This study focused on the design of fillet welds fabricated using high-strength steel (HSS). Herein, we present a numerical design calculation (NDC) method for assessing weld strength using the regular inclined shell element model (RISEM) in the finite element method (FEM). The NDC uses the FEM to design joints according to standardised procedures. The inclined shell element in the RISEM represents the geometry and stiffness of the welds, which controls the stresses in the plane. We recommend using a common inclined shell element with rigid links to accurately represent the geometry and rigidity of fillet welds. A new strength criterion is proposed based on the stresses on the inclined shell element, using the maximum equivalent stress as an indicator. An experiment was conducted on the transverse fillet lap-welded connection from the S700 MC Plus steel plate and OK AristoRod 13.12 electrode at room temperature. The deformation capacity of the welded connection was measured via a non-contact process using the digital image correlation (DIC) technique. This method was validated by comparing the RISEM and test results using finite element (FE) simulations. Additionally, the weld resistance computed from the analytical model of prEN 1993–1–8:2020 was in good agreement.