steel

Steel Connections -II Welding

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Design of Steel Structures Durgesh C. Rai Department of Civil Engineering, IIT Kanpur

Basics • Field welded truss

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Basics…

• Welds seem simpler, but… – Large welding required at each connection – Need for following a predetermined weld sequence

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Basics…

• Types of welded joints Butt

Lap

Tee 4

Edge

Corner

Basics…

• Types of welds

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Basics…

• Types of Groove Welds

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Basics…

• Types of Fillet welds

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Basics…

• Types of welding technology – Shielded metal arc welding (SMAW)

Electrical Electricalcircuit circuit 8

Basics…

• Types of welding technology… – Submerged arc welding (SAW)

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Basics…

• Types of welding technology… – Gas Metal Arc Welding (GMAW) – Metal Inert Gas (MIG) Welding

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Basics…

• Types of welding technology… – Gas Tungsten Arc Welding (GTAW) – Tungsten Inert Gas (TIG) Welding

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Basics…

• Welding machines … – Manual to Fully Automatic Equipment

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Basics…

• Choosing an Electrode

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Basics…

• Positions of welding electrode

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Basics…

• Welding Symbols

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Basics…

• Welding Symbols

6

150

6

150

8 12

150 150

6

6

16

150

Process OF Welding • Edge preparations for groove welds

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Quality of WeldING • Possible weld defects

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Quality OF Welding…

• Preferred weld profile – For better flow of forces

Poor Poor

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Good Good


• Weld profile… Fillet FilletWeld Weld

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Groove GrooveWeld Weld


• Weld profile…

1.5-2mm

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• Weld profile…

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• Weld problems…

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• Checking size of fillet welds – Weld inspection gauge

Tolerance 24

Problems of welding • Heat affected zone – Material properties are changed

Base metal

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Fusion zone

Heat affected zone

Problems of welding…

• Distortion and dimensional changes – Unsymmetric welds

Angular distortion

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Process OF Welding…

• Distortion due to welding

Curvature Curvaturedeveloped developedafter afterwelding welding

– Sequence of intermittent welds to avoid weld-induced curvature

or

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or


• 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

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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

Weld Access Hole

Cope

Beam Column

CJP Weld

Detail A

Backup Bar Un-Fused Interface Detail A

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• Performance of Welds – Tri-axial state of stress at column face • Avoid high stresses in welds Yield stress σy/2 is not reached in shear; brittle fracture τ

τ σ2

σu/2 σy/2 σ1 σ2 = σ3 = 0

σy/2 τmax

σ 1 = σy

σ σ1 = σu

σ3

σ2

σy

σu

σ1

σ

σ3

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Uni-axial Uni-axialStresses Stresses

Tri-axial Tri-axialStresses Stresses


• 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

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Cover plate I-section

Proper welds possible

Structural welds • Design of welds Butt ButtWeld Weld

Fillet Fillet Weld Weld 40

Structural Design of welds …

• Weld sizes – Fillet Welds

Cl. 10.5.8

• Max. size: smax = t-1.5 mm for square edges of t > 6 mm smax = t for square edges of t < 6 mm smax = 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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[ Cl. 10.5.8.5 ]


• Weld sizes… – Fillet Welds • Min. size: smin = 3 mm for tmax ≤ 10 mm smin = 5 mm for 10 mm ≤ tmax ≤ 20 mm smin = 6 mm for 20 mm ≤ tmax ≤ 32 mm smin = 8 mm for the first run and 10 mm for 32 mm ≤ tmax ≤ 40 mm

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[ Cl. 10.5.2.3 ] Table 21


• Weld sizes … – Butt Welds • Min. groove depths for different situations applicable

– End returns: min of 2 times weld size – Min length Lmin = max (4 s, 40 mm) – Lap Joints: min. lap length Llap = 4t min or 40 mm

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Structural Design of welds…

• Stresses in Fillet Welds s

s

s s

s

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lw


• Stresses in Fillet Welds …

N

– Due to individual forces

f a = N /(lwtt ) q = Q /(lwtt )

s

s

Axial force

s

Shear force Q

[ Cl. 10.5.9 ]

s

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s =sK

lw


• Stresses in Fillet Welds … [ Cl. 10.5.10 ]

– Due to combination of stresses

Combined normal and shear stresses

fe =

fu f + 3q ≤ 3γ mw 2 a

2

[ Cl. 10.5.10.1.1 ]

Fillet Fillet Weld Weld 47


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

[ Cl. 10.5.10 ]

Butt ButtWeld Weld Combined bearing, shear and bending

fe =

f b2 + f br2 + f b f br + 3q 2 [ Cl. 10.5.10.1.1 ]

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Structural design of welds…

• Design of fillet weld connection – Design strength Rwdf = (lwtt ) f wdf β lw Effective throat area

Design stress

f wd = f wn γ mw

γ mf = f wn 49

[ Cl. 10.5.7.1.1 ]

1.25

Shop welds

1.50

Field welds

fu = ; f u = m in( f uw , f up ) 3


• Design of fillet weld connection … – Design strength Rwdf = (lwtt ) f wdf β lw

[ Cl. 10.5.7.3 ]

Reduction factor for long joints

β lw = 1.2 −

50

0.2 l j 150tt

≤ 1.0


• Design of butt weld connection – Design strength Rwdb = (lwtt ) f wdb β lw Effective throat area With throat thickness equal to tickness of plate

[ Cl. 10.5.7.1.1 ]

Design stress

f wdb = f wnb γ mw

γ mf =

1.25

Shop welds

1.50

Field welds

f wn = f u = m in( f uw , f up ) 51


• 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

Rwdf = (lwtt ) f wdf β lw Rwdf = 1 × (0.7 × 6mm) ×

410 MPa = 0.8 kN / mm 3 × 1.25

160 kN

Eqm. requires

FAB + FGF = 160 kN LAB + LGF = 160 kN /0.8kN / mm = 200mm

Moment condition requires

A 0.8kN/mm LAB

FAB (75mm) = FGF (125mm) LAB = (5 /3) LGF

B

75 mm 160 200 mm 125mm

Solving A and B 52

LAB = 125mm , LGF = 75mm

0.8kN/mm LGF

kN

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

P

P

M=Pe

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Shear-only Weld Group

Shear + Tension Weld Group

Eccentric connection …

• Shear-only weld group P

P

y tt z M=Pe

Resultant at point of interest

f R t t = ( t t f m )2 + ( t t f c ) 2

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Direct Shear

Rotation Effect

P fc = lw t t

( Pe ) ri ( Pe ) ri ( Pe ) ri = 2 = fm = J ∫ r dA I z + I y A

Based on shaft torsion analogy

Eccentric connection …

• Shear +Tension bolt group e P

P c

NA

= Resultant at a point of interest

f R t t = ( t t f m )2 + ( t t f c )2

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Pe

+

Bending behaviour (elastic)

Direct Shear

Locate NA, i.e., c

P fc = lw t t

∫t x t

wt

dx = ∫ tt x wc dx

Tensile stress at a point ( Pe )y fm = I na

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steel

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