Weld Design to Eurocode 3 - Throat, Stresses & EN 1993-1-8 Methods

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$\,\sigma_\perp = \tau_\perp$.

Directional method (EN 1993-1-8 §4.5.3.2)

The directional method combines the throat stresses and checks two conditions against the design weld strength$\,f_{vw,d}$:

Because the longitudinal shear $\tau_\parallel$ sits under the factor of 3 rather than acting on its own, the directional method rewards welds loaded along their length and is generally less conservative than the simplified method below - useful when a weld is fully utilised and you need every bit of capacity. The second condition caps the normal stress $\sigma_\perp$ alone, so a weld loaded purely in tension across its throat never benefits from the combined formula.

Simplified method (§4.5.3.3)

The simplified method ignores direction: the resultant design force per unit length $F_{w,Ed}$ must not exceed the design resistance per unit length, $F_{w,Rd} = f_{vw,d}\,a$. It is faster and always at least as safe as the directional method.

The correlation factor β_w

The correlation factor $\beta_w$ (EN 1993-1-8 Table 4.1) depends on the steel grade:

Butt (groove) welds (§4.7)

A full-penetration butt weld made with matching consumables develops the weaker parent metal - no separate weld check is needed; verify the parent (von Mises against $f_y/\gamma_{M0}$). A partial-penetrationbutt weld is treated as a deep-penetration fillet on its effective throat $a_{eff}$.

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.