Fillet Weld Strength Calculation

A fillet weld strength calculation determines the maximum load a fillet weld can carry based on its throat dimension, length, and the allowable shear stress of the weld metal. The fillet weld is assumed to fail in shear along its throat, which is the weakest cross-section. This calculation is required for designing pipe support attachments, structural connections, and verifying socket weld joint adequacy in piping systems.

Fillet Weld Strength Formulas

The allowable load per unit length of a fillet weld depends on the method (ASD or LRFD) and the electrode classification.

Parameter	Formula	Description
Theoretical throat (a)	a = 0.707 x L (equal-leg)	Shortest distance from root to face
Effective area	A = a x weld length	Throat area resisting the load
ASD shear capacity	Fv = 0.30 x FEXX	Allowable shear stress (AWS D1.1 / AISC)
ASD load per unit length	q = a x 0.30 x FEXX	Allowable force per mm or per inch
LRFD design strength	phi x Rn = 0.75 x 0.60 x FEXX x a x L	Factored resistance (AISC 360)

Where:

• L = fillet weld leg size
• a = effective throat
• FEXX = electrode tensile strength (e.g., 482 MPa for E70xx)

Allowable Load per Unit Length (ASD Method)

Leg Size (mm)	Throat (mm)	E60xx (414 MPa) kN/mm	E70xx (482 MPa) kN/mm	E80xx (551 MPa) kN/mm
3	2.1	0.26	0.30	0.35
5	3.5	0.44	0.51	0.58
6	4.2	0.52	0.61	0.70
8	5.7	0.71	0.82	0.94
10	7.1	0.88	1.03	1.17
13	9.2	1.14	1.33	1.52
16	11.3	1.40	1.63	1.87
19	13.4	1.67	1.94	2.22
25	17.7	2.20	2.56	2.93

Worked Example

Problem: Determine the allowable tensile load for two 8 mm fillet welds, each 150 mm long, on a pipe trunnion support. Electrode: E70xx (FEXX = 482 MPa).

Step 1: Throat = 0.707 x 8 = 5.66 mm

Step 2: Allowable shear stress = 0.30 x 482 = 144.6 MPa

Step 3: Total effective area = 2 welds x 5.66 x 150 = 1,698 mm2

Step 4: Allowable load = 1,698 x 144.6 = 245,571 N = 245.6 kN

Strength Check Considerations

When calculating fillet weld strength, the engineer must verify both the weld metal and the base metal. The governing capacity is the lower of:

1. Weld metal shear strength (calculated above)
2. Base metal shear yield on the fusion face: 0.60 x Fy x effective area
3. Base metal tensile rupture: 0.60 x Fu x net area

For carbon steel base metals (e.g., ASTM A36, Fy = 250 MPa), the weld metal typically governs when E70xx electrodes are used. For higher-strength base metals, the base metal capacity may control.

Direction of Loading

The strength of a fillet weld varies with the angle of the applied force relative to the weld axis:

Load Direction	Strength Factor	Notes
Longitudinal (parallel to weld)	1.0 (baseline)	Pure shear along throat
Transverse (perpendicular to weld)	1.5	Higher resistance per AISC 360
45-degree angle	1.21	Interpolated

AISC 360 Section J2.4 allows a directional strength increase for fillet welds loaded at angles other than parallel. AWS D1.1 also permits this approach when the engineer specifically invokes the directional strength provisions.

Common Mistake

Neglecting to check the base metal shear capacity on the fusion face. If the base metal is thin relative to the weld size, the base metal may fail before the weld does. This is common with light-gauge attachments welded to heavy pipe or structural members. Always verify both weld and base metal capacity.

Read the full guide to fillet welds