In many cases, fire endurance tests of seemingly comparable floor and roof assemblies have yielded different results. The author reports on a current study which, thus far, has involved analyzing thermal moments developed in floor assemblies due to fire exposure, evaluating the distribution of thermal forces to the adjacent building frame, and deriving the resultant reaction of the structure. He discusses techniques for determining the onset of structural collapse mechanisms, using mathematical expressions for the change in modulus of elasticity and yield stress as a function of temperature.