How to Use the Fillet Weld Throat Strength Calculator
A fillet weld does not carry load on its visible leg but on its effective throat. This calculator takes the leg size, the length of each weld line, the number of lines and the electrode classification, then returns the effective throat, the throat area and the weld capacity in both LRFD and ASD formats.
The effective throat of an equal-leg fillet weld is 0.707 times the leg size, and the nominal shear strength of the weld metal is Rn = 0.60 FEXX Aw per AISC 360 Section J2.4. LRFD uses φ = 0.75 and ASD uses Ω = 2.00. For E70XX electrodes this works out to the familiar shop rule of 1.392 kips per inch for each sixteenth of leg size.
When a fillet weld is loaded along its length in an end-loaded connection such as a lap splice or a long gusset, AISC J2.2b reduces the effective length once the weld runs longer than 100 times the leg size, using β = 1.2 − 0.002(l/w). Beyond 300 times the leg size the effective length is capped at 180w. Selecting the end-loaded option applies that reduction automatically.
The result covers the weld metal only. Base metal rupture and yielding at the fusion face, eccentricity and torsion in a weld group, and the minimum and maximum leg sizes tied to the thickness of the joined parts all have to be checked separately. Have a licensed structural engineer review the connection before the design is finalized.
Frequently Asked Questions
Capacity is proportional to the effective throat, so it does scale roughly with leg size. The volume of deposited metal, however, grows with the square of the leg, so welding time and heat distortion rise much faster. Adding length or extra weld lines is usually cheaper than increasing the leg.
Stress in a long end-loaded fillet weld is not uniform; the ends pick up far more load than the middle. AISC J2.2b accounts for that with the β reduction factor once the weld exceeds 100 times the leg size, so a longer weld does not gain capacity in direct proportion to its length.