How to design Steel Plate-I Girder as per AASHTO LRFD. Kindly explain if different steel grades can be assigned to flange and web plates. 

In midas Civil, the steel design is supported only for the sections defined from the 'DB/User' tab in the Section dialog in which the built-up sections can be defined by the 'User' type as below. Both rolled sections and built-up sections can be designed by the 'Steel Design' function.


As for the hybrid steel sections (different steel grade for flange and web), it is not supported for the steel design. Just in case it is a composite plate girder (steel beam & concrete slab), the hybrid steel sections are supported for the composite girder design.