Shear Behaviour of Reinforced Concrete Deep Beams

RC deep beams are key safety critical structural systems carrying heavy loads over short span, such as transfer girders in tall buildings and bridges. Current design provisions in codes of practice fail to predict accurately and reliably the shear capacity of RC deep beams and in some cases they are unsafe. This work aims to develop a better understanding of the behaviour of RC deep beams and governing parameters, and to improve existing design methods to more accurately predict the shear capacity of such members.
An extensive experimental programme examining 24 RC deep beams is carried out. The investigated parameters include concrete strength, shear span to depth ratio, shear reinforcement and member depth. To develop a better insight on the distribution and magnitude of developed stresses in the shear span, finite element analysis is also performed. The microplane model M4 is implemented as a VUMAT code in ABAQUS to represent the behaviour of concrete in a more reliable manner and validated against experimental tests on RC deep beams. This model is utilised in a parametric study to further investigate the effect of concrete strength, shear span to depth ratio and shear reinforcement.
The experimental and numerical results show that concrete strength and shear span to depth ratio are the two most important parameters in controlling the behaviour of RC deep beams, and that shear strength is size dependent. The analysis also shows that minimum amount of shear reinforcement can increase the shear capacity of RC deep beams by around 20% but more shear reinforcement does not provide significant additional capacity.
A lateral tensile strain based effectiveness factor is proposed to estimate the strength of the inclined strut to be used in strut-and-tie model. Additionally, node factors to estimate the developed strength in different type of nodes are proposed. The proposed model is evaluated against a large experimental database and the results show that it yields more accurate and reliable results than any of the existing models. The model is characterized by the lowest standard deviations of 0.26 for both RC deep beams with and without shear reinforcement and accounts more accurately for all influencing parameters.