steel

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Design of Steel Structures Durgesh C. Rai

Department of Civil Engineering, IIT Kanpur

Steel

Connections

-

_-

II

_II

Welding

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Basics

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• Welds seem simpler, but…

– Large welding required at each connection

– Need for following a predetermined weld sequence

Basics

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• Types of welded joints

Butt

Lap Edge

Basics

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• Types of welds

Basics

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• Types of Groove Welds

Basics

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• Types of Fillet welds

Basics

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• Types of welding technology

– Shielded metal arc welding (SMAW)

Basics

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• Types of welding technology…

– Submerged arc welding (SAW)

Basics

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• Types of welding technology…

– Gas Metal Arc Welding (GMAW)

– Metal Inert Gas (MIG) Welding

Basics

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• Types of welding technology…

– Gas Tungsten Arc Welding (GTAW) – Tungsten Inert Gas (TIG) Welding

Basics

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• Welding machines …

– Manual to Fully Automatic Equipment

Basics

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• Choosing an Electrode

Basics

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• Positions of welding electrode

Basics

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• Welding Symbols

Basics

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• Welding Symbols

Basics Basics…… 6 6 8 12 6 6 150 150 150 150 150

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Process OF Welding

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Quality of

WeldING

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• Preferred weld profile

– For better flow of forces

Quality OF Welding Quality OF Welding…… Poor Poor Good Good

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• Weld profile…

Quality OF Welding

Quality OF Welding……

Fillet Weld

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• Weld profile…

Quality OF Welding……

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• Weld profile…

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• Weld problems…

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• Checking size of fillet welds

– Weld inspection gauge

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Problems of welding

• Heat affected zone

– Material properties are changed

Base metal

Fusion zone

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• Distortion and dimensional changes

– Unsymmetric welds

Problems of welding

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Curvature developed after welding Curvature developed after welding

• Distortion due to welding

– Sequence of intermittent welds to avoid weld-induced curvature

Process OF Welding

Process OF Welding……

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• Distortion and dimensional changes…

– Unsymmetric welds

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• Internal stresses

– Weld restraints

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• Internal stresses

– Weld restraints

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• Internal stresses

– Weld restraints • One solution Problems of welding Problems of welding……

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• Closing welds in indeterminate structures

– Weld and base metals contract on cooling

• Accompanied by yielding, cracking or elongation of

members

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• Avoid problems of closing welds…

– Use proper weld sequence

– Adopt prescribed number of passes for a required total weld size

– Allow the prescribed cooling time after each weld

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• Lamellar tearing due to shrinkage of welds

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• How to reduce lamellar tearing

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• Residual stresses in welded sections

– Comparable to that in hot-rolled sections

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• Beam bottom flange welding - a challenge

– Weld access hole, cope and backup bar required

– Un-fused interface at bottom of back-up bar • Potential crack initiation of CJP weld

Detail A Beam Column Detail A Weld Access Hole CJP Weld Cope Un-Fused Interface Backup Bar Problems of welding Problems of welding……

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• Performance of Welds

– Tri-axial state of stress at column face • Avoid high stresses in welds

σ2 σy/2 τ σ σu/2 τ σ σ σ σ σu σy/2 σy τmax

Yield stress σ_y/2 is not reached in shear; brittle fracture

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• Welding is difficult in tapered sections

– Only obtuse-angled small-thickness weld possible

at tapered tip

• Use parallel flange sections

Cover plate I-section

Only small thickness

weld possible _{Proper welds} possible

Cover plate I-section

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Structural welds

• Design of welds

Butt Weld Butt Weld

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Structural Design of welds

Structural Design of welds ……

• Weld sizes

– Fillet Welds • Max. size:

s_max = t-1.5 mm for square edges of t > 6 mm s_max = t for square edges of t < 6 mm

_s_max _{= 0.75t for the rounded edges of rolled sections}

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• Weld sizes …

– Fillet Welds • Max. size:

End fillet weld normal to force direction

Throat thickness not less than 0.5t

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• Weld sizes…

– Fillet Welds • Min. size:

s_min = 3 mm for t_max ≤ 10 mm

s_min = 5 mm for 10 mm ≤ t_max ≤ 20 mm

s_min = 6 mm for 20 mm ≤ t_max ≤ 32 mm

s_min = 8 mm for the first run and 10 mm for 32 mm ≤ t_max≤ 40 mm

[ Cl. 10.5.2.3 ] Table 21

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• Weld sizes …

– Butt Welds

• Min. groove depths for different situations applicable – End returns: min of 2 times weld size

– Min length L_min = max (4 s, 40 mm)

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Structural Design of welds……

• Stresses in Fillet Welds

s

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• Stresses in Fillet Welds … – Due to individual forces

s s s s s lw /( ) a w t f = N l t /( _{w t} ) q Q l t= N Q Axial force Shear force [ Cl. 10.5.9 ] = s K

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• Stresses in Fillet Welds … – Due to combination of stresses

Combined normal and shear stresses

Fillet Weld Fillet Weld 2 ₃ 2 3 u e a mw f f f q γ = + ≤ [ Cl. 10.5.10.1.1 ] [ Cl. 10.5.10 ]

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• Stresses in Butt Welds …

– Due to combination of stresses [ Cl. 10.5.10 ]

Butt Weld Butt Weld

Combined bearing, shear and bending

2 2 ₃ 2

br

e b b br

f = f + f + f f + q

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Structural design of welds

Structural design of welds……

• Design of fillet weld connection

– Design strength ( ) wdf w t wdf lw R = l t f β Effective throat area Design stress [ Cl. 10.5.7.1.1 ] wd wn mw

f

=

f

γ

;

min(

,

)

3

u wn u uw up

f

=

f

=

f

Field welds Shop welds

1.25

1.50

mf

γ

=

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• Design of fillet weld connection …

– Design strength

( )

wdf w t wdf lw R = l t f β

Reduction factor for long joints

[ Cl. 10.5.7.3 ]

0.2

1.2

1.0

150

j lw t

l

t

β

=

−

≤

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• Design of butt weld connection

– Design strength ( ) wdb w t wdb lw R = l t f β Effective throat area With throat thickness equal to tickness of plate Design stress [ Cl. 10.5.7.1.1 ] wdb wnb mw

f

=

f

γ

min(

,

)

wn u uw up

f

=

f

=

f

Field welds Shop welds

1.25

1.50

mf

γ

=

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• Design Example of fillet weld connection

Design weld on face AB and GF with no eccentricity, plate thickness is 16mm

Strength per unit length for 6 mm weld Ex50xx and E250(Fe410) plates, shop welds

410 1 (0.7 6 ) 0.8 / 3 1.25 wdf MPa R = × × mm × = kN mm × Eqm. requires 160 kN 160 kN 75 mm ( ) wdf w t wdf lw R = l t f β 0.8kN/mm L_AB 160 AB GF F + F = kN 160 /0.8 / 200 AB GF L +L = kN kN mm = mm

Moment condition requires

(75 ) (125 )

AB GF

F mm = F mm

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Eccentric connection

• Definition

– Resultant of applied forces does not pass through the c.g. of

weld group

– Two types

• Cause only shear in fasteners

• Cause shear + tension in fasteners

Shear-only Weld Group Shear + Tension Weld Group

M=Pe P

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Eccentric connection

Eccentric connection ……

• Shear-only weld group

Rotation Effect Direct Shear P f = m ( ) i ( ) i ( ) i Pe r Pe r Pe r f = = = +

∫

M=Pe P P t_t y z Resultant at point of interest

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Eccentric connection

Eccentric connection ……

• Shear +Tension bolt group

Bending behaviour (elastic) Direct Shear

t wt t wc

t x dx = t x dx

∫

Locate NA, i.e., c

Tensile stress at a point

c Pe

=

NA

+

e _P P c w t P f l t = ( ) m na Pe y f I = Resultant at a point of interest 2 2 ( ) ( ) R t t m t c f t = t f + t f

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