Shaft enclosures that connect 4 or more stories must have a minimum fire-resistance rating of how many hours?

Shaft enclosures connecting four or more stories are critical fire protection features in commercial buildings. They provide a means of vertical transport for both people and emergency services while containing potential smoke and fire. A fire-resistance rating of 2 hours for these enclosures is mandated to ensure that, in the event of a fire, the structure can maintain its integrity long enough to allow for safe evacuation and to protect adjacent areas from fire spread.

This 2-hour requirement is based on building codes that reflect extensive research and historical data on fire behavior and structural performance. The enclosure must withstand high temperatures and prevent the passage of flames and smoke for a significant duration, which is essential for life safety and property protection. This rating allows sufficient time for emergency responders to address the fire, thereby improving the overall safety and reliability of the building's design.

Other options reflect lower timeframes that would not provide adequate protection in such critical scenarios, which is why they would not comply with the established standards for shaft enclosures in multi-story buildings.