I seem to recall the AISC having a bit of a 'blurb' in one of their journals (be about 10 years ago) and it was likely an article where yield line was used in conjunction with the main topic; I couldn't find it in a quick search of their index.
You can try a regular yield line approach with the sagging lines following the web and flanges and 'fiddle' with the hogging ones...
The plastic section modulus for a plate (rectangular section) is 1.5 times the elastic section modulus and there is a significant strength gain (no stabillity issue for plates in this application, the ultimate in compact sections).
The report is authored by Murray ( I think Thomas). Another AISC article, by Dranger, offers a little more detail and arrives at the same solution. There are some yield line pattern web sites.
