Mixed-Use Podium Height and the Fire Wall That Wasn't
A developer proposes a 6-storey, 21 m tall building. The ground floor and second floor (combined 7.2 m floor-to-floor, 2,900 m² per storey) hold retail shells and a restaurant. Floors 3 through 6 (3.0 m floor-to-floor, 1,450 m² per storey) hold 48 residential suites. The architect argues the building is a single mixed-use building because the two occupancies are separated by a 2-hour horizontal assembly at the third-floor slab. The fire official notes that the exits from the residential floors pass through a shared lobby on the second floor before reaching the street, and that the retail tenants want open cooking in two suites. The developer wants to avoid sprinklering the residential floors to save cost and asks whether the 2-hour slab alone permits treating the upper floors as a separate building for code purposes.
1. Is the 2-hour horizontal assembly alone sufficient to treat the upper residential floors as a separate building? What additional conditions would be required?
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No. A horizontal assembly separates occupancies or creates separate fire areas, but treating part of a building as a separate building for height/area purposes requires a genuine fire wall or equivalent separation with independent structural stability.
Reasoning:
- The slab is a horizontal fire separation; it does not create two buildings. The building remains one structure of 6 storeys and 21 m, so height and area must be evaluated for the whole.
- The residential exit path discharges through a shared second-floor lobby, so egress is interdependent. A true separation would require independent exits from the residential portion directly to the exterior or to a protected exit enclosure.
- The open cooking in retail suites is a hazard that increases the required separation and likely triggers sprinkler and hood/duct requirements.
- Conclusion: the developer cannot delete residential sprinklers on the strength of the slab. Either provide full sprinkler protection throughout, or redesign with independent exits and a compliant fire wall.
2. The retail portion has open cooking. What fire-protection and coordination issues does this raise for the separation and for the residential floors above?
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Open cooking introduces a commercial cooking hazard requiring:
- Type I hoods, grease duct enclosure, and listed extinguishing systems for the cooking lines.
- Automatic sprinkler protection in the cooking and retail areas; the residential floors above are exposed to a higher fire load and smoke potential through the shared lobby and any penetrations.
- Careful sealing of all penetrations through the 2-hour slab (drains, ducts, conduits) with listed through-penetration firestop systems.
- Smoke control or at minimum smoke-detection coordination so residential occupants are alerted before smoke migrates via the shared lobby.
Because the residential exit path passes through the retail level, the separation cannot be treated as a clean break. The practical outcome is full sprinklering of the building plus upgraded separation at the lobby interface.