From what I understand...

The "Worst Case Stress"  in a beam analysis the software calculates the highest stress at a critical point on the cross-section by combines the axial and bending stresses due to moments M1 and M2.

In a beam only model - the worst case stress does a good job of calculating the absolute value of the axial stress, bending dir 1, and bending dir 2.

This value is ALWAYS positive for all elements.

Ill give an example:

For element # 300:

Axial Stress = -500 psi

Bending Stress 1 = -20,000 psi

Bending Stress 2 = +2500 psi

Worst Case Stress = |-500| + |-20,000| + 2500 = 23,000 psi

In a mixed mesh model (beam and shell) -  I have placed an shell part in the model (this part has no impact on element 300).  I am confused.  I have worst case stress values that are negative.  Even if I have an insignificant shell part in the model, the Worst Case Stress scale is now in the negative region.

For element # 300:

Axial Stress = -500 psi

Bending Stress 1 = -20,000 psi

Bending Stress 2 = +2500 psi

Worst Case Stress = -500 + -20,000 + (-1) * 2500 = -23,000 psi

Can someone help with what is going on here?