Applying simplified design code provisions for the shear assessment of existing concrete girders may result in unnecessary strengthening, justifying the effort to apply more refined approaches. Based on the cross-sectional analysis for continuous girder zones (B-regions), this paper discusses the theoretical influences of transverse reinforcement ductility and effective web concrete-compressive strength on shear resistance. It is shown that the plastic deformation capacity is fundamentally influenced by the hardening behavior of the transverse reinforcement and by the bond properties between reinforcement and concrete. For typical examples of stirrup configurations, allowable compression field inclinations are derived and are also compared to the provisions of the fib Model Code 2010 and the Generalized Stress Field Approach. This paper concludes with an outlook on practical considerations and how the findings have been integrated in the recently published Swiss code SIA 269/221 on existing concrete structures.