| name | weld-analysis |
| description | Weld design and analysis — fillet and groove welds, weld groups under combined loading, AWS/AISC allowable stresses, throat area, fatigue of welds, preheat requirements. |
| metadata | {"priority":7,"promptSignals":{"phrases":["weld","fillet weld","groove weld","weld group","weld stress","weld size","AWS"],"minScore":4}} |
Weld Analysis — Complete Skill
Fillet Weld Geometry
- w = leg size (equal legs assumed)
- t = throat = 0.707w (45° throat for equal-leg fillet)
- Effective throat for partial penetration groove: t_e < t_groove
- Minimum fillet weld size (AISC Table J2.4): from 3mm (1/8") to 8mm (5/16") depending on thicker plate
Weld Area Properties (per unit throat)
Treat weld as lines (1mm throat) for group analysis:
- A_w = Σ(length) mm
- I_w = Σ(bd³/12 + Ad²) about centroid
- J_w = I_x + I_y (polar moment of inertia)
Common weld groups (per unit throat):
| Pattern | A | I_x | I_y | J |
|---|
| Both sides, width b | 2b | b³/6 | — | — |
| Rectangle b×d | 2(b+d) | d²(3b+d)/6 | b²(3d+b)/6 | I_x+I_y |
| Circle radius r | 2πr | πr³ | πr³ | 2πr³ |
Allowable Stresses (AWS D1.1)
Fillet weld — shear on throat (governs):
τ_allow = 0.30·F_EXX (based on electrode classification)
E60 (SMAW): τ = 18 ksi (124 MPa)
E70 (most common): τ = 21 ksi (145 MPa)
E80: τ = 24 ksi (165 MPa)
Groove weld (full penetration): Same as base metal allowable
AISC LRFD:
φR_n = φ·0.6·F_EXX·A_eff (φ = 0.75)
R_n = 0.6·F_EXX·A_eff
Directional strength increase (AISC):
R_n = 0.6·F_EXX·A_eff·(1 + 0.50·sin^1.5(θ))
θ = angle between load and weld axis
θ=0° (longitudinal): factor=1.0, θ=90° (transverse): factor=1.5
Stress Calculation — Direct Loading
Tension/compression:
σ = F/(t·L) where L = total weld length, t = throat
Shear (in-plane):
τ = V/(t·L)
Bending:
σ_b = M·c/I_w (c = distance to farthest fiber)
Resultant: σ_r = √(σ_b² + τ²) ≤ 0.30·F_EXX
Eccentric Shear (Weld Group in Torsion)
- Find centroid of weld group
- Calculate J_w (polar moment, unit throat)
- Direct shear: τ_d = V/(A_w·t) [direction of V]
- Torsional shear: τ_t = V·e·r/(J_w·t) [perpendicular to radius r]
- Vector sum: τ_r = √(τ_dx_total² + τ_dy_total²)
- Find worst point (usually farthest from centroid AND component adds)
Eccentric Normal Load (Out-of-Plane)
τ = V/A_w (direct shear)
σ = M·y/I_w (bending, if moment = V·e)
τ_resultant = √(σ² + τ²) / (0.707) ≤ 0.30·F_EXX [check on throat]
Fatigue of Welds (AWS D1.1 Annex K)
Welds have dramatically reduced fatigue strength vs. base metal.
Category C (transverse butt weld, ground flush): FSFS = 69 MPa at 2×10⁶ cycles
Category E (fillet weld toe): FSFS = 31 MPa (18 ksi) — very low!
Category E' (partial penetration): FSFS = 18 MPa
Fatigue life check: Δσ ≤ FTH/n (FTH = fatigue threshold from AWS table)
If cyclic: prefer full-penetration welds, grind flush, avoid undercutting
Preheat Requirements (AWS D1.1 Table 3.2)
Based on carbon equivalent: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
CE < 0.40: no preheat (usually)
CE 0.40-0.60: preheat 38-107°C
CE > 0.60: preheat 107°C+
Plate thickness also matters: t > 25mm → more preheat
Minimum Weld Size Selection
- Governed by larger of: fatigue/stress requirement OR minimum size from code
- Maximum fillet weld: t_thinner_plate - 2mm (European), t_thinner_plate - 1/16" (US)
- Minimum fillet: from code table based on thicker plate thickness
Output
Provide: weld size w, throat t, stress on throat, allowable stress, utilization ratio, fatigue category (if cyclic), governing failure mode.
