Size effect on shear strength of deep beams: Investigating with strut-and-tie model

Since a diagonally cracked deep beam behaves as a tied arch, the conventional plane section remaining plane approach is not applicable to analyses of deep beams. Besides, for beams without web reinforcement, it has been shown that shear strength decreases as member size increases. This is associated with a phenomenon called size effect. In this study, the causes of size effect on shear strength of deep beams are investigated using both the strut-and-tie model (STM) and finite element model. The study shows that size effect in concrete beam arises primarily from an inappropriate adoption of the shear transfer concept for steel beams. In addition, size effect also depends on secondary factors such as the geometry of strut, and the spacing and diameter of web reinforcement. The modified STM is verified with three published case studies comprising a total of 36 beams. Generally, the predictions are accurate and consistent, with a uniform safety margin for different member sizes considered.