Abending momentis the reaction induced in astructural elementwhen an externalforceormomentis applied to the element causing the element tobend.[1][2]The most common or simplest structural element subjected to bending moments is the beam. The example shows a beam which is simply supported at both ends. Simply supported means that each end of the beam can rotate, therefore each end support has no bending moment. The ends can only react the shear load. Other beams can have both ends fixed, therefore each end support has both bending moment and shear reaction loads. Beams can also have one end fixed and one end simply supported. 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). In reality, beam supports are usually neither absolutely fixed nor absolutely rotating freely.The internal reaction loads in across-sectionof the structural element can be resolved into aresultant forceand a resultantcouple. For equilibrium, the moment created by external forces (and external moments) must be balanced by the couple induced by the internal loads. The resultant internal couple is called thebending momentwhile the resultant internal force is called theshear force(if it is transverse to the plane of element) or thenormal force(if it is along the plane of the element).The bending moment at asectionthrough a structural element may be defined as "the sum of the moments about that section of all external forces acting to one side of that section". The forces and moments on either side of the section must be equal in order to counteract each other and maintain a state ofequilibriumso the same bending moment will result from summing the moments, regardless of which side of the section is selected. If clockwise bending moments are taken as negative, then a negative bending moment within an element will cause "sagging", and a positive moment will cause "hogging". It is therefore clear that a point of zero bending moment within abeamis a point ofcontraflexurethat is the point of transition from hogging to sagging or vice versa.Moments andtorquesare measured as a force multiplied by a distance so they have as unitnewton-metres(Nm), orpound-foot or foot-pound(ftlb). The concept of bending moment is very important inengineering(particularly incivilandmechanical engineering) andphysics.Contents[hide] 1Background 2Computing the moment of force 2.1Example 2.2Sign conventions 3Computing the bending moment 3.1Example 3.2Sign convention 4See also 5References 6External linksBackground[edit]Tensileandcompressivestresses increase proportionally with bending moment, but are also dependent on thesecond moment of areaof the cross-section of a beam (that is, the shape of the cross-section, such as a circle, square or I-beam being common structural shapes). Failure in bending will occur when the bending moment is sufficient to induce tensile stresses greater than theyieldstress of the material throughout the entire cross-section. In structural analysis, this bending failure is called a plastic hinge, since the full load carrying ability of the structural element is not reached until the full cross-section is past the yield stress. It is possible that failure of a structural element inshearmay occur before failure in bending, however the mechanics of failure in shear and in bending are different.Moments are calculated by multiplying the externalvectorforces(loads or reactions) by the vector distance at which they are applied. When analysing an entire element, it is sensible to calculate moments at both ends of the element, at the beginning, centre and end of any uniformly distributed loads, and directly underneath any point loads. Of course any "pin-joints" within a structure allow free rotation, and so zero moment occurs at these points as there is no way of transmitting turning forces from one side to the other.It is more common to use the convention that a clockwise bending moment to the left of the point under consideration is taken as positive. This then corresponds to the second derivative of a function which, when positive, indicates a curvature that is 'lower at the centre' i.e. sagging. When defining moments and curvatures in this way calculus can be more readily used to find slopes and deflections.Critical values within the beam are most commonly annotated using abending moment diagram, where negative moments are plotted to scale above a horizontal line and positive below. Bending moment varies linearly over unloaded sections, and parabolically over uniformly loaded sections.Engineering descriptions of the computation of bending moments can be confusing because of unexplained sign conventions and implicit assumptions. The descriptions below use vector mechanics to compute moments of force and bending moments in an attempt to explain, from first principles, why particular sign conventions are chosen.Computing the moment of force[edit]

Computing the moment of force in a beam.An important part of determining bending moments in practical problems is the computation of moments of force. Letbe a force vector acting at a pointAin a body. The moment of this force about a reference point (O) is defined as[2]

whereis the moment vector andis the position vector from the reference point (O) to the point of application of the force (A). Thesymbol indicates the vector cross product. For many problems, it is more convenient to compute the moment of force about an axis that passes through the reference pointO. If the unit vector along the axis is, the moment of force about the axis is defined as

whereindicates the vector dot product.Example[edit]The adjacent figure shows a beam that is acted upon by a force. If the coordinate system is defined by the three unit vectors, we have the following

Therefore,

The moment about the axisis then

Sign conventions[edit]The negative value suggests that a moment that tends to rotate a body clockwise around an axis should have anegativesign. However, the actual sign depends on the choice of the three axes. For instance, if we choose another right handed coordinate system with, we have

Then,

For this new choice of axes, apositivemoment tends to rotate body clockwise around an axis. ...Computing the bending moment[edit]In a rigid body or in an unconstrained deformable body, the application of a moment of force causes a pure rotation. But if a deformable body is constrained, it develops internal forces in response to the external force so that equilibrium is maintained. An example is shown in the figure below. These internal forces will cause local deformations in the body.For equilibrium, the sum of the internal force vectors is equal to the applied external force and the sum of the moment vectors created by the internal forces is equal to the moment of the external force. The internal force and moment vectors are oriented in such a way that the total force (internal + external) and moment (external + internal) of the system is zero. The internal moment vector is called thebending moment.[1]Though bending moments have been used to determine the stress states in arbitrary shaped structures, the physical interpretation of the computed stresses is problematic. However, physical interpretations of bending moments in beams and plates have a straightforward interpretation as thestress resultantsin a cross-section of the structural element. For example, in a beam in the figure, the bending moment vector due to stresses in the cross-sectionAperpendicular to thex-axis is given by

Expanding this expression we have,

We define the bending moment components as