About this calculator
This tool determines the LRFD design strength of a fillet weld, comparing the weld metal's own shear strength against the base-metal shear rupture strength of the thinner connected part, and reports the controlling capacity, the demand/capacity ratio, and the weld length required to carry a given factored load. It is intended for structural engineers and fabricators sizing fillet welds on steel connections.
Method & formulas
Effective throat: te = 0.707·w, for an equal-leg fillet weld of leg size w.
Weld metal nominal strength (AISC LRFD): FEXX is the electrode classification strength; θ is the angle between the load line and the weld's axis (0° = parallel, 90° = transverse — transverse welds carry up to 50% more per unit length).
te = 0.707·w
Fnw = 0.6·FEXX·(1 + 0.50·sin1.5θ)
φRw = φ·Fnw·te (φ = 0.75)
Base metal shear rupture per unit length: φRbm = φ·0.6·Fu·t (φ = 0.75), with Fu = base metal tensile strength, t = thickness of the thinner connected part.
Governing strength. Controlling strength = min(φRw, φRbm). Total capacity = controlling × L; demand/capacity = Pu / (controlling × L); required length Lreq = Pu / controlling.
Worked example
Inputs: w = 8 mm fillet, E70XX electrode (FEXX = 485 MPa), θ = 90° (transverse), Fu = 450 MPa (A992 base metal), t = 12 mm, Pu = 250 kN.
te = 0.707 × 8 ≈ 5.66 mm.
Fnw = 0.6 × 485 × (1 + 0.50×1) = 436.5 MPa (sin 90° = 1).
φRw = 0.75 × 436.5 × 5.66 ≈ 1852 N/mm.
φRbm = 0.75 × 0.6 × 450 × 12 = 2430 N/mm → weld controls.
Required length: Lreq = 250,000 / 1852 ≈ 135 mm (e.g. two 70 mm-long fillet welds, one each side of the connected element).
Assumptions & limitations
- Assumes a uniform, equal-leg fillet weld applied along its full effective length, with no separate allowance for weld start/stop craters or end returns.
- The load-angle increase factor (1 + 0.50 sin1.5θ) applies to a weld loaded at a constant angle θ along its whole length; weld groups under eccentric or varying-angle shear need an instantaneous-center or elastic vector analysis instead.
- Minimum and maximum fillet weld size limits (based on connected material thickness) are not enforced by this tool — check them separately.
- Only static LRFD strength is evaluated; fatigue-loaded welds require a separate fatigue category check.
- Verify results against the governing code and a licensed engineer's judgment before fabrication.
FAQ
- Why is a transverse (90°) fillet weld stronger per unit length than a longitudinal (0°) weld of the same size?
- Test data shows fillet welds loaded transverse to their axis fail at a higher unit stress than those loaded longitudinally; the (1 + 0.50 sin1.5θ) factor captures that directional strength increase, up to 50% higher at θ = 90°.
- When does the base metal control instead of the weld?
- When the connected material is thick relative to the weld leg size — once the connected part's own shear rupture strength drops below the weld's strength, undersized fillets rarely control and the plate or shape becomes the weak link.
- How does "required length" differ from "total capacity"?
- Total capacity multiplies the controlling unit strength by whatever length you entered; required length instead solves backward for the length needed so controlling strength × length just equals your applied factored load — it sizes the weld for you.