The fire safety assessment of steel structures is a critical aspect of civil engineering, especially regarding optimized structural solutions such as cellular beams. Featuring circular web openings, these beams are efficient for long-span and facilitate the integration of building services. However, their complex geometry increases susceptibility to instabilities and local failure modes, which are aggravated at elevated temperatures due to the degradation of steel mechanical properties. One of the most critical phenomena is the Vierendeel collapse mechanism, characterized by the formation of four plastic hinges around the openings [1]. Despite its importance, the current Eurocode for the fire design of steel structures (EN 1993-1-2 [2]) is essentially limited to material-property reduction factors and does not provide specific formulations for this failure mode under fire conditions. This work addresses this limitation through a parametric study based on a numerical model developed in Ansys, with shell elements. Nonlinear analyses were performed considering material nonlinearity, initial geometric imperfections, and residual stresses. The reliability and accuracy of the model was confirmed by validation against experimental test results. The study provides a detailed assessment of the combined shear and bending effects acting on the inclined critical section associated with Vierendeel collapse. Results show that opening dimensions, cross-sectional properties, and temperature are key parameters governing structural response and the onset of this mechanism. The findings also highlight limitations of current Eurocode requirements and provide a basis for improved design rules that ensure structural fire safety while promoting cost-effective material use.