Weld Joint Types — Butt, Fillet, Groove, Plug and Slot Welds Reference

Structural steel connections rely on five basic joint types -- butt, tee, corner, lap, and edge -- defined by AWS D1.1 and AISC 360-22 Chapter J2. Each joint type can be joined with fillet welds, groove welds (complete or partial joint penetration), or plug and slot welds. This reference covers every joint classification, design strengths, preparation requirements, preheat tables, and a worked example comparing CJP versus fillet weld capacity.

Quick access:

• The five basic joint types
• AWS D1.1 joint classification table
• Fillet welds
• Complete joint penetration groove welds
• Partial joint penetration groove welds
• Plug and slot welds
• Preheat and interpass temperature
• Worked example: CJP vs fillet weld
• Common mistakes
• Frequently asked questions
• Run this calculation

The five basic joint types per AWS D1.1

AWS D1.1 Section 2 defines joint types based on the geometric relationship between the members being connected, not the weld type. Understanding joint geometry is the first step in selecting the correct weld and preparation.

Butt joint (B)

Two members aligned approximately in the same plane, edge to edge. The most common joint in structural splices.

• Description: Members meet end-to-end or edge-to-edge in the same plane.
• Typical applications: Column splices, girder flange splices, groove-welded beam connections, plate girder web splices.
• Preparation requirements: Square, single-V, double-V, single-bevel, double-bevel, single-U, double-U, or single-J, double-J groove preparation depending on thickness and access. Backing bars required for single-sided CJP welds.
• Common weld types: CJP groove welds (most common), PJP groove welds, square groove welds for thin material.

Tee joint (T)

Two members where the edge of one meets the surface of the other at approximately 90 degrees, forming a T shape.

• Description: One member is perpendicular to the surface of another.
• Typical applications: Beam-to-column connections, stiffener-to-flange connections, base plate to column connections, built-up cross sections (web-to-flange).
• Preparation requirements: Generally no edge preparation for fillet welds. For CJP, bevel or J-groove preparation on the through-member, often with a backing bar on the opposite side.
• Common weld types: Fillet welds (most common), PJP groove welds, CJP groove welds for demand-critical connections.

Corner joint (C)

Two members located approximately at right angles to each other, forming an L shape at the corner of a component.

• Description: Members meet at an angle (typically 90 degrees) at a corner.
• Typical applications: Box column fabrication, hollow structural section (HSS) connections, end-plate fabrication, machinery frames, lintel angles.
• Preparation requirements: Similar to tee joints. Fillet welds require no edge prep. Groove welds require bevel preparation on one or both members.
• Common weld types: Fillet welds (outside corner), groove welds (closed corner), flare-bevel groove welds for round-to-flat connections.

Lap joint (Lap)

Two overlapping members in parallel planes. One of the simplest joint geometries.

• Description: One member overlaps the surface of another.
• Typical applications: Gusset plate-to-member connections, angle clip connections, cover plate attachments, bracket connections, composite deck shear studs.
• Preparation requirements: No edge preparation needed. Fillet welds are deposited along the overlapping edges. Plug or slot welds may be used to prevent separation of overlapping plies.
• Common weld types: Fillet welds (dominant), plug welds, slot welds, PJP groove welds (rare).

Edge joint (E)

Two parallel or nearly parallel members joined at their edges. The least common joint type in structural steel construction.

• Description: Edges of two parallel members are joined.
• Typical applications: Flange-to-web connections in cold-formed steel, sheet metal ductwork, tanks and vessels (non-structural), standing seam roof decking.
• Preparation requirements: Square or slight bevel groove preparation. Often welded from one side only.
• Common weld types: Square groove welds, flare-groove welds, fillet welds (if offset), spot welds (sheet metal).

AWS D1.1 joint classification table

AWS D1.1 assigns alphanumeric designations to prequalified joint geometries. The letter indicates the joint type and the number indicates the specific groove configuration.

The "P" in the designation stands for prequalified -- these joint geometries do not require procedure qualification testing provided all requirements of AWS D1.1 Section 3 are met, including welding process, electrode classification, and position.

Fillet welds -- the workhorse of steel construction

Fillet welds account for approximately 80% of all structural welds. They require no joint preparation, are economical, and can be deposited in all positions. Design strength per AISC 360-22 Eq. J2-4:

phiRn = 0.75 x 0.60 x FEXX x a x L

Where phi = 0.75 (AISC resistance factor), 0.60 = shear coefficient, FEXX = electrode classification tensile strength (ksi), a = effective throat (in), and L = weld length (in). For equal-leg fillets, the effective throat a = 0.707 x w, where w is the leg size.

Fillet weld capacity per inch of length (E70XX, FEXX = 70 ksi)

Directional strength increase

For loads applied at angle theta to the weld longitudinal axis, AISC 360-22 permits a strength increase per Eq. J2-5:

Rn = 0.60 x FEXX x (1.0 + 0.50 x sin^1.5(theta)) x a x L

At theta = 0 degrees (longitudinal): factor = 1.0. At theta = 90 degrees (transverse): factor = 1.50. This means transversely loaded fillet welds are 50% stronger than longitudinally loaded welds of the same size. However, the weld must also be checked for base metal shear on the fusion face.

Minimum fillet weld size (AISC Table J2.4)

These minimum sizes ensure sufficient heat input to prevent lack of fusion and brittle microstructures.

Maximum fillet weld size

Along edges of material less than 1/4 in (6 mm) thick, the maximum fillet weld size equals the material thickness. For material 1/4 in (6 mm) and thicker, the maximum along an edge is the material thickness minus 1/16 in (2 mm). This ensures the weld does not melt away the parent edge.

Effective length and end returns

Minimum effective length of a fillet weld is four times the nominal weld size. If the length is less than 4w, the weld size is taken as L/4 for capacity calculations.

End returns (also called wrap-around welds) are fillet welds that continue around the corner of a connection for a minimum of 2w. AISC J2.2b requires end returns on certain connections to reduce stress concentrations and prevent notch effects at weld terminations. End returns are required for:

• Welds subject to cyclic (fatigue) loading
• Connections with calculated weld capacities
• Lap joints where the weld terminates at the end of the connected part

Complete joint penetration (CJP) groove welds

CJP groove welds achieve full penetration through the entire member thickness, developing the full strength of the base metal. Per AISC 360-22 J2.6, the design strength of a CJP groove weld is governed by the base metal, not the weld metal -- no separate weld capacity calculation is needed when matching or overmatching filler metal is used.

Groove weld preparation types

CJP requirements

• Backing bars: Required for single-sided groove welds. Steel backing (minimum 1/4 in / 6 mm thick) must be continuous. Remove backing and backgouge to sound metal for cyclic loading applications (AISC 360-22 Appendix 3).
• Weld tabs: Used at weld terminations to contain starting and stopping defects. Remove and grind smooth after welding for connections subject to fatigue.
• Inspection: UT (ultrasonic testing) or RT (radiographic testing) required per AWS D1.1 Table 6.2. Visual inspection (VT) is always required. MT (magnetic particle testing) may supplement UT for surface and near-surface defects.
• Demand-critical welds: Seismic force-resisting systems require CJP welds meeting AWS D1.1 seismic supplement requirements, including notch-tough electrodes (minimum Charpy V-Notch of 20 ft-lb at -20 F / -29 C).

Use CJP welds for: Moment connections, column splices, truss chord splices, demand-critical seismic connections, and any joint requiring the full member strength.

Cost: 3-5x more per linear inch than fillet welds due to joint preparation, fit-up, backing, and inspection.

Partial joint penetration (PJP) groove welds

PJP groove welds penetrate only partway through the connected member. The effective throat depends on the groove geometry, weld process, and groove angle per AISC 360-22 Table J2.1.

Effective throat determination

Design strength of a PJP groove weld in tension or shear:

phiRn = 0.75 x 0.60 x FEXX x te x L

Where te is the effective throat and L is the weld length. Note that for PJP welds in compression normal to the weld axis, phiRn = 0.90 x Fy x te x L (base metal yielding governs).

Minimum effective throat (AISC Table J2.3)

Use PJP welds for: Column splices not requiring full CJP, built-up section flange-to-web connections, connections where fillet welds cannot achieve the required throat, heavy structural members with limited access.

Plug and slot welds

Plug welds fill circular holes and slot welds fill elongated holes in one ply of a lap joint. They transfer shear between overlapping plies through the fused area on the faying surface.

Dimensional requirements

Where w = weld size (leg dimension) and t = thickness of the ply containing the hole.

Design strength

Plug and slot weld capacity is based on the nominal shear area on the faying surface:

phiRn = 0.75 x 0.60 x FEXX x Af

Where Af = fusing area (faying surface area) = pi x d^2 / 4 for plug welds, and Af = width x length for slot welds.

Use plug and slot welds for: Cover plate attachments, stitch welding of overlapping plies, composite beam shear transfer, retrofit connections, resisting uplift in bracket connections.

Preheat and interpass temperature requirements

Preheat slows the cooling rate, reducing hydrogen-induced cracking and improving weld metal microstructure. AWS D1.1 Table 3.3 specifies minimum preheat and interpass temperatures based on steel grade, thickness, and welding process.

Preheat requirements by steel grade and thickness

Interpass temperature must be maintained within the range specified in the Welding Procedure Specification (WPS). Maximum interpass temperature limits prevent excessive heat input that degrades toughness. Typical maximum: 350 F to 600 F (177 C to 316 C) depending on the electrode and steel grade. For quenched-and-tempered steels (A514, A517), maximum interpass is typically 400 F (204 C).

Worked example -- CJP vs fillet weld capacity for a beam-to-column connection

Problem

A W18x50 beam (Fy = 50 ksi / 345 MPa) frames into a W12x96 column. The beam flange reaction is 120 kips (534 kN) in tension from a moment connection. Compare the required weld for a CJP groove weld versus fillet welds on the beam flange.

Given

• Beam flange width: bf = 7.495 in (190 mm)
• Beam flange thickness: tf = 0.570 in (14.5 mm)
• Required strength: Ru = 120 kips (534 kN) in tension
• Electrode: E70XX (FEXX = 70 ksi / 483 MPa)

Solution A: CJP groove weld

For a CJP groove weld with matching filler metal (E70XX for A992 Gr 50), the weld is stronger than the base metal. No weld capacity calculation is required per AISC J2.6.

• phiRn = phi x Fy x bf x tf = 0.90 x 50 ksi x 7.495 in x 0.570 in = 192.3 kips
• 192.3 kips > 120 kips -- CJP is adequate

Required preparation: single-bevel groove (B-P4 or T-P2) with backing bar. Requires UT inspection.

Solution B: Fillet welds on both sides of flange

For fillet welds along both edges of the beam flange, loaded transversely:

• Available length per side: L = bf = 7.495 in (ignoring end returns)
• Total weld length: 2 x 7.495 = 14.99 in
• Required phiRn = 120 kips

For transversely loaded fillet welds, phiRn = 0.75 x 0.60 x FEXX x (1.0 + 0.50 x sin^1.5(90 deg)) x a x L

With theta = 90 deg (transverse), the factor = 1.50:

phiRn = 0.75 x 0.60 x 70 x 1.50 x a x 14.99 = 707.3 x a

Required throat: a = 120 / 707.3 = 0.170 in

Required leg size: w = 0.170 / 0.707 = 0.240 in, use 1/4 in fillet weld.

Check minimum size per AISC Table J2.4: tf = 0.570 in falls in the 1/2 to 3/4 in range, minimum = 1/4 in. The 1/4 in fillet satisfies the minimum requirement.

phiRn = 0.75 x 0.60 x 70 x 1.50 x 0.707 x 0.25 x 14.99 = 125.2 kips > 120 kips -- Fillet welds are adequate

Comparison

Conclusion: For this connection, fillet welds provide adequate capacity at significantly lower cost. CJP is only required if the connection is demand-critical (seismic) or if full-flange strength development is specified by the engineer of record.

Common mistakes

1. Specifying CJP when fillets are adequate. CJP costs 3-5x more and requires UT/RT inspection. For shear connections and many moment connections, properly sized fillet welds provide sufficient capacity.

2. Using leg size as the throat dimension. The effective throat of a fillet weld is 0.707 x leg size for equal-leg welds. Using leg size directly overestimates capacity by approximately 41%. For unequal-leg fillets, the throat is the shortest distance from the root to the face.

3. Ignoring base metal shear on the fusion face. The weld metal may be adequate, but the base metal adjacent to the weld must also be checked per AISC J2. The base metal check uses phiRn = 0.75 x 0.60 x Fu x t x L, where Fu is the base metal tensile strength.

4. Neglecting preheat requirements. Welding on thick material without adequate preheat causes hydrogen-induced cold cracking. The preheat must be maintained for a distance of at least 3 in (75 mm) in all directions from the weld joint.

5. Undersized welds relative to the connected part. AISC Table J2.4 minimum sizes exist to ensure adequate heat input. A weld that is too small on thick material will cool too rapidly, producing brittle martensitic zones.

6. Not accounting for weld access and position. Overhead and vertical welds have reduced effective throat due to gravity working against the molten pool. The WPS must specify the welding position and corresponding parameters.

7. Mixing electrode grades without recalculation. E70XX is standard for A572 Gr 50 and A992. Using E60XX reduces weld metal capacity by approximately 14%. Using E80XX on A572 Gr 50 provides no benefit for fillet welds because the base metal still limits the connection.

8. Ignoring weld length reductions for start-and-stop defects. For very short welds (less than about 2 in / 50 mm), the defective start and stop regions represent a significant fraction of the total length. AISC J2.2b limits the effective length to the actual length minus 2 x weld size at each end for intermittent welds.

Multi-code comparison

AS 4100-2020 (Australia)

• SP category (structural purpose): Requires UT/RT inspection for butt welds. Fillet weld design: phi x vw x tt, where vw = 0.6 x fuw x kr. phi = 0.80 for SP welds, 0.60 for GP (general purpose) welds. fuw = weld metal ultimate strength. kr = 1.0 for weld lengths up to 70 times the throat.
• Weld categories: SP requires testing and qualified procedures. GP is for non-critical welds with reduced capacity factors.

EN 1993-1-8 (Eurocode 3)

• Fillet weld design: Fw,Rd = fvw.d x a x L, where fvw.d = fu / (sqrt(3) x beta_w x gamma_M2).
• Beta_w correlation factors: 0.80 (S235), 0.85 (S275), 0.90 (S355), 1.00 (S420 and S460).
• gamma_M2 = 1.25 (partial safety factor for welds).
• Groove welds: Full-penetration butt welds with matching filler metal are governed by base metal strength, similar to AISC.

CSA S16-19 (Canada)

• Fillet welds: Vr = 0.67 x phi_w x X_u x 0.707 x w x L, where phi_w = 0.67, X_u = electrode tensile strength (MPa).
• Groove welds: CJP with matching filler metal develops full base metal strength. PJP uses effective throat similar to AISC.

Frequently asked questions

What is the strongest weld type?

A CJP groove weld with matching filler metal develops the full strength of the connected members. The weld itself is stronger than the base metal. However, "strength" depends on the failure mode -- a properly designed fillet weld in transverse loading can also develop significant capacity at far lower cost.

When should I use fillet welds versus groove welds?

Use fillet welds whenever they provide adequate strength. They require no joint preparation, are faster to deposit, and require less stringent inspection. Use CJP groove welds only when full member strength must be developed (demand-critical connections, column splices, moment frames in high-seismic regions). Use PJP groove welds when intermediate strength is needed and fillets cannot achieve the required throat.

What is the minimum fillet weld size?

Per AISC 360-22 Table J2.4: 1/8 in (3 mm) for material up to 1/4 in (6 mm) thick, 3/16 in (5 mm) for 1/4 to 1/2 in (6 to 13 mm), 1/4 in (6 mm) for 1/2 to 3/4 in (13 to 19 mm), and 5/16 in (8 mm) for material over 3/4 in (19 mm). These minimums ensure adequate heat input.

What preheat temperature do I need?

Preheat depends on the steel grade, thickness, and welding process per AWS D1.1 Table 3.3. For A36 and A572 Gr 50 up to 1-1/2 in (38 mm) thick, a minimum of 50 F (10 C) ambient is sufficient. For thicker material, 150 F (65 C) to 225 F (107 C) may be required. Quenched-and-tempered steels have specific preheat ranges that must not be exceeded.

How do I calculate fillet weld capacity?

The basic formula is phiRn = 0.75 x 0.60 x FEXX x 0.707w x L for longitudinally loaded welds. For loads at angle theta, multiply by (1.0 + 0.50 x sin^1.5(theta)). Always check both the weld metal and the base metal shear on the fusion face. Use the Welded Connections Calculator to run the calculation instantly.

What is the difference between CJP and PJP groove welds?

CJP (complete joint penetration) welds fuse through the entire member thickness, developing full base metal strength. PJP (partial joint penetration) welds penetrate only part of the thickness, and capacity is limited by the effective throat. CJP requires backing bars for single-sided welds and UT/RT inspection. PJP may or may not require NDE depending on the application.

Can plug welds replace fillet welds?

Not generally. Plug welds have lower design strength per unit area than fillet welds and are limited by the hole geometry. They are best used to supplement fillet welds, prevent separation of overlapping plies, or resist uplift forces. AISC limits plug welds to specific applications and they should not be the primary load path in structural connections.

Weld selection guide

Run this calculation

Use the free Steel Calculator tools to compute weld capacity for your specific connection:

• Welded Connections Calculator -- fillet weld capacity, CJP and PJP design, multi-directional loading, base metal checks
• Gusset Plate Calculator -- gusset plate weld design, Whitmore section, block shear
• Base Plate Calculator -- base plate fillet welds to column, anchor bolt design

Related references

• Fillet Weld Size Chart -- complete table of fillet weld capacities for E70XX and E80XX electrodes
• Weld Electrodes -- electrode classification, storage, and selection guide (E60XX through E110XX)
• Weld Symbol Chart -- AWS weld symbol notation and interpretation
• Bolt Grade Reference -- ASTM bolt grades for bolted-welded combination connections
• Steel Grades and Properties -- A36, A572, A992, A514 mechanical properties
• Connection Design Guide -- overview of structural steel connection types and selection
• How to Verify Calculations -- step-by-step verification of structural calculations
• Base Plate Design Guide -- base plate sizing and weld design for columns
• Beam Connection Design -- simple and moment beam-to-column connections

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 D1.1-2020, and the governing project specification. The Steel Calculator disclaims liability for any loss, damage, or injury arising from the use of this information. Always engage a licensed structural engineer for connection design on actual projects.

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