Rule Of Thumb For determining the Fillet Weld Size

How to find minimum and maximum fillet weld size

This post gives simple calculations for determining the minimum and maximum fillet weld sizes to withstand the maximum stresses at the weld. The fillet weld size will be the optimum weld size using this calculation and fits most of the weld application. Any weld size above this value will not add any strength to the joint but instead goes wasted as additional added weld metal, man power and other costs.

So, I advise not to weld more than the size you obtain with this calculation.

Formula to calculate the Fillet weld size

To make the full strength of a plate using fillet welds, it is essential that the leg size of the fillet be 3/4 or 1/7 of the of the plate thickness. In situation of dissimilar plate thickness, consider the thinner member.

A fillet weld with this leg size will resist the maximum stress for a T joint under any scale and direction of loading.

This rule of thumb takes into consideration the following:

  1. fillet weld on both sides of the plate.
  2. fillet weld for the full length of the plate.
  3. If the plates are different thicknesses, the thinner plate thickness should be used.

An example t-joint is shown below with a weld size of 3/8″ on 1/2″ plates:

If designing for rigidity, meaning the stress in the plate is lower than 1/3 to 1/2 of the yield strength, a leg size of only 1/4 to 3/8 of the thickness of the plate is required. Intermittent welds may also be used to reduce the total volume of weld for a rigidity design.

If a weld can only be made on one side of the plate, then the weld size should be doubled.

The following table gives the leg size of fillet welds for various plate thicknesses based on the previous formula. It’s easy to calculate them yourself, but this table is a useful resource nevertheless:

If there is not enough welding heat input into a thick plate, a weld can cool too rapidly, resulting in a joint with poor penetration (a weak joint). The American Welding Society (AWS) has established minimum weld sizes for thick plates. In practicality, very thick plates should be preheated. The AWS table is shown below:

The approaches above are a fast and reliable method to size simple fillet weld joints. Complex critical joints, however, should be considered carefully. Check my other article on fillet weld sizing, stresses under tension, compression, vertical shear, bending, and twisting in the weld as given below:

Stress calculation in fillet and butt weld joints

How to calculate throat size or leg length size in a fillet weld?

The required fillet size for this design application was determined to be 5/16″. This technique can be used to size any fillet weld application.

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