It is quite difficult to calculate the plastic section modulus for compound sections, unless some experimental data are available in which case the user must enter manually the determined Wpl values.
For compound sections, Advance Design use by default the elastic values for plastic sectional modulus (Wpl). If Wpl cell has the information entered manually by user, then it uses this value instead of Wel.
Also, for compound sections the user must impose manually the cross-section class as Eurocode classification doesn't include these types of cross-sections. You can impose the class in the linear element properties window (Design Experts section > Steelwork Design > Cross section class).
