In structural steel design, what is the maximum permissible size of a fillet weld placed on the square edge of a plate, considering proper fusion and avoidance of overwelding at the root?

Introduction / Context:
Fillet weld sizing on square plate edges must respect code rules to ensure sound fusion and to prevent lack-of-fusion defects at the root. Overly large fillets on square edges can trap slag and cause incomplete penetration, compromising connection performance.

Given Data / Assumptions:

• Square (unprepared) plate edge with fillet weld on the edge.
• Standard structural steel practice and code-based detailing rules.
• Objective: determine the upper bound of the leg size for such a fillet weld.

Concept / Approach:
Codes and good practice limit the maximum leg size of a fillet weld on a square edge to slightly less than the plate thickness, typically by 1.5 mm. This provides a small root face to avoid overwelding and assists proper heat distribution for fusion without excessive melt-through or distortion.

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