Shear capacity of reinforced concrete beams

Reinforced concrete beams are weak in carrying shear and tensile stresses through the web. As the result of various combinations of shearing and flexural stresses, tensile stresses are developed in diagonal directions. Failures due to the inability of a reinforced concrete beam to resist diagonal tensile stresses are characterized by their sudden and violent nature. Laboratory tests have shown that simply supported under-reinforced rectangular concrete beams under concentrated loads may fail by either splitting-bond when no end anchorage is furnished; by diagonal tension when no web reinforcement is supplied or by a "ductile" flexural failure when adequate web reinforcement is provided. It was found that the first diagonal cracking load is not affected by the presence of hooks, or by the use of vertical stirrups. The concrete strength and the shear span ratio a/d are factors influencing most the first diagonal cracking load. It was also found that vertical stirrups are not effective until the first diagonal crack develops and do not prevent the development of diagonal cracks but help in keeping cracks from operating excessively. It is recommended that the nominal shearing stress, as a measure of diagonal tension in simply supported rectangular reinforced concrete beams, be calculated by the equation [See PDF for Equation]