AISC Minimum Fillet Weld Size — Table J2.4 by Material Thickness
Fillet weld size is bounded by two code limits: a minimum to ensure adequate heat input for proper fusion, and a maximum to prevent edge notching and excessive weld metal. AISC 360-22 Table J2.4 specifies the minimum fillet weld leg size based on the thickness of the thicker part being joined. This page reproduces Table J2.4, the maximum size limits, single-pass limits, and design implications.
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AISC 360 Table J2.4 — Minimum Fillet Weld Size
The minimum fillet weld leg size depends on the thickness of the thicker connected part. The purpose is metallurgical: a thicker plate acts as a heat sink, so a larger weld is needed to deliver enough heat for proper fusion.
| Thickness of Thicker Part (t) | Minimum Fillet Weld Leg Size |
|---|---|
| t ≤ 1/4 in (≤ 6 mm) | 1/8 in (3 mm) |
| 1/4 in < t ≤ 1/2 in (6–13 mm) | 3/16 in (5 mm) |
| 1/2 in < t ≤ 3/4 in (13–19 mm) | 1/4 in (6 mm) |
| t > 3/4 in (> 19 mm) | 5/16 in (8 mm) |
Important exception (AISC 360 Section J2.2b): Where the minimum fillet weld size from the table exceeds the thickness of the thinner part, the minimum is limited to the thickness of the thinner part.
Quick Lookup by Common Plate Thickness
| Plate Thickness (in) | Minimum Weld Size (in) |
|---|---|
| 3/16 | 1/8 |
| 1/4 | 1/8 |
| 3/8 | 3/16 |
| 1/2 | 3/16 |
| 5/8 | 1/4 |
| 3/4 | 1/4 |
| 7/8 | 5/16 |
| 1 | 5/16 |
| 1-1/4 | 5/16 |
| 1-1/2 | 5/16 |
| 2 | 5/16 |
Note: The minimum does not increase beyond 5/16 in for any thickness. A 2-in plate and a 6-in plate both require a minimum 5/16-in fillet weld.
Maximum Fillet Weld Size — AISC 360 Section J2.2b
The maximum fillet weld size along the edge of a connected part prevents notching and stress concentration:
| Edge Condition | Maximum Fillet Weld Leg Size |
|---|---|
| Material thickness < 1/4 in | Equal to material thickness |
| Material thickness ≥ 1/4 in | Material thickness − 1/16 in |
Examples:
- 1/2-in plate edge → max weld = 1/2 − 1/16 = 7/16 in
- 3/4-in plate edge → max weld = 3/4 − 1/16 = 11/16 in
- 3/16-in angle leg → max weld = 3/16 in (equal to thickness)
This limit only applies to welds along the edge of a part. Welds on the flat face of a plate are not subject to the maximum edge restriction.
Single-Pass Weld Size Limits
Per AWS D1.1 Clause 5.23, single-pass fillet welds are limited by position:
| Welding Position | Maximum Single-Pass Leg Size |
|---|---|
| Flat (1F) | 1/4 in |
| Horizontal (2F) | 5/16 in |
| Vertical (3F) | 1/2 in |
| Overhead (4F) | 5/16 in |
When the required fillet weld size exceeds the single-pass limit, multiple passes are required. Preheating between passes and interpass temperature control per AWS D1.1 Table 3.2 applies for material thicker than 3/4 in.
Minimum Weld Size Comparison — AISC vs AS 4100 vs EN 1993
| Base Metal Thickness | AISC 360 (US) | AS 4100 (Australia) | EN 1993-1-8 (Europe) |
|---|---|---|---|
| ≤ 6 mm (1/4 in) | 3 mm (1/8 in) | 3 mm | 3 mm |
| 6–10 mm (1/4–3/8 in) | 5 mm (3/16 in) | 6 mm | 3 mm |
| 10–13 mm (3/8–1/2 in) | 5 mm (3/16 in) | 8 mm | 4 mm |
| 13–19 mm (1/2–3/4 in) | 6 mm (1/4 in) | 8 mm | 5 mm |
| > 19 mm (> 3/4 in) | 8 mm (5/16 in) | 10 mm | 6 mm |
Key difference: AISC bases the minimum on the thicker part; AS 4100 and EN 1993 base it on the thinner part. When joining a thin plate to a thick plate, AISC requires a larger minimum weld.
Effective Throat and Capacity
Design capacity uses the effective throat (E), not the leg size (a):
E = a × cos(45°) = a / √2 = 0.707 × a
For E70XX electrodes (F_EXX = 70 ksi), the design shear strength per inch of weld:
| Leg Size (in) | Effective Throat (in) | φRn per inch (kips/in) | φRn per ft (kips/ft) |
|---|---|---|---|
| 1/8 (2/16) | 0.088 | 2.78 | 33.4 |
| 3/16 (3/16) | 0.133 | 4.18 | 50.1 |
| 1/4 (4/16) | 0.177 | 5.57 | 66.9 |
| 5/16 (5/16) | 0.221 | 6.96 | 83.6 |
| 3/8 (6/16) | 0.265 | 8.36 | 100.3 |
| 7/16 (7/16) | 0.309 | 9.75 | 117.0 |
| 1/2 (8/16) | 0.354 | 11.14 | 133.7 |
Calculated as φRn = 0.75 × 0.60 × 70 ksi × E = 31.5 × E kips/in.
Worked Example — Minimum Weld Size Check
Problem: A 3/8-in gusset plate is welded to a 1-in column flange. The connection is designed for a 5/16-in fillet weld.
Step 1: Minimum weld size check Thicker part = 1-in column flange. Per Table J2.4, t > 3/4 in → minimum = 5/16 in. Our weld = 5/16 in ≥ 5/16 in — OK.
Step 2: Maximum weld size (edge condition) Edge of 3/8-in plate: max = 3/8 − 1/16 = 5/16 in. Our weld = 5/16 in ≤ 5/16 in — OK.
Step 3: Check thinner part exception The 5/16-in minimum from the table does not exceed the thinner part thickness (3/8 in), so no exception applies.
Step 4: Capacity check Effective throat = 5/16 × 0.707 = 0.221 in. φRn = 31.5 × 0.221 = 6.96 kips/in per weld.
Prequalified Weld Details (AWS D1.1)
AWS D1.1 defines prequalified joint details that do not require welding procedure qualification testing. The fillet weld details depend on joint configuration:
| Joint Type | AWS Detail | Description | Min. Weld Size |
|---|---|---|---|
| Lap joint | B-L1a | Fillet, single or double | Per Table J2.4 |
| T-joint | TC-P1 | Fillet both sides | Per Table J2.4 |
| T-joint | TC-P4 | Single-V groove + fillet | Per thickness |
| Corner joint | BC-P1 | Fillet both sides | Per Table J2.4 |
Prequalified Fillet Weld Requirements
- Minimum effective throat per AISC Table J2.4 (above)
- Maximum concavity: 1/16 in below theoretical throat
- Minimum convexity: per AWS D1.1 Table 6.1
- Interpass temperature minimum: 50–150°F for A992 per Table 3.2
- Preheat: per AWS D1.1 Table 3.2 for thickness > 3/4 in
Common Design Mistakes
Mistake 1: Using thinner part thickness for minimum weld size. AISC Table J2.4 bases the minimum on the thicker part, not the thinner part. A common error is using the thinner gusset plate thickness instead of the thicker column flange thickness. This results in an undersized weld.
Mistake 2: Exceeding maximum weld size at plate edges. A 1/2-in fillet weld on the edge of a 1/2-in plate exceeds the maximum of 7/16 in. This creates a notch at the plate edge that reduces fatigue life and creates a stress riser.
Mistake 3: Confusing leg size with effective throat. Design capacity uses effective throat (0.707 × leg size), not leg size. A 1/4-in fillet weld has only 0.177 in of effective throat — approximately 30% less than novice designers often assume.
Mistake 4: Ignoring base metal shear rupture. The connection capacity may be limited by base metal shear rupture (φ × 0.60 × Fu × t) rather than weld metal strength, especially for thin connected parts.
Frequently Asked Questions
What is the minimum fillet weld size for 1/2-in thick steel? Per AISC 360 Table J2.4, if the thicker part is 1/2 in (between 1/4 in and 1/2 in), the minimum fillet weld is 3/16 in. If the thicker part is greater than 1/2 in (e.g., a 5/8-in plate), the minimum increases to 1/4 in.
What is the maximum fillet weld size along the edge of a plate? For plate thickness ≥ 1/4 in: maximum leg size = material thickness − 1/16 in. For a 1/2-in plate, max = 7/16 in. For plate thickness < 1/4 in: maximum = material thickness.
Why does AISC use the thicker part for minimum weld size? The thicker part acts as a larger heat sink. A small weld on a thick plate may cool too quickly, preventing adequate fusion between the weld metal and base metal. The minimum weld size ensures sufficient heat input for metallurgical bonding.
Do I need matching electrodes for high-strength steel fillet welds? No. AISC 360 Section J2.4 permits undermatched electrodes for fillet welds. E70XX electrodes are routinely used on all steel grades up to 100 ksi yield strength.
What happens if my weld size is between two standard values? Specify the next larger standard weld size. Fillet weld sizes are typically specified in 1/16-in increments (3/16, 1/4, 5/16, 3/8, etc.). Non-standard sizes require special inspection criteria and are generally avoided in practice.
Related Pages
- Fillet Weld Size Chart — Capacity Multi-Code
- Weld Symbol Chart
- Weld Joint Types
- Welded Connections Calculator
- Steel Fy & Fu Table
- Bolt Hole Sizes — Table J3.3
- Steel Connection Design
- Weld Electrodes Reference
Educational reference only. Verify against AISC 360-22 Table J2.4 and governing project specification.