Modified Strut-and-Tie Model for RC Deep Beams Considering Size Effect and Longitudinal Reinforcement

Accurate prediction of the shear capacity of reinforced concrete (RC) deep beams remains challenging due to the complex interaction of multiple load transfer mechanisms and the pronounced size effect in quasi-brittle materials. Existing strut-and-tie-based models are widely used in practice; however, they often neglect the coupled influence of structural size and longitudinal reinforcement, leading to reduced reliability for large-scale members. In this study, a modified simplified strut-and-tie model (M-SSSTM) is proposed in order to achieve a fracture mechanics-inspired empirical enhancement in shear strength prediction; in the model, a size effect coefficient and a reinforcement-related term accounting for dowel action are explicitly incorporated. The size effect coefficient is calibrated using an extensive database comprising 572 test results collected from the literature, ensuring that the formulation captures the general trend of size-dependent behavior. To verify the predictive capability of the proposed model, nine RC deep-beam specimens were tested, and the comparison between predicted and measured results demonstrates improved accuracy and reduced scatter relative to existing methods. The results indicate that incorporating the coupled effects of size and longitudinal reinforcement is essential to rational shear design, and the proposed model provides a robust and practical tool for the analysis and design of RC deep beams, particularly for large-scale structures.