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Steel Connections -II Welding 1 Design of Steel Structures Durgesh C. Rai Department of Civil Engineering, IIT Kanpur
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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
Quality OF Welding…
• Weld profile… Fillet FilletWeld Weld
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Groove GrooveWeld Weld
Quality OF Welding…
• Weld profile…
1.5-2mm
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Quality OF Welding…
• Weld profile…
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Quality OF Welding…
• Weld problems…
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Quality OF Welding…
• 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
Problems of welding…
• Distortion and dimensional changes… – Unsymmetric welds
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Problems of welding…
• Internal stresses – Weld restraints
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Problems of welding…
• Internal stresses – Weld restraints
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Problems of welding…
• Internal stresses – Weld restraints • One solution
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Problems of welding…
• Closing welds in indeterminate structures – Weld and base metals contract on cooling • Accompanied by yielding, cracking or elongation of members
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Problems of welding…
• 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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Problems of welding…
• Lamellar tearing due to shrinkage of welds
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Problems of welding…
• How to reduce lamellar tearing
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Problems of welding…
• Residual stresses in welded sections – Comparable to that in hot-rolled sections
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Problems of welding…
• 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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Problems of welding…
• 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
Problems of welding…
• 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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Structural Design of welds …
• 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 ]
Structural Design of welds …
• 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
Structural Design of welds …
• 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
Structural Design of welds…
• 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
Structural Design of welds…
• 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
Structural Design of welds…
• 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
Structural design of welds…
• 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
Structural design of welds…
• 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
Structural design of welds…
• 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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