<p>T-shaped reinforced concrete (RC) frame buildings are more vulnerable to seismic forces due to their geometric irregularities, uneven mass and stiffness distributions, and susceptibility to torsional forces. These factors make them prone to higher damage during earthquakes as compared to symmetrical structures. There is a lack of comprehensive studies that simultaneously address the effects of the response reduction (R)-factor and building height on the seismic vulnerability of T-shaped reinforced concrete frame buildings, which better represent real-world structures with irregular geometries. The present study examines the seismic vulnerability of T-shaped RC buildings with combined effect of height (ranging from 5 to 20 storeys) and R (ranging from 3 to 6). All the buildings of different heights are designed for various R-factors, and then the seismic vulnerability of the buildings is obtained. The results are illustrated through pushover curves, fragility curves and assessment of damage probability (DP), damage index (DI) and seismic vulnerability index (SVI) for varying R-factors and building heights under both design basis earthquake (DBE) and maximum considered earthquake (MCE) scenarios. A unique relationship is also established between DI and SVI. A three-dimensional diagram has been developed to depict the combined effects of building height and R-factor on the potential damage and seismic vulnerability. For the study purpose, pushover analysis has been performed to evaluate critical parameters such as base shear capacity and ductility demand across various building heights and R-factors. Furthermore, time history analyses have also been conducted to compare the variation in seismic demand with pushover analysis. The study indicates that increasing R-factors from 3 to 6 leads up to a 31% increase in the probability of CD, while increasing building height from 5 to 20 storeys results in 30% increase in CD. Finally, the study advises that selected T-shaped buildings up to 10 storeys can be safely designed up to a R-factor of 6 and 15 storeys can be designed up to R = 5. Designing buildings of 15 storeys or more with a R-factor of 6 or higher is not recommended, as these structures are experiencing over 30% CD, with DI values exceeding 3 and SVI values exceeding 0.7 during MCE-level earthquakes. These findings can result in improved seismic design considerations for irregular T-shaped RC buildings by considering the selection of the R-factor based on the height and seismic vulnerability of the buildings.</p>