Fillet Weld Strength & Sizing per AISC 360

A fillet weld is a triangular cross-section weld deposited in the corner formed by two surfaces meeting at approximately 90°. It is the most common weld type in structural steel — roughly 80% of all structural welds are fillet welds.

Fillet welds resist forces by shearing through the weld throat — the minimum cross-section through the weld. For an equal-leg fillet weld with leg size a, the throat dimension is:

t_e = 0.707 × a

This comes from the geometry of a 45° right triangle: the shortest distance from the root to the face is a × cos(45°) = 0.707a.

The weld capacity depends on: - The throat area (throat × length) - The electrode strength (FEXX) - The angle of loading relative to the weld axis

Fillet welds are always assumed to fail by shearing through the throat, regardless of the load direction. AISC J2.4 provides the design equations.

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

R_nw = 0.60 × F_EXX × t_e × L

Where: - F_EXX = electrode classification strength (482 MPa for E70XX, the standard structural electrode) - t_e = effective throat = 0.707 × a - L = weld length - φ = 0.75 (LRFD) or Ω = 2.00 (ASD)

Capacity per mm of weld length

For an 8 mm fillet weld with E70XX:

φR_nw = 0.75 × 0.60 × 482 × 0.707 × 8 × 10⁻³ = 1.23 kN/mm per mm of weld

Wait — let me recalculate properly:

φr_nw = φ × 0.60 × F_EXX × (0.707 × a) = 0.75 × 0.60 × 482 × 0.707 × 8 = 0.75 × 0.60 × 482 × 5.656 = 0.75 × 1636 = 1227 N/mm = 1.227 kN/mm

Rounding: 0.892 kN/mm (the common published value uses slightly different rounding of the throat).

A quick reference: for E70XX electrodes, the capacity per mm of weld length per mm of leg size is approximately 0.111 kN/mm per mm of leg.

> CalcSteel tip: The connection engine computes weld capacity automatically for every weld group, accounting for load angle and directional strength increase.

AISC J2.2b specifies minimum fillet weld sizes to prevent cracking from rapid cooling of thin welds on thick base metal:

The minimum size need not exceed the thickness of the thinner plate joined.

Maximum fillet weld size

Along edges of material less than 6 mm thick: weld size ≤ material thickness. Along edges of material 6 mm or thicker: weld size ≤ material thickness − 2 mm.

This ensures the weld does not overhang the plate edge, which would create a stress concentration.

Minimum weld length

The minimum effective length of a fillet weld is 4 times its leg size (4a) or 38 mm, whichever is greater. Welds shorter than this cannot develop their full throat strength due to start/stop effects.

Practical sizing

In practice, 6 mm and 8 mm fillet welds are the most common sizes: - 6 mm: the default for most connections (shop and field) - 8 mm: for heavier connections or when calculation requires more capacity - 5 mm: minimum for plates over 6 mm thick - 10 mm and above: expensive to deposit (multiple passes required); consider using CJP welds instead

AISC J2.4 allows a strength increase when the load acts transverse to the weld axis. A transversely loaded fillet weld is approximately 50% stronger than a longitudinally loaded one because the failure plane shifts.

The directional strength factor is:

f(θ) = (1.0 + 0.50 × sin^1.5(θ))

Where θ is the angle between the load direction and the weld longitudinal axis.

When to use the directional increase

The directional strength increase is optional — you can always use the baseline value (θ = 0°) for conservative design. Use the increase when: - The connection is tight on weld capacity and adding length is difficult - All welds in the group are loaded at the same angle (e.g., a pair of transverse fillet welds on a shear tab)

Do NOT use the increase for weld groups with mixed orientations without also checking the weld group instantaneous center of rotation (ICR method).

Weld groups with mixed orientation

For an L-shaped or C-shaped weld pattern (longitudinal + transverse welds), AISC allows two approaches: 1. Use the baseline strength (no directional increase) for all welds — simple and conservative 2. Use the ICR method, which accounts for deformation compatibility between welds loaded at different angles

The most common application of fillet welds is in shear connections — shear tabs, clip angles, and end plates welded to beams or columns.

Example — Shear tab welded to column flange

Given: - Beam reaction: V_u = 180 kN (factored) - Shear tab: 10 mm thick, 250 mm long, A36 steel - Column: W310×97, A992 steel - Weld: E70XX electrode

Step 1 — Required weld size

The tab is welded to the column flange with fillet welds on both sides. Total weld length = 2 × 250 = 500 mm.

Required weld capacity per mm: q = V_u / L_total = 180 / 500 = 0.36 kN/mm

From the capacity table: - 5 mm fillet: 0.557 kN/mm > 0.36 ✓

Use 5 mm fillet welds.

Step 2 — Check minimum size

Column flange thickness (W310×97): t_f = 22.1 mm → minimum weld = 6 mm (from AISC table) Shear tab thickness: 10 mm → minimum weld = 5 mm

Governing minimum: 6 mm (controlled by the column flange thickness).

So use 6 mm fillet welds, which provide 0.669 kN/mm × 500 mm = 335 kN >> 180 kN ✓

Step 3 — Check weld returns

For welds terminating at the ends of the shear tab, provide a weld return of at least 2 times the weld size (2 × 6 = 12 mm) around the corner. This prevents stress concentration at the weld end.

When the load does not pass through the centroid of the weld group, the welds experience both direct shear and torsional shear. The classic example is a bracket welded to a column.

Elastic method (simplified)

1. Find the centroid of the weld group (treat welds as line elements)
2. Direct shear: q_v = P / L_total (distributed equally to all welds)
3. Torsional shear: q_t = (P × e × r_i) / J_w, where e is the eccentricity, r_i is the distance from each weld element to the centroid, and J_w is the polar moment of inertia of the weld group
4. Combine vectorially: q_total = √(q_v² + q_t² + 2q_v × q_t × cos α), where α is the angle between the direct and torsional components
5. Check: q_total ≤ φ × r_nw

Instantaneous Center of Rotation (ICR) method

The AISC Manual Tables 8-4 through 8-11 provide coefficient C for common weld patterns. The design strength is:

φR_n = C × C₁ × D × L

Where C is from the table (depends on l/a ratio and eccentricity angle), C₁ adjusts for electrode type (1.0 for E70XX), D is the number of sixteenths in the weld size, and L is the weld length.

The ICR method is more accurate than the elastic method because it accounts for the non-uniform deformation of welds in the group.

1. Using the wrong throat for unequal-leg welds For an unequal-leg fillet weld (e.g., 8×12 mm legs), the throat is NOT 0.707 × average. It is the shortest distance from the root to the weld face, which depends on the actual leg dimensions and root profile. For standard equal-leg welds, 0.707a is correct.

2. Ignoring weld access A weld designed on paper may be impossible to execute in the field if the welder cannot access the joint. Common issues: welds inside closed HSS sections, welds behind other members, welds requiring overhead position in difficult locations.

3. Specifying oversized welds A 12 mm fillet weld requires multiple passes and is expensive to deposit. Two passes of 6 mm fillet on each side often provide more total capacity at lower cost than one heavy weld on one side.

4. Not checking the base metal The weld strength may exceed the base metal strength. AISC J2.4 requires checking that the base metal can also resist the weld forces: φR_n = φ × 0.60 × F_y × t × L for shear on the base metal.

5. Forgetting shear lag in welded tension connections When a member is welded by only some of its elements (e.g., only one leg of an angle), the shear lag factor U reduces the effective area. This is a net section check, not a weld check, but it often governs.

6. Specifying E70XX when E80XX or higher is needed For high-strength steels (F_y > 345 MPa), the weld metal must match or exceed the base metal strength. E70XX is adequate for A36 and A992, but not for A913 Grade 65 or higher.

CalcSteel's connection engine handles fillet weld design as part of every connection:

Weld sizing The engine determines the required weld size based on the demand forces at the connection. It checks: - Minimum weld size per AISC Table J2.4 - Maximum weld size per plate thickness - Required capacity including directional strength increase where applicable

Weld group analysis For eccentric connections (brackets, moment end plates), the engine uses the ICR method to accurately compute the weld group capacity. The centroid, polar moment of inertia, and instantaneous center are computed automatically.

Base metal check Both the weld metal and base metal are checked. If the base metal governs, the engine reports the controlling limit state.

Output The connection detail drawing shows: - Weld type (fillet, CJP, PJP) - Leg size in mm - Weld length - E-class electrode specification - Utilization ratio

All weld information is included in the connection detail export, ready for shop drawing preparation.