--- | ------------------ | ------------- | ---------------------- | | Fillet weld | General connection (lap, T, corner) | Directional strength or simplified | Weld shear, base metal strength | | Complete joint penetration (CJP) groove weld | Moment connections, splices | Equal to base metal strength | Base metal (matching electrode) | | Partial joint penetration (PJP) groove weld | Column splices, stiffeners | Effective throat shear | Weld shear, base metal | | Plug/slot weld | Lap splices, composite | Shear on plug area | Weld shear | | Flare-bevel groove | HSS connections | Effective throat | Weld shear |

Fillet Weld Effective Throat

The effective throat is the shortest distance from the root to the face of the fillet weld. For equal-leg fillet welds:

te = 0.707 x w (where w = weld leg size)

For unequal-leg fillet welds (legs of size w1 and w2): te = w2 x sin(alpha) / sqrt(1 + (w2/w1)^2 - 2 x (w2/w1) x cos(alpha))

For a 90-degree joint with fillet weld, alpha = 45 degrees.

Standard Fillet Weld Sizes and Throat Dimensions

Fillet Weld Strength per AISC 360 (Table J2.5)

LRFD Method (phi = 0.75)

Rn = 0.60 x FEXX x te x L (per unit length: phi x 0.60 x FEXX x te)

For weld metal: phi x Rn = 0.75 x 0.60 x FEXX x te = 0.45 x FEXX x te (per unit length)

For base metal: phi x Rn = 0.75 x 0.60 x Fu x t_base (shear yielding of base metal)

ASD Method (Omega = 2.00)

Rn / Omega = 0.60 x FEXX x te / 2.00 = 0.30 x FEXX x te (per unit length)

E70XX Electrode Strength

FEXX = 70 ksi (482 MPa)

Other common electrodes: E60XX (FEXX = 60 ksi), E80XX (FEXX = 80 ksi), E90XX (FEXX = 90 ksi), E100XX (FEXX = 100 ksi), E110XX (FEXX = 110 ksi).

For E80: phiRn = 0.45 x 80 x te = 36.0 x te kips/in For E90: phiRn = 0.45 x 90 x te = 40.5 x te kips/in

Fillet Weld Strength per EN 1993-1-8 (Table 4.1)

Directional Method

Fw,Rd = fvw,d x a (per unit length)

fvw,d = fu / (sqrt(3) x beta_w x gamma_M2)

Where:

• a = effective throat thickness
• fu = nominal tensile strength of the weaker part
• beta_w = correlation factor (Table 4.1)
• gamma_M2 = 1.25

Simplified Method

Alternative: Fw,Rd = 0.90 x fu x a / gamma_M2 (independent of direction)

EN 1993 Fillet Weld Capacity per mm throat (S355, S235)

Fillet Weld Strength per AS 4100 (Clause 9.7)

Design Capacity

phi_vw = phi x 0.6 x fuff x tt x L (Clause 9.7.3.10(1))

Where:

• phi = 0.80 (capacity factor)
• fu_w = nominal tensile strength of weld metal
• tt = design throat thickness

SP Weld (E41XX) — fu_w = 410 MPa

W50XX Weld — fu_w = 490 MPa

Fillet Weld Strength per CSA S16:24 (Clause 13.13)

Vu = phi_w x 0.67 x phi_w x X_u x A_w (where phi_w = 0.67)

For E49XX electrodes (Fu = 490 MPa): Vu = 0.67 x 0.67 x 490 x A_w = 219.6 x A_w (in N, with Aw in mm^2)

Complete Joint Penetration (CJP) Groove Welds

CJP groove welds develop the full strength of the base metal — no capacity reduction.

Tension: phi x Rn = phi x Fy x Ag (phi = 0.90 LRFD) or Fy x Ag / 1.67 (ASD) Compression: phi x Rn = phi x Fy x Ag (phi = 0.90) Shear: phi x Rn = phi x 0.60 x Fy x Aw (phi = 1.00 LRFD)

Design requirement: use matching or stronger electrode per AISC Table J2.6 / AWS D1.1.

Matching Electrode Requirements (AISC Table J2.6)

Partial Joint Penetration (PJP) Groove Welds

PJP groove weld capacity is based on the effective throat (depth of penetration).

phi x Rn = phi x 0.60 x FEXX x te (shear on effective throat) phi = 0.80 (AISC LRFD, higher than fillet due to reduced uncertainty)

Weld Electrode Specifications

International Electrode Equivalents

Minimum Fillet Weld Sizes (AISC Table J2.4)

Note: Maximum fillet weld size along edges less than 1/4 in thick equals the plate thickness. For plate edges 1/4 in or thicker, maximum fillet weld size = plate thickness - 1/16 in.

Maximum Weld Size Limitations

• Along edges of plates < 1/4 in (6 mm) thick: weld leg = plate thickness
• Along edges of plates >= 1/4 in (6 mm) thick: weld leg = plate thickness - 1/16 in (1.5 mm)
• In lap joints: minimum lap = 5 x t_min (AISC J2.2b)
• Return welds: at least 2 x weld size (AWS D1.1 Clause 2.8.3)
• Intermittent fillet welds: minimum length = 4 x weld size (AWS D1.1)

Base Metal Check

The weld design strength is the lesser of:

1. Weld metal strength (shear on effective throat)
2. Base metal strength (shear rupture of connected material)

For base metal shear: phi x Rn = phi x 0.60 x Fu x t_base x L (LRFD) Where phi = 0.75 (AISC) or phi = 0.80 (AS 4100 / CSA S16)

Frequently Asked Questions

What is the difference between a fillet weld and a groove weld? A fillet weld is a triangular weld deposited in a corner formed by two surfaces — no joint preparation is needed. A groove weld is deposited in a pre-prepared groove between two members. Fillet welds are more economical for smaller members and thinner plates; groove welds (especially CJP) develop the full base metal strength and are used in moment connections and high-stress applications.

When should I use the directional method vs the simplified method for EN 1993 fillet welds? The directional method (EN 1993-1-8 Clause 4.5.3) resolves weld stresses into longitudinal and transverse components. It typically gives 15-25% higher capacity when the force is primarily transverse (perpendicular to the weld axis). The simplified method is conservative and independent of load direction. Use directional method for economy, simplified method for simplicity or when load direction is uncertain.

What electrode should I use for welding A992 steel? For A992 (Fy = 50 ksi, Fu = 65 ksi), use E70XX electrodes per AISC Table J2.6. E70XX provides matching or overmatching strength. For 50 ksi base metal, E70XX is standard. Do not use E60XX for A992 — the weld strength would be lower than the base metal, and connection capacity would be limited by the weld.

How do I determine the required fillet weld size for a given load? Divide the required strength per unit length by the design strength per unit length for the electrode. For example, if the demand is 12 kips/in on E70XX using LRFD, and a 5/16 in fillet provides 6.96 kips/in, the required weld length per inch is L = 12/6.96 = 1.72 in. For a continuous weld, round up to the next standard weld size: 3/8 in (8.36 kips/in) would be adequate.

What is the minimum fillet weld size restriction based on? Minimum fillet weld sizes (AISC Table J2.4) exist to ensure sufficient cooling rate and fusion. Too small a weld on a thick plate creates a quenching effect that can produce hard, brittle weld metal and reduce ductility. The minimum also ensures adequate throat area for the connection to transfer forces. Always use at least the minimum from Table J2.4 regardless of calculated demand.

How does weld inspection affect design? Weld inspection requirements are specified by AWS D1.1 and differ by weld type. CJP groove welds require 100% ultrasonic testing (UT) or radiographic testing (RT) per AWS D1.1 Clause 6. PJP groove welds and fillet welds typically require visual inspection (VT). For critical connections, NDT beyond VT should be specified. Higher inspection levels reduce the uncertainty in weld quality, which is reflected in the resistance factors.

What is the economic difference between a 1/4 in fillet weld and a 3/8 in fillet weld? A 3/8 in fillet weld requires roughly 2.25 times the weld metal volume of a 1/4 in fillet weld for the same length. However, the time to deposit a 3/8 in weld in a single pass is only about 1.5 times longer. The total cost difference is approximately 1.5-2.0x considering labor, electrode consumption, and preheat requirements. Optimizing weld size can significantly reduce fabrication cost.

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Related Pages

• Fillet Weld Size Chart — minimum and maximum fillet weld sizes
• Weld Electrodes Reference — detailed electrode selection and properties
• Weld Symbols Chart — AWS welding symbol interpretation
• Steel Grades Reference — base metal material properties
• AISC Steel Manual Reference — AISC 360 specification chapters
• Minimum Weld Size Guide — AISC Table J2.4 minimum sizes
• Steel Fy and Fu Table — yield and tensile strengths

Design Resources

Calculator tools

• Welded Connection Calculator
• Weld Group Calculator
• Connection Design Calculator
• Splice Connection Calculator

Design guides

• Weld Design Workflow
• Fillet Weld Capacity Worked Example
• CJP Weld Design Guide
• Welded Connection Checklist

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