von mis stress, principal stress, and yield stress


I try to design a bin using the plate element. When I check the stress summary, Staadpro provided the max von mis stress (top and bottom) and max principal stress (top and bottom). My questions are:

 1) The yield stress Fy of the material I used is 245Mpa. Do both the max von mis stress and max principal stress have to be less than 0.9 Fy to consider the plate will not yield? I did some research. Someone says the for ductile material, we only need to ensure the max von mis stress is less than 0.9 Fy. The max principal stress is to be compared with tensile strength. Is this correct?

2) How are the top and bottom von mis or principal stress calculated? I know how the von mis stress and principal stress are calculated from Sx, Sy, Sxy, Mx, My and Mxy. This should only result in one value. Why the top and bottom values are different? I found the top stress calculation here 'EX. US-18 Stress Calculation for Plate Elements EX. US-18 Stress Calculation for Plate Elements (bentley.com)' Is the bottom stress calculated in a different way?

Thank you very much for the help. I am looking forward for your answer.