Understanding the performance of steel frames under fire loading is crucial, as numerous recent instances have revealed vulnerability of steel frames to such hazard. The present study is carried out to evaluate the vulnerability of regular and irregular steel structures in plan to fire event. Irregular buildings are encountered in practice due to various reasons. In this regard, three dimensional four storey steel moment resisting frame structures with rectangular, C-shape, H-shape and L-shape are considered for the analysis. All moment resisting frames are subjected to five different fire cases in each floor and the most critical member is identified. The effect on stress resultants of the surrounding structural elements is investigated at critical temperatures applied to the critical member. Linear static analysis is performed and the results in terms of DCR, axial forces, bending moments and shear forces are obtained and variation in internal forces are calculated. Significant variations in the internal forces indicate that the affected column distributes force on the neighbouring members that are greater than the forces initially carried by them, potentially resulting in member failure and gradual collapse. It is observed that irregularity considerably affects the performance of the structure. The critical components of irregular structures fail at a much lower temperature when compared to regular structures. Among the various structures analysed, maximum difference in axial force, bending moment and shear forces before and after fire exposure are observed in H-shape structure.