AISC 360-22 Weld Capacity — Fillet & Butt Weld Design Guide

Complete reference for welded connection design per AISC 360-22 Chapter J2. Covers fillet weld capacity per inch of length, the directional strength increase (load angle effect), matching electrodes for common US steel grades, base metal rupture and yield checks, CJP (complete joint penetration) and PJP (partial joint penetration) butt welds, weld symbol interpretation per AWS A2.4, minimum and maximum fillet weld sizes, and two fully worked examples including a gusset plate connection.

PRELIMINARY — NOT FOR CONSTRUCTION. All results are for educational and reference use only. Must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in any project.

Related pages: US Bolt Group Design | US Column K-Factor Guide | Welded Connection Calculator

Fillet Weld Design Basis — AISC 360-22 Section J2.4

The design strength of a fillet weld is based on the effective throat dimension and the weld metal classification strength. The effective throat of a fillet weld is the shortest distance from the root of the weld to the face of the weld diagram — for a standard 45-degree fillet weld with equal legs, the throat = leg size x sin(45 degrees) = 0.707 x leg size.

Weld metal strength (nominal):

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

Where:

• FEXX = electrode classification strength (e.g., 70 ksi for E70XX)
• theta = angle between the weld axis and the line of applied force (degrees)
• Aw = effective throat area = 0.707 x leg_size x length

For longitudinal loading (theta = 0 degrees): Rn = 0.60 x FEXX x Aw For transverse loading (theta = 90 degrees): Rn = 0.60 x FEXX x 1.50 x Aw = 0.90 x FEXX x Aw

The 1.50 multiplier for transverse welds — known as the directional strength increase — reflects the 50% higher strength of fillet welds loaded perpendicular to their axis. This is a significant economy driver in weld design. The sin^1.5(theta) function provides a smooth transition between longitudinal and transverse orientations, following experimental data by Lesik and Kennedy (1990).

LRFD: phi = 0.75, phi_Rn = 0.75 x Rn ASD: Omega = 2.00, Rn/Omega = Rn / 2.00

Electrode Selection — E70XX Standard

E70XX electrodes (FEXX = 70 ksi / 483 MPa) are the standard for AISC structural steel welding, suitable for ASTM A36 (Fy = 36 ksi, Fu = 58 ksi), A572 Gr. 50 (Fy = 50 ksi, Fu = 65 ksi), and A992 (Fy = 50 ksi, Fu = 65 ksi).

Per AWS A5.1 / A5.20, "XX" denotes the coating type and welding position (e.g., E7018 = low-hydrogen, all-position; E7024 = iron powder, flat/horizontal only). E70XX electrodes provide a weld deposit tensile strength of 70 ksi minimum.

E80XX (80 ksi) and E90XX (90 ksi) electrodes are available for higher-strength steels such as A913 Gr. 65/70, but are less common in building structures. Table J2.5 of AISC 360-22 provides matching electrode recommendations:

Base Metal Checks — Section J2.4(b) and J4

The weld is only as strong as the base metal it connects. AISC 360-22 requires the following limit states:

Base metal shear yield (Section J4.2): Rn = 0.60 x Fy x Ag (gross area in shear) phi = 1.00 (LRFD), Omega = 1.50 (ASD)

Base metal shear rupture (Section J4.2): Rn = 0.60 x Fu x Anv (net area in shear) phi = 0.75 (LRFD), Omega = 2.00 (ASD)

For a fillet weld on plate edge loaded parallel to the weld, the base metal shear rupture check governs the weld size independent of the electrode strength. The design rule: the base metal capacity must equal or exceed the weld metal capacity. This often controls the minimum plate thickness at the connection.

Base metal tension rupture (Section J4.1): Rn = Fu x Ae (effective net area) phi = 0.75 (LRFD), Omega = 2.00 (ASD)

Base metal tension yield (Section J4.1): Rn = Fy x Ag (gross area) phi = 0.90 (LRFD), Omega = 1.67 (ASD)

Minimum and Maximum Fillet Weld Sizes

Per AISC 360-22 Table J2.4, the minimum fillet weld size depends on the thickness of the thicker connected part:

The maximum fillet weld size along an edge is:

• Along edges of connected parts < 1/4 in. thick: weld leg size = plate thickness
• Along edges of connected parts >= 1/4 in. thick: weld leg size = plate thickness - 1/16 in.

These limits prevent edge melting and undercut. For welds on the toe of a sloping flange (channel or angle), the maximum size is 3/4 of the flange thickness at the toe.

Complete Joint Penetration (CJP) Butt Welds

CJP groove welds develop the full strength of the connected parts. Per Section J2.1, no capacity calculation is required — the strength equals that of the base metal. CJP welds require weld access holes, backing bars, back-gouging, and NDT (ultrasonic testing) for tension applications under cyclic loading.

PJP groove welds (Section J2.3) are designed as deep fillet welds. The effective throat equals the depth of the prepared groove. Capacity per unit length follows:

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

PJP welds are common for column splices where full-strength welds are not needed. The required effective throat is determined by the required strength divided by the weld capacity per unit length. PJP welds in tension applications must consider the root notch effect and may require back-gouging.

Fillet Weld Capacity Per Inch Tables (E70XX, LRFD)

For quick preliminary sizing, the following table gives the LRFD design strength per inch of weld for common fillet weld sizes:

These values assume FEXX = 70 ksi. For E80XX electrodes, multiply by 80/70 = 1.143. For E90XX, multiply by 90/70 = 1.286.

Weld Symbols per AWS A2.4

Understanding weld symbols is essential for communicating weld requirements on structural drawings. The basic fillet weld symbol consists of:

• Reference line (horizontal)
• Arrow pointing to the weld location
• Weld symbol (triangle for fillet weld)
• Size to the left of the symbol
• Length to the right of the symbol
• Pitch (center-to-center spacing) for intermittent welds after the length

A fillet weld symbol with "1/4" on the left and "6" on the right means a 1/4 in. fillet weld, 6 in. long. If the weld symbol is below the reference line, the weld is on the "arrow side." If above the reference line, the weld is on the "other side."

For field welds vs. shop welds: a filled flag at the junction of the arrow and reference line indicates a field weld. No flag means a shop weld. All-around welds are indicated by a circle at the junction.

Weld Detail Considerations for Fatigue

While Chapter J2 governs static strength, AISC 360-22 Appendix 3 provides fatigue provisions for welded connections subject to repeated loading. Key considerations:

• Weld profile: Convex fillet welds have higher stress concentration at the toe than flat or concave profiles. Specify "flat profile" or "slightly convex" for fatigue-critical connections.
• Weld toes: Grinding the weld toe smooth reduces notch effects and increases fatigue life by 2-3x.
• Weld access holes: Required for CJP welds in tension flanges of beams and girders per AISC 360-22 J1.6. The hole detail must not create sharp re-entrant corners.
• Backing bars: For CJP welds requiring full strength, backing bars must be removed and the root back-gouged and re-welded for tension applications, unless the backing bar detail is designed as a permanent part of the connection.

Fatigue design of welded connections is a specialized topic beyond the scope of this reference. For fatigue-critical applications, consult AISC Design Guide 21 and AISC 360-22 Appendix 3.

Worked Example 1 — Web-to-Flange Weld

A W21x50 beam web (tw = 0.38 in.) is fillet-welded to a column flange. Design shear Vu = 60 kips. Specify the fillet weld size using E70XX electrodes (LRFD).

Throat area per inch: Aw_per_inch = 0.707 x leg_size x 1.0 in.

Nominal strength per inch (longitudinal): Rn_per_inch = 0.60 x 70 x 0.707 x leg_size = 29.7 x leg_size kips/in

Try 1/4 in. fillet: Rn_per_inch = 29.7 x 0.25 = 7.43 kips/in phi_Rn_per_inch = 0.75 x 7.43 = 5.57 kips/in

Required length: Lw = Vu / phi_Rn_per_inch = 60 / 5.57 = 10.8 in.

Check base metal shear rupture (A36 web, Fu = 58 ksi): Rn_base = 0.60 x 58 x 0.38 = 13.2 kips/in (nominal) phi_Rn_base = 0.75 x 13.2 = 9.9 kips/in > 5.57 kips/in — OK, weld metal governs.

Minimum fillet weld size per Table J2.4: for tw = 0.38 in. (between 1/4 and 1/2 in.), min = 3/16 in. Max fillet weld along web edge: web thickness = 0.38 in. (less than 1/4 in.? No, 0.38 in. > 0.25 in., so max leg = 0.38 - 1/16 = 0.38 - 0.0625 = 0.318 in. — use 5/16 in. max. Our 1/4 in. is between 3/16 and 5/16 — OK.

Use 1/4 in. fillet welds, minimum 11 in. total length (e.g., 5.5 in. each side of web).

Worked Example 2 — Gusset Plate Connection

A 1/2 in. thick A36 gusset plate (Fy = 36 ksi, Fu = 58 ksi) is welded to a column flange. The gusset transfers a diagonal brace force of 100 kips (LRFD) at a 30-degree angle to the weld line. The weld is on both sides of the gusset. Design using E70XX electrodes.

Step 1: Resolve loads parallel and perpendicular to the weld line.

Force parallel to weld (longitudinal component): Pl = 100 x cos(30 deg) = 86.6 kips Force perpendicular to weld (transverse component): Pt = 100 x sin(30 deg) = 50.0 kips

Both sides of plate: Pl per side = 43.3 kips, Pt per side = 25.0 kips.

Step 2: Select weld size and check limits.

Max fillet weld along gusset edge (1/2 in. thick plate): 1/2 - 1/16 = 7/16 in. Min fillet weld (1/2 in. plate = over 1/4 to 1/2 range): 3/16 in. Try 3/8 in. fillet weld.

Step 3: Calculate capacity per inch.

Throat = 0.707 x 0.375 = 0.265 in. FEXX = 70 ksi. The resultant force angle varies along the weld. Conservatively, use the longitudinal strength for design:

phi_Rn_parallel = 0.75 x 0.60 x 70 x 0.265 = 8.36 kips/in per side

Step 4: Required weld length for parallel load component.

Lw_parallel = 43.3 / 8.36 = 5.2 in.

Step 5: Check transverse strength at the end returns.

If the weld wraps around the gusset end, the transverse strength is 1.5 x longitudinal = 12.54 kips/in. A 4 in. end weld provides: 4 x 12.54 = 50.2 kips > 25.0 kips — OK.

Step 6: Combined check using vector sum.

For a weld return of 4 in. on each end plus 6 in. along each side: Total longitudinal capacity = 6 x 8.36 = 50.2 kips > 43.3 kips — OK End weld transverse capacity = 4 x 12.54 = 50.2 kips > 25.0 kips — OK

Step 7: Check base metal.

Gusset shear rupture along weld line: Anv = 2 sides x 6 in. x 0.5 in. = 6.0 in^2 phi_Rn_base = 0.75 x 0.60 x 58 x 6.0 = 156.6 kips > 100 kips — OK

Block shear in gusset (Section J4.3): for the welded gusset end, the block shear path is the sum of the weld lengths. The smaller of:

• Fu x Ant = 58 x (0.5 x 4) = 116 kips (tension rupture)
• 0.60 x Fu x Anv = 0.60 x 58 x 10.0 = 348 kips (shear rupture)

Per J4.3: Since Fu x Ant (116) < 0.60 x Fu x Anv (348), use the second case: phi_Rn = 0.75 x (116 + 0.60 x 58 x 10.0) = 0.75 x (116 + 348) = 0.75 x 464 = 348 kips > 100 kips — OK

Selected: 3/8 in. fillet weld (E70XX), 6 in. along each side of gusset with 4 in. end returns. Weld on both sides of plate.

Frequently Asked Questions

What is the difference between LRFD and ASD for weld design? LRFD uses phi = 0.75 applied to the nominal weld strength, while ASD uses Omega = 2.00 (equivalent to dividing nominal strength by 2.00). The LRFD/ASD conversion ratio is approximately 1.5 (0.75 x 1.5 = 1.125, close to the average live/dead load ratio of 1.5). Both methods produce similar weld sizes for typical load ratios, but LRFD is more commonly used in US practice.

When should I increase the fillet weld size above the minimum? Minimum fillet weld sizes per Table J2.4 ensure adequate cooling and fusion. Increase the weld size when: (1) the required weld length exceeds the available connection length, (2) base metal checks govern (thin plate with high-strength weld), (3) the connection is subject to fatigue loading requiring larger throat area for stress range reduction, or (4) corrosion allowance is needed for exposed structures.

Can fillet welds be used for tension connections? Yes, but with limitations. Fillet welds loaded transversely (perpendicular to the weld axis) are 50% stronger than longitudinally loaded welds, but they are also less ductile. For connections where the load path is primarily perpendicular to the welds (such as gusset plate end welds), the directional strength increase provides economy. However, fillet welds should not be the sole means of transferring tension in butt splices — use CJP or PJP groove welds for full-strength tension splices.

How do I specify a field weld on structural drawings? Per AWS A2.4, add a filled flag at the junction of the arrow and the reference line. The flag should point toward the tail of the arrow. An unfilled (open) flag does not have a standard meaning in AWS A2.4 — use only filled flags. When all welds on a drawing are field welds, a general note such as "ALL WELDS ARE FIELD WELDS UNLESS NOTED OTHERWISE" is more efficient than adding a flag to every weld symbol.

What is the minimum effective length of a fillet weld? Per AISC 360-22 Section J2.2(b), the minimum effective length of a fillet weld is 4 times the weld size. For a 1/4 in. fillet weld, the minimum effective length = 1.0 in. Weld lengths below this are considered incapable of developing the full weld strength due to the irregular stress distribution at weld ends.

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.