Shear size effect in simply supported RC deep beams

The shear size effect refers to the phenomenon that the shear strength of reinforced concrete (RC) beams decreases as the beam depth increases. The shear strength of RC deep beams is sensitive to boundary conditions (in this case, load-and support-bearing plate (or column) sizes), which in turn affect the shear size effect of deep beams. In this study, to separate and identify the influences of the bearing plate size on the shear size effect, existing deep beam tests on shear size effect are classified. It is verified that the shear size effect of deep beams with a fixed bearing plate size is stronger compared to deep beams with proportionally varied plate sizes. By using a non-linear analysis software ATENA based on concrete fracture and plasticity theory and a mechanical model called cracking strut-and-tie model (CSTM), the shear size effects of the classified test groups are accurately predicted, and the maximum height of each beam group is extrapolated to 4 m. Through in-depth analysis of the finite element model and CSTM results, it is inferred that the possible reasons that lead to the shear size effect of RC deep beams are: (1) bearing plate size effect, that is the reduced relative strut width caused by the disproportionately varied bearing plate size with the beam height; and (2) beam depth effect, which refers to the deterioration of the shear transfer strength by aggregate interlock of the critical shear crack due to the increase of the beam depth. In addition, based on the prediction results for the 4 m high beams and the existing test results, the STM in the ACI 318-14 is evaluated. The results show that the ACI STM can not inherently consider the beam depth effect, resulting in the safety of large-size deep beams designed according to the ACI STM is lower than that of small-size deep beams. For this reason, proposals for considering the beam depth effect in STM design are put forward.