Bending Moment

When the external force is applied on the object, the reaction occurs (In the structure) which force the object to bend is known as Bending Moment. It is a reaction induced in a structural element by the moment and forces the element to bend. The simplest structural member which mostly faces the bending moment is the beam. In short form, it is termed as BM.

In simply supported beams, the bending moment is maximum at the mid of the beam span and minimum or zero at the end support of the beam. It is measured in (kNm).

In Beam, the different types of bending moment occur in different sections of the span. In simply supported (UDL) beams the moment is maximum at the mid and decreases toward the ends and become zero when reaches at the support.

Different types of the moments we face in the beams which are,

  1. Hogging (Negative)
  2. Sagging (Positive)


The hogging is also called negative moment and defines a beam which curves upward in the middle direction.

  • Mainly occur in cantilever beams.
  • In multi-span continuous beam, this moment occurs at its intermediate supports.
  • This one side moment is in anti-clockwise (L.H.S) and the other side is the clockwise direction (R.H.S).




The sagging moment is also known as a positive moment. Bending moment is the most important factor that should be considered and calculate when you are designing a beam, etc.

In simple words, the bending moment forces the member to bend due to an external or internal moment and it can be calculated as the perpendicular force multiplying by the distance from the force. If the tensile stresses are greater than the yield stresses than failure in bending will occur. When forces are applied on beam than the beams deform and this deformation is called as bending of a beam due to the shear force and bending moment. These stresses are produced due to bending is called flexure stresses.


Point of Contraflexure:

In beams, a point of zero or minimum bending moment is a point of contraflexure. It is a location at which the no bending moment will occur or where BM changes its sign is called a point of contraflexure.

point of contraflexure

Types of bending stresses in beams:

1.Pure bending stresses:

When the bending occurs solely because of coupling on its ends, it is called pure bending. In pure bending stresses, the no shear force stresses are produced in a beam But the normal stress should be produced in this case, the normal stress causes failure to beam.

pure bending moment

2.Simple bending stress:

Bending will occur due to the self-load and external load of the beam, it is called as simple bending stress. Both shear force stress and normal stress are produced in simple bending stress. It is also known as ordinary bending.

simple bending moment

Result both shear and bending stresses.

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