How do we define a  composite section (prestressed concrete I girder and concrete slab) in Midas Civil with unequal slab widths on either side of girder centerline? 

Answer:

For this purpose, the Section Property Calculator (SPC) would have to be used.  You can either import a dxf drawing file with such unsymmetric section to SPC or directly generate the geometry in SPC.
A tutorial on how SPC could be used is attached. Also, please note that only "lines" from a dxf file could be imported in Midas. Thus, if our model has polyline, we have to change them into lines and the geometries can be imported.