Welded Joints Complete

Ch-4: Design of Welded Joints

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints

 Welded Joint is a permanent joint which is obtained by the fusion of the edges of the two parts to be joined together, with or without the application of pressure and a filler material

 Heat required for the fusion of the material may be obtained by burning of Gas (in case of Gas Welding) or by an Electric Arc (in case of Electric Arc Welding)

Advantages and Disadvantages of Welded Joints over Riveted Joints Advantages

 Welded Structures are usually Lighter than riveted structures (Gussets or other connecting components are not used)

 provide maximum Efficiency (up to 100%), not possible in case of riveted joints  Alterations and Additions can be easily made in the existing structures  smooth in Appearance, therefore looks pleasing Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Advantages and Disadvantages of Welded Joints over Riveted Joints Advantages--contd--

 Welded Joint has a great Strength, usually has the strength of the parent metal itself

 members are of such a shape (i.e. Circular Steel Pipes) that they afford difficulty for riveting. But they can be easily welded

 Welding provides very Rigid Joints  It is possible to weld any part of a structure at any point. But riveting requires enough clearance

 Process of welding takes less time than the riveting

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Advantages and Disadvantages of Welded Joints over Riveted Joints Disadvantages

 Uneven Heating and Cooling during fabrication, therefore the members may get distorted or additional stresses may develop

 Requires a Highly Skilled Labor and supervision  Inspection of welding work is more difficult than riveting work  No provision is kept for expansion and contraction in the frame → there is a possibility of cracks developing in it

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Types of Welded Joints Lap Joint

 Lap

Joint or the Fillet Joint is obtained by overlapping the plates and then welding the edges of the plates

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Cross-section of the fillet is approximately Triangular

Single Transverse Fillet

Double Transverse Fillet

Parallel Fillet Joints

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Types of Welded Joints Butt Joint

 Butt Joint is obtained by placing the plates edge to edge  plate edges do not require Beveling if the thickness of plate is less than 5 mm

 If the Plate Thickness is 5 mm to 12.5 mm, the edges should be beveled to V or U-groove on both sides

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Types of Welded Joints Other Joints

 Other type of Welded Joints are Corner Joint, Edge Joint and T-joint

 Main considerations involved in the selection of weld type are: o o o

Shape of the welded component required Thickness of the plates to be welded Direction of the forces applied

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Basic Weld Symbols

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Elements of a Weld Symbols 1. Reference line

5. Supplementary symbols

2. Arrow

6. Finish symbols

3. Basic weld symbols

7. Tail

4. Dimensions and other data

8. Specification, process or other references

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Representation of welding symbols

 Fillet-weld each side of Tee-convex contour

 Single V-butt weld –machining finish  Double V- butt weld  Plug weld - 30° Groove angle- flush contour Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Representation of welding symbols

 Staggered Intermittent Fillet Welds o

welds are intermittent and staggered 40 mm along on 100-mm centers

Circle on the weld symbol → welding is to go all around

Further details on Types of Welding Symbols: Engineering Dept. CEME NUST 11 Chap-9 (page: 478), Book:Mechanical Shigley’s Mechanical Engineering Design, 9th ed.

Design of Welded Joints Strength of Transverse Fillet Welded Joints

 Transverse Fillet welds are designed for Tensile Strength Single Transverse Fillet

Double Transverse Fillet

 Assumption:

section of fillet is a Right Angled Triangle ABC with hypotenuse AC making equal angles with other two sides AB and BC

 Leg Or Size Of The Weld: Length of each side (AB or BC)

 Throat

Thickness: Perpendicular distance of the

hypotenuse from the intersection of legs (i.e. BD) Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Strength of Transverse Fillet Welded Joints t = Throat thickness (BD) s = Leg or size of weld= Thickness of plate l = Length of weld

⇒

Throat Thickness = t = s × sin 45° = 0.707 s

 Minimum Area of the weld is taken because the stress is maximum at the minimum area

⇒

Minimum area of the weld or throat area= A = Throat thickness × Length of weld = t × l = 0.707 s × l

 Tensile Strength of the joint for Single Fillet Weld: P = Throat area × Allowable tensile stress = 0.707 s × l × σt

 Tensile Strength of the joint for Double Fillet Weld: P = 2 × 0.707 s × l × σt = 0.707 s × l × σt

weld is weaker than the plate due to slag and blow holes, therefore the weld is given a 13 Mechanical Engineering CEME NUST Reinforcement which may be takenDept. as 10% of the Plate Thickness

Design of Welded Joints Strength of Parallel Fillet Welded Joints

 Parallel Fillet Welded Joints are designed for Shear Strength Double Parallel Fillet Weld

Combination of transverse and parallel fillet weld

τ = Allowable Shear Stress for the weld metal

 Shear Strength of the joint for Single Parallel Fillet Weld: P = Throat area × Allowable Shear Stress = 0.707 s × l × τ

 Shear Strength of the joint for Double Parallel Fillet Weld: P = Throat area × Allowable Shear Stress = 2 × 0.707 × s × l × τ = 1.414 s × l × τ

 For Combination of Single Transverse and Double Parallel Fillet Welds: P = 0.707s × l1 × σt + 1.414 s ×Mechanical l2 × τ Engineering Dept. CEME NUST

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Polar Moment Of Inertia and section modulus of welds

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Special Cases of Fillet Welded Joints 1. Circular fillet weld subjected to torsion d = Diameter of rod, T = Torque acting on the rod, s = Size (or leg) of weld, t = Throat thickness, J = Polar moment of inertia of the weld section =

Shear Stress of the material is:

maximum shear occurs on the throat of weld which is inclined at 45° to the horizontal plane Length of throat: t = s sin 45° = 0.707 s ⇒

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Special Cases of Fillet Welded Joints 2. Circular fillet weld subjected to Bending Moment d = Diameter of rod, M = Bending moment acting on the rod, s = Size (or leg) of weld, t = Throat thickness, Z = Section modulus of the weld section

Bending Stress:

maximum shear occurs on the throat of weld which is inclined at 45° to the horizontal plane Length of throat: t = s sin 45° = 0.707 s

⇒

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Strength of Butt Joints

 Butt Joints are designed for tension or compression  In case of butt joint, length of leg or size of weld is equal to throat thickness which is equal to thickness of plates

 Tensile Strength of the butt joint (single-V or square butt joint) P = t × l × σt  Tensile Strength for Double-v Butt Joint P = (t1 + t2) l × σt t1 = Throat thickness at the top t2 = Throat thickness at the bottom

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Example 2.1 Figure shows a horizontal steel bar of thickness h loaded in steady tension and welded to a vertical support. Find the load F that will cause an allowable shear stress, τallow, in the throats of the welds.

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Eccentrically Loaded Welded Joints

 Eccentric Load may be imposed on welded joints in many ways  induced stresses are combined depending upon the nature of stresses  When the shear and bending stresses are simultaneously present in a joint, then maximum stresses are as follows Maximum Normal Stress Maximum Shear Stress

σb = Bending stress τ = Shear stress

T-joint fixed at one end and subjected to eccentric load

 a T-joint fixed at one end and subjected to an eccentric load P at a distance e

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joint will be subjected to the following two types of stresses 1. Direct shear stress due to the shear force P acting at the welds 2. Bending stress due to the Mechanical bendingEngineering moment P× e. NUST Dept. CEME

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Design of Welded Joints Eccentrically Loaded Welded Joints T-joint fixed at one end and subjected to eccentric load —contd-A = Throat thickness × Length of weld = t × l × 2 = 2 t × l ... (For Double Fillet Weld) = 2 × 0.707 s × l = 1.414 s × l ... (t = s cos 45° = 0.707 s)

Shear Stress in the weld (assuming uniformly distributed) Section Modulus of the weld metal through the throat ...(For both sides weld)

Bending moment, M = P × e Bending Stress Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Stresses in Welded Joints in Torsion

 A Cantilever of length l welded to a column by two Fillet Welds  Eccentric Load F can be replaced by a Shearing Force V and a Moment M

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Shear Force produces a Primary Shear τ/ in the welds of magnitude:

A is the Throat Area of all the welds Primary Shear τ/ is always directed parallel to P

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Moment at the support produces Secondary Shear or Torsion of the welds: r = Distance from the Centroid of Weld Group to the point in the weld of interest J = Second Polar Moment of Area of Weld Group about the Centroid τ// is proportional to its distance from the center of twist (r), ⇒ (τ//)max will occur at the corners of the weld

 Secondary Shear Stress τ// can be added vectorially to the Primary Shear Stress τ/ determine the Maximum Shear StressEngineering τmax Mechanical Dept. CEME NUST

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to

Design of Welded Joints Stresses in Welded Joints in Torsion Two Welds in a Group

 Rectangles

represent the Throat Areas of the welds

Throat Thickness of Weld-1 = t1 = 0.707h1 Throat Thickness of Weld-2 = t2 = 0.707h2 h1 and h2 are the respective Weld Sizes

⇒ Throat Area of both welds together A = A1 + A2 = t1d + t2b Second Moment of Area of Weld-1 through G1 about x-axis is: Second Moment of Area of Weld-1 through G1 about y-axis is:

Second Polar Moment of Area of Weld-1 about its own centroid: Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Stresses in Welded Joints in Torsion Two Welds in a Group--contd-Second Polar Moment of Area of Weld-2 about its own centroid:

Centroid G of the Weld Group is located at

distances r1 and r2 from G1 and G2 to G

using the Parallel-axis Theorem, Second Polar Moment of Area of the Weld Group This is to be used in Torsion Eq.

 In a Reverse Procedure, Weld Size can be found for which the Allowable Shear Stress is given Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Stresses in Welded Joints in Torsion Two Welds in a Group--contd--

 Setting the weld thicknesses t1 and t2 to Unity leads to the idea of treating each fillet weld as a Line

 Resulting Second Moment of Area is then a Unit Second Polar Moment of Area  Since Throat Width of a fillet weld is 0.707h, the relationship between J and Ju is:

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Torsional Properties of Fillet Welds

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Example 2.2 A steel bar of thickness h, to be used as a beam, is welded to a vertical support as shown in the figure. Find the safe bending force F if the allowable shear stress in the welds is 140 MPa

Mechanical Engineering Dept. CEME NUST

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Design of Welded Joints Example 2.3

A 50-kN load is transferred from a welded fitting into a 200-mm steel channel as illustrated in Fig. Estimate the maximum stress in the weld.

Mechanical Engineering Dept. CEME NUST

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