I understand that you have derived the MOI of this section in AUTOCAD and then you have defined this as a prismatic section in Staad. You have only taken the IY or IZ value from AUTOCAD and then provided thes information in Staad Prismatic section, so Staad does not know its actual geometric information. Moreover, the actual geometry for any non standard section even if you would have generated in STAAD USER TABLE would have been discarded by the program. Staad extracts and uses only the sectional properties like IX, IY, IZ, AX, AY and AZ which are only required for performing the analysis and hence you can't see the cross section to know the stress at different profile point in that section. Now, to know if AUTOCAD determines the correct sectional properties or not, you may want to redraw this profile in Staad USER TABLE (GENERAL) or in the Sectional Wizard-Free Sketch mode and compute the properties.
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