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 126.96.36.199.5, 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.