This study numerically investigates the influence of non-uniform diaphragm stiffness distribution across different stories on the formation of torsional vibration modes in reinforced concrete buildings. Although floor diaphragms are commonly idealized as rigid in conventional structural analysis, their actual in-plane stiffness and deformation characteristics may influence the dynamic properties and seismic response of a structure. To investigate this issue, eight three-dimensional four-story reinforced concrete building models were developed and analyzed using ETABS. The models were divided into two groups. The first group examined different combinations of rigid and semi-rigid diaphragm assumptions across the stories, while the second group investigated the effects of varying slab thicknesses, considering 15 cm and 20 cm slabs in different story arrangements. The dynamic behavior of the models was evaluated by comparing their natural periods, modal shapes, modal mass participation ratios, and displacement responses. The results indicated that the third vibration mode was predominantly torsional in the analyzed models, with a reported rotational mass participation ratio of approximately 0.82. The vibration period was approximately 0.75 s. For the structural configurations and parameter ranges considered, variations in diaphragm modeling assumptions and slab thickness distribution did not substantially change the formation of torsional modes. These findings suggest that, for the investigated low-rise reinforced concrete buildings, the examined variations in diaphragm stiffness were not sufficient to produce a notable change in the identified torsional mode. However, the results should be interpreted within the limitations of the building configurations and modeling assumptions considered.