Weld Design to TCVN 5575 - Throat, Stresses & the Two-Plane Check

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

TCVN 5575 - two-plane fillet check

The Vietnamese code TCVN 5575 checks a fillet weld on two planes: through the weld metal (subscript$\,f$) and along the fusion boundary with the parent metal (subscript $\,z$). Each plane uses its own throat coefficient ($\beta_f$, $\,\beta_z$, which depend on the welding method - manual, semi-automatic or automatic) and its own design strength, all factored by the working-condition factor $\gamma_c$:

The governing plane is whichever gives the higher utilisation: a relatively soft weld metal makes the through-weld plane ($\,f$) critical, while a strong electrode on a low-grade parent steel shifts the check to the fusion boundary ($\,z$). Both planes must be satisfied for every weld in the group, which is why TCVN 5575 welds are often sized by the fusion-boundary strength $\,f_{ws} = 0.45\,f_u$ rather than by the electrode.

Butt (groove) welds

A full-penetration butt weld is checked as the parent metal (von Mises against the design strength$\,f = f_y/\gamma_M$, with $\gamma_c$); a partial-penetration weld is checked as a fillet on its effective throat $a_{eff}$, and a double-sided 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.