Weld Design to AISC 360 - Nominal Weld Stress, LRFD & ASD

The effective throat

A fillet weld is checked not on its visible leg but on its effective throat - the narrowest plane through the weld, where failure is assumed to occur. For an equal-leg fillet of leg size $s$ the throat is

All design stresses act over the throat area $A_w = a\,L_{eff}$, where $L_{eff}$is the effective length (the full length for a continuous run; doubled for a double-sided fillet). The throat - not the leg - is the quantity that sets the weld's capacity.

The leg $s$ is what the drawing specifies - typically 4 to 12 mm, and at least comparable to the thickness of the thinner connected part so the joint cools without cracking - and the throat follows from the 0.707 factor. The effective length leaves out the start and stop craters where the weld is not full size: for an end that is not returned around the corner, a deduction of one throat at each end is common, and a fillet shorter than the greater of 30 mm or six times the leg is treated as non-load-bearing. These detailing limits are why a longer, smaller weld is often more efficient than a short, heavy one - capacity scales with length, while a thick fillet adds heat, distortion and cost.

Stresses on the throat

The applied force is resolved into three components on the throat plane: the normal stress $\sigma_\perp$, the transverse shear $\tau_\perp$ (perpendicular to the weld axis) and the longitudinal shear$\,\tau_\parallel$ (along the weld axis). A force perpendicular to the weld at 45° to the throat produces$\,\sigma_\perp = \tau_\perp$.

AISC 360 - nominal weld stress (Chapter J2)

The American code AISC 360 sizes a fillet weld from the nominal weld stress$\,F_{nw} = 0.60\,F_{EXX}$, where $F_{EXX}$ is the electrode classification strength (E60–E90). The demand is the resultant stress on the effective throat $\,a = 0.707\,w$ (leg $w$), and the available strength is reported in both design philosophies:

Both the LRFD ($\phi = 0.75$) and ASD ($\Omega = 2.0$) rows are shown so you can read either; the chosen design basis selects which governs the verdict. AISC inputs may be entered in imperial units (in · kip · ksi) via the unit toggle.

Groove welds - CJP and PJP

A CJP (complete-joint-penetration) groove weld develops the base metal - checked against$\,\phi F_y$ (LRFD) or $F_y/\Omega$ (ASD). A PJP (partial-joint-penetration) weld is checked as weld metal ($F_{nw}=0.60\,F_{EXX}$) on its effective throat; a double-V groove gives two throats.

Weld groups - the "weld as a line" method

When a pattern of welds (a box, a C-shape, two lines) resists an in-plane force and a torsion moment, the classic approach treats the weld as a line of unit throat. Compute the group length $L$, its centroid and the polar moment of the weld line $I_p = I_y + I_z$. The direct force per unit length is$\,f_{dir} = F/L$; torsion adds $\,f_{tor} = M\,r_{\max}/I_p$ at the farthest point. The two combine as vectors at the worst corner to give the peak resultant per unit length, divided by the real throat to a stress checked against the same code limit:

Select a weld pattern in the calculator's Weld group tab and enter the box size and the in-plane actions to get the governing utilisation.