Stiffened extended shear tab connections (either in full-depth or partial-depth configurations) are widely used to connect simply supported beams to the web of supporting girders or columns. Full-scale laboratory tests of stiffened extended shear tab connections underscored the differences between their observed and expected design strength calculated according to current design specifications. In particular, the design procedure of such connections neglects the influence of the out-of-plane deformation of the supporting girderweb on yielding and inelastic buckling of the shear plate. These are the main governing failure modes for the full-depth configurations of stiffened extended shear tabs, when placed on one side of a supporting girder or column. The research described in this paper aims to develop a better understanding of the load transfer mechanism and failure modes of extended beam-to-girder shear tab connections. The findings are based on finite element (FE) simulations validated with full-scale experiments on beam-to-girder shear tab connections. The influence of girder web flexibility on the behaviour of single-and double-sided shear tabs is assessed. The stiffened portion of the full-depth extended shear tabs yielded due to the interaction of horizontal shear and vertical axial force. Due to the flexibility of the girder web of the single-sided shear tab, its stiffened portion experienced much larger vertical axial force in comparison to that of the double-sided configuration.