Shear and flexural strengths resulting from confinement of the compression regions in circular section structural concrete beams

This paper examines the effect of confinement of the compression regions on the behaviour of structural concrete beams with circular cross-sections. The programme of experimental work included eight 200 mm diameter beams traditionally designed to resist shear; however, the stimy, configurations inseveral of the beams were determined by the shape of the compressive force path. The results obtained from the test programme confirmed that the confined compression concrete regions in such beams are the main contributors to shear resistance and that the development of truss action is not necessarily required to transmit the applied loads to the support. It is concluded that the enhancement of the concrete strength in the compression regions due to the confining influence of the stirrups offsets the reduction in the flexural capacity of the beams due to the shear effects. The flexural capacity and ductility of the beams resulting from confinement were found to be higher than the corresponding values for traditionally detailed beams. A flexural-shear interaction model has been derived from a more general approach to determine the number of confinement stirrups required to prevent diagonal failures and also to evaluate the flexural strength of such beams. There was good agreement between the results obtained from the model and those obtained from the test programme. Eight structural concrete 200 mm diameter beams reinforced with semi-circular and/or circular shaped stirrups, were included in the investigation. The shape of the compressive force path was used to determine the stirrup configurations in several of the beams. The experimental results indicated that the ductility and strength of these beams were more than 50% and up to 32 % (depending on the level of confinement) greater than found in beams detailed according to current Code provisions. A flexure—shear interactive analytical model which recognizes the importance of the compression concrete in resisting lateral loads was found to closely predict the results obtained from the investigation.