Weld Strength Calculator Formula

Understand the math behind the weld strength calculator. Each variable explained with a worked example.

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Formulas Used

Effective Throat

throat_mm = throat

Weld Load Capacity

capacity = throat * weld_length * allowable_shear / 1000

Strength per mm Length

per_mm = throat * allowable_shear / 1000

Variables

VariableDescriptionDefault
weld_sizeFillet Weld Leg Size (a)(mm)6
weld_lengthWeld Length (L)(mm)200
allowable_shearAllowable Weld Shear Stress(MPa)124
throatDerived value= weld_size * 0.707calculated

How It Works

Fillet Weld Strength

Fillet welds fail through the throat, which is the shortest path through the weld cross-section.

Key Relationships

Effective throat = 0.707 x leg size (for equal-leg fillets at 45 degrees)

Capacity = throat x length x allowable shear stress

The 0.707 factor is cos(45 degrees) = sin(45 degrees), representing the minimum cross-section through the triangular fillet.

Worked Example

A 6 mm fillet weld, 200 mm long, with 124 MPa allowable shear (E70 electrode).

weld_size = 6weld_length = 200allowable_shear = 124
  1. 01Throat = 0.707 x 6 = 4.24 mm
  2. 02Capacity = 4.24 x 200 x 124 = 105,152 N = 105.2 kN
  3. 03Strength per mm = 4.24 x 124 / 1000 = 0.526 kN/mm

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