What is the maximum size of the fillet weld in a lap joint for a member that is 3/8" thick?

The maximum size of a fillet weld in a lap joint is generally specified based on the thickness of the material being joined. According to welding codes and standards, the maximum leg size of a fillet weld should typically not exceed the thickness of the thinner member being joined.

In this case, with a member that is 3/8" thick, the applicable guideline suggests that the maximum size for the fillet weld would be 3/8" itself. However, standard practice often recommends that the preferred maximum size for such a weld is limited to be slightly less than the base material thickness to ensure proper fusion and strength, which corresponds to 5/16".

The correct answer reflects this guideline. The consideration of effective weld size impacts the overall joint strength, since it's vital to ensure both penetration and control over the weld's physical properties. Thus, for a member thickness of 3/8", the maximum recommended size for a fillet weld in a lap joint is 5/16".