Floor load with a floor group

I'm trying to add load on a platform for the cross section below.

I'm wondering, since drawing this platform on STAAD mean that i must draw the platform with the member centred in the middle of the HSS and add the HSS width in the Properties. Does floor load and/or floor group take into account only the part that is drawn, or does it also take into account the width of the members of Properties (in this case, it will be a length difference of 2 half HSS)?

Thanks !