Here is the FEMA Page (I hope). The only pressures on the floor are +- GCpi, internal pressures. No external uplift or downward force, according to FEMA.http://files.engineering.com/getfile.aspx?folder=e357d272-70c5-49d4-a7be-e3a60658cdf8&file=fema55_volii_ch8.pdf
Look at ASCE 7-10 pp 267- 270. This has design notes for Free roofs. I would try using monoslope free roof with 0 (zero) roof angle.
The Corps of Engineers have some design guides for open structures, but I can't locate it right now.