The purpose of this study is to investigate the effect of lap joint length on structural performance under ultimate load using both nonlinear numerical analysis and laboratory experiments. Three-dimensional (3D) finite element models were employed for the numerical simulations, which were carried out using LUSAS software. A typical four-point bending system was modelled in order to analyze the impact of lap length. In total, 24 cases of lap length were studied across three concrete grades: C30, C45, and C60. The study also examined the influence of beam depth, shear link spacing, and concrete grade on lap joint performance. In order to validate the reliability of the 3D model, experimental data from RC beam test specimens were used.