According to the International Building Code (IBC), every required exit stairway that extends more than 75 feet ABOVE the lowest level of fire department vehicle access (high rise buildings), and every required exit stairway that serves floor levels more than 30 feet BELOW the level of exit discharge must comply with the referenced sections on smokeproof exit enclosures. (IBC 2009 – 403.5.4 & 405.7.2, IBC 2003 & 2006 – 403.13 & 405.8.2)

A smokeproof enclosure is defined in the IBC as: “An exit stairway designed and constructed so that the movement of the products of combustion produced by a fire occurring in any part of the building into the enclosure is limited.”

An alternative to a smokeproof enclosure is allowed by the IBC where the building has a sprinkler system:

“909.20.5 Stair pressurization alternative. Where the building is equipped throughout with an automatic sprinkler system in accordance with Section 903.3.1.1, the vestibule is not required, provided that interior exit stairways are pressurized to a minimum of 0.10 inches of water (25 Pa) and a maximum of 0.35 inches of water (87 Pa) in the shaft relative to the building measured with all stairway doors closed under maximum anticipated conditions of stack effect and wind effect.”
(The 2006 and 2003 editions of the IBC required a minimum of .15 inch of water but the requirements are otherwise the same.)

If the stair pressurization alternative isn’t used and a smokeproof enclosure is required, the entrance to the enclosure must be via a ventilated vestibule or an open exterior balcony.  The doors in a smokeproof enclosure must be self-closing or automatic-closing by actuation of a smoke detector mounted on the “floor-side”  entrance to the smokeproof enclosure (in the corridor, not in the vestibule or stair).  The activation of a smoke detector on one door must result in closing all of the doors in the smokeproof enclosure on all levels.  The door from the corridor to the vestibule is 90-minute rated, and the door from the vestibule to the stair is 20-minute rated.

Using the mechanical ventilation alternative (IBC 2009 & 2006 – 909.20.4.1) the door from the building to the vestibule has to comply with section 715.4.3, and the door assembly from the vestibule to the stairway shall not have less than a 20-minute fire protection rating and meet the requirements for a smoke door assembly in accordance with Section 715.4.3. The door shall be installed in accordance with NFPA 105. Section 715.4.3 requires fire door assemblies to meet the requirements of smoke and draft control assemblies, therefore, perimeter smoke gasketing would be required for both of these doors.  (The 2003 edition of the IBC specifically requires gasketing on the door from the building to the vestibule.  909.20.4.1)

Mechanically Ventilated Vestibule

Using the natural ventilation alternative (IBC 2009 – 909.20.3), the IBC says that the doors have to meet the requirements of section 715.4.  In this case, the smoke may enter the vestibule and then dissipate through the unenclosed openings to the exterior.  It’s not clear to me whether gasketing would be required for these doors, but since the doors open to a vestibule that is open to the exterior, gasketing makes sense for HVAC purposes even if it’s not required by code.

The other question for either smokeproof enclosure or pressurized stair doors is whether or not door bottoms (door sweeps or automatic door bottoms) are required.  They’re not required in order to comply with the UL 1784 requirement, but we’ve had several projects where sweeps had to be added in the field because the stair pressurization couldn’t be maintained without sealing the gap below the door.  Now that I’ve done more research, this seems hard to believe since during a fire, the system is expected to maintain the stair pressurization even with several doors fully open as building occupants are exiting.

If you’re interested, here’s a report about the testing of pressurized stairs: Performance Assessment of Pressurized Stairs in High Rise Buildings.  And if you have experience or input that you could share about doors to smokeproof enclosures or pressurized stairs, I’m all ears.

Graphic Source:  IBC 2003 Commentary.

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