Fillet Weld
What is the difference between leg size and effective throat in a fillet weld?
Leg size is the visible dimension from the root to the toe along the joint faces, while the effective throat is the shortest load-bearing distance through the weld metal. Design strength is based on throat area, not the apparent bead size.
Why is fillet weld strength often considered in shear?
Many fillet welds in lap and T-joints carry load parallel to the weld axis, so the governing design check uses the effective throat area under shear loading. This is why code-based calculations treat fillet welds as shear planes.
When does a fillet weld become a bad choice?
When the joint requires deep penetration, a critical root seal, or geometry that demands a groove weld for full fusion and code compliance, a fillet weld may be insufficient by design intent.
A fillet weld is a weld bead deposited in the inside corner of a joint, typically a T-joint, lap joint, or corner joint. Its specified leg size defines the cross-section, while its effective throat governs load capacity. Because the cross-section is roughly a right isosceles triangle, throat is approximated as leg multiplied by 0.7.
In a CNC or fabrication shop, fillet welds are the default choice when two members meet at a right angle and full groove preparation is not required. They are economical because they need no beveling and can be deposited quickly along the intersection. However, practical success depends on joint fit-up and process control. The operator must balance travel speed, amperage, wire feed or filler deposition, torch angle, and shielding gas flow so that the bead wets both toes and develops the intended throat. If travel is too fast, the weld becomes undersized; if too slow, the profile becomes excessive or misshapen. Structural welders verify fillet leg size with purpose-built gauges, and large specified legs may require multiple passes. In production, heat input and shielding must be managed carefully to avoid porosity and inconsistent toe blend, since any of these defects can force grinding, rework, or rejection during visual inspection.
Undersized throat hiding behind a full bead: A weld can look substantial while the effective throat remains too small, slashing load capacity when leg size is judged only by face width rather than gauge measurement.
Lack of fusion at the root and toes: Fast travel, low amperage, or a poor torch angle leaves weld metal sitting on the surface, and the joint fails despite a continuous bead. Poor fit-up worsens it.
Porosity and distorted bead profile: Excessive shielding gas turbulence or heat input traps gas in the weld and produces an overly convex bead that fails visual acceptance and needs rework.