Open Stair in Mixed-Use Building
A three-story building has a ground-floor restaurant (A-2) and offices (B) on the second and third floors. The ground floor is 12,000 sq ft with an occupant load of 240. The upper floors are 10,000 sq ft each, with occupant loads of 100 per floor. The building is Type V-B construction and fully sprinklered. The owner wants to remove the existing 1-hour fire-resistance-rated separation between the ground floor and the second floor to create an open communicating stair. The owner argues that because the building is sprinklered, the separation is not required. The designer proposes to treat the building as a nonseparated mixed occupancy. However, the open stair would connect the restaurant to the office space, potentially allowing smoke to spread. The building official must evaluate the code provisions for mixed occupancies, vertical openings, and exit access stairways to decide whether the open stair can be approved.
1. Under what conditions, if any, can the 1-hour separation between the A-2 and B occupancies be removed and the building be classified as a nonseparated mixed occupancy?
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The building official should first determine whether the building qualifies as a nonseparated mixed occupancy under IBC Section 508.3. For nonseparated occupancies, the entire building must comply with the most restrictive requirements of the occupancies involved, including fire-resistance, egress, and fire protection systems. Here, A-2 is generally more restrictive than B for fire safety. Because the building is fully sprinklered, some trade-offs may be permitted, but the official must verify that the structural frame, exterior walls, and egress capacity meet the A-2 requirements. Removing the 1-hour separation also creates a vertical opening. The open stair would be an exit access stairway and must comply with IBC Section 1019. If the building is sprinklered and the stair connects only two stories, an exception may allow it to be open. However, the official must confirm that the stair is not a required exit, that smoke spread is controlled, and that the restaurant’s cooking smoke does not compromise the office space. The official should require a smoke control or detection system and may limit the open stair to a single-story connection.
2. What life-safety concerns should the building official evaluate before approving the open stair, particularly regarding egress and smoke spread?
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The building official must evaluate egress capacity and smoke spread. The open stair could serve as a second means of egress from the restaurant, but the official must confirm that the stair width and travel distance comply with the occupant load of 240 on the ground floor. If the stair is part of the required egress, it must be enclosed or protected as an exit. For smoke spread, the official should require smoke detection and sprinkler coverage at the opening and may require a smoke barrier or draft curtain. The restaurant produces grease-laden vapors, so the stair must not be located where smoke can accumulate. The official should also verify that the open stair does not create a path for smoke to reach the office floors through the unsealed floor opening. If the building is fully sprinklered and a smoke control system is provided, the risk may be acceptable, but the official has discretion to impose conditions. The final decision should balance the owner’s desire for openness with the life-safety of occupants on all floors.