This paper presents a preliminary numerical study aimed at replicating and extending an experimental investigation on diagonally stiffened steel I-girders predominantly subjected to shear. The numerical models were developed to correspond to the experimental specimens: one model representing a reference I-girder without diagonal stiffeners, and another model incorporating a welded diagonal stiffener. This stiffener was placed along the diagonal of the girder web to enhance stability and load-carrying capacity under shear. The primary objectives of this numerical study are: i) to quantitatively assess the shear capacity of diagonally stiffened I-girders under varying stiffening strategies, and ii) to evaluate the accuracy and consistency of existing analytical methods for predicting the shear resistance of these girders. The numerical study is calibrated against the experimental data, forming the basis for a wider parametric study to be undertaken in the near future by the authors. This parametric investigation will explore a broader range of geometries, material properties, and stiffening configurations, with the ultimate goal of developing an analytical formulation capable of accurately predicting the ultimate strength of diagonally stiffened girders under shear.