NFPA 92 defines design, testing of smoke control systems

NFPA 92: Standard for Smoke Control Systems provides fire protection engineers with guidance for the design and testing of smoke control systems.

03/21/2014


Over the past few decades, building, fire, and life safety codes have been forced to continuously adapt to changing architectural trends. While smoke control systems are required to be provided in certain situations, they are sometimes provided as an alternative to having to comply with other requirements, usually for aesthetic or financial reasons. As the prevalence of design features such as large open spaces and open corridors without vestibules continues to increase, so do the number of new smoke control system installations and, consequently, the need for experienced individuals who understand and know how to correctly apply the applicable codes and standards.

NFPA 92: Standard for Smoke Control Systems is a standard published by the NFPA that provides requirements, recommendations, and guidance regarding the design, installation, acceptance testing, operation, and ongoing periodic testing of smoke control systems. An important distinction to recall is that a “code” tells us “when” or “where” something is required, while a “standard" tells us “how” it is designed, installed, tested, maintained, and so on. In this case, NFPA 92 tells us “how” to design smoke control systems such as stair pressurization, large volume exhaust, and elevator hoistway pressurization systems that are required to be provided in buildings by “codes” such as the International Building Code (IBC) or the NFPA 101: Life Safety Code.

Figure 1: This shows the organization of the major smoke control system design elements as defined in NFPA 92. All graphics courtesy: Koffel Associates

Creation of NFPA 92

NFPA 92 was created during the NFPA Annual 2011 code cycle as a result of merging two predecessors: NFPA 92A: Standard for Smoke-Control Systems Utilizing Barriers and Pressure Differences and NFPA 92B: Standard for Smoke Management Systems in Malls, Atria, and Large Spaces. These two were maintained as separate documents from 1991 until 2009. The NFPA Technical Committee on Smoke Management Systems then decided to combine the two into a single document, in part to remediate the use of confusing terminology and duplicate provisions.

Much confusion existed due to the fact that NFPA 92A referred to pressurization systems as “smoke control systems” and NFPA 92B referred to systems used in large spaces such as malls and atria as “smoke management systems,” while at the same time, building codes and other standards recognized no distinction between these two terms. Building codes and standards simply referred to both pressurization (or “smoke control” as designated by NFPA) systems and systems used to maintain tenability in large spaces (or “smoke management” systems as designated by NFPA) as “smoke control systems.”

Therefore, to create consistency between the building codes and NFPA 92, the convention of referring to all systems used to address the impact of smoke from a fire as “smoke control systems” was adopted. Pressurization systems now fall under the smoke control sub-classification of “smoke containment systems,” while systems used in large spaces fall under the sub-classification of “smoke management systems.”

Figure 2: This computational fluid dynamics (CFD) model of an atrium smoke control system was created using Fire Dynamics Simulator.Chapters 1 through 4

The 2012 edition of NFPA 92 consists of 8 chapters and 13 Annexes. Chapters one through three cover the typical NFPA standardized introductory topics: Administration (scope, purpose, retroactivity, and units), Referenced Publications, and Definitions, respectively. Chapter 4, Design Fundamentals, contains exactly what the title implies, the fundamentals of smoke control design. The chapter walks users through a logical design process, which first involves selecting the desired smoke control method or methods to be used based on the selection of the specific design objectives.

As mentioned previously, the two smoke control “methods” (or sub-classifications) recognized by NFPA 92 include smoke containment, which involves establishing and maintaining pressure differences to contain smoke to the zone of origin, and smoke management, which involves removing smoke or managing smoke spread in large volume spaces to maintain tenable conditions. The ideal smoke control method for a particular application depends on the desired design objectives, four of which are listed in Section 4.1.2 (see Figure 1).

Three additional objectives are listed in Annex A and include providing increased visibility for fire department personnel, limiting the spread of toxic gases, and limiting the spread of combustion products to protect building contents. These are sometimes referred to as secondary objectives because, like anything contained in the Annex of an NFPA code or standard, they are not part of the mandatory requirements unless adopted so by the authority having jurisdiction (AHJ). (An example of this is where the AHJ indicates that the Annex is to be part of the mandatory requirements and the word “should” is to be replaced with “shall.”) Nevertheless, most of these secondary objectives are inherently met by systems designed to meet one or more of the primary required objectives. For example, a system designed to maintain the smoke layer interface at a predetermined elevation will usually meet all three of these objectives to some extent or for some specified period of time.

As you may have already guessed, the hierarchy of terminology used in this chapter is often misunderstood and misrepresented. After the selection of design objectives and methods, comes the selection of the design “approaches.” Smoke containment system “approaches” include stair, elevator, zoned, vestibule, and smoke refuge area pressurization. These approaches, along with the smoke management system approaches, are contained in Figure 1, which should help to clarify the major design terminology used in NFPA 92.


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