For buildings that are four stories or more in height, what is the required fire resistance rating for enclosure?

The required fire resistance rating for the enclosure of buildings that are four stories or more in height is typically specified as two hours. This requirement is grounded in the need to provide a higher level of protection due to the increased risks associated with taller structures. Buildings of this height often contain larger quantities of combustible materials and present greater hazards in the event of a fire, thus necessitating a more robust fire-resistance approach to help contain fires, protect structural integrity, and provide occupants with sufficient time to evacuate safely.

The two-hour rating ensures that structural elements such as walls, columns, and floors have the necessary durability to withstand the impact of fire for a longer duration, minimizing the potential for structural failure and allowing fire suppression efforts to take place more effectively. In tall buildings, this requirement is crucial because it assists in controlling the spread of flames and smoke between different floors, which could be critical in protecting the lives of occupants and the firefighting personnel involved.