How It Works: How do you determine the adequate size of a fillet weld

Question: How do you determine the adequate size of a fillet weld (if there is no drawing, this is typical for maintenance welding)? How is it measured?

Author: Jim Galloway Professor & Coordinator of Welding Programs Conestoga College - Cambridge Campus

Answer: I’m reluctant to answer a question with questions, but this one often comes up and my response is: “What do you want the weld to do?”  “What are the worst-case consequences if it fails?”  “Who will take the responsibility if it fails?”

Fillet welds can either be too small (or short) and these may lead to an unanticipated failure due to undesirable metallurgical changes in steels.  Even if the weld is only holding a small item or acting as a seal for liquid, this weld may act like an arc strike in a steel structure and lead to a catastrophic crack.

If a fillet weld can be too small, the typical response would be, “Let’s just make it bigger!”  Unfortunately, an oversized weld can lead to other problems such as distortion, residual stresses, or grain growth which can weaken or embrittle a steel structure.

In North America we measure the size of fillet welds by the leg-size.  These legs form a triangle, and the effective throat determines the effective area of the weld after multiplying this distance by the effective length (see figure).  In this way a fillet weld with small legs and a long length could carry more load than a large leg fillet that is short; the effective area is the key.

After the effective area is determined, the allowable strength of the deposited weld metal is factored in.  If the steel weld metal is only the minimum strength for standard electrodes, e.g., E43xx (E60xx), and the allowable strength is specified to 30% of this (as a safety factor), a 5 mm (3/16 in.) fillet weld that is 38 mm (1.5 in.) long can support a static load of over 17 kN (3,500 lbf).

Therefore, the load capacity of the weld is determined by the Effective Throat x Effective Length x Allowable Strength, however it’s not that simple!

There are still too many unknowns:  Will the steel around weld support the load?  Was the joint fit-up tight?  How can we be assured that the weld was properly applied?  Will the weld be subjected to cyclic loading, twisting, impacts, temperature extremes, or corrosion that could cause premature failure?

The bottom line is that weld design can be a complex problem with many hidden pitfalls and risks.  In Canada structural steel or aluminum welds must be designed and approved by a professional engineer (P.Eng.) licensed to practice in your province or jurisdiction.  Beyond this, a welding engineer must have a strong background in welding procedures and processes, welding metallurgy, electrode and filler metal selection, and welding codes and standards.  This professional will interpret the design standard requirements and specify a suitable fillet weld size to meet the requirements.