In this example, we have a beam with a bending force Fz = -50KN. If the beam is placed on these supports, the superior fiber is compressed, the inferior fiber is tensioned and the middle part does not take the normal force Fxx = 0 or Sxx = 0. In this situation, if the beam is off center, the distribution of the normal force is different.
This way it is possible to view SxxMax, that represents the most tensioned part (or less compressed) of an abscissa along the element and SxxMin, that represents the most compressed part (or the less tensioned), of an abscissa along the element.
As can be seen in the two graphs of SxxMax and SxxMin, the Sxx cross-section stresses at different abscissas, respectively X = 0m, X = 2m and X = 6m.