Experiment to mitigate size effect on deep beams

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.

Size effect in RC deep beams: Experimental investigation and STM verification

“…It indicates that strut geometry, which is determined by the dimensions of loading and support plates and the strut length, will influence ultimate shear strength and lead to apparent size effect. This conclusion has also been confirmed in a recent experimental study (Tan et al [16]). …”

“…It was argued (Kotsovos and Pavlovic [9,23], Tan and Cheng [15], Tan et al [16]) that the conventional definition of ultimate shear strength of V /(bd) as indicative of size effect is inappropriate for concrete deep beams. This equation originating from steel beams, with more or less uniform shear flow in the steel web, does not reflect the arch action in a deep beam.…”

“…The loading and support plate size will affect the dimension of diagonal strut and nodal zone (potential concrete failure volume), thereby considerably influencing the ultimate shear strength. Some researchers (Tan and Cheng [15], Tan et al [16]) had identified the dimensions of loading and support plates as one of the key factors that influence size effect. They explained it through the concept of a strut-and-tie approach.…”

“…But this enhancement varies with the effectiveness of arch action, which in turn depends on the geometry and boundary conditions of diagonal struts [15]. The presence of web reinforcement can mitigate this variation of shear strength in deep beams [16,17].…”

Experimental Study on Shear Capacity of High Strength Reinforcement Concrete Deep Beams with Small Shear Span–Depth Ratio

“…In recent years, many experiments and numerical analysis have been carried out the influence of different design parameters on the shear capacity of deep beams. In reference [20][21][22][23], based on the research of the failure mode of deep beams, the influence of concrete strength, shear span-depth ratio, reinforcement ratio, loading plate and other factors on the shear capacity of deep beams has been conducted. The influence of high strength concrete on the shear capacity of deep beams has been discussed in references [24,25].…”