The shear buckling and post-buckling behaviour of welded plated I girders with slender webs plays a critical role in the safety and economy of steel bridge and building structures. However, experimental data and validated numerical models for multi-span girders with slender webs remain limited, and the accuracy of current design provisions is not yet fully established. This paper presents the results of an experimental and numerical investigation on ten two-span welded plated I girders with transversely stiffened webs. By placing transverse stiffeners on both sides, the web panels were divided into four or eight rectangular segments with an aspect ratio of a/h=2.5. The web slenderness ratios h/t ranged from 175 to 350, while the flange slenderness ratios b/t varied between 8 and 40. The specimens exhibited shear failure governed by tension-field action near the internal support or flange buckling in the middle of the span. To further examine the shear buckling behaviour, geometrically and materially nonlinear analyses with imperfections were performed using advanced full-shell finite element models. The validated numerical models were then employed in a comprehensive parametric study covering a wide range of geometric configurations. The numerical results were used to assess the accuracy of the shear strength formula specified in EN 1993-1-5:2024 and AISC 360-22. Based on the results of the numerical simulations new formulas are proposed for the contribution of the web and flanges to determine the shear strength of plated I girders.