Fillet Weld Strength Calculator — Transverse & Longitudinal Loading, Throat & Leg Size Design
Calculate fillet weld capacity under transverse and longitudinal loads. Determine required leg size from design load, electrode classification, and loading direction.
Quick Answer
For a T-joint with combined loading, E70XX electrode, 8mm leg fillet both sides, 150mm length: Longitudinal Shear Capacity = 204 kN, Transverse Capacity = 306 kN (1.5× increase per AWS). The 1.5× transverse factor makes side fillets 50% stronger than end fillets.
Fillet Weld Design Deep Dive
Fillet welds are the workhorse of structural fabrication — 90%+ of all welds are fillets. Understanding loading direction is critical because capacity changes dramatically.
1. Longitudinal Loading (End Fillet)
Load parallel to weld axis — shear on throat. Capacity: P_L = 0.707 × L × τ_allow. This is the “standard” fillet capacity you’ll find in most tables. It’s conservative for all load directions.
2. Transverse Loading (Side Fillet)
Load perpendicular to weld axis — combined shear+tension on throat. AWS D1.1 permits 1.5× increase over longitudinal capacity: P_T = 1.5 × P_L. Eurocode (EN 1993-1-8) uses a directional method: √(σ²+3τ²) ≤ f_u/(β_w×γ_M2). Both give similar results.
3. Minimum Fillet Size
AWS D1.1 Table 5.8: plate 6mm → min 3mm fillet, plate 12mm → min 5mm, plate 20mm → min 6mm, plate >20mm → min 8mm. The minimum prevents cracking from rapid cooling — thin fillets on thick plates quench too fast.
Common Mistakes
- Using longitudinal capacity for all directions — If your weld sees only transverse load, you’re wasting 33% capacity by not applying the direction factor. Conversely, if you design for transverse but the actual load goes longitudinal, you’re under-designed by 33%.
- Not considering combined loading — A fillet weld in a beam-to-column connection sees both shear (longitudinal) and moment-induced tension (transverse). Calculate separately and combine: √(f_L² + f_T²) ≤ F_w. The interaction may govern.
- Neglecting eccentricity in single-sided fillets — A single fillet on one side of a plate creates a moment arm = plate thickness/2. This bending stress adds to the weld stress. Single-sided fillets should only be used for non-structural attachment, not primary load path.
- Forgetting that fillet welds have no penetration credit — A fillet weld sits on the surface. Deeper penetration from high-current processes (SAW, spray transfer GMAW) gives extra throat, but AWS doesn’t allow credit unless you qualify the procedure. Don’t assume penetration.
- Specifying fillet size = plate thickness — A 10mm fillet on 10mm plate doesn’t add strength vs 8mm fillet — the base metal governs. Maximum fillet: 1mm less than plate for plates <6mm, 2mm less for >6mm. Beyond that, you’re just burning rod.
Frequently Asked Questions
How do I calculate the required fillet leg size?
From design load: D = P / (0.707 × L × 0.30 × F_EXX × β). For E70XX: F_EXX=480 MPa, τ_allow=144 MPa. Example: 200 kN on 400mm total weld length, transverse: D = 200,000/(0.707×400×144×1.5) = 3.3mm → specify 5mm fillet (round up to standard size). Always round up — partial millimeters aren’t practical.
What is the difference between AWS D1.1 and Eurocode 3 for fillet welds?
AWS: working stress design, 0.30× electrode strength, 1.5× transverse increase. Eurocode: directional method, separate σ⊥, τ⊥, τ∥ checks, more complex but more accurate. For typical structural steel with E70XX/ER70S-6, both give similar results within 5-10%. AWS is simpler; EC3 gives more optimization.
Can I combine fillet and groove welds?
Yes — common in moment connections: full-penetration groove for flange (tension/compression), fillet for web (shear). But don’t add their capacities — the groove weld alone carries the flange force, the fillet handles shear. They’re separate load paths.
What is a PJP weld and how does it differ from fillet?
Partial Joint Penetration groove: some penetration into root, specified by effective throat (E). PJP is stronger than fillet of same leg because it goes INTO the joint. But PJP requires bevel preparation — more machining cost. Use PJP when fillet size would be impractical. Check with our Weld Strength Calculator.
How do I design for fatigue loading?
Fillet welds are terrible in fatigue — the toe is a natural notch (stress concentration 2-3×). For cyclic loading: (1) Grind toe to smooth radius, (2) TIG dress (re-melt toe), (3) Use full-penetration groove instead of fillet, (4) Reduce allowable stress to 1/3-1/2 of static. AWS D1.1 fatigue curves (Appendix 3) give life vs stress range.
How does multi-pass welding affect fillet strength?
Multi-pass has no inherent strength penalty — the throat area is what matters regardless of pass count. But: multi-pass adds heat, distortion, and cost. For thick fillets (>10mm leg), multi-pass is mandatory (single-pass can’t fill). Interpass cleaning between passes is critical — slag inclusions in multi-pass are the #1 defect.
What about unequal-leg fillet welds?
Used to match the acute angle joint or favor one plate. Throat is based on the inscribed isosceles triangle, not the visible legs. For an 8×10mm unequal fillet, throat ≈ 0.707 × 8 = 5.7mm (based on shorter leg). The longer leg doesn’t increase strength — it just adds filler.
How do I inspect fillet welds for quality?
Visual (VT): check leg size with fillet gauge, surface profile (flat or slightly convex, no overlap, no undercut >1mm). For critical: MT (magnetic particle) for surface cracks, UT (ultrasonic) for internal defects. Fillet weld throat can be verified with a bridge cam gauge. AWS D1.1 Table 6.1 gives acceptance criteria — know them.