Weld Joint Types — Fillet, Groove, Plug and Slot Welds Reference
Structural steel connections use four primary weld types: fillet welds, complete joint penetration (CJP) groove welds, partial joint penetration (PJP) groove welds, and plug/slot welds. Each has distinct strengths, fabrication requirements, and applications per AISC 360-22 Chapter J2 and AWS D1.1.
Fillet welds — the workhorse of steel construction
Fillet welds account for approximately 80% of all structural welds. They require no joint preparation and can be made in all positions. Design strength: phiRn = 0.75 _ 0.60 _ FEXX _ a _ L, where a = 0.707*w (throat), w = weld leg, FEXX = 70 ksi for E70XX.
Fillet weld capacity per inch of length
| Weld Size (in) | Throat a (in) | phiRn/in (kips/in) |
|---|---|---|
| 3/16 | 0.133 | 4.18 |
| 1/4 | 0.177 | 5.57 |
| 5/16 | 0.221 | 6.96 |
| 3/8 | 0.265 | 8.35 |
| 1/2 | 0.354 | 11.14 |
| 5/8 | 0.442 | 13.92 |
For loads at angle theta to the weld axis, directional strength increase: (1 + 0.50*sin^1.5(theta)).
Minimum fillet weld size (AISC Table J2.4)
| Thinner Part | Min Size |
|---|---|
| t <= 1/4" | 1/8" |
| 1/4" < t <= 1/2" | 3/16" |
| 1/2" < t <= 3/4" | 1/4" |
| t > 3/4" | 5/16" |
Maximum along edges: material thickness minus 1/16" for plates 1/4" or thicker.
Complete joint penetration (CJP) groove welds
CJP welds achieve full penetration through the entire thickness, developing the full strength of the thinner member. No separate weld capacity calculation needed. Requires joint preparation (V-groove, bevel, J-groove, or U-groove), backing bars for single-sided access, and UT or RT inspection.
Use for: Moment connections, column splices, truss chord splices, demand-critical seismic welds, any joint requiring full member strength. Cost: 3-5x more per linear inch than fillet welds.
Partial joint penetration (PJP) groove welds
PJP welds penetrate partway through. Effective throat depends on groove angle and process per AISC Table J2.1. For groove angle >= 60 degrees: effective throat = groove depth. For 45-59 degrees with SMAW/FCAW: effective throat = groove depth minus 1/8". Design strength: phiRn = 0.75 _ 0.60 _ FEXX _ te _ L.
Use for: Column splices not requiring full CJP, built-up sections (flange-to-web), connections where fillets cannot reach required throat.
Plug and slot welds
Plug welds fill circular holes; slot welds fill elongated holes in one ply of a lap joint. Used for cover plates, stitch welding, and composite shear transfer. Design strength based on faying surface area.
Weld selection guide
| Criteria | Fillet | CJP | PJP |
|---|---|---|---|
| Cost | Low | High | Medium |
| Joint prep | None | Bevel + fit-up | Bevel |
| Inspection | Visual + MT | UT/RT required | UT/RT sometimes |
| Capacity | Limited by leg size | Full member | Intermediate |
| Best for | Shear connections | Moment frames | Column splices |
Multi-code comparison
AS 4100-2020: SP category requires UT/RT for butt welds. Fillet: phivwtt, vw = 0.6fuwkr. phi = 0.80 (SP), 0.60 (GP).
EN 1993-1-8: Fillet Fw,Rd = fvw.daL, fvw.d = fu/(sqrt(3)beta_wgamma_M2). beta_w: 0.80 (S235), 0.85 (S275), 0.90 (S355). gamma_M2 = 1.25.
Common mistakes
- Specifying CJP when fillets are adequate. CJP costs 3-5x more and is unnecessary for shear connections.
- Using leg size as throat dimension. Throat = 0.707*leg. Using leg overestimates capacity by 41%.
- Ignoring base metal capacity. The base metal adjacent to the weld must also be checked.
- Not accounting for weld access. Tight corners and overhead positions affect weld quality.
- Mixing electrode grades. E70XX is standard. E60XX reduces capacity by 14%.
Frequently asked questions
What is the strongest weld type? CJP groove weld -- develops the full member thickness and is stronger than the base metal.
Fillet vs. groove weld? Use fillets whenever they provide adequate strength (cheaper, no prep). Use CJP only when full member strength is required.
What is the minimum fillet weld size? Per AISC Table J2.4: 1/8" for material up to 1/4", 3/16" for 1/4"-1/2", 1/4" for 1/2"-3/4", 5/16" over 3/4".
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Related references
Disclaimer
This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against AISC 360-22 Chapter J2, AWS D1.1, and the governing project specification. The site operator disclaims liability for any loss arising from the use of this information.