This paper challenges the design provisions that rely on the traditional assumptions of simply supported web boundary conditions for shear. While some literature discuss the flange stiffening effect on the web, the detrimental impact of flexible flanges on the shear strength has not been addressed before. Through a detailed analysis of excessive deformations at the web-flange junctions, this study shows that the assumption of simply supported web boundary conditions may be unsafe, contradicting expectations of conservative design in some girders that comply with current design specifications. Accordingly, minimum flange dimensions are proposed to ensure adequate support at the web edges. In addition to categorizing cross-sections that do not achieve simply supported boundary conditions, the paper proposes a modified elastic shear buckling coefficient, which can enhance the estimate of elastic shear buckling stresses of I-girders by up to 60% compared to simply supported web plates. The improved elastic buckling stress is also shown to improve the overall shear strength predictions of I-girders by means of geometric and material nonlinear analyses. This revised coefficient considers the intricate interplay between the flanges, transverse stiffeners, and the web, which shall enable a more mechanistic and consistent interpretation of the ultimate shear capacities in future studies. The findings presented in this paper have implications for cross-sectional proportioning and shear design of plate girders, given that the ultimate design shear capacities of I-girders are typically estimated by summing the web elastic shear buckling stresses with their post-buckling strengths.