Horizontal Shearing Stress
Let us consider a differential length dx of the beam shown
For the upper shaded portion of the beam, the forces acting are the total normal forces FR and FL due to the bending stresses to the left and to the right of the beam. These forces will be resisted by the shearing force fvb dx acting at the boundary surface between the shaded and the unshaded portions.
For equilibrium of the upper shaded portion
But where represents the shear at the section in , and represents the first moment of an area of the shaded section about N.A. in mm3 which we will denote as , then