For a structure to remain in static equilibrium, the bending moments induced by external forces must not exceed the moment capacity of the elements upon which they act.
A bending moment will cause both compressive and tensile stress in a cross section. Because plain concrete is much weaker in tension than it is in compression, it will tend to fail on its tension face when subjected to a bending moment. This is why tensile reinforcement is used in a lot of concrete structural elements.
A bending moment diagram is a useful way to visualise the distribution of bending moments in a structural element. It gives the magnitude of the bending moment at any given point in the element.
In a horizontal member, such as a beam or floor, the following statements are true: