Internal forces

The ability to determine internal forces is a very important element of our knowledge, because objects such as strength of materials or various types of mechanics are based on these calculations.
Calculation of internal forces in structures results in drawing diagrams of these forces. In the next stages of learning, you will have to draw such charts from memory (that is, without making calculations). It is good to understand the basics so that you do not have problems later.
In the further part of the guide, I will put the rules for drawing internal force charts from memory, but do not run so far forward. Let’s see what internal forces are and how they work.

Determination of internal (cross-sectional) forces in structural elements.

During calculations, we must determine the values of the three following cross-sectional forces, namely N, T, M, these are: the normal, cutting and bending moment.

Let’s see how the normal force works on the beam.

It is very easy to see that normal forces are those that work parallel to the beam’s axis. When the force causes the bar to stretch, then we get positive values, when the force causes the compression of the bar, the values of force N will be negative.

Now let’s see how shear forces act on a beam.

In the case of cutting cross-sectional forces, we see that they act perpendicular to the axis of the beam. When the arrowheads are facing the beam, we get negative values, and when they are reversed we have positive values.

Let’s see how the last internal force M works – the bending moment.

In this case, we see that forces cause bending of the beam. The value of the bending moment depends on how the beam is bent. I recall tension(+), compression (-).

I suggest to remember these drawings, because when calculating beams, frames, trusses or arches, usually students make a mistake, that is, they take turns of individual forces with bad marks. So much for the entrance to the internal forces, in the next step we will learn the basics of drawing diagrams of cross-sectional forces.