For one-hour fire-resistance-rated wood joist assemblies, how do I compensate using a 2×8 rather than a 2×10?

One-hour fire-resistance-rated floor/ceiling assemblies that are derived by ASTM E119 testing are typically constructed with 2×10 or 2×12 joists. Joists of lesser depths, such as 2×8, 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 AWC’s DCA 4 can be used to estimate fire endurance times for 2×8 floor/ceiling assemblies without testing.

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