weld-analysis

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)

1. Find centroid of weld group
2. Calculate J_w (polar moment, unit throat)
3. Direct shear: τ_d = V/(A_w·t) [direction of V]
4. Torsional shear: τ_t = V·e·r/(J_w·t) [perpendicular to radius r]
5. Vector sum: τ_r = √(τ_dx_total² + τ_dy_total²)
6. 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.