This video is of a test burn conducted on April 28, 2012 by the Dutchess Community College Fire Science program, in Poughkeepsie, NY. The intent of the test burn was to compare the different floor assembly's ability to withstand the ravages of fire. The wooden i beam floor was built to support the same weight load as the 2 x 12" floor. The fireload for each was exactly the same amount of hay and wooden pallets. They were ignited at the same time.
Granted, neither floor assembly was protected by sheetrock but this test indicates how much quicker a wooden i beam can fail in a fire. Also notice the heavy/black smoke that is immediately produced as the wood I beam gets heated up. This smoke is the glue in the OSB off gassing. This smoke is very toxic contains formaldehyde) and is very flammable.
Wood I beams are very strong, until a fire starts....... Then they can fail very quickly and without any warning.