In a high rise, smoke proof enclosures must be enclosed from highest point to lowest point by fire barriers with a rating of at least:

• A. 3 - 5 hours.
• B. 1-2 hours.
• C. 3-4 hours.
• D. 4-5 hours

Answer: (B)

The key idea is that a smokeproof enclosure acts as a protected vertical shaft for egress, and it must be surrounded by fire barriers that can withstand fire along the entire height of the building. This continuity from top to bottom keeps smoke and heat out of the enclosure long enough for people to evacuate and for responders to work.

The minimum rating of 1–2 hours provides a practical balance: it ensures the enclosure remains intact for a sufficient period (at least an hour, up to two) to maintain a smoke-free path during the critical early phase of a fire. Higher durations, like 3–5 hours, are typically beyond what is required for standard smokeproof enclosures and are used for other, more demanding fire-resistance scenarios.