Clause 10.5.7 gives the design strength of fillet welds based on the shear strength of the weld throat. The weld is assumed to fail in shear along the throat plane regardless of the direction of applied force. The design strength depends on the weld size, electrode grade, and fabrication quality (shop vs. site).
Key Requirements
•Design strength of fillet weld per unit length: Rw = fwd × tt × Lw
•tt = throat thickness = K × s, where s = weld size (leg length)
•K = 0.7 for 60–90° angle between fusion faces (most common); K varies for other angles per Table 28
•Minimum weld size depends on thicker plate thickness (Table 21): 3 mm for t ≤ 10 mm; 5 mm for 10 < t ≤ 20 mm; 6 mm for 20 < t ≤ 32 mm
•fu = smaller of fu of the base metal or fu of the weld electrode (usually weld electrode fuw ≥ base metal fu)
Reference Tables
Table 21 — Minimum Size of First Run of Fillet Weld (Clause 10.5.2)
Thickness of Thicker Part (mm)
Minimum Weld Size (mm)
Up to and including 10
3
Over 10 to 20
5
Over 20 to 32
6
Over 32
First run 8, min 6 after
Design Strength of Fillet Weld per mm Length (E250 steel, E41XX electrodes)
Weld Size s (mm)
Throat tt (mm)
Strength (kN/mm)
3
2.1
0.4
4
2.8
0.53
5
3.5
0.66
6
4.2
0.8
8
5.6
1.06
10
7
1.33
12
8.4
1.59
Based on fwd = 410/(√3 × 1.25) = 189.4 MPa for E250 steel (fu = 410 MPa) with shop welds (γmw = 1.25). Strength = fwd × tt. For site welds, use γmw = 1.50 and reduce by 17%.
Formulas
fwd = fu / (√3 × γmw)
Design shear stress of fillet weld
fwd = Design weld shear stress (MPa)fu = Smaller of ultimate stress of base metal or weld metal (MPa)γmw = Partial safety factor for welds = 1.25 (shop) or 1.50 (site)
tt = 0.7 × s
Throat thickness for standard fillet weld (90° included angle)
✓For quick estimation with E250 steel: a 6 mm fillet weld (most common site weld) has strength ≈ 0.80 kN/mm (shop) or 0.67 kN/mm (site). A 200 mm long weld provides ≈ 160 kN (shop).
✓Maximum weld size along the edge of a plate = plate thickness − 1.5 mm (Cl. 10.5.3.3). For a 10 mm thick plate, maximum weld = 8.5 mm.
✓In Indian practice, E41XX (now E 4111 IS 814) electrodes are standard for E250 steel. For E350 steel, use E49XX electrodes to ensure weld metal strength ≥ base metal strength.
Common Mistakes
⚠Using the weld leg length instead of the throat thickness in strength calculations — the throat (0.7 × leg) is the critical dimension, not the leg.
⚠Not reducing effective length by 2 × s for welds that do not return around corners — this accounts for crater effects at weld ends.
⚠Using shop weld γmw = 1.25 for site welds — site welds must use γmw = 1.50, reducing capacity by about 17%.