High-Rise Office: Smoke Control or Natural Venting?
A 12-storey office building (about 38 m building height) has a central windowless exit stair and a single interior corridor serving tenant spaces. The building is fully sprinklered and has a fire alarm system. The designer proposes to satisfy smoke control for the exit stair by providing a 1.5 m2 openable vent at the top of the stair and a 1.0 m2 opening at the bottom, relying on the stack effect for natural venting. The corridor has no smoke control. The building official is concerned about the stair's ability to remain tenable during a fire on a floor and about the corridor serving the floor of fire origin. The mechanical consultant suggests a pressurized stair instead but notes budget constraints.
1. Assess the natural venting proposal for the exit stair and explain what the code expects for a windowless stair in a building of this height.
Read the worked answer
Natural venting of an exit stair by top and bottom openings is a recognized approach in some circumstances, but its adequacy depends on the building height, the stair's geometry, and whether the openings are reliably operable and of sufficient size. For a 12-storey, 38 m building, stack effect is strong and can be beneficial, but natural venting alone is difficult to demonstrate as maintaining tenability on all floors, particularly where the stair door on the fire floor is opened and smoke can enter.
A windowless stair in a building of this height is typically expected to be protected by a smoke control system - most commonly stair pressurization - or by an equivalently demonstrated alternative. The 1.5 m2 top vent and 1.0 m2 bottom opening may be acceptable only if the code's venting provisions for the stair are met and the design demonstrates that smoke will not compromise the stair. The proposal should be supported by an engineering analysis of stack effect, wind, and door-opening conditions.
The safer and more defensible solution is a pressurized stair with a rated enclosure and self-closing doors, with the natural vent as a supplement, not the sole measure.
2. The corridor has no smoke control. Is that acceptable, and what factors determine whether it is?
Read the worked answer
Whether the corridor needs smoke control depends on the building's height, the occupancy, whether the corridor is a required exit, and whether the corridor serves sleeping occupants or is a means of egress that must remain tenable. For a 12-storey office building, the corridor is part of the means of egress and must remain usable during evacuation. The code's smoke control requirements for corridors typically apply to certain building heights and occupancies (notably residential and care occupancies), and office corridors may be addressed by compartmentation, sprinklers, and closing of doors rather than full smoke control.
The assessment should consider: (1) whether the corridor is a public corridor requiring protection; (2) whether doors to tenant spaces are self-closing and smoke-tight; (3) whether the sprinkler system controls the fire before smoke spreads; and (4) whether the corridor has adequate separation from the floor of fire origin. If the corridor is a required exit and the floor of fire origin can produce smoke that enters it, smoke control or equivalent protection is needed. The designer should document the basis and, if relying on sprinklers and door control, demonstrate that the corridor remains tenable for the evacuation time.