Experiment to mitigate size effect on deep beams

Size effect is a phenomenon in reinforced-concrete beams associated with a reduction of shear strength owing to an increase in member size. It is postulated here that the size effect is related to the effectiveness of strut-and-tie model (STM), the mobilisation of which largely depends on the strut geometry and the strut boundary conditions. Strut geometry is governed by the size of the loading and support plates as well as the beam depth, while strut boundary conditions are governed by the spacing and diameter of web reinforcement transverse to the inclined struts. The test programme provided experimental evidence to the hypotheses obtained from STM. Test specimens consisted of two groups of four geometrically similar specimens. The first group consisted of beams without any web reinforcement; the dimensions of the loading and support plates were kept constant. The second group corresponded to the first group, except that they had (a) orthogonal web reinforcement and (b) proportionally increased loading and support plate widths. It was observed that the ultimate shear strengths reduced sharply for the first group of beams without web reinforcement, showing significant size effect. For the second group of beams the ultimate shear strengths were, however, size independent.