Smoke control design considerations

Learn from this overview of NFPA 92: Standard for Smoke Control Systems, and how fire protection engineers should apply it in design.

05/20/2016


This article is peer-reviewed.

Learning Objectives:

  • Illustrate NFPA 92: Standard for Smoke Control Systems and its basic guidelines.
  • Compare the various smoke control terminology and design parameters.
  • Recall the various equations and calculations when designing smoke control systems. 

If your fire protection engineering firm has been tasked with performing a smoke control system design, there are several design decisions and considerations that need to be addressed along the way. The primary document that deals with smoke control systems is NFPA 92: Standard for Smoke Control Systems. The latest edition of this standard, 2015, was issued by the NFPA Standards Council on Nov. 11, 2014. While some jurisdictions may directly mandate compliance with NFPA 92 via local codes or amendments, many jurisdictions reference NFPA 92 indirectly by mandating compliance with the International Building Code (IBC) or NFPA 101: Life Safety Code.

Figure 1: NFPA 92 Chapter 4 hierarchy of smoke control design terminology is shown. Courtesy: Koffel Associates

Several jurisdictions, including the state of Maryland, already have adopted the latest edition of the IBC (2015 edition), which references the 2015 edition of NFPA 92 for the design of opposed airflow and smoke exhaust systems (see Sections 909.7 and 909.8, respectively). The 2015 IBC also contains additional design criteria for these system types. Note that the codes do not always require compliance with NFPA 92 whenever a smoke control system is required. For example, for smokeproof enclosures (e.g., pressurized stairs and elevator hoistways), the IBC has self-contained criteria and does not reference NFPA 92. Please note that all references within this article are based on the 2015 editions of NFPA 92, NFPA 101, and the IBC.

Smoke control systems

The NFPA 92-2012 created a new hierarchy of terminology, which has not been changed for the 2015 edition. The term "smoke control system" is now used as a broad classification to include two subclassifications, or methods, of smoke control: smoke management and smoke containment (see Figure 1).

Smoke containment systems include stairwell, elevator, vestibule, and refuge-area pressurization systems, as well as zoned smoke control systems. These system types are referred to as "design approaches" within the context of NFPA 92 (Chapter 4). Smoke containment systems generally involve using fans to either inject air into protected areas of a building or exhaust air from fire areas to create pressure differences with respect to adjacent areas, thus "containing" the smoke outside of the protected area.

Building codes generally dictate when a smoke containment system is required, or in many cases, offer a smoke containment system as an alternative design feature. For example, the IBC requires all interior exit stairways serving floors more than 75 ft above the lowest level of fire department vehicle access to be designed as smokeproof enclosures. The code also permits stairwell pressurization systems to be used as an alternative to providing an open exterior balcony or ventilated vestibule (see Sections 403.5.4 and 909.20).

Smoke-management systems involve those used to manage smoke within large-volume spaces, such as atriums or smoke-protected assembly seating. Approaches for smoke management permitted by NFPA 92 include:

  • Mechanical smoke exhaust
  • Natural smoke filling (simply allowing smoke to fill a large void space above)
  • Opposed airflow
  • Gravity smoke venting (providing pathways for the smoke to naturally leave the space).

Choosing a smoke control system

Building owners and designers are often quick to jump to a decision by providing a mechanical smoke exhaust system without considering other potential design alternatives that may be easier and less expensive.

Once the type of system is selected, the designer must determine the applicable design criteria. In many cases, the design criteria for smoke control systems are the same in both the IBC and NFPA 92; however, this is not always the case, particularly with stairway or elevator hoistway pressurization systems. For example, Table 1 illustrates some of the differences between the IBC and NFPA 92 that must be taken into account when designing a stair pressurization system where both documents are applicable.

When evaluating the maximum pressure difference between a stairway and the interior of a building, the IBC specifically limits the design to 0.35 in. wc, whereas NFPA 92 relies more directly on door-opening forces. It should be noted, however, that for a standard-sized door, the maximum design pressure difference stated in the IBC can be shown to cause door-opening forces roughly equal to the maximums permitted by NFPA 92.

The equation for resolving door-opening force, given the design pressure difference, is given in the IBC equation 9-1:

F = Fdc + K(WADP)/2(W-d)

Where:

A = Door area, square feet (square meters)

d = Distance from door handle to latch edge of door, feet (meters)

F = Total door-opening force, pounds (Newtons)

Fdc = Force required to overcome closing device, pounds (Newtons)

K = Coefficient 5.2 (1.0)

W = Door width, feet (meters)

DP = Design pressure difference, inches of water column (Pascals).

Assuming it is a 3x7-ft door, with a 10-lb self-closer, a 0.35-in.-wc pressure difference causes about a 30-lb opening force. It should be noted that the force to overcome the door-closing device varies and can affect the maximum pressure difference some. Therefore, the 0.35-in.-wc maximum pressure difference should be considered a guide.

Calculating pressure and airflows

Figure 2: The interior of a 4-story atrium has complex geometry and a mechanical smoke exhaust system. Courtesy: Koffel Associates

To calculate pressure differences and airflows, the codes and standards allow the designer flexibility in determining the most appropriate calculation methodology for a particular system. While algebraic calculations and/or spreadsheets can be used to design and analyze smoke containment systems, the Multizone Airflow and Contaminant Transport Analysis Software (CONTAM), published by the National Institute of Standards and Technology (NIST), is a viable solution. This software can be used to calculate the expected pressure difference across an opening such as a stair or elevator door based on leakage, mechanical injection, and exhaust airflow rates into and out of the stair and adjacent spaces. In addition to open-airflow pathways like open doors and windows, objects such as walls, floors, closed doors, and other elements may have varying leakage rates depending on their age, construction type, condition, undercut, side gap, etc. Airflow rates and pressure differences across all of these elements can be modeled and calculated in CONTAM.

The impacts of weather, stack effect, HVAC systems, locations of injection points, and other variables should also be analyzed and documented as required by the applicable code or standard as part of the rational analysis for the smoke control system. CONTAM can especially be useful for taller buildings, which are more susceptible to stack effect, and/or buildings with multiple smoke control systems, which may operate simultaneously and create complex interrelationships.

For example, if a stairwell and/or elevator hoistway protected by a pressurization system has a door that opens into an atrium provided with a mechanical smoke exhaust system, the atrium exhaust rate can have a significant impact on the pressure difference across the stair door, hence, the door-opening force and required fan size. Software programs such as CONTAM also are useful for performing sensitivity analyses to determine which variables have the greatest impact on the design. For example, the designer can do multiple trials to get a handle on the stack effect on a stair or hoistway pressurization system based on various outside and inside temperatures.

Determining the design number of doors open

One important consideration in any stair pressurization system design is the "design number of doors open"; that is, how many doors are anticipated to be open at any one point for a reasonable amount of time. Generally, the determination of the design number of doors open is the responsibility of the designer. He/she must consider the use of the building, egress configuration, occupant loads, and any other characteristics that may impact occupant movement.

NFPA 92 states that the pressure-difference calculations must take the design number of doors to be opened simultaneously into account (see Section 4.4.2.1.5). This means that the minimum pressure-difference requirements listed in NFPA 92 Table 4.4.2.1.1 must be maintained with the design number of doors open, which can include both interior and exterior doors.

The design number of doors open for NFPA 92 compliance is left to the discretion of the designer. In contrast, the 2015 IBC minimum and maximum pressure differences—0.1 in. wc and 0.35 in. wc, respectively—are required to occur with all interior doors closed. This allows the potential for leaving an exterior door open. Previously, the 2012 edition of the IBC required these pressure differences to be maintained with all doors closed.


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