When welding to members of dissimilar thickness, what is the maximum fillet size allowed?

• A. The fillet size must not exceed the thickness of the smallest member.
• B. The fillet size must equal the thickness of the smallest member.
• C. The fillet size must be twice the thickness of the smallest member.
• D. The fillet size must not exceed the thickness of the smallest member unless larger is required by calculated stress.

Answer: (D)

When joining members of different thickness, the fillet weld size is controlled by the thinner member. This ensures the weld legs fit within the edge geometry of the thinner plate and provides proper weld geometry and load transfer without overbuilding the thinner part. In practice, the fillet size should not exceed the thickness of the smallest member, because that keeps the weld feasible and properly formed.

However, design work can require a larger weld to meet calculated stresses or required strength. If the structural analysis shows the joint needs more weld metal to carry the expected loads, the fillet size can be increased accordingly, within what the code and design permit.

So the best approach is to limit the fillet to the thinner member unless the design calculations justify a larger weld.