Right that’s external forces but what about forces acting internally?

Take this column for example:

What are the forces acting within it to stop it crumbling into a heap, whats keeping it in equilibrium?

The column is under compression so it pushes up against the ceiling and down against the floor to balance the forces acting on it:

As it’s under compression it is classes as a strut.

You can work out the exact forces required in a similar way to the graphical analysis I used in a previous post. As they’re acting at right angles to each other, the results are all the same, if they were at an angle you’d need to use a triangle of forces to work it out.