7/23/2019 Fillet Weld Size Calcul

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Weld Strength Calculation Example for Bending Moment Application

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Fillet Weld Size Calculation Equations for Welding Joint

Subjected to Bending Moment

The basic steps required, as per BS 5950, for fillet weld design calculations where the welded jointed structure is subjected to

bending momentload are:

1. Calculation of unit throat weld area (Au): If you have a fillet weld line of, say, 50 mm long (L), then the unit throat

area of the welding joint can be calculated as:

Au =L*1=50 * 1=50 sq. mm.eq.1

Similarly, if your joint is in between a plate and a rectangular cross sectional tube with two sides of the rectangle are 10mmand 20mm. Then the periphery of the rectangle is 2x(10+20)=60 mm. So, the unit throat area for this case will be 60 sq.mm.

2. Design strength of the welded joint (Pw): It is the maximum permitted weld stress value for a particular welding

joint . Design your fillet joint in such a manner that the stress developed on the joint does not cross the design strength(Pw).

The values of the design strength are fixed and depends upon the kind of electrode used and the kind of parentmaterial.

You should get the Pw value as a input while performing the fillet weld size calculation. However, if yo dont get thePw value as an design input then use the following rule of thumb:

Pw=0.5*fu..eq.2

Where, fuis the ultimate tensile strength of the parent material.

3. Calculate unit area moment of inertia (Iu) for the welded joint: This is the area moment of inertia value of the weld

section considering throat length as one. For details refer How to calculate unit area moment of inertia for circularwelded joint.

4. Calculate direct shear stress (s): Two types of shear stress will be developed in the joint subjected to only bending:direct shear stress (s)and shear stress due to bending (b).

s=F/Au..eq.3

Where,

Fis the applied load

Page 1 of 4Weld Stress Calculation of Welding Joint Design in Bending Loads

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7/23/2019 Fillet Weld Size Calcul

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5. Calculate shear stress due to bending (b): The following equation is used here:

b=M*Y/Iueq.4

Where,

Mis the applied bending moment to the welded joint

Y is the distance between the centre axis and the extreme fibre of the section of the welded joint. For, the rectangular cross

section, the value of the Y is equal to the half of the depth of the cross section.

6. Calculate the resultant stress (): The resultant stress of the bending and shear stress is calculated by:

= (s* s + b* b).eq.5

7. Calculate the weld throat size (t):The resultant stress calculated above ()above are for the unit throat size of theweld. The throat size required for keeping the resultant stress below (or equal) to the design stress ( Pw) is:

t= /Pw.eq.6

Where,

is the resulted stress experienced by the joint

Pwis the design stress for the fillet welding

8. Calculate leg length of the weld (L):The leg length of the weld can be found from the below equation:

L=1.4141*t.eq.7

Where,

t is the weld throat size

For the sake of practicality, the next higher value to the value of the weld leg length obtained from the weld strength designcalculationis to be take into account for the weld connection design.

About Shibashis Ghosh

Shibashis is a mechanical design engineer by profession and writer/blogger by passion. He has worked invarious design projects in automobile, power plant and heavy earth moving equipment domain since 2005.

In his spare time apart from maintaining this blog, heenjoy investing time building his first RC plane

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Page 2 of 4Weld Stress Calculation of Welding Joint Design in Bending Loads

04/11/2015http://blog.mechguru.com/machine-design/fillet-weld-size-calculation-equations-for-...