For one-hour fire-rated wood joist assemblies, how do I compensate using a 2x8 rather than a 2x10?

• One-hour rated floor/ceiling assemblies that are derived by ASTM E119 testing are typically constructed with 2x10 or 2x12 joists. Joists of lesser depths, such as 2x8, are generally not permitted to be substituted unless the assembly is retested. However, with approval of the building official, the empirical methods in IBC 721.6 and AF&PA's DCA4 can be used to estimate fire endurance times for 2x8 floor/ceiling assemblies without testing.