Fillet Weld Throat Thickness — Leg Size & Effective Throat
The throat thickness of a fillet weld is the shortest distance from the root of the weld to the face — the minimum cross-sectional dimension through which the weld resists shear. The throat, not the leg size, governs the weld's strength. The relationship between leg size (the visible external dimension) and effective throat (the internal failure plane) is the most fundamental concept in weld design.
For equal-leg fillet weld (theoretical flat profile):
te = 0.707 × L (where L = leg size)
Aw = te × Lw (effective throat area = throat × weld length)
PRELIMINARY — NOT FOR CONSTRUCTION. All content is 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.
Geometry of the Fillet Weld Throat
A fillet weld is a triangular bead deposited in the corner between two surfaces. The theoretical profile is a right isosceles triangle:
___ Leg (L) ___
/| |
/ | |
/ | |
/ | Te (throat) | Leg (L)
/ | |
/_____|______________|
L
Te = L × sin(45°) = L / √2 = 0.7071 × L
The throat is the altitude to the hypotenuse of the right isosceles triangle. Since the angle between legs is 90° and both legs are equal, the throat bisects the 90° angle at 45°. The sine of 45° is 1/√2 = 0.7071.
Why the Throat Governs Strength
The fillet weld is assumed to fail in shear through the throat — the shortest path through the weld metal. The resistance of a fillet weld is:
Rn = 0.60 × FEXX × Aw (AISC 360 J2-4)
where:
FEXX = electrode classification strength (e.g., 70 ksi for E70XX)
Aw = effective throat area = te × Lw
0.60 = shear strength factor (von Mises criterion: τyield = Fy/√3 ≈ 0.577Fy)
The nominal load capacity of a 1-inch-long segment of a 1/4" fillet weld (E70XX):
te = 0.707 × 0.25 = 0.177 in
Aw = 0.177 × 1.0 = 0.177 in² per inch of weld
Rn = 0.60 × 70 × 0.177 = 7.42 kips/inch
φRn = 0.75 × 7.42 = 5.57 kips/inch (LRFD)
Leg Size vs Throat: Common Misconceptions
| Statement | Correct? | Explanation |
|---|---|---|
| "A 1/4" weld has a 1/4" throat" | No | Leg = 1/4", throat = 0.707 × 1/4" = 0.177" |
| "Bigger leg always means stronger weld" | Partly | Strength scales with throat, not leg directly |
| "Convex weld is stronger than flat" | No | Extra reinforcement does NOT increase throated area |
| "Throat = leg for partial-penetration welds" | No | PJP weld throat depends on groove depth, not leg |
The leg size is the dimension you specify on drawings and measure with a fillet weld gage. The throat is the dimension that determines strength. Specifying a larger leg to increase strength is valid but the strength gain is proportional to the throat (0.707×leg), not the leg directly.
Unequal-Leg Fillet Welds
When the two legs are unequal (common when welding plates of different thickness), the effective throat is determined by inscribing the largest right triangle within the weld cross-section:
___ L1 ___
/|
/ |
/ |
/ | Te (throat — shortest path to weld root)
/ |
/_____|__________
L2
Te ≈ (L1 × L2) / √(L1² + L2²)
For an unequal-leg weld with L1 = 1/4" and L2 = 3/8":
Te = (0.25 × 0.375) / √(0.25² + 0.375²)
= 0.09375 / √(0.0625 + 0.1406)
= 0.09375 / 0.4507
= 0.208 in
Concave vs Convex Profiles
| Profile | Appearance | Actual Throat | Effective Throat (Design) | Strength Basis |
|---|---|---|---|---|
| Flat | Straight face | 0.707 × leg | 0.707 × leg | Theoretical throat |
| Concave | Dished (re-entrant) face | Less than 0.707L | 0.707 × leg | Theoretical (conservative) |
| Convex | Bulging (reinforced) face | More than 0.707L | 0.707 × leg | Theoretical (ignore extra) |
Critical point: The design effective throat is always based on the theoretical flat profile, regardless of the actual weld profile. A convex weld with extra reinforcement does NOT increase design strength — the extra metal contributes nothing to the calculated capacity because the failure path remains in the throat of the inscribed triangle. Convex welds may fatigue faster due to stress concentration at the toe.
AISC 360 — Minimum and Maximum Fillet Weld Sizes
Minimum Fillet Weld Size (AISC Table J2.4)
| Thickness of Thinner Part Joined (t) | Minimum Leg Size (L) |
|---|---|
| t ≤ 1/4" | 1/8" |
| 1/4" < t ≤ 1/2" | 3/16" |
| 1/2" < t ≤ 3/4" | 1/4" |
| 3/4" < t | 5/16" |
The minimum size ensures sufficient heat input to prevent rapid cooling and cracking. It is based on the thinner part joined, not the thicker.
Maximum Fillet Weld Size (AISC J2.2b)
Along edges t < 1/4" thick: L_max = t (material thickness)
Along edges t ≥ 1/4" thick: L_max = t − 1/16"
The maximum prevents melting away the plate edge. For a 3/8" plate, maximum leg = 3/8" − 1/16" = 5/16".
AS 4100 — Australian Fillet Weld Throat
AS 4100 Clause 9.7.3 uses similar throat geometry but different notation:
Design throat thickness: tt = Lw / √2 (same 0.707 × leg)
Design capacity: φVw = φ × 0.6 × fuw × tt × Lw
φ = 0.80 for fillet welds
Where fuw is the nominal tensile strength of the weld metal (480 MPa for E48XX electrodes).
EN 1993-1-8 — European Effective Throat
EN 1993-1-8 Clause 4.5.3 defines the effective throat thickness a:
a = throat thickness of the inscribed isosceles triangle
For equal-leg welds: a = 0.7 × leg (simplified, close to 0.707)
Design resistance of a fillet weld:
Fw,Rd = (fu / √3) / (βw × γM2) × a × Lw
Where βw is a correlation factor (0.80-1.00 depending on steel grade) and γM2 = 1.25.
Worked Example — Weld Capacity Comparison
Problem: Determine the design strength of a 5/16" fillet weld, 6 inches long, E70XX electrode, A36 base metal.
Leg size L = 5/16" = 0.3125 in
te = 0.707 × 0.3125 = 0.221 in
Aw = 0.221 × 6.0 = 1.325 in²
Rn = 0.60 × 70 × 1.325 = 55.7 kips
φRn = 0.75 × 55.7 = 41.7 kips (LRFD)
Strength per inch: φRn / Lw = 41.7 / 6 = 6.96 kips/in
Frequently Asked Questions
What's the difference between a 1/4" fillet weld and a 1/4" effective throat? A 1/4" fillet weld has a 1/4" leg — the effective throat is 0.707 × 0.25 = 0.177". A 1/4" effective throat corresponds to a leg size of 0.25/0.707 = 0.354" (approximately 3/8"). Always clarify whether the dimension refers to leg size or throat in specifications — the leg is the shop-measurable dimension, the throat is the design-critical dimension.
Can I increase weld strength by making a convex profile bead? No — design codes base strength on the theoretical throat of the inscribed triangle, not the actual deposited metal. Convexity provides no calculable strength increase and may increase stress concentration at the toe, reducing fatigue life. If higher strength is needed, specify a larger leg size.
Why do fillet welds fail in shear through the throat? The 45° throat plane is the minimum cross-sectional area through the weld. The resolved force on this plane is primarily shear (with some tension/compression components). Fracture mechanics and testing confirm that the critical failure surface follows this shortest path. The 0.60×FEXX factor accounts for the shear-dominant failure mode.
Related Terms and Pages
- Weld Capacity — Complete Design Guide
- Weld Symbols — AWS A2.4 Reference
- Fillet Weld Size Guide — Selection & Limits
- Block Shear — Definition & Failure Mode
- Welded Connection Calculator — Free Online Tool
- Welding Procedure — AWS D1.1 Requirements
Educational reference only. Weld design must be performed per the governing welding code (AWS D1.1, AS/NZS 1554, EN 1090-2) and design standard by a licensed Professional Engineer. All designs must be independently verified.
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.