by Brian D. Kuhn Jr., PE
Most fire deaths are not caused by burns, but by smoke inhalation. Often, smoke incapacitates so quickly people are overcome and cannot make it to an otherwise accessible exit, as discussed in the National Fire Protection Association’s (NFPA’s) “A Reporter’s Guide to Fire and the NFPA” (accessible through www.nfpa.org). Given the nature of many modern buildings (e.g. high-rise) where it will take several minutes, if not hours, to evacuate the building during an emergency, occupants need all the tools at their disposal to help them get out during a fire.
The International Building Code (IBC) calls for smoke control for several high-risk building conditions. Typically, it is required in large-volume spaces (e.g. shopping malls, theaters, airport terminals, entrance lobbies, and sports arenas) where many occupants may be exposed to the effects of fire. High-risk compartmentalized spaces—such as laboratories, high-rise buildings, and underground facilities—may also require smoke control systems.
Unfortunately, designing and installing the smoke control system correctly and efficiently can be difficult; identifying the most appropriate system-type and configuration can also be confusing. Coordination is challenging because it is a multi-disciplinary affair. Engaging a fire protection engineer to perform computer fire/smoke modeling can seem like overkill to an architect, although it is necessary for the interconnectedness of modern buildings. Architects and owners do not want bigger fans or more equipment than what is absolutely necessary. At the end of it all, the building official and fire department need to sign off on the system.
Designing and implementing smoke control does not have to be a headache. Strong collaboration among the design team is a must, and a fire protection engineer should be included from the beginning. Having proper understanding of system types, code requirements, and available analysis tools is also necessary.
There can be flexibility in achieving a balance between the use of active smoke control and passively zoned spaces. With this resilience, designers can integrate smoke control designs into the building fabric, helping achieve the often grand visions of interior, interconnected, and open-air spaces, whether for new construction or for historic buildings where design is limited by existing features.
Types of systems
There are two basic types of smoke control systems: passive and active. Passive systems use smoke barriers or partitions to limit and control the movement of smoke in certain directions or allow it to accumulate in a properly sized reservoir (e.g. the top of an atrium).
Active smoke control can be divided into three subcategories: pressurization, exhaust, and airflow. This discussion focuses on the first two, as airflow is typically used in tunnels, rather than buildings.
A pressurization system supplies and exhausts air at strategic locations to create pressure differentials across smoke barriers for either keeping smoke in or out of an area. It does not necessarily remove smoke; it simply keeps the smoke in the zone of origin and out of adjacent sectors. It assumes that occupants are in close proximity to the fire egress from the area before conditions become untenable.
Used for large-volume spaces (e.g. malls, theaters, and atria), the exhaust method uses mechanical smoke control or natural venting to maintain the accumulating smoke layer above the heads of occupants exiting the building. The IBC suggests the smoke layer must not descend below 1.8 m (6 ft) above the highest walking surface. Simple hand calculations assume there is a clean line at the bottom of the smoke layer, although in reality there is a lot of mixing between the lower and upper zones, especially after sprinkler activation. That is why performance-based design methods often evaluate occupants’ visibility within a space to determine how much smoke exhaust is required.
For both the pressurization and exhaust methods, IBC references National Fire Protection Association (NFPA) 92, Standard for Smoke Control Systems, for the design of the systems. NFPA 92 is a combination of the formerly distinct 92A, Standard for Smoke-Control Systems Utilizing Barriers and Pressure Differences, and 92B, Standard for Smoke Management Systems in Malls, Atria, and Large Spaces, which some local codes may still reference independently.