How It Works
Fillet Welds vs Groove Welds
Welded joints are classified by geometry. The two primary types are:
- Fillet Welds: Triangular cross-section joining two surfaces at approximately right angles. Most common weld type (~80% of structural welds). Used for lap joints, tee joints, and corner joints.
- Groove Welds: Made in a groove between workpieces. Provides full or partial penetration. Types include V-groove, bevel, U-groove, J-groove. Used when full strength of base metal is required.
- Plug/Slot Welds: Circular or elongated welds made through holes in one member to join to another. Used when fillet welds are impractical.
Throat Thickness and Effective Area
The critical dimension for weld strength is the effective throat, which is the shortest distance from the root to the face of the weld:
- Fillet Weld: For equal-leg 45-degree fillets:
a = 0.707 x wwhere w = leg size - Groove Weld (CJP): Throat = thickness of thinner member joined
- Groove Weld (PJP): Throat = depth of preparation minus allowance for incomplete fusion
Effective weld area: Awe = throat x effective length
For fillet welds, the effective length excludes crater lengths (typically 2w at each end for intermittent welds).
Allowable Stresses per AWS D1.1
AWS D1.1 Structural Welding Code provides allowable stresses for different weld types and loading conditions:
- Fillet Welds (shear on effective throat): Fw = 0.30 x FEXX (ASD) or phi x 0.60 x FEXX (LRFD, phi=0.75)
- CJP Groove Welds: Same as base metal allowable stress for tension, compression, and shear
- PJP Groove Welds (tension/compression): Fw = 0.30 x FEXX (perpendicular to axis)
- Plug and Slot Welds: Fw = 0.30 x FEXX on effective area
Note: For matching electrodes, the base metal typically governs. For undermatching, the weld metal strength controls.
Electrode Classifications
| Electrode | FEXX (ksi) | FEXX (MPa) | Common Use |
|---|---|---|---|
| E60XX | 60 | 414 | Mild steel, low-carbon |
| E70XX | 70 | 482 | Most common - A36, A992 steel |
| E80XX | 80 | 551 | Higher strength steel |
| E90XX | 90 | 620 | Q&T steels, A514 |
| E100XX | 100 | 689 | High-strength applications |
| E110XX | 110 | 758 | Very high-strength steel |
Weld Symbols
AWS standard welding symbols communicate weld requirements on drawings:
Minimum and Maximum Fillet Weld Sizes
AWS D1.1 specifies minimum fillet sizes based on thicker plate thickness to prevent cracking, and maximum sizes to ensure proper fusion:
- Minimum: Prevents rapid cooling and hydrogen cracking
- Maximum: At plate edge = plate thickness - 1.5mm (1/16"). Away from edge = unlimited (practical limit = plate thickness)
Weld Sizing Calculator
Calculate fillet and groove weld capacity, required sizes, and stress analysis per AWS D1.1 and AISC standards.
Weld Geometry
Loading
Material Properties
Strength Utilization
Weld Symbol
Weld Analysis Results
Minimum Fillet Weld Sizes (AWS D1.1)
| Base Metal Thickness | Min Weld Size |
|---|---|
| t ≤ 6 mm (1/4") | 3 mm (1/8") |
| 6 < t ≤ 13 mm (1/4-1/2") | 5 mm (3/16") |
| 13 < t ≤ 19 mm (1/2-3/4") | 6 mm (1/4") |
| 19 < t ≤ 38 mm (3/4-1.5") | 8 mm (5/16") |
| 38 < t ≤ 57 mm (1.5-2.25") | 10 mm (3/8") |
| t > 57 mm (2.25") | 13 mm (1/2") |
Maximum at edge = plate thickness - 1.5mm (1/16"). Based on thicker member joined.
Design Guidelines
- Minimum effective length = 4 x leg size or 40mm (1.5"), whichever is greater
- For lap joints, minimum overlap = 5 x thinner plate thickness
- Intermittent welds: min segment = 40mm, max pitch = 24 x thinner plate or 300mm
- End returns: extend around corners at least 2 x leg size
- Avoid placing welds on highly stressed areas subject to fatigue
- For dynamic/cyclic loading, use SF ≥ 3.0 and continuous welds
Reference Formulas
Throat Size (equal-leg fillet):
a = 0.707 x w
Effective Weld Area:
Awe = a x L_eff
AISC LRFD Capacity:
Rn = phi x 0.60 x FEXX x Awe
phi = 0.75 for fillet welds
AWS Allowable Stress:
Fw = 0.30 x FEXX
Capacity per Unit Length (E70XX):
q = 1.392 x D kips/in
D = leg size in 1/16 inch increments