The characteristics of internal reaction for a beam section are, as stated in the earlier chapters, the elementary internal reactions transmitted by the section itself. In the case of a plane beam (line of axis contained in the plane), there are three characteristics of internal reaction (Figure 5.1):
1. Axial force, which is the component of the force tangential to the axis of the beam 2. Shearing force (or, more simply, shear), which is the component of the force perpen-
dicular to the axis of the beam 3. Bending moment, which is the moment of the force that the two portions of beam
transmit to one another, with respect to the section being considered
The usual conventions regarding the signs of the characteristics in the plane are the following:
1. Axial force is taken to be positive when it is tensile. 2. Shearing force is taken to be positive when it tends to rotate the segment of the beam
on which it acts in a clockwise direction. 3. Bending moment is taken to be positive when it stretches the lower fibres and com-
presses the upper fibres of the beam.