AISC Weld Design Basics — Fillet, Groove, and Plug Welds

AISC 360-22 Chapter J2 covers the design of welded connections for structural steel buildings. Welds are the primary method for transferring forces between connected parts when bolting is impractical or when a continuous load path is needed. The three most common weld types — fillet welds, complete joint penetration (CJP) groove welds, and partial joint penetration (PJP) groove welds — each have distinct strength calculations, inspection requirements, and practical applications. This page covers the fundamentals of each weld type, their design strengths, and worked examples per AISC 360-22.

Quick access:

• Weld Types Overview
• Fillet Weld Design (Section J2.5)
• Complete Joint Penetration (CJP) Groove Welds
• Partial Joint Penetration (PJP) Groove Welds
• Plug and Slot Welds
• Minimum and Maximum Weld Sizes
• Effective Weld Length
• Worked Examples
• Weld Electrode Selection
• Frequently Asked Questions

Weld Types Overview

Weld Strength Hierarchy

In general, weld strength follows this order (strongest to weakest):

1. CJP groove weld — matches or exceeds base metal strength (the weld metal and base metal are fused through the full thickness)
2. PJP groove weld — partial penetration reduces the effective throat
3. Fillet weld — depends on leg size and electrode strength
4. Plug/slot weld — limited by the hole diameter and weld depth

Fillet Weld Design — Section J2.5

Fillet welds are the most common weld type in structural steel connections. They are triangular in cross-section and are placed at the junction of two parts (tee joints, lap joints, corner joints).

Design Strength

The nominal shear strength of a fillet weld per unit length is:

R_n/l = 0.6 x F_EXX x t_e

Where:

• F_EXX = electrode classification strength (ksi) — e.g., 70 ksi for E70XX electrodes
• t_e = effective throat thickness (inches)

LRFD design strength:

phi R_n/l = 0.75 x 0.6 x F_EXX x t_e = 0.45 x F_EXX x t_e

Effective Throat Thickness

The effective throat is the shortest distance from the root of the weld to the face of the weld, measured on a plane perpendicular to the weld face:

For an equal-leg fillet weld with leg size W:

t_e = W / sqrt(2) = 0.707 W

Fillet Weld Strength Table (E70 Electrodes, phi = 0.75)

Quick rule for E70 electrodes: Design strength (kips/inch) = 3.34 x W (where W = weld leg size in inches). This is a convenient approximation: phi R_n/l = 0.75 x 0.6 x 70 x 0.707W = 0.75 x 29.69W = 22.27W, and the kip/inch value is 22.27 x W / 1 = 22.27W... let me correct: for a 1/4" weld, phi R_n/l = 0.45 x 70 x 0.177 = 5.57 kip/in.

Simplified formula: phi R_n/l = 22.27 x W (kips per inch of weld, E70 electrodes)

Where W is the leg size in inches. For a 1/4" weld: 22.27 x 0.25 = 5.57 kip/in ✓

Base Metal Check

The weld strength must not exceed the base metal strength. Per AISC 360 Section J2.5, check:

phi R_n (weld) <= phi R_n (base metal shear rupture)

Base metal shear rupture strength:

phi R_n (base metal) = 0.75 x 0.6 x F_u x t_b x l

Where t_b = base metal thickness and l = weld length.

When base metal governs: For high-strength electrodes (E110XX) welding thin A36 plate, the weld may be stronger than the base metal. In this case, the base metal strength governs the connection.

Complete Joint Penetration (CJP) Groove Welds

CJP groove welds fully penetrate the joint — the weld metal fills the entire joint preparation from root to face. When properly executed, a CJP groove weld is as strong as (or stronger than) the base metal.

Design Strength

CJP groove welds are designed as the base metal. There is no separate weld strength calculation. The governing limit state is:

• Tension perpendicular to weld: phi R_n = 0.75 x F_u x A_g (base metal rupture)
• Compression: phi R_n = 0.90 x F_y x A_g (base metal yielding)
• Shear on weld: phi R_n = 0.75 x 0.6 x F_u x A_wv (base metal shear rupture)

The weld metal (electrode) must match or exceed the base metal strength per AWS D1.1 requirements:

• For A36 steel (F_u = 58 ksi): use E70XX electrodes (F_EXX = 70 ksi)
• For A992 steel (F_u = 65 ksi): use E70XX electrodes (F_EXX = 70 ksi)
• For A572 Gr. 65 (F_u = 80 ksi): use E80XX or higher electrodes

When to Use CJP Groove Welds

• Moment connections: Flange-to-column connections in moment frames
• Full-strength butt splices: Where the splice must develop the full capacity of the member
• Seismic connections: AISC 341 requires CJP groove welds for many seismic details
• Column splices: Where full axial load transfer is needed

Inspection Requirements

CJP groove welds in structural steel require non-destructive testing (NDT) — typically ultrasonic testing (UT) per AWS D1.1. This adds cost but ensures the weld is fully fused through the thickness.

Partial Joint Penetration (PJP) Groove Welds

PJP groove welds do not fully penetrate the joint. The weld fills only a portion of the joint thickness, leaving an unwelded root.

Design Strength

PJP groove weld strength depends on the effective throat (the depth of weld metal from the face to the root):

R_n/l = 0.6 x F_EXX x t_e     (for shear on the effective throat)

The effective throat for PJP groove welds is the depth of the joint preparation (bevel depth, J-groove depth, etc.):

PJP vs. CJP Selection

Plug and Slot Welds

Plug and slot welds fill circular or elongated holes in one plate to connect it to the underlying plate.

Design Strength

Shear on the contact surface:

phi R_n = 0.75 x 0.6 x F_EXX x A_e

Where A_e = effective area = 0.707 x W x (circumference of weld), simplified to:

phi R_n = 0.75 x 0.6 x F_EXX x (0.707 x D_min x pi/4 x D_min)   (for plug welds)

More practically, for a circular plug weld of diameter d and depth equal to the plate thickness t:

A_e = pi x d^2 / 4    (effective area for shear)

Restrictions on Plug and Slot Welds

• Minimum hole diameter: the greater of the thickness of the part containing the hole plus 5/16" or 3/4" (AISC Section J2.2c)
• Maximum hole diameter: the lesser of 2.25 times the thickness of the part or 1"
• Minimum depth of filling: at least 1/2 the thickness of the part containing the hole
• Slot weld length: at least 2 times the width

Minimum and Maximum Weld Sizes

Minimum Fillet Weld Size (AISC Table J2.4)

The minimum fillet weld size depends on the thickness of the thicker connected part:

Exception: Where the minimum weld size exceeds the thickness of the thinner part, the minimum is limited to the thickness of the thinner part. A 3/16" minimum weld is the practical lower bound for most field welding.

Maximum Fillet Weld Size (Section J2.2b)

Along the edge of a connected part:

Example: For a 1/2" plate edge, max weld = 1/2 - 1/16 = 7/16".

This limit prevents edge notching and undercut from the weld arc.

Effective Weld Length

The effective length of a fillet weld is the actual weld length minus any craters, starts, or unwelded segments:

• Effective length of a fillet weld: The overall length of the weld (including returns around corners)
• Minimum effective length: 4 times the nominal weld size (per AISC Section J2.2b). A 1/4" fillet weld must be at least 1" long.
• End returns: Fillet welds that are loaded in the longitudinal direction should have returns around the corner of at least 2W, but the return length is not counted in the effective length for strength calculations.

Intermittent (Stitch) Welds

Intermittent fillet welds are a series of short weld segments along a joint. They are used for:

• Cover plates
• Stiffener-to-web connections
• Connections where continuous welds are not required

Minimum segment length: 4 times the weld size (1.5" minimum practical) Maximum gap between segments: 16 times the thickness of the thinner part (to prevent buckling)

Worked Examples

Example 1 — Fillet Weld Capacity

Problem: Determine the capacity of a 1/4" fillet weld (E70 electrodes) on both sides of a lap joint, 6 inches long.

Given:

• Weld leg size W = 1/4"
• Weld length per side = 6"
• Total weld length = 12" (both sides)
• E70 electrodes: F_EXX = 70 ksi

Effective throat:

t_e = 0.707 x 0.25 = 0.177"

Design strength per inch:

phi R_n/l = 0.75 x 0.6 x 70 x 0.177 = 5.57 kip/in

Total design strength:

phi R_n = 5.57 x 12 = 66.9 kips

Base metal check (assuming 3/8" plate, A36):

phi R_n (base) = 0.75 x 0.6 x 58 x 0.375 x 12 = 117.5 kips

Weld governs (66.9 kips < 117.5 kips) ✓

Example 2 — Required Weld Size for a Given Load

Problem: A 150 kip factored load must be transferred through fillet welds (E70) on a 1/2" plate. The available weld length is 10 inches on each side (20 inches total).

Required strength per inch:

R_n/l = 150 / 20 = 7.50 kip/in

Required effective throat:

t_e = R_n/l / (0.75 x 0.6 x 70) = 7.50 / 31.5 = 0.238"

Required leg size:

W = t_e / 0.707 = 0.238 / 0.707 = 0.337" → use 3/8" weld

Check minimum weld size: 1/2" plate → minimum 3/16" (Table J2.4). 3/8" > 3/16" ✓

Check maximum weld size: 1/2" plate edge → max = 1/2 - 1/16 = 7/16". 3/8" < 7/16" ✓

Check base metal: 0.75 x 0.6 x 58 x 0.50 x 20 = 261 kips > 150 kips ✓

Result: Use 3/8" fillet welds, 10" long on each side.

Example 3 — CJP Groove Weld Splice

Problem: Splice a W14x43 (A992, F_u = 65 ksi) with CJP groove welds at the flanges and web.

Design strength (tension, governed by base metal rupture):

Flanges:

A_f = 2 x 7.995 x 0.530 = 8.47 in^2
phi P_n = 0.75 x 65 x 8.47 = 413 kips

Web:

A_w = (13.66 - 2 x 0.530) x 0.305 = 3.82 in^2
phi P_n = 0.75 x 65 x 3.82 = 186 kips

Total splice capacity: 413 + 186 = 599 kips

This matches the gross section tension capacity of the W14x43 (the CJP weld develops the full section).

Weld Electrode Selection

Most common: E70XX (E7018 for SMAW, E71T-1 for FCAW, ER70S-6 for GMAW). This electrode covers the vast majority of structural steel connections.

AWS D1.1 requires the electrode to match or exceed the base metal minimum tensile strength. For A992 (F_u = 65 ksi), E70XX (F_EXX = 70 ksi) satisfies this requirement.

Calculator

Check fillet and groove weld capacity automatically:

• Welded Connections Calculator — Fillet and groove weld capacity per AISC 360-22
• Weld Group Properties — Elastic and plastic neutral axis for weld groups
• Weld Symbol Chart — AWS A2.4 weld symbols for drawings
• Weld Inspection Reference — Visual and NDT acceptance criteria

FAQ

Q: What is the strength of a 1/4" fillet weld per inch? A: With E70 electrodes (the most common), the LRFD design strength is 5.57 kips per inch of weld. The nominal (unfactored) strength is 7.43 kips per inch.

Q: What is the difference between CJP and PJP groove welds? A: CJP (Complete Joint Penetration) welds fill the entire joint thickness — the weld is as strong as the base metal. PJP (Partial Joint Penetration) welds fill only part of the joint — the strength is based on the reduced effective throat. CJP welds require NDT inspection; PJP welds typically only need visual inspection.

Q: How do I determine the minimum fillet weld size? A: Use AISC 360 Table J2.4. The minimum depends on the thickness of the thicker connected part: 1/8" for material up to 1/4", 3/16" for up to 1/2", 1/4" for up to 3/4", and 5/16" for anything over 3/4". If the minimum exceeds the thinner part thickness, use the thinner part thickness instead.

Q: Can I use a fillet weld larger than the plate thickness? A: No. The maximum fillet weld leg along a plate edge is the material thickness minus 1/16" (for material 1/4" and thicker). For material less than 1/4", the maximum equals the material thickness. This prevents edge notching and undercut.

Q: What electrode should I use for A992 steel? A: E70XX electrodes (such as E7018 for SMAW, E71T-1 for FCAW, or ER70S-6 for GMAW). The electrode tensile strength (70 ksi) exceeds the base metal tensile strength (65 ksi), satisfying AWS D1.1 requirements.

Q: Do CJP groove welds need inspection? A: Yes. CJP groove welds in structural steel must be tested by non-destructive methods (typically ultrasonic testing, UT) per AWS D1.1. PJP groove welds and fillet welds are typically inspected visually, with additional NDT only when specified by the engineer or the code.

Q: What is the effective throat of a 1/4" fillet weld? A: 0.707 x 0.25 = 0.177 inches (approximately 3/16"). The effective throat is the shortest distance from the root to the face of the weld.

Related: AISC Minimum Weld Size — Table J2.4 | Fillet Weld Size Chart | Weld Symbols | Weld Electrodes | Weld Inspection | AISC 360-22 Steel Design Overview | AISC Block Shear