What type of assembly is usually required for a 3-4 hour FT rating?

For a 3-4 hour fire-resistance rating, an assembly classified as an “EJ” or expansion joint is often required. This type of assembly is engineered specifically to accommodate structural movement and reduce the potential for fire spread, while maintaining integrity in protecting against fire for an extended duration. Expansion joints are typically detailed to ensure that they can withstand thermal expansion and contraction, which can be critical in longer duration ratings.

A typical drywall assembly, while useful for various fire ratings, may not provide the necessary durability and fire resistance required for longer durations like 3-4 hours without additional enhancements or specifications. Furthermore, simply having no assembly or relying on a 2-hour assembly would not meet the necessary criteria for a 3-4 hour fire-resistance rating, as they do not offer sufficient protection or structural integrity according to code requirements. Therefore, an expansion joint assembly is the correct approach for achieving the desired fire rating in construction projects where such specifications are required.