How to use NFPA 92 to design 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.

05/26/2017


This article is peer-reviewed.Learning objectives:

  • Understand NFPA 92: Standard for Smoke Control Systems, which is a combined standard to be used in the design of smoke control systems.
  • Realize the International Building Code is the basis of smoke control system design.
  • Apply calculations to model the smoke control solution.

When designing smoke control systems, the 2015 edition of NFPA 92: Standard for Smoke Control Systems is a standard you need to know. Historically, HVAC engineers designed these systems using spreadsheets and the prescriptive calculations in the building codes. This resulted in oversized systems having a major impact on construction costs and the building architecture, as well as unpredictable results in a real fire condition. The days of designing a smoke control system based on the volume of space as the only factor have gone away, making way for a scientific process using the latest fire science information to more accurately determine the protection needed for various smoke control scenarios.

In the evolution of codes, NFPA 92 is a relatively new document, first appearing in 1988 and 1991 as two separate documents, NFPA 92A and NFPA 92B, respectively. With the 2012 edition being published, the NFPA Technical Committee on smoke-management systems combined NFPA 92A and NFPA 92B into one standard to be used for various systems.

As a standard, NFPA 92 is a document that is referenced by other codes for application purposes. It is intended to outline the process for designing various smoke control systems when those systems are required to be installed by various adopted codes. These include the International Code Council’s International Building Code (IBC) as well as NFPA 101: Life Safety Code and NFPA 5000: Building Construction and Safety Code. NFPA 92 does not dictate when a smoke control system is required, but dictates how to design the system.

Figure 1: This model shows mesh boundaries, smoke production, vent locations, and an axisymmetric fire location. All graphics courtesy: WSPInternational Building Code and smoke control systems

Most local jurisdictions have adopted the IBC; therefore, it is the most commonly used starting point for determining the need for a smoke control system. The two most common systems that are required by the IBC are atrium smoke control (IBC Section 402) and smokeproof enclosures (IBC Section 403) required for stairwells in high-rise buildings.

Specifically, the IBC requires an atrium smoke control system when an atrium is connecting more than two floors. (Note the differences with NFPA 101 described later on.) The IBC requires smokeproof enclosures for stairs that serve floors that exceed the threshold for high-rise floors. Another smoke control system, not as commonly used but required as an alternative to providing elevator lobbies, is an elevator pressurization system. Another type of system is a smoke control system for underground buildings or portions of buildings with a floor level more than 30 ft. below the level of exit discharge. Also, smoke-protected assembly occupancies may require a smoke control system.

The way the IBC requirements are organized, NFPA 92 is only referenced for the airflow design of permanent openings in rated barriers across smoke boundaries and for the exhaust of large-volume spaces (atria or malls). The design criteria for the other systems, although addressed in NFPA 92, are specifically spelled out in the IBC. In these cases, NFPA 92 can be used as guidance to further understand how to design these other types of systems; however, the requirements of the IBC will need to be met.

Using NFPA codes

For projects that use NFPA 101 or NFPA 5000, there is a more direct correlation between the code and use of NFPA 92. NFPA 5000 requires smoke control systems for underground buildings, smoke-protected assembly occupancies and atria, and smokeproof enclosures for high-rise buildings. NFPA 101 requirements are similar, but different in that smokeproof enclosures are not required for most buildings and a smoke control system is not required for underground buildings but rather just smoke venting.

One significant difference between the needs for atrium smoke control in the NFPA codes and IBC is that, with NFPA codes, an analysis is required to be conducted to show that the smoke can be maintained for all atria. There is no exception for atria with only 2 stories as there is in the IBC. This can have a significant cost impact on projects that are required to comply with the NFPA codes (e.g., health care facilities).

Types of smoke control systems

NFPA 92 breaks down the types of smoke control systems into two major categories: smoke containment and smoke management. A smoke-containment system is one that uses pressure differentials across a barrier using mechanical means. A smoke-management system is one that uses natural or mechanical systems to maintain a tenable environment for large-volume spaces or reduces smoke migration between the area of origin and any spaces that have direct communication with that area of origin.

These two different categories have several different design approaches for each type of system. Under the smoke-containment category, system types can include the following: stair pressurization, elevator pressurization, zoned pressurization, vestibule pressurization, and refuge area pressurization. Examples of smoke-management systems include atrium exhaust, smoke filling, natural ventilation, and opposed airflow.

The most commonly used systems are stair pressurization and atrium exhaust to meet the requirements of smokeproof stair enclosures and atria, respectively. Open vestibules are a less commonly used option permitted in codes for smokeproof enclosures.  Also, natural ventilation can be used instead of mechanical systems for atrium tenability. However, this approach requires a specific set of conditions regarding the architecture of the space and external factors, especially wind, that would need to be favorable for this type of system to provide adequate protection.

Elevator pressurization is something that can be used in lieu of passive smoke-protected lobbies at each elevator lobby. However, these systems are very challenging to employ due to the stack effect, piston effect, and leakage. Separate shafts adjacent to the elevator shafts are required, with a means of balancing at multiple levels to maintain the required differential pressure ranges at each level. It is strongly encouraged that buildings be planned with enclosed elevator lobbies to avoid the need for this system. If the system is required, detailed smoke modeling should be used to determine the criteria for the design of an elevator-hoistway pressurization system.

Zoned pressurization systems were required in many jurisdictions for high-rise buildings under some of the older codes. However, the base codes no longer require this type of system, which uses pressurization and exhaust to create a “sandwich-like” condition to keep smoke from migrating from the compartment of origin. These zoned systems are now only required in underground buildings, but may be required in some jurisdictions that have modified the base IBC requirements.

Vestibule pressurization or ventilation systems are alternatives to stair pressurization permitted in codes where a separated enclosure is provided between the stair enclosure and the rest of the floor and pressurization or ventilation of the vestibule creates a pressure-differential gap between the stair and the floor. This also can be combined with the stair pressurization.

Prior to beginning the design process, there are several important considerations that will need to be reviewed and discussed with the design team and the authority having jurisdiction (AHJ). This includes determining the design objectives and what type of system is to be used to achieve those objectives. As the designer designs the smoke-containment system, they will need to determine the pressure differentials that need to be obtained. NFPA 92 outlines different pressure differentials based on the presence of sprinklers and the ceiling height of the space. Additionally, these pressure differentials cannot exceed the IBC or NFPA 101 requirements for maximum door forces to be exceeded. For smoke-management systems, the designer has to determine if the smoke will be managed by either maintaining the smoke layer above the level of occupancy to allow safe egress, using smoke barriers to separate communicating spaces, providing airflow to prohibit smoke spread, or a combination of these. NFPA 92 requires that these factors be governed by engineering analysis and calculations.

When preparing this evaluation, the designer has to take into consideration both the tenability of the environment as well as egress time. This could require conducting an egress analysis to show that the occupants can safely egress the space prior to getting to untenable conditions. It is beyond the scope of NFPA 92 to perform the egress analysis. The designer will need to consult other references for this information such as the Society of Fire Protection Engineers’ Engineering Guide to Performance-Based Fire Protection.


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