Calculating the throat thickness of a fillet weld

Throat thickness is the design dimension that governs the strength of a fillet weld. Yet on the shop floor, and even on drawings, the leg length z is regularly meant where a is written, or the other way round. The difference is a factor of 1.41, enough to make a joint substantially over or under designed.

This article explains exactly what throat thickness is, how to calculate it, how it relates to leg length, and why a bigger weld is not always a better weld.

What throat thickness is

Throat thickness is the height of the largest isosceles triangle that fits within the cross section of the fillet weld, measured from the theoretical root to the weld face. Leg length z is the length of the triangle leg along the parent material.

For an equal leg fillet, a = z times 0.7 (more precisely 1 divided by the square root of 2). A fillet with z = 10 mm therefore has a throat of 7 mm. Conversely, if the calculation requires a = 5 mm, the welder must deposit a leg of roughly 7 mm.

The calculation to EN 1993-1-8

Eurocode 3 part 1-8 uses either the directional method or the simplified method. The simplified method checks the resultant force per unit length of weld against the design weld resistance, derived from the tensile strength of the parent material, a correlation factor beta-w and the partial factor.

For S235 beta-w is 0.8, for S355 it is 0.9 and for S460 it is 1.0. That means stronger steel does not proportionally allow a smaller weld: the correlation factor partly offsets the gain. In practice the required throat is therefore always calculated, never estimated.

Minimum and maximum values

There is a practical minimum: too small a weld cools so quickly that hardness and cracking risk increase, particularly on heavy material. Many specifications set a minimum throat of 3 mm, and more on heavy sections, derived from the thickness of the thinner part.

At the upper end, a fillet on thin material need not be larger than the thickness justifies. A throat above roughly 0.7 times the thinner plate thickness adds little strength while costing disproportionate weld metal, arc time and shrinkage.

Why bigger is not better

Weld metal volume increases with the square of the throat. Going from a = 4 to a = 6 mm means over twice the weld metal, so over twice the arc time, wire and heat. That heat translates directly into more shrinkage and more distortion.

A fillet routinely laid a few millimetres oversize is therefore one of the most expensive habits in a workshop. Measuring with a fillet gauge and holding the specified size saves material and straightening. See controlling weld distortion.