No abstract

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.

This paper investigates the effects of the shear span:depth a/d ratio and effective depth d on the behaviour and shear strength of large reinforced concrete deep and shallow beams. A total of seven large- and medium-sized specimens were tested to failure under two-point symmetric top loading to determine their diagonal cracking and ultimate shear strengths. It was found that the beams' ultimate shear strength reduced sharply when the overall height increased from 500 to 1000 mm. The diagonal cracking strength was, however, not size dependent. The shear strength reduction is related to the a/d ratio: that is, at a critical a/d value, the size effect becomes very pronounced; beyond it, the size effect is not as apparent. After diagonal cracking has occurred, deep beams behave like a tied arch, with uncracked concrete as compression struts and main longitudinal reinforcement as tension ties. A finite element program ‘WCOMD’ was used to supplement the experimental investigation on size effect. Based on test results, finite element modelling (FEM) and a tied arch analogy, the myth of size effect is unravelled.