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


Design of Fillet Welded Connections

The size of a fillet weld should be calculated by the designer of the welded structure, based on the nature and magnitude of applied loads, on the material, and on the design basis.

The strength of a fillet weld is based, in the design, on the product (effective area of the weld: T x W) of the theoretical throat (design throat thickness) and effective weld length as shown in the below figure.

throat size
Figure 1. Nomenclature of fillet weld dimensions

Concave or Convex or flat fillet weld

Concave fillet weld is favored because they offer a smoother path for the flow of stress. But concave fillet welds during weld solidification shrink and causes tension to the surface which may cause cracks in the joint after the weld solidifies.

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fillet weld throat size
Figure 2. Mitre, Convex, and Concave fillet weld

On the other hand, shrinkage of the weld will cause compression in the case of convex fillet weld. Concave fillet welds are more suitable under alternating stresses.

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Minimum size of fillet weld

Often, in fabrication, it happens many instances where a fillet weld size is unclear on the given in the drawings, or not reported, or simply the information has been lost. In such cases, we should consult the designer for the required fillet weld size. Otherwise, as a rule-of-thumb at least two passes should be deposited, with a minimum leg length of 6mm (1/4in), considering that the first run is likely to not have full fusion in weld length.

Minimum fillet size required according to AWS D1.1: If fillet weld size is not specified in the drawing, the general guidelines of AWS D1.1, Table 7.7 shall be followed. The table below gives the minimum fillet size required for different base metal thicknesses.

minimum size of fillet weld
Table 1. Minimum fillet size requirement

American Institute of Steel Construction (AISC) 360-16 Specification for Structural Steel Buildings you’ll find this on Table J2.4 as given below:

minimum size of fillet weld

Maximum size of fillet weld

The maximum size of fillet weld will be= 0.7 x T, where T is the base metal thickness. In case of dissimilar thickness, we should consider the thinner base metal thickness.

Types of loading in fillet welds

Fillet welds joints are most commonly used in fabrication and structures. A fillet weld joint can be loaded in any direction in shear, compression, or tension loading. However, fillet weld always fails in shear loading.

The shear failure of the fillet weld happens along a plane through the throat of the weld, as shown in the Figure below.

minimum size of fillet weld

Throat size (a) to Leg length (Z) conversion formula

The ‘a’ size is calculated from the previous corner of the workpiece before welding until the 45° created middle of the welding seam. The sidewall – connection between the welding seam and the base material is called – ‘z’. The ‘a’ size is nothing more than the Hypotenuse of an isosceles triangle.

For a fillet weld with equal leg lengths, the cross section triangle is a right-angle triangle with angles of 45 degrees in each corner. The relationship between weld throat, a and leg length z is given by:
a ≈ 0.7z and z ≈ 1.4 a
(For the maths-minded, 0.7 is 1/√2 and 1.4 is √2).

minimum size of fillet weld

So, if we know the value of shear strength required as per design, we can calculate the throat size required a fillet weld joint.

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