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- Summary:
- Signs (directions) of forces on FBDs are not matching up with the conventions used for tensile and compressive stresses

I have recently learned that, by convention, axial tensile stresses are positive and axial compressive stresses are negative. I am having some trouble incorporating this information into my FBDs that are used to find the axial forces leading to these stresses.

For instance, from the attached FBD, I ended up finding Fab to be -500 N, which indicates that the force is actually compressive (directed toward point A) and not tensile as drawn in the FBD. However, if I had originally drawn Fab to be in compression, then wouldn't I find Fab = +500 N?

If I had drawn the latter kind of FBD and used Fab = +500 N to find the axial stress in Fab, I would end up with a positive value for the stress, which I know is wrong because the stress is compressive. In this case, how can I make the stress a negative value?

Could someone please help me figure out what I am missing here?

Thanks!

For instance, from the attached FBD, I ended up finding Fab to be -500 N, which indicates that the force is actually compressive (directed toward point A) and not tensile as drawn in the FBD. However, if I had originally drawn Fab to be in compression, then wouldn't I find Fab = +500 N?

If I had drawn the latter kind of FBD and used Fab = +500 N to find the axial stress in Fab, I would end up with a positive value for the stress, which I know is wrong because the stress is compressive. In this case, how can I make the stress a negative value?

Could someone please help me figure out what I am missing here?

Thanks!