<p>Castellated beams are widely used in structural applications because of their efficiency in providing higher strength-to-weight ratios, while accommodating services within their openings. The shape and geometry of these openings, particularly variations in tee length and tee depth, significantly influence the structural behaviour and performance of the beams. This study investigates the impact of the opening shape on the structural behaviour of castellated beams by evaluating the key limit states, including axial and flexural strength, web post buckling, and shear performance. This study adopts a parametric study by varying the tee lengths and depths to examine their effects on the critical strength ratio (<i>R</i>_<i>r</i><i>/R</i>_<i>c</i>) and deflection characteristics. Analytical models were developed to assess the structural capacity under different configurations. The results reveal that axial and flexural strengths are the most critical factors among all limit states, with minimal sensitivity to tee length but a significant dependence on tee depth. Larger tee depths enhance web post buckling and vertical shear resistance but slightly reduce the horizontal shear capacity. Tee depth variations also influence deflection performance, with deeper tees contributing to greater stiffness. These findings provide valuable insights into optimising the opening geometry of castellated beams to achieve improved structural performance and stability, thereby offering guidance for efficient and robust designs in structural engineering applications.</p>