Handbook›Weld Sizes & Strength

Weld Sizes & Strength

IS 800:2007 / IS 816:1969 · IS 800 Cl 10.5 — Welded connections

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.

IS 800 →IS 816 →Try Steel Section Table →

27 items shown

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

← Previous

Effective Length Factors for Columns

Ductile Detailing for Seismic (IS 13920)

Generated by InfraLens · infralens.in