Fillet Weld Throat Shear Stress
Fillet Weld Throat Shear Stress
Fillet welds are checked not on their leg size directly but on their effective throat — the shortest distance through the weld cross-section, which for an equal-leg fillet weld is 0.707 (that is, 1/sqrt(2)) times the leg size, since the weld fails along this plane under load. Under both AISC and most other structural welding codes, fillet welds are conservatively treated as loaded purely in shear on this throat plane regardless of the actual load direction, which is why weld capacity tables report an allowable shear stress rather than a tensile or bending stress — undersizing the throat by misjudging leg size is a common weld-design error.
Fillet weld throat shear stress is tau = F/(0.707*leg*L). where F_weld is the applied load, leg_w is the fillet weld leg size, and L_w2 is the total effective weld length.
Inputs
F_weld := 80 kN=80000 N
leg_w := 10 mm=10 mm
L_w2 := 200 mm=200 mm
The 0.707 factor converts the leg dimension into the effective throat thickness, and multiplying by the weld length gives the throat area the load is spread across.
tau_weld := F_weld/(0.707·leg_w·L_w2)=56.58 MPa
Results
A throat shear stress in the range of 50-60 MPa here is a moderate value that should still be checked directly against the weld metal's allowable shear stress (commonly around 0.3 times the electrode's classification strength, on the order of 100-150 MPa for common electrodes) before accepting the design — if it exceeds that allowable, the fix is almost always to add weld length or increase leg size rather than change the base metal, since the weld itself is normally the weaker link.
Assumptions & limitations
- Equal-leg fillet weld with the standard 0.707 effective-throat factor.
- Weld assumed uniformly loaded along its full effective length.
- Stress concentration at weld ends/start-stop points not explicitly modeled.
- Static loading — fatigue loading on welds requires a separate S-N based check.