Behaviour of reinforced concrete beams having recesses in tension and compression sides

A reinforced concrete beam with a recess is an example of a non-prismatic beam, which develops stress concentrations at the corner of the recess owing to the abrupt change in its depth. This entails appropriate detailing of internal reinforcement in order to avoid premature failure. This study presents the load transfer mechanism employing the strut-and-tie model (STM) for four non-prismatic beams with recesses in the shear span zone at both compression and tension sides, tested under a four-point loading scheme. The proposed STMs for the tested beams were developed based on both the cracking patterns near failure of experimentally tested beams and the developed stress contours based on linear static analysis. In addition, a simplified method is proposed to estimate the maximum mid-span deflection of simply supported beams with recesses. The results obtained reveal that the STM gave good predictions, with an average of about 86% of the ultimate experimental loads. Based on the results of the parametric study conducted, it was found that the most significant parameter was the recess depth from the viewpoint of stress concentration factor, cracking load and maximum crack width. Finally, recommendations and guidelines for the design of reinforced concrete beams with recesses are presented.