Fillet Weld

A fillet weld is a triangular cross-section weld used to join two metal pieces meeting at right angles or oblique. The weld fills the corner formed at the junction of the two pieces — hence the name 'fillet'. Fillet welds are the most common weld type in structural steel fabrication and erection, used for nearly all T-joints, lap joints, corner joints, and angle-to-plate connections. Indian standards: IS 816:1969 (manual metal arc welding code of practice — design clauses for fillet welds), IS 9595:1996 (manual metal arc welding of carbon steels), IS 800:2007 Section 10 (design of welded connections in structural steel).

Geometric parameters: 'leg size' is the distance from the corner to the toe of the fillet, measured along each connected face. 'Throat thickness' is the perpendicular distance from the corner to the hypotenuse of the triangular weld cross-section — for an equal-leg fillet at 90°, throat = 0.7 × leg size. The throat thickness is the design parameter — design strength is computed at the throat. Minimum leg size per IS 800:2007 Cl. 10.5.2: 3 mm (plates ≤ 10 mm thick), 5 mm (10-20 mm), 6 mm (20-32 mm), 8 mm (32-50 mm). Maximum effective length per Cl. 10.5.7: not more than 12× leg size in unbroken length; longer welds reduced for stress concentration.

Design per IS 800:2007 Cl. 10.5: fillet weld design strength = 0.7 × leg × length × (fu/√3) ÷ γmw, where γmw = 1.25 (shop weld, controlled environment) or 1.5 (site weld, field conditions). For Fe-410 steel (fu = 410 MPa) with 6 mm leg: shop weld strength ≈ 800 N/mm of weld length. Site weld with same leg: strength ≈ 663 N/mm, 17% lower due to field conditions. The most common Indian fabrication mistake is under-sizing site welds — fabricator drawings often specify 6 mm legs designed for shop quality, but actual site welds are field-applied with γmw = 1.5; design must specify the leg size with the appropriate γmw factor.