Historically type VA wood portion of any construction had been limited to the maximum 4 levels of wood. In the past few years, this condition has been changed to +5 stories of wood in Washington State along with a few other states such as Oregon. The trade off for this exception has mainly provisions for additional criteria for automatic sprinkler systems and an addition of pressurization fans to the main egress exit stairs that occupants will have to use to pass through these floors to the outside.
IBC-909 defines the smoke control fire protection life safety criteria for different buildings and where smoke control is deemed to be required by the local and state jurisdictions. Historically IBC with recommendations from NFPA-92 along with IFC defines the conditions where a building may require provisions for life safety exciting of occupants to the outside (or a safe relocation) through maintaining tenable smoke-free enclosures.
The provisions mainly applicable to these standards are IBC-909.20 (smoke proof stairs) and 909.11 (standby Power). The intent is to keep the stair enclosures in positive (+) pressure in relation to the adjacent floors in order to avoid smoke migration. IBC-909 defines many aspects of a sound smoke control system such as pressure rating Max/Min ranges along with maximum pound force door knob limitations to allow occupants to open the doors safely including accessibility provisions.
Some of the main references that will assist in designing a sound smoke control system along with having a qualified fire protection engineer (FPE) designer are: IBC (International Building Code), NFPA-5000 (Building construction and Safety Code), NFPA-92 (Standard for smoke control systems), A guide to smoke control in the 2006 (or latest) IBC and Handbook of Smoke Control Engineering-AHRAE 2012 along with several other useful related design guidelines.
It is good to note that smoke control systems are highly recommended and have shown to be effective in saving lives especially in those about +/-10% cases where sprinklers have been reported to fail but at least the building happened to have a smoke control system. However, since a number of combustible materials used by the tenants are not monitored, there may be situations where two separate neighboring 5 story buildings, one with 5 levels of wood and pressurization and the other with 4 levels of wood having more combustibles but without pressurization posing higher risks. It is probably safe to assume that adding stair pressurization in exchange for taking advantage of the wood products for an extra story of wood is a step toward better life safety under the circumstances.
Original Article: http://www.abossein.com/stair-pressurization-4-stories-wood-construction/
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