WELDED CONNECTIONS I�

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Advantages of welding

• Direct transfer of stress - minimum weight , efficiency
• Less fabrication
• Economy - 15% saving in weight in bridges, less labor
• Neat appearance
• More rigid

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INTRODUCTION

BASIC WELDING PROCESSES

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Gas welding – Oxy-acetelene welding(a jet of burning O2 and acetylene) , simple , slow, repair and maintenance work(inert gases are used)

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Electric Arc welding - All structural welding

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Electric arc by use of electric energy

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• Electric-arc welding :- Heat is applied by means of an

electric arc struck between the parts to be welded and an

electrode melts and fills the gap at the joint.

TYPES OF JOINTS OR WELDS

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• Joints: Lap, Tee, Butt and Corner
• Welds: Groove, fillet, plug and slot
• Welded joint description - Type of joint and weld
• Position of welding

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COMMON TYPES OF WELDS

Types of welds:-

• Fillet weld
• Butt weld
• Plug weld

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Groove welds

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Selection of a particular type of groove weld depends

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• Size of the plate to be joined
• welding by hand or automatic
• Type of welding equipment
• Accessibility of both sides
• Position of weld

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• Size of butt weld
• Thickness of connected plate for full penetration
• Depth of penetration for partial penetration

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• Advantages

High strength, high resistance to impact and cyclic

stress

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• Disadvantages

High residual stress , edge preparation and proper

aligning

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GROOVE WELD DETAILS

Welded from both side shell not be greater then ¼

thickness of thinner part to be jointed

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• Fillet welds

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• Ease of fabrication and adaptability
• Less precision
• No special edge preparation
• Throat of a weld
• Concave and convex surfaces

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QUALITY OF WELDED CONNECTIONS

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• Proper electrodes , welding apparatus and procedures
• Welding sequence

Doubling up method

Planned wandering method

Step back method

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WELD SYMBOLS

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Symbolic representation of welds

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(Ref. IS:813 - 1986 ‘ Scheme of symbols for welding’ )

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• Welds and there symbol :

DEFECTS IN WELDS

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• Incomplete fusion
• Porosity
• Inadequate preparation
• Undercutting - Excessive current or long arc
• Slag inclusion - Failure to remove slag between runs
• Cracks - Breaks in the weld metal
• Lamellar tearing - Occurs in the base metal beneath the weld

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• Due to uneven heating and cooling of the members

during the welding. The member may distort resulting

in additional stress.

• No provision for expansion and contraction is kept in

welded connection and therefore, there is possibility

of cracks.

• Defects like lack of fusion, slag inclusion and

incomplete penetration

are difficult to detect.

WELD DEFECT TOLERANCE

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For joints welded from both sides

• Incomplete penetration - 5% of parent metal thickness < 2mm
• Length of flaw < 200 mm / meter length

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Welded on one side

• Incomplete penetration - 15% of thickness < 3mm
• Slag inclusion < 200 mm / meter weld length
• Total gas pores < 5 per square centimeter of weld
• Thickness upto 10mm , undercut < 0.5mm
• For thickness > 10mm undercut < 1mm

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WELD INSPECTION

Visual inspection

Liquid penetrants

Magnetic particles

Ultrasonic testing

Radiography

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CONNECTION DESIGN

Static strength of welded joint

• Type and size of the weld
• Manner of welding
• Type of electrode used

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BUTT WELDS

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• Critical form of loading - Tension in transverse direction

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• Yield stress of weld metal and parent metal in HAZ (Heat affected Zone) is much higher

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• Failure always occurs away from the weld

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• Toughness and ductility properties are affected

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DIFFERENT TYPES OF BUTT JOINTS

DESIGN

• Direct tension or compression
• Design strength same as parent metal strength
• Effective area equals effective length times throat size
• For full penetration,thickness of weld ,equals thickness of thinner part of connection
• Partial penetration welds are avoided
• Throat thickness - 5/8 thickness of thinner part
• Average stress concept
• Permissible stresses - Parent metal values
• Site welds – lower design strengths

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FILLET WELDS

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Behaviour

• Lap joints splices
• Shear is the main design consideration
• Side fillets and end fillets
• End fillet loaded in tension - high strength and low ductility
• Side fillet loaded - Limited to weld shear strength (50% tensile strength) Improved ductility
• Average stress in weld throat
• Fillet weld shape is important for end fillets.

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Size of fillet weld (S) :

• The sides containing the right angle of the

fillet weld are called legs. The size of the

weld is specified by the minimum leg

length.

S – is the size of the weld

From, Annex A.2.1.8 of IS 9595 :1996

• Maximum size of weld

• For square edge :-

• For rounded edge :-

t – thickness of thinner plate

S = t – 1.5

S should not exceed ¾ t

Effective throat thickness(r) :-

• The effective throat thickness of a fillet weld is the

perpendicular distance from the root to the hypotenuse

joining the two end of the legs.

From, IS 816, Table - 2

For the angle other than

Right-angle, the value of “K”

r should not be less then 3 mm

r should not be more then 0.7r or 1.0r mm

• Effective length of fillet :-

• It is the length of the weld for which the specified size and the effective throat thickness of the weld exist.

The effective length of fillet weld should not be less than four times the size of the weld. End returns are made 2 times size of weld to relieve member from high stress concentration at their ends.

d < 16 times the thickness of thinner member. If l exceeds 150 t throat thickness the reduction in weld length as per long joint is done

Β = 1.2 –(0.2lj / 150 tt ) ≤ 0.1

Effective Area : The effective area of a fillet weld is taken as product of effective length and effective throat thickness.

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A fillet weld is not used for joining parts, if the angle between fusion faces is greater than 120 degree or less than 60 degree

Design procedure:

1. Assume the size of weld from the thickness of members to be jointed.
2. A fillet weld fails by shear at an angle 45degree.
3. Design strength of fillet weld for a length L

Pdw = Lw tt Fwd, where t = KS

Fwd = fu/√3 γmw shear stress in N/sqmm

γmw 1.25 for shop welding and 1.5 site welding

3. Pull or thrust to be transmitted by connections is estimated

4. Calculate effective length = pull/strength of weld per mm

• The length can either be adjusted as longitudinal fillet weld(parallel to load axis) or as transverse fillet weld(perpendicular to fillet weld)
• Note- transverse weld having 30% more strength as compare to longitudinal weld.

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Butt welding

• Size of the weld :-

Size of the weld is specified by the effective throat thickness

as follow :

• The size of the butt weld is the thickness of the thinner

plate.

• The effective throat thickness in case of complete

penetration is taken as the thickness of the thinner plate

e.g. Double-V, Double – U, Double – J and Double Bevel

butt joint are the examples of completely penetration butt

weld.

• In case of incomplete penetration of butt weld the effective

throat thickness is taken as the 7/8^th of the thickness of the

thinner part. But for stress calculation the effective throat

should not exceeding 5/8^th of the thickness of the thinner part.

• Reinforcement- is the extra weld metal which makes the throat dimension at least 10% greater then the thickness of the welded material. Reason- to increase efficiency of the joint.
• In no case reinforcement exceed 3 mm.

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Design strength

• Design strength of groove or butt joint in tension or compression
• Tdw = fy Lw te /γmw

Fy = smaller of yield stress of the weld fyw & the parent metal Fy in Mpa

Le= effective length of the weld in mm

γ mw= 1.25 for shop welding & 1.5 for site welding

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• Design strength of groove or butt joint in shear also governed by yield
• Vdw = fyw1 Lw te /γmw
• Fyw1= smaller of shear stress of the weld fyw/√3 & the parent metal fy/√3

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(A) CONNECTIONS WITH SIMPLE WELD DESIGN,

(B) CONNECTIONS WITH DIRECTION- DEPENDENT WELD DESIGN

DESIGN

Simple approach - Uniform strength

Size of fillet weld  3mm or thickness of thinner part

Effective throat thickness  3 mm

< 0.7t and 1.0t

= k  fillet size

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SLOT AND PLUG WELDS

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• Provided along with fillet welds in lap joints
• Strength of a plug or slot weld - allowable stress

and nominal area in the shearing plane

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(a) WELDS SUBJECTED TO SHEAR AND TORSION,

(b) WELDS SUBJECTED TO SHEAR AND BENDING