# 4.2: Basic Definitions

- Page ID
- 42957

## Normal Force

The normal force at any section of a structure is defined as the algebraic sum of the axial forces acting on either side of the section.

## Shearing Force

The shearing force (SF) is defined as the algebraic sum of all the transverse forces acting on either side of the section of a beam or a frame. The phrase “on either side” is important, as it implies that at any particular instance the shearing force can be obtained by summing up the transverse forces on the left side of the section or on the right side of the section.

## Bending Moment

The bending moment (BM) is defined as the algebraic sum of all the forces’ moments acting on either side of the section of a beam or a frame.

## Shearing Force Diagram

This is a graphical representation of the variation of the shearing force on a portion or the entire length of a beam or frame. As a convention, the shearing force diagram can be drawn above or below the *x*-centroidal axis of the structure, but it must be indicated if it is a positive or negative shear force.

## Bending Moment Diagram

This is a graphical representation of the variation of the bending moment on a segment or the entire length of a beam or frame. As a convention, the positive bending moments are drawn above the *x*-centroidal axis of the structure, while the negative bending moments are drawn below the axis.