For equilibrium, the moment created by external forces/moments must be balanced by the couple induced by the internal loads. The internal reaction loads in a cross-section of the structural element can be resolved into a resultant force and a resultant couple. In reality, beam supports are usually neither absolutely fixed nor absolutely rotating freely. The simplest type of beam is the cantilever, which is fixed at one end and is free at the other end (neither simple or fixed). Beams can also have one end fixed and one end simply supported. Other beams can have both ends fixed (known as encastre beam) therefore each end support has both bending moments and shear reaction loads. The diagram shows a beam which is simply supported (free to rotate and therefore lacking bending moments) at both ends the ends can only react to the shear loads. The most common or simplest structural element subjected to bending moments is the beam. In solid mechanics, a bending moment is the reaction induced in a structural element when an external force or moment is applied to the element, causing the element to bend. Force tending to bend a structural element Shear and moment diagram for a simply supported beam with a concentrated load at mid-span.
