The beam element is assumed to have a constant cross-section, which means that the cross-sectional area and the moment of inertia will both be constant (i.e., the beam element is a prismatic member). If a beam is stepped, then it must be divided up into sections of constant cross-section, in order to obtain an exact solution. If a beam is tapered, then the beam can be approximated by using many small beam elements, each having the same cross-section as the middle of the tapered length it is approximating. The more sections that are used to approximate a tapered beam, the more accurate the solution will be.
The moment of inertia is a geometric property of a beam element, which describes the beams resistance to bending and is assumed to be constant through the length of the element. The moment of inertia can be different along different axes if the beam element is not symmetric, we use the moment of inertia (I) of the axis about which the bending of the beam occurs
Where (Iz) refers to the moment of inertia, resisting bending about the "z" axis and (Iy) about the "y" axis.