Question 1

How is fillet weld throat thickness calculated?

Accepted Answer

For an equal-leg fillet weld, the effective throat thickness is a_eff = 0.707 × a (= a/√2), where a is the leg size. This corresponds to the height of the right-isosceles triangle formed by the weld bead cross-section. The effective weld area is then A = a_eff × L, where L is the weld length. AWS D1.1 and ISO 5817 both use this formula as the basis for stress calculations.

Question 2

What is the allowable shear stress for fillet welds per AWS D1.1?

Accepted Answer

Per AWS D1.1, the allowable shear stress on the effective throat of a fillet weld is 0.30 times the nominal tensile strength of the filler metal (F_EXX). For example, with E70XX electrodes (F_EXX = 483 MPa / 70 ksi), τ_allow = 0.30 × 483 ≈ 145 MPa. The base metal allowable (approximately 0.40 × F_y) must also not be exceeded.

Question 3

How do you calculate weld stress under combined shear, bending, and torsion?

Accepted Answer

For combined loading, each stress component is calculated on the effective throat area: shear stress τ = F/(a_eff × L × n), normal stress σ = F_normal/(A), bending stress σ_b = M/Z_w, and torsional shear τ_T = T·r/J_w. The resultant stress is τ_res = √(τ_total² + σ_total²). This is compared to the allowable stress to determine the safety factor SF = τ_allow / τ_res.

Question 4

What is the difference between ISO 5817 quality classes B, C, and D?

Accepted Answer

ISO 5817 classifies weld quality by allowable imperfection limits: Class B (highest quality) has the strictest limits for undercut, excess weld metal, and porosity, used for fatigue-loaded and safety-critical structures. Class C is intermediate quality for general structural use. Class D (lowest) permits larger imperfections and is used for non-critical static applications. Higher safety factors correlate with better quality class requirements.

Fillet Weld Joint Strength Calculator

Theory