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Basic Design of Steel Structures

CE 3050

P. S. Lakshmi Priya
LAKSHMIPRIYA@IITM.AC.IN
STR 303

WELDED CONNECTIONS
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What is welding? Welding Processes

Method of connecting two pieces of metal by heating to a plastic/ fluid state to enable fusion

Welding may be electric (for structural applications) and sometimes gas (acetylene + oxygen)

Shielded metal arc welding

Submerged arc welding

Gas shielded metal arc welding

Gas welding Electric arc welding Flux core arc welding

Electroslag welding

Stud welding

Shielded Metal Arc Welding (SMAW) – Stick Welding

Manual process and most common – low capital cost & flexibility

Manual processes must comply with IS 2879, IS 1395, IS 814

Electrodes are usually stronger than the metal

Flux coating decomposes and

creates a gaseous shield to protect
electrode tip, metal and molten
pool from atm. contamination. This
slag is lighter and can later be
brushed off.

Low hydrogen for structural appl.

as H2 causes weld to crack
Weld thickness ≥ 3 mm
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Submerged Arc Welding (SAW)

Hand-held semi-automatic SAW machines/ motor driven carriage

Automatically feeds the electrode from large coils, distributes the flux
ahead of the weld area, and also gathers and reuses unused flux

2-6mm dia electrodes – upto three electrodes

Deposition rates up to 90 kg/hr with 2-electrode automatic machines

In ship and bridge building, where long straight welds are common

Equipment not portable and difficult to position

Restricted in shop welding

Gas Shielded Metal Arc Welding (GMAW)/ Metal-active gas Welding (MAG) or
Metal-inert Gas Welding (MIG)
Low-hydrogen, manually operated, great range of electrode strengths

Welding wire touched to base metal; as wire is consumed feed

mechanism supplies more electrode wire at a steady rate

For Al, carbon steel, copper, low-alloy steel, magnesium, nickel,

stainless steel, and titanium

Mainly used for fillet welded joints

All positions can be used, higher deposition rates, deeper penetration

Metals as thin as 0.58mm (24 gauge) may be welded!

Welding equipment is more expensive, less portable than SMAW, difficult to use in tight quarters
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Flux-Core Arc Welding (FCAW)

Structure and chemical composition of the FCAW electrode wire differentiates FCAW and GMAW

Low-hydrogen, semi-automatic, great range of electrode strengths, can be used in all positions

Great penetration, groove angles as narrow as 30º,saving as much as 50% of filler metal as SMAW

Easier to carry out than SMAW, high deposition rates (>11.3 kg/hr)

Work better in windy conditions than GMAW

Unlimited thicknesses can be joined with multiple passes, and can use in all positions

Generates large volumes of fumes and smoke, requiring additional ventilation indoors and reducing
visibility for welders

Electroslag Welding (ESW)

Process starts with an arc, but then continued by the heat generated from flow of electrode current
through the molten slag

Was developed to join thick sections (25 – 450 mm in single pass)

Multiple solid or flux-cored electrodes speed up the process

High deposition rates, minimum joint preparation, low distortion

Flat or vertical joints only, complicated setup, requires cooling water

Used to shop weld components, in bridge work

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Stud Welding (SW)

Permits rapid attachment of studs used in composite construction, without piercing structure metal

Insert a stud into the stud gun, and position gun perpendicular to the structure

Simple, fast and can be automated

Needs clean surfaces, equipment sensitive to adjustment

Location of welding operation Thickness of parts and costs

Field: SMAW, Shop: Rest

Accuracy of setup Steel Composition

Choice of GMAW, SAW less likely to heat-

SAW, GMAW, ESW require accuracy
affected zone cracking
welding
process
Penetration of weld Access to Joint
Easy access: SAW, GMAW
FCAW & SAW better than SMAW
Cramped: SMAW

Volume of weld to be deposited Position of welding

Overhead: not SAW/ ESW, SMAW best
FCAW, GMAW, ESW high rates
All positions: FCAW, GMAW
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Eliminates holes

Airtight and watertight More silent process than

joining of plates riveting or bolting

Why More efficient than bolted

More rigid than bolted
connections Welding? joints

Used to make built-up Practical for complicated

sections: design flexibility Continuous integral joints
structure

Economical: splice plates, bolts eliminated, less expensive alterations

Highly skilled labour

Inspection difficult and Prone to brittle fracture

expensive

Problems
Members may distort
with Connections prone to
Welding? cracking

Costly equipment
Large residual stresses

Field welding is expensive, and difficult in vertical and overhead positions

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TYPES AND PROPERTIES

OF WELDS
Groove welds

Fillet welds

Slot welds

Plug welds

Groove Welds

Connect structural members aligned in the same plane and often used in butt joints, Tee connections

Grooves have slope of 30 - 60º Doub. bevel or doub.

V for plates > 12mm

Doub. U or doub. J for

plates > 40 mm

Sing. J or Sing. U for

plates : 10- 40 mm

Groove welds transmit the full load of joining members: should have
same strength as the members (Usually only full penetration welds)

Backing for full penetration Backing may or may not be removed after welding, not to be used
and sound weld when root face is provided
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Fillet Welds

Economical, easy fabrication even on site: most-widely used

Require less precision in fitting up 2 sections unlike groove welds due to overlapping of pieces

Donot require edge preparation like groove welds

Intersection angles between 60º - 120º can be used,

provided correct throat size is used in design

Assumed to fail in shear

Slot and Plug Welds

Not used extensively in steel construction: when impossible to use fillet welds or when the length of
fillet weld is limited

Plug welds occasionally to fill holes in construction (e.g. temporary erection bolts)

Assumed to fail in shear: design similar to fillet welds

Difficult to inspect these welds and penetration difficult to

confirm

Not used for tensile members

Useful to prevent
overlapping parts from
buckling
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Structure and Properties of Weld Metal

Weld metal is a mixture of parent metal and steel melted from electrode

Solidified weld metal has characteristics of cast steel (has higher yield/ ultimate ratio, low ductility)

Parent material near joint is subjected to heating and cooling cycles and the metallurgical structure here
will be different: HEAT AFFECTED ZONE (HAZ)

Change in structure in HAZ can be considered by selecting a suitable Charpy V-Impact value for the
electrode (greater than parent metal), or preheating joints

Weld Defects

Weld quality affected by:

1. Type of joint, its preparation and fit up, root opening, etc.
2. Choice of electrode, welding poition, welding current and
voltage, arc length, rate of travel
3. Accessibility of the weld

Incomplete fusion: surfaces not cleaned properly/ insufficient current

supplied/ high rate of welding

Incomplete penetration: incompatible groove design & welding process

Porosity: formed when gas pockets or voids draped during cooling;

Due to excessively high current, longer arc length, adjacent to back-up
plate. Results in stress concentration & low ductility

Undercutting: local decrease in cross-section, detect visually

Slag inclusion: slag gets trapped in rapid cooling; multi pass welds

Cracks: most harmful weld defect. May extend from line of weld to the
base metal or may appear entirely in base metal in the HAZ.
Prevent by uniform heating, slow cooling, low-hydrogen electrodes
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Weld Defects

Porosity

Spatter

Lamellar Tearing (From Tensile Stresses in the Through-Thickness Dir’n of Base Metal)

Brittle fracture in the base metal beneath the weld (Poor ductility in parent metal in through-thickness)

Occurs in T-butt and fillet welds parallel to weld fusion boundary and plate surface

Surface of the fracture is fibrous and woody with long parallel section

Conditions for lamellar tearing to occur:

Transverse strain – shrinkage strains on welding must act in dir’n of plate thickness

Weld orientation and size

Material susceptibility

Loading perpendicular to mill rolling

direction
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Design to Avoid Lamellar Tearing

1. Full penetration butt welds will be more susceptible: use two fillet welds instead

2. Double sided welds rather than single-sided welds and balanced welding

3. Large single-side fillet welds to be replaced with smaller double-sided fillets

4. Redesign joint such that fusion boundary is more normal to the susceptible
plate surface

Non-Destructive Tests on Welds for Quality Control

Small errors in welds

lead to catastrophic
Liquid penetrant inspection collapse. Magnetic particle inspection
Checks should be made
before welding, during
welding and after
welding

Radiographic inspection Ultrasonic inspection

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TYPES OF WELDED JOINTS

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Butt Joints

Does not have the eccentricity of a lap joint, also more aesthetic: full/ partial penetration welds

Face reinforcement plates makes connection stronger under static loads, but stress concentrations
under cyclic loads and potentially failure

If unequal plates: wider or thicker part should be reduced at the butt joint by a slope of 1 in 5.

Both plates need to be carefully aligned and specially prepared

High residual stresses

Lap Joints

Ease of fitting, ease of joining, no special preparation, most-commonly used

Utilize fillet welds, suited for both shop and field welding

May require erection bolts, which could be removed

or left in place

Different thickness plates can be easily joined

Introduces eccentricity in loads, unless double

lap joint is used
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Tee Joints

Plates at right angles: T-shapes, I-shapes, plate girders, hangers, brackets and stiffeners

Corner Joints and Edge Joints

Corner joints used to form built-up rectangular box sections, which may be used as columns or beams

Edge joints not used in structural applications: used to keep two plates in a given plane
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Shrinkage and Distortion

While weld cools, it contracts both along and transverse to its axis: tensile residual stress

If surrounding structure is less rigid, it will cause distortion

If welding is eccentric, it will cause distortion

Longitudinal distortion cause slender elements to buckle

Shrinkage and Distortion

Transverse shrinkage produces both angular and out of plane distortions

Shrinkage forces may be reduced by proper welding practices. Balance shrinkage forces by:

Symmetry in welding Scattered and intermittent welds Peening Clamps to force cooling
weld to stretch
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Weld Symbols

Weld Symbols
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FILLET WELDS
BEHAVIOUR AND DESIGN

Failure of Welds

Failure of a weld = fracture of weld metal Strength of a weld (load to cause fracture):
= (effective area of weld) x (stress at fracture)
P
P Effective area = effective throat thickness (te) x
Fracture along throat of the weld length of weld (Lw)

P Effective throat thickness = K x s

K = constant depending on angle
P
between faces
s = size of the weld
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Stress at Fracture of Welds

Stress distribution is complex: most critical is the direction of loading on the weld

Applied load produces shear stress on the effective

area of long. welds. These welds are controlled by the
shear strength of the weld
Shear strength = fu/ 3

Longitudinal fillet weld

Applied load produces both shear and tensile stress on

the effective area of trans. welds. The stress at fracture
of weld will be intermediate between shear strength
and tensile strength of the weld metal

Strength of trans. weld > str. of long weld (40 – 60%)

Transverse fillet weld

Load Deformation Response of Fillet Welds

Strength of trans. weld > str. of long weld (40 – 60%)

But, there is a loss of stiffness and ductility

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Design Strength of Fillet Welds: Assumptions

Welds are homogeneous, isotropic and elastic

Parts are connected by the welds are rigid and their deformation is neglected

Only stresses due to external forces are considered. The effects of residual stresses, stress
concentrations and the shape of the weld are neglected. (residual stresses are considered in member
design, not in weld design)

All loads on fillet welds are assumed to be carried as pure shear stress on the effective area of the weld.
Thus, the failure of a fillet weld is defined as the shear fracture through the effective throat thickness

Design Strength of Fillet Welds

Shear fracture of the weld metal occurs at ~ fu 3

IS 800 defines fu = min(fu of parent material, fu of weld metal)

However, weld metal is always of a higher strength. Why?

Pdw  L wKsfu  
3 mw , s = weld size (Table 21 of IS 800: 2007), K (Table 22 of IS 800), γmw (Table 5 of IS 800)

Design of slot/ plug welds uses same equations. In addition, see below: (IS816: 1969)

Width or diameter of the weld should be less than max (three times the thickness , 25 mm)

Corners at enclosed ends of slots should be rounded with radius ≥ max (1.5 times thickness, 12 mm)

Distance from edge of slot to edge of part or between adjacent slots ≥ ( twice the thickness, 12 mm)
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Local Shear Fracture of Base Metal

Shear fracture of the metal occurs at ~ fu 3

IS 800 defines fu = min(fu of parent material, fu of weld metal)

Fracture of weld

Local shear fracture of base metal

w

Design strength of weld based on local shear fracture of base metal: = w. Lw ( fu 3)

Design strength of weld based on fracture through throat of weld: = (0.707w). Lw (fuw 3)

For shear fracture of base metal to control: = fu ≤ 0.707fuw ; never satisfied for “matching” weld metal

Can only govern when over matched electrodes are used

Design Specifications on Fillet Welds

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Problem 1: A tie member in a truss girder is 250 mm x 14mm in size. It is welded to a

10mm thick gusset plate by a fillet weld. The overlap of the member is 300 mm and the
weld size is 6 mm. Determine the design strength of the joint, if the welding is done as
shown.
What is the increase in strength if welding is done all around (shop welding)
300 mm
For E 250 steel, fu = 410 MPa, fy = 250 MPa Shop welding, γmw =1.25

Effective length of weld = 2 x 300 +250 = 850 mm

250 mm Effective throat thickness = Ks = 0.7 x 6 = 4.2mm

Design weld strength, Pdw  L wKsfu  3 mw   850  4.2  410  

3x1.25  103

= 676 kN

Effective length of weld = 2 x (300 +250) = 1100 mm

For all around welding:
Design weld strength, Pdw  LwKsfu  3mw   1150  4.2  410  
3x1.25  103

= 875 kN
Increase in strength = 875 – 676 = 199 kN

Design Specifications on Fillet Welds

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Design Procedure for Fillet Welds

1. Assume the size of the weld based on member thicknesses

2. Equate design strength of the weld to external factored load and calculate effective length of weld

Longitudinal and transverse welds are assumed to be of equal strength

Apply reduction for long weld as shown previously in Clause 10.5.7.3

If only longitudinal welds are provided, perpendicular distance between welds must be greater than the
length of individual welds

3. End returns must be provided at end of each longitudinal fillet weld

End return = 2 x size of weld

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Problem 2: A 75mm x 8mm tie member is to transmit a factored load of 145 kN. Design
fillet welds necessary overlaps for the cases shown. The steel grade is E250. Gusset
plates are 12mm thick
For E 250 steel, fu = 410 MPa, fy = 250 MPa Shop welding, γmw =1.25 Field welding, γmw =1.5

Design strength of weld ≥ factored load = 145 kN

Min. size of weld for 12mm thick plate = 5mm Min. size of weld for 8mm thick plate = 8 – 1.5 = 6.5 mm

Provide weld size of 5mm Effective throat thickness = Ks = 0.7 x 5 = 3.5mm

Design weld strength, Pdw  L wKsfu  3 mw  145  lw  3.5  410  

3x1.25  103

lw= 220 mm

Length of weld on each side = 220/2 = 110mm , not < 75 mm

Provide 5mm fillet size of length 110 mm on two sides. End returns = (2s) = 2x 5 = 10mm each

Total length of weld = 2 x 110 + 4 x 10 = 260 mm

Problem 2: A 75mm x 8mm tie member is to transmit a factored load of 145 kN. Design
fillet welds necessary overlaps for the cases shown. The steel grade is E250. Gusset
plates are 12mm thick
Design weld strength, Pdw LwKsfu  3mw  145  lw  3.5  410  
3x1.25  103 lw= 220 mm

Length of weld possible on transverse side = 75mm

Required overlap : 220 = 2x overlap +75 Overlap = 73 mm

Provide 5mm fillet size of length 73 mm on two sides. End returns = (2s) = 2x 5 = 10mm each

Total length of weld = 2 x 73 + 2 x 10 +75 = 241 mm

Design weld strength, Pdw LwKsfu  3mw  145  lw  3.5  410  

3x1.5  103 lw= 262.5 mm

Required overlap : 262.5 = 2 x overlap + 2 x 75 Overlap = 57 mm

Total length of weld = 2 x 75 + 2 x 57 = 264 mm

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Intermittent Fillet Welds

Used when required strength < than strength of a continuous weld of smallest practical size

(a) better due to balancing nature of the welds: reduces distortion

Not economical unless smallest size weld is used (weight of weld metal increases with square of size)

Difficult when automatic process is used, more difficult to maintain, not suitable for dynamic/ repetitive
loads

Procedure:
Assume weld size, and calculate required total length of fillet weld

Minimum effective length clauses of IS codes to be followed

At ends, length of int. fillet weld > width of member. If not, transverse welds must
be provided. Then total length (long + transverse) should not be less than twice
the width of the member

Methods of Providing Fillet Welds

Increases length Failure of joint

of weld by using shown in (b)
slot with fillet
Stress concentration
weld
at re-entrant angles
of the notches
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Combination of Stresses

2Ys 2  (f11  f22 )2  (f22  f33 )2  (f33  f11 )2  6(f122  f232  f312 )

f112  3f122  fu 3 f22, f33, f23, f31 =0

Balanced Fillet Welds

Unbalanced fillet weld
L C.g. of weld group
Eccentricity “e” will introduce a moment on the weld group, in
addition to axial force
Centroidal axis of
e
member
Centroid of weld group should coincide with the centroidal axis

l1
Treat each weld as a line element
C.G li = length of weld
l2
xili or xi  li w i  wi = size of weld
x x x
li li xi = distance to c.g. of weld i
l3
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Fillet Welds for Truss Members

h = length of end fillet weld fu

P3 = factored load shared by end fillet weld = hte
3 mw

Total weld length = L1 + L2 + h

Take moment about line through L1

L1, L2 = length of longitudinal fillet welds on two sides
P1, P2 = factored design loads along lengths L1 and L2 P2h + P3 h/2 – Ph2 = 0
P = factored load acting on the centroid of the section
Take moment about line through L2
Take moment about line through L1
P1h + P3h/2 – Ph1 = 0
P2h – Ph2 = 0 P2 = Ph2/h P1 = Ph1/h
Solve for P1 and P2

Once P1,P2 and P3 are calculated, fillet weld length can be designed

Providing transverse weld can reduce required size of gusset plate

Problem 3: A tie member consisting of an ISA 80 x 50 x 8 (mm), Fe 410 grade steel is

fillet welded to a 12mm thick gusset plat on site. Design welds to transmit loads equal
to the design strength of the member
For E 250 steel, fu = 410 MPa, fy = 250 MPa Field welding, γmw =1.5
Partial safety factor against yielding = γmo =1.1

Properties of ISA 80 x 50 x 8: Ag = 978 mm2, Czz = 27.3 mm

Design strength of ISA by yielding of gross-section = Agfy/ γmo = 223 kN

Weld must transmit load 222.3 kN P1 = 222.3 x (80-27.3)/80 = 146.4 kN P2 = 222.3 x 27.3/80 = 75.9 kN

Assuming weld size = 6mm, te = Ks =0.7 x 6 = 4.2 mm (>3 mm)

Design weld strength, Pdw LwKsfu  3mw  146.4  lw 1  4.2  410  

3x1.5  103 lw1 = 221 mm

221  lw2  4.2  410  

3x1.5  103 lw1 = 115 mm
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GROOVE WELDS
BEHAVIOUR AND DESIGN

Effective Throat Dimensions of Groove Welds

Complete joint penetration (CJP) weld:

Thickness of thinner part

Partial penetration weld (PJP): In unsealed single groove welds

of V, U, J and bevel type groove
IS 816: 1969 : te = 5/8 (thinner part thickness) welds: throat thickness must be
≥ 7/8 (thinner part thickness)

The unwelded portion of incomplete penetration welds, welded from both sides must be ≤ 0.25 (thinner part thk)

Groove welds with thickness less than specified here (in butt welds due to accessibility) should be considered as
non-load carrying
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Design Strength of Groove Welds: 10.5.7.1.2

Groove welds subjected to axial/ bending tension or compression & sometimes shear

Groove weld failure governed by yielding

Design Strength in tension/ compression: Design Strength in shear:

In CJP , weld strength at joint = strength of member (no design calculations are required)

In PJP , find effective throat dimension, and required effective length to make weld strength = member strength
Use equations (1) or (2) above appropriately

Problem 4: Two plates of 16mm and 14 mm thickness are to be joined by a groove weld
as shown. The joint is subjected to a factored tensile force of 430 kN. Due to some
reasons, the effective length of the weld that could be provided was 175 mm only.
Check the safety of the joint if (joints are shop welded)
(a) Single-V groove weld is provided
(b) Double- V groove weld is provided
For E 250 steel, fu = 410 MPa, fy = 250 MPa

te = 5t/8 = (5 x14)/8 = 8.75 mm lw = 175 mm

(Incomplete penetration)

Strength of the weld, Tdw = lwtefy/γmw = 175 x 8.75 x 250/1.25 = 306.25 kN < 430 kN

te = thickness of thinner plate = 14 mm

Strength of the weld, Tdw = lwtefy/γmw = 175 x 14 x 250/1.25 = 490 kN > 430 kN

(Complete penetration)