Atrium Smoke Filling and the Egress Path
A 5-storey office building has a rectangular atrium 18 m by 24 m, open from the ground floor to a skylight at 22 m. The atrium is the only route from the third-floor tenant space to the main exit stair; occupants must cross a 6 m wide balcony that rings the atrium. The mechanical engineer proposes a smoke exhaust of 40,000 L/s at the skylight and argues this is adequate because the atrium volume is large. The architect has specified 1.1 m high glass guardrails at the balcony edge and no smoke barriers. The fire official is concerned about a design fire on the ground-floor retail kiosk and asks for a tenability analysis. The owner wants to avoid a smoke control system entirely by arguing the atrium is 'open and large enough to vent naturally'.
1. Can the owner rely on natural venting through the skylight alone? What must the tenability analysis demonstrate?
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Natural venting through a skylight is a recognized strategy only if the analysis shows the smoke layer stays above the tenable egress height for the time occupants need to cross the balcony.
The analysis must demonstrate:
- A design fire (size, growth rate, location at the kiosk) and the resulting smoke production rate.
- The descending smoke layer interface remains above the balcony egress path (typically at least 2 m above the walking surface, adjusted for the guardrail) for the required egress time.
- Makeup air is provided at low level; a skylight alone with no inlet will not vent reliably.
- The balcony crossing time plus detection and response time is less than the time the layer descends to the tenability limit.
Because the balcony is the only egress route, the margin must be conservative. A 40,000 L/s mechanical exhaust is not automatically adequate either; the number must come from the analysis, not from the atrium volume.
2. What egress and smoke-barrier deficiencies should the plan reviewer cite regarding the balcony and guardrail?
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Deficiencies to cite:
- The balcony is the sole means of egress from the third floor. A single exit route across an atrium is generally not permitted without an engineered alternative; a second independent exit or a protected path is needed.
- The 1.1 m glass guardrail must be evaluated for both fall protection and smoke behavior; glass that shatters under fire exposure could compromise the path.
- No smoke barriers separate the atrium from the tenant floors, so smoke can enter tenant spaces and vice versa. Smoke barriers or an engineered smoke-control boundary are required.
- The kiosk on the ground floor should be limited in size and separated, or the design fire must account for it.
Recommendation: require a second exit, smoke barriers at the atrium perimeter, and a validated smoke-control design with low-level makeup air.