RE: plate girder design -AISC-360-05/10

The answers to your questions are

You can design plate girders in STAAD.Pro using the AISC 360-05 or AISC 360-10 ( CODE AISC UNIFIED 2005 or CODE AISC UNIFIED 2010 ). It was not possible in the older AISC 9th edition ( CODE AISC ) implementation.

The cross sectional properties at all intermediate section locations are calculated by the software for tapered members and the code provisions are checked along the member length, considering the corresponding properties at those locations.

Please ensure that you have specified the correct Ly and Lz. If it still fails in slenderness, the only option would be use a bigger section. If you think slenderness check is not important for the members in question and would like to bypass it, you may define a parameter called MAIN, set it to a value of 1 and assign it to those members. In that case STAAD.Pro would bypass the slenderness check for those members and would only check the other applicable design criteria.