Assessment of the size effect on shear strength in reinforced concrete beams and slabs

Abstract

Failures due to shear, unlike flexure, are sudden and catastropic. The design recommendations of several codes of practice are based on empirical relations derived from laboratory tests. It has been well established that the shear strength of concrete members decreases as the overall depth of the element increases-commonly referred to as the size effect. Over the years, several theories, have been proposed to explain this behavior, such as the energetic size effect law (SEL), critical Shear crack theory (CSCT), modified compression field theory (MCFT), and the and Weibull-based approaches. These theories are compared and their limitations have also been identified. Of these theories, only the energetic size effect law (SEL) is grounded in fracture mechanics and energy release principles. The SEL has been rigorously validated through experimental and theoretical studies and has been incorporated in ACI 318: (2019) for both shear and punching shear design. The provisions in different national codes, based on different theories, for determining the size effect factor are briefly explained and compared. It is seen that the size effect factor used in ACI 318: (2019) code has a sound theoretical basis. The values of size effect factor as per different codes has been compared by taking a beam of depth 600 mm, and also for different depths of beams. Based on these comparisons, the SEL theory is recommended to be used in future codes, such as the Draft IS 456 codes.