Fillet Weld Group Shear Strength
Fillet Weld Group Shear Strength
Fillet welds are the workhorse connection in steel fabrication, joining plates and shapes without the precision fit-up a full-penetration groove weld requires. Their strength is governed not by the visible leg size but by the smaller effective throat dimension, since that is the plane where the weld metal actually shears under load.A fillet weld that looks adequately sized on the leg can still be under-strength if the throat is miscalculated, and because a fillet weld fails by shearing along its throat rather than yielding gradually, under-sizing a structural weld is one of the more common causes of connection failure in fabricated steelwork — which is why the throat, not the leg, is what gets checked against the applied load.
The weld throat carries the applied shear as an average stress tau = P/A_w, where A_w is the throat area computed from the 0.707 factor applied to the leg size. where P is the applied shear load, leg is the fillet weld leg size, throat is the effective throat thickness, L_w is the total weld length, A_w is the throat area, tau is the resulting shear stress, and f_w is the allowable weld metal shear stress.
Inputs
P := 150 kN=150000 N
L_w := 300 mm=300 mm
leg := 8 mm=8 mm
f_w := 200 MPa=200 MPa
For a fillet weld the failure plane is the throat, at 45 degrees to the legs, which is smaller than the leg size by the geometric factor 0.707.
throat := 0.707·leg=5.66 mm
The throat area over the full weld length is what actually resists the applied shear.
A_w := throat·L_w=1.7e-03 m^2
Dividing the load by the throat area gives the average shear stress the weld metal must sustain.
tau := P/A_w=88.4 MPa
Comparing the allowable weld shear stress with the demand gives the factor of safety on the weld.
SF := f_w/tau=2.26
Results
A factor of safety near or above 2, as found here, is typical because fillet welds are usually sized generously since increasing the leg by even 1–2 mm is cheap relative to redesigning the joint. If SF drops close to 1, the fix is almost always a bigger leg size or a longer weld run rather than a different weld process, since throat area scales directly with both. Note that this check only covers weld metal shear — the base metal at the fusion line and the connected plates still need their own checks.
Assumptions & limitations
- Load is assumed to act uniformly along the full weld length (no eccentricity or bending on the weld group).
- Only the weld metal shear failure mode is checked — base metal and plate checks are separate.
- The 0.707 throat factor assumes an equal-leg fillet weld with a flat weld face.
- Weld quality (no undercut, adequate fusion) is assumed per code inspection.