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Welded Connections | Design Strength of Fillet Weld and Butt Weld

Welding is a type of connecting two members (pieces of metal) by heating to a plastic or fluid state so that fusion occurs and a rigid connection is formed between the members. Types of weld and design of butt and fillet weld are discussed further.


Type of Welded Joint


Based on the position of weld on the members there are four different types of weld that are used in practice.

  • Fillet welds

  • Groove welds

  • Slot welds

  • Plug welds


1. Fillet Welds


These are the most commonly used weld due to their economy, ease of fabrication, and site adaptability. They are approximately triangular in cross-section and requires overlapping of pieces that are to be welded. Because of this overlapping, they require less precision while fitting.


Specifications for a Fillet Weld


Listed below are the specifications of a fillet weld according to IS 800: 2007, and IS 816.


1. Size of a Fillet Weld (S)


  • The size of the fillet weld is represented by S. The size of a fillet weld is the length of the smaller side of the fillet weld (S1/Smin) or it is taken between S1(S min) and S2 (S max).

  • According to IS800: 2007 the minimum size (S1) of a fillet weld is based on the thickness of the thicker plate as given below.

The thickness of the thicker plate (mm)

Min size of the weld (S min/S1) (mm)

​t ≤ 10

3

​10 < t ≤ 20

5

20 < t ≤ 32

6

32 < t ≤ 50

8 (first run) + 2 = 10

  • Note: If S1 (Smin) > thickness of the thinner plate (t thin), then S1 is taken as t thin.

  • S2 or S max is based on the thickness of the thinner plate;

  • S2 = t thin - 1.5 (for square edge)

  • S2 = (3/4) * t thin (for round edge)

Size of fillet weld (S) is therefore either adopted between S1 (S min) and S2 (S max) or simply taken as S1 (S min) in most cases.


2. Throat Thickness of the Fillet Weld (tw)


As the fillet weld is in the form of a triangle in cross-section, the effective size of a fillet weld is required for designing purposes. This effective size is expressed in terms of throat thickness and is measured as the perpendicular distance drawn on the hypotenuse of the triangular cross-section from the root of the fillet.


According to IS800: 2007, the throat thickness (tw) of a fillet weld is totally dependent on the fushion angle (the angle at which two members are joined), as shown below.

Fushion Angle

Throat thickness (tw) (in mm)

60 - 90

0.7*S

91 - 100

0.65*S

101 - 106

0.6*S

107 - 113

0.55*S

114 - 120

0.5*S

Note:

  • Throat thickness (tw) shall not be less than 3mm, and

  • Throat thickness (tw) shall not be greater than 0.7 * thickness of the thinner plate (t thin).


3. Effective Length of Fillet Weld (leff)


The effective length of a fillet weld is the total length of fillet weld (l) - (2 * weld size (S)) i.e., leff = l - (2*S)


Note: According to IS 800: 2007,

  • leff must have a minimum value of 4 times the weld size (S), i.e., leff > 4*S always

  • In case of intermittent weld, the minimum leff value shall be greater of 4*S or 40 mm

Note: The length of the weld shown in the diagram is always taken as the effective length (leff) and the extra length i.e., 2 * S, is provided by the welder.


4. Spacing of Intermittent Weld


Spacing of intermittent weld according to IS 800: 2007, shall not exceed

  • for tension member - 16 * thickness of thinner plate (t thin) or 200mm

  • for compression member - 12 * thickness of thinner plate (t thin) or 200 mm


5. Maximum Permissible Shear Stress in Fillet Weld (f weld)


According to IS 800: 2007 the maximum permissible shear stress (f weld) or (fw) in a fillet weld shall be,


f weld = fu/(√3*γmw),

where,

fu - smaller of the ultimate street of the weld or of the parent metal (but mostly taken as the ultimate tensile strength of the parent metal),

γmw = 1.25 for shop welding (refer table 5 of IS 800: 2007),

γmw = 1.5 for field welding (refer table 5 of IS 800: 2007).


Note: The direct shear force applied in the weld is always less than or equal to the tensile strength of the smaller plate (given by ((fy/1.1) * (b*t))).


Design Strength of a Fillet Weld


P = fw * leff * tw


As said in the above note, P must be less than or equal to the tensile strength of the smaller plate. Therefore, according to the Limit State Method (IS 800), the design strength of a fillet weld per mm length of the weld is given by,


P = (fy/1.1) * (b * t (smaller)) = (fu/√3*γmw) * leff * tw,

where,

fy - yield strength of the smaller plate,

b - breadth of the smaller plate,

t - thickness of the smaller plate.


2. Groove / Butt Weld


A groove weld is also called a butt weld as this type of weld is often used for forming butt joints. Edge preparation is necessary for making groove welds. A single V-butt weld is shown below.




Design Strength of a Butt Weld


Clause 10.5.7.1.2 Butt welds of IS 800: 2007 says, "Butt welds shall be treated as parent metal with a thickness equal to the throat thickness, and the stresses shall not exceed those permitted in the parent metal. Therefore, according to the Limit state method, the design strength of a butt weld (Pw) is given by,


Pw = (fy/γmw) * lw * tw,

where,

tw - (5/8) * thickness of thinner plate, for single V- Butt weld;

tw - thickness of the thinner plate, for double V-Butt weld;

fy - yield strength of the plate,

γmw = 1.25 for shop welding (refer table 5 of IS 800: 2007),

γmw = 1.5 for field welding (refer table 5 of IS 800: 2007).


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