2007

Section 10 (page 73 …)• 10.1 General• 10.2 Location Details of Fasteners• 10.3 Bearing Type Bolts• 10.4 Friction Grip Type Bolting

Bolting in Tapered Flange SectionsBolting in Tapered Flange SectionsBolting in Tapered Flange Sections


Indian Hot-Rolled SectionsIndian Hot-Rolled Sections

Bolting is poorly executed Shank gets bent due to tapered flange

Bent bolt-shank

I-section

Plain (flat) washers

Cover plate


Use taper washers IS:5372 for ISMC IS:5374 for ISMB

• Bolt diameter: 10 – 39 mm

Straight bolt-shank

Cover plate

I-section

Taper washers

Indian Hot-Rolled Sections…Indian Hot-Rolled Sections…

General Issues in Connection DesignGeneral Issues in Connection DesignGeneral Issues in Connection Design


Traditional AnalysisTraditional Analysis

Assumptions:

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


10.11 – Analysis of Bolt / Weld Group10.11 – Analysis of Bolt / Weld Group

In-plane Loading:: The design force in a bolt/weld shall be determined by

a) Considering the connection plates to be rigid and to rotate relative to each other about a point known as the instantaneous centre of rotation ICR of the group

b) In the case of a group subject to a pure couple only, the ICR coincides with the group centroid. In the case of in-plane shear force applied at the group centroid, the ICR is at infinity and the design force is uniformly distributed throughout the group.

In all other cases, either the results of independent analyses for a pure couple alone and for an in-plane shear force applied at the group centroid shall be superposed, or a recognized method of analysis shall be used


10.11 – Analysis of Bolt / Weld Group10.11 – Analysis of Bolt / Weld Group

In-plane Loading:: The design force in a bolt/weld shall be determined by

c) The design force in a bolt or design force per unit length at any point in the group shall be assumed to act at right angles to the radius from that point to the instantaneous centre, and shall be taken as proportional to that radius


10.11 – Analysis of Bolt / Weld Group…10.11 – Analysis of Bolt / Weld Group…

Combined In-plane Shear and Moment Bolt shear due to Px and Py

• Rxi = Px / N; Ryi = Py / N

M = Px × y’ + Py × x’• Rmi = k × ri

• Mi = k × ri2

• M = Σ(k × ri2) = k Σri

2

Bolt shear due to MRmi= Mri / Σri

2

Combined Shear

22 sincos imiyiimixii RRRRR

Bolt group eccentrically loaded in shear


2

22

2

22 )()( ii

iy

ii

ixi yx

Mxn

P

yxMy

nP

R

riP

x’

y’

Rmi


10.11 – Analysis of Bolt / Weld Group…10.11 – Analysis of Bolt / Weld Group…

Bolted Bracket Determine Px and Py

Identify the CG of bolt group, say (xcg, ycg) Determine x’ and y’ from (xcg, ycg) Calculate twisting moment M = Px y’ + Py x’ Determine coordinates of ALL bolts (xi,yi) wrt (xcg, ycg) Calculate Identify the critical bolt (xi,yi) Compute Ri

Ensure Ri to be LESS THAN bolt value



2

22

2

22 )()( ii

iy

ii

ixi yx

Mxn

P

yxMy

nP

R

n

iii yx

1

22

riP

x’

y’

Rmi


Thank You

Bolted Connections[1]

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