Design data for fillet and butt welds in structural steel. Fillet welds are used at right-angle joints (lap, T, corner); butt welds are used end-to-end at the same level. Capacity = effective throat × length × design strength fwd. For E250 steel + matching electrode, fwd ≈ 158 MPa.
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Fillet Weld — Leg Size & Throat Thickness
Throat = 0.7 × leg size for equal-leg fillet welds (Cl 10.5.3.1)
Leg / Item Throat Capacity / Formula Notes Limit Comment 3 mm 2.1 mm 0.332 kN/mm ≥ 3 mm parts — Smallest practical fillet 4 mm 2.8 mm 0.442 kN/mm ≥ 5 mm parts — Light frames 5 mm 3.5 mm 0.553 kN/mm ≥ 6 mm parts — General building work — most common 6 mm 4.2 mm 0.664 kN/mm ≥ 8 mm parts — — 8 mm 5.6 mm 0.885 kN/mm ≥ 10 mm parts — Heavy frames 10 mm 7.0 mm 1.106 kN/mm ≥ 12 mm parts — Heavy plate work 12 mm 8.4 mm 1.327 kN/mm ≥ 16 mm parts — Multi-pass welds typically required 16 mm 11.2 mm 1.770 kN/mm ≥ 20 mm parts — Heavy industrial 20 mm 14.0 mm 2.212 kN/mm ≥ 24 mm parts — Very heavy — multiple passes
Minimum Fillet Weld Size by Plate Thickness (IS 800 Table 21)
Minimum size to avoid cracking from rapid cooling
Leg / Item Throat Capacity / Formula Notes Limit Comment Min 3 mm — — Up to 10 mm thicker plate — — Min 5 mm — — 10–20 mm thicker plate — — Min 6 mm — — 20–32 mm thicker plate — — Min 8 mm (1st run) — — 32–50 mm thicker plate — — Maximum size — — Thinner plate − 1.5 mm Or full thickness if rounded Cl 10.5.3.2
Fillet Weld — Length Limits
Leg / Item Throat Capacity / Formula Notes Limit Comment Minimum effective length — ≥ 4 × leg size AND ≥ 40 mm — Cl 10.5.4.1 Maximum effective length (lap joint) — ≤ 16 × thinner plate t Or 200 mm — Beyond this, longitudinal stress is non-uniform End return — ≥ 2 × leg size Around corner — Required at end of intermittent welds Intermittent fillet — longitudinal spacing — ≤ 12 t (compression) ≤ 16 t (tension) ≤ 200 mm t = thinner plate
Butt Weld — Effective Throat
Leg / Item Throat Capacity / Formula Notes Limit Comment Single V (60° prep) Full plate t = t × fwd — Up to 16 mm Plate butt joint, single side prep Double V (60° prep) Full plate t = t × fwd — Plates > 16 mm Both sides — required for thicker plates Single U Full plate t = t × fwd — Heavy plates Less filler metal than V Square butt Up to 4 mm — — Plates ≤ 5 mm No edge prep needed Incomplete penetration 0.625 × T (max) — — — Cl 10.5.7.1.2
Design Strength fwd & Capacity Calc
Leg / Item Throat Capacity / Formula Notes Limit Comment fwd (E250 + matching electrode) — fu / (√3 × γmw) = 410 / (1.732 × 1.50) = 158 MPa Shop weld γmw = 1.25 Site weld γmw = 1.50 fwd (E350 + E70 electrode) — = 490 / (1.732 × 1.50) = 188 MPa — — Strength of weld — Pdw = lw × tt × fwd lw = effective length tt = throat thickness — Match electrode to parent metal — — E250 → E41/E43 electrode E350 → E51/E55 electrode IS 814 / IS 7307
Notes
• Effective throat = 0.70 × leg size for equal-leg fillet welds (Cl 10.5.3.1)
• Always design weld strength ≥ design force, not the parent metal capacity
• Throat is measured perpendicular to the hypotenuse of the fillet triangle
• Single-pass fillet welds: 6 mm leg in vertical/overhead, 8 mm in flat — beyond this use multi-pass
• End returns are mandatory for intermittent fillet welds — improves fatigue strength
• Pre-heat is required for thick plates (>32 mm) and high-carbon steel — prevents cracking
• All site welds: γmw = 1.50; shop welds: γmw = 1.25 (better quality control)
• Inspect every weld visually; do MPI/UT for critical joints (≥ 6 mm fillet, all butt welds in tension)
• Welder qualification per IS 7318 / IS 817 is mandatory before structural welding
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