bolted connections

Dr S R Satish Kumar, IIT Madras 1

BOLTED CONNECTIONS


• Introduction• Bolted Connections• Bolts and Bolting• Force Transfer Mechanism• Failure of Connections

In shearIn tensionCombined shear and tensionBlock shear

CONTENTS


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


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


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


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


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


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


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


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


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


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


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)



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


(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



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….)


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


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….)


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


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


• 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


Concentric Connections

(a) (b)

Moment Connections

(a) (b)

TYPES OF CONNECTIONS

Classification based on type of resultant force transferred


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


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


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


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.


V


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)



Rigid beam-to-column connections a) Short end plateb) Extended end plate c) Haunched

column webstiffeners

diagonalstiffener

web plate

(a) (b) (c)


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


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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bolted connections

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