To find the average normal stress in any fixed member, only the forces acting on the member which are between the point of desire and the free end are taken into account. For example, to find the average normal stress in section BC of the member, only F2 needs to be considered, NOT F1. To find the average normal stress in section AB of the member forces F1 AND F2 must be taken into consideration. For multiple forces, take the summation. Obviously if forces are going in opposite directions, take the difference. If they are going in the same directions, add them. The average normal stress is then found by simply dividing the resulting force by the area at the spot where the average normal stress is desired. At member BC the average normal stress is abs(F2/A2) and at member AB the average normal stress is -abs((F1-F2)/A1). Typically, compression implies a negative value for the stress - it has a negative sign. Conversely, tension always implies a positive value for the stress.
