Figure represents the distribution of principal stresses across the span of a homogeneous concrete beam. The direction of the principal compressive stresses takes the form of an arch while the tensile stresses have the curve of a catenary or suspended chain. Towards mid-span, where the shear is low and the bending stresses are dominant, the direction of the stresses tends to be parallel to the beam axis. Near the supports, where the shearing forces are greater, the principal stresses are inclined at a steeper angle, so that the tensile stresses are liable to cause diagonal cracking. If the diagonal tension exceeds the limited tensile strength of the concrete then shear reinforcement must be provided.
