Designing smoke control systems
A smoke control system should be designed by a fire protection engineer who can tailor the system to the characteristics of the building and its occupants.
- Understand the codes and standards that define the design of smoke control systems.
- Learn how fire suppression and smoke control systems are integrated.
- Identify at least four design considerations for smoke control systems.
Smoke control is not just an engineered system, but rather is a comprehensive approach to fire safety in a building, established by the integration of various building characteristics, features, or systems. Effective smoke control starts with providing automatic sprinkler protection. Properly designed, installed, and maintained automatic sprinkler systems will assist in accomplishing any smoke control, limiting fire growth and therefore the quantity of smoke produced.
Modern building codes also address smoke control by varying degrees of compartmentation ranging from corridor walls to smoke barriers and fire barriers. Control features required in air handling systems also address smoke control by reducing the likelihood that smoke will spread through the air handling systems to other parts of the building. Lastly, and the focus of this article, is the incorporation of smoke control systems consisting of natural venting or mechanical systems.
Smoke control is typically accomplished by either containing smoke to the zone of origin or managing the smoke with a defined space, typically a large volume. As defined in NFPA 92: Standard for Smoke Control Systems, the specific design objectives for a smoke control system will include one or more of the following:
- Containing the smoke to the zone of fire origin
- Maintaining a tenable environment within exits
- Maintaining a tenable environment with exit access and smoke refuge areas
- Maintaining the smoke layer interface to a predetermined elevation.
The International Building Code (IBC) provides criteria for smoke control systems, but the provisions are limited to providing a tenable environment for the evacuation or relocation of the occupants. In fact, the IBC specifically states that the design criteria are not intended for the preservation of contents, the timely restoration of operations, or to assist fire suppression personnel. The design engineer should keep these constraints in mind because the owner’s fire safety objectives may include property protection and continuity of operations.
It should be noted that the owner or occupants may also expect a level of protection that is beyond maintaining tenability. For example, would occupants on the 20th floor of a high-rise building think the level of protection is adequate if fire on a floor several stories below resulted in smoke spread to their area, causing property damage or impacting their evacuation, even if the evacuation occurs in a tenable environment? This is not to say that current building codes and design standards are inadequate, but rather, and as stated in those documents, they provide minimum requirements for an acceptable level of safety as required by the code/standard.
Basis of design
The basis of design (BOD) report should begin with a description of the system’s purpose and design objectives. The engineer should then indicate if the objectives will be accomplished with smoke containment systems (e.g., stair pressurization) or smoke management systems (smoke exhaust). While often overlooked, natural smoke filling and gravity smoke venting may be used as smoke management systems (see “Fire and smoke modeling” sidebar).
The BOD then needs to address the design assumptions and design criteria. Design assumptions include items such as ambient conditions (wind effect, climate, temperature), leakage rates, and the impact and reliability of other fire protection systems (impact of automatic sprinkler protection on the heat release rate). With respect to the effect of wind, the IBC requires that the design considerations are consistent with the wind loading provisions in the IBC.
In some cases, the applicable codes and standards may have varying criteria with respect to specific design assumptions. For example, for stair pressurization systems the IBC requires a specific minimum and maximum pressure to be achieved with all interior stair doors closed (Section 909.20.5, IBC-2015). NFPA 92 requires that the design calculations take into account the design number of doors to be opened simultaneously (Paragraph 18.104.22.168.5, NFPA 92-2012) However, consistent with the varying assumptions regarding door position, the required pressure differences are also different between the IBC and NFPA 92.
The design fire(s) must be identified in the BOD. Engineers may use either steady fires with constant heat release rates, unsteady fires with heat release rates that vary with time, or a combination thereof. The design fire(s) should be determined by considering the type of fuel, fuel spacing, and configuration. Another critical factor to be determined is the location of the fire(s), which also will impact the rate of smoke mass production depending on the type of plume resulting from the fire location.
The required duration for which the system performance is evaluated will vary depending on the edition of the applicable code and standard. For example, U.S. codes typical refer to a 20-minute operational time, but that can then be modified and typically must be increased when 1.5 times the calculated egress time is greater than 20 minutes. Some editions of the IBC permit the use of the calculated egress time when it is less than 20 minutes, while the 2015 edition of the IBC now requires a minimum of 20 minutes of operation and that shall be increased if 1.5 times the calculated egress time exceeds 20 minutes.
Additional design considerations
While the applicable codes and standards prescribe criteria by which smoke control systems are to be designed, a number of common oversights frequently occur.
System start-up time: The system start-up time needs to include the time for detection of the fire, signal processing time, and the system activation time. Specific oversights involve improperly estimating the time to detection or assuming that system activation commences immediately upon activation, thereby ignoring signal processing time, fan start-up times, or the time needed for dampers to open or close.
Big fires dictate the design: While bigger fires may result in higher smoke production rates and pressure differences, smaller and slower developing fires may result in delayed detection times or a longer time for the sprinkler system to activate. There are some instances in which a smaller or slower developing fire may present challenges to the system.
Makeup air: While some designs fail to even consider the need for providing makeup air for exhaust systems, others fail to consider the impact of the makeup air. What will be the impact of makeup air in climates with extremely hot or cold temperatures? What impact will the velocity have on plume dynamics and doors opening or closing? When answering these questions, consider the periodic testing and inadvertent operations of the system.
Interaction with other systems: These may include other systems, typically HVAC systems, in the building and other smoke control systems. The IBC requires one to consider the interaction effects of multiple smoke control systems (Section 909.4.7, IBC 2015).
System reliability: Reliability data for various fire protection systems and the components of such systems is often hard to obtain. Even where data may exist, the range of reliability of such systems documented in various studies may be significant. As an alternative, the design engineer might consider evaluating fire scenarios in which individual systems and features fail. This is consistent with a design fire scenario often required for performance-based designs (see Paragraph 22.214.171.124, NFPA 101: Life Safety Code-2015).
Coordination of design documents: Frequently, the design of a smoke control system is not prepared by a single engineer, such as a fire protection engineer, but instead by multiple engineers, each practicing within their areas of expertise. For example, this was very apparent on a project in which a third party was tasked to develop and oversee an acceptance test protocol for an atrium smoke exhaust system. Unfortunately, the design documents were not properly coordinated and smoke detectors around the perimeter of the atrium were not programmed to activate the smoke control system, only the detectors at the ceiling of the atrium. The approved test protocol, which involved a smoke bomb test, was performed and it was discovered that many of the smoke detectors that one engineer assumed would activate the smoke control system were not programmed to activate the system due to lack of coordination between the engineers involved. After the programming oversight was corrected, a second smoke bomb test was performed with favorable results.
Improper acceptance test protocols: Despite the previous discussion regarding smoke bomb tests, it must be recognized that smoke bomb tests to not provide the same heat, buoyancy, and entrainment of a real fire. As such, a properly designed system may not pass a smoke bomb test. It also is possible that a system that does pass a smoke bomb test may not perform as intended during a real fire. The reason this is included in the design oversight section is that the design engineer should identify the appropriate system commissioning procedures in the BOD.
Effective smoke control in buildings requires coordination between passive and active fire protection features and fire protection systems. The requirement to provide smoke control in a building is dictated either by building and fire codes or by the fire safety objectives for a specific project. Even when the system is provided based upon a code requirement, the design engineer should consider other design objectives the owner might desire. Some prescriptive code criteria are based on the need to maintain a tenable environment and do not consider property protection or continuity of operations. The design engineer should prepare a BOD report that not only identifies the design objectives, code requirements, and system calculations, but should also explicitly state the design assumptions, system operation logic, and commissioning procedures. An operations and maintenance manual should also be developed for all smoke control systems to ensure proper operation of the system over the useful life of the building.
William E. Koffel is president of Koffel Associates. He is chair of the NFPA Correlating Committee on Life Safety and a member of numerous NFPA technical committees. He is a member of the Consulting-Specifying Engineer editorial advisory board.