Firstly, let's suppose we have bars with intermediate actual loads.

So in this case, we have a rod here A B C D.

Which is subject to an upward load PB at this point here,

a downward load PC and another downward load PD.

So this we would say is piece wise constant.

In other words, the conditions are constant within each segment AB, BC, CD.

The loading is the same cross sectional area material properties are the same.

So it's piecewise constant and in this case,

we analyze this by separating it, drawing a free body diagram for

each segment and finding the force in each segment by statics.

So for example, in the segment CD here.

Shown here, the internal force N3 in that segment

is just equal to the downward load PD and et cetera.

We can find it in the other segments as I'll show it in example soon.