Mechanisms of Shear Resistance of Concrete Beams

The nature of shear resistance of reinforced concrete beams is examined, and it is shown that shear may be resisted in two different ways, namely by beam and arch actions. At the diagonal cracking load of the shear span, beam action breaks down. Unless the beam contains pre stressed reinforcement arch action cannot develop to an appreciable extent prior to the diagonal cracking. It is shown that the strength of beam action depends largely on the mechanism of shear transfer across the crack by interlocking of the aggregate particles, and to a lesser extent by dowel action of the reinforcement. Direct tests on aggregate interlock action show that this shear transfer increases with concrete strength, and a reduction in crack width. From tests the dowel capacity of bottom bars is found to be considerably greater than that of top bars. Beam tests illustrate the contradictions of these different mechanisms.