Smoke is the major killer in building fires, and smoke control
provides significant protection from the threat of smoke.
The Official Website of Dr. John H. Klote, PE - Updated January 9, 2018
Smoke Control Systems
The purpose of the following discussion is to provide a general overview of smoke control systems. These systems are based on recognized principles of engineering that have been verified experimentally. The design of smoke control systems needs to be done by registered professional engineers familiar with smoke control technology. For detailed technical information about this technology, see the Handbook of Smoke Control Engineering. In the U.S., many smoke control systems are designed in accordance with NFPA 92, Standard for Smoke Control Systems.
To have confidence that life safety systems including smoke control will work as intended, they need to have commissioning, acceptance testing and periodic testing. The commissioning process needs to begin at the start of a project and continue throughout the project. ASHRAE Guideline 1.5, Commissioning Process for Smoke Control Systems, provides methods for verifying and documenting that the performance of smoke control systems conforms to the intent of the design.
In the U.S., the most common pressurization smoke control systems are pressurized stairwells and pressurized elevators. The intent of a pressurized stairwell system is to provide a tenable environment in the stairwell during a building fire. The intent of a pressurized elevator system is to prevent smoke from traveling through the elevator shaft to floors remote from the fire where it can threaten life. In the event of a fire, occupants should not use elevators, but the fire service often uses elevators to rescue occupants and move men and equipment during firefighting.
Zoned smoke control uses pressurization in conjunction with passive compartmentation to divide the building into a number of zones with the objective of minimizing smoke flow beyond the zone of the fire. Typically zones are one or more floors of a building. Zoned smoke control systems are not as common as they once were.
For simple buildings, analysis of pressurization smoke control systems can be done with algebraic equations or a rule of thumb. For complex buildings, analysis using network modeling may be needed to determine if a particular smoke control system in a particular building is capable of being balanced such that it will perform as intended. If analysis shows that successful balancing is unlikely, the building can be modified or an alternate smoke control system can be used. Depending on local codes, some alternate systems may need code approval.
For this discussion, the term “atrium” is used to mean any high bay space such as an atrium, sports arena, airplane hangar or even a very large hotel lobby. Atrium smoke exhaust is probably the most common type of atrium smoke control used in the U.S. Smoke flows upward from the fire and forms a smoke layer under the ceiling of the atrium. Because the smoke plume pulls air into itself as it rises, the smoke layer temperature is much lower than that of the smoke near the fire. Smoke is exhausted from the smoke layer to provide a “smoke free” lower layer for occupant evacuation.
Atrium smoke control design should include a design fire analysis, and this analysis needs to consider both materials normally expected in the space and transient fuels. Transient fuels are materials that are in a space temporarily. A few examples of transient fuels are Christmas decorations, paint and solvents in stairwells during redecorating, cut-up cardboard boxes awaiting removal, and new upholstered furniture awaiting delivery to an executive office.
A design fire in an atrium does not normally take into account any benefit of sprinklers. In spaces with high ceilings, the temperature of the smoke plume near the ceiling can drop so much that sprinklers may not activate or activation may be so delayed that the spray may evaporate before it reaches the fire. There is a critical ceiling height in large spaces above which the design fire should not take into account any benefits of the sprinkler system, but this is not to say that sprinklers will not work above this critical height. Many professionals believe that this critical height is in the range of 35 to 45 feet (10.7 to 13.7 meters).
For spaces open to the atrium, design fires should take into account sprinkler action when the ceiling height is less than the critical height. Often the design fires for such spaces are shielded fires. A shielded fire is one where an object prevents the sprinkler spray from reaching the fire, and there are many shielded fire scenarios. An example of a shielded fire is burning boxes filled with paper under a table or desk.
An engineering analysis of atrium smoke control is needed, and this engineering analysis often includes more than one design fire. The engineering analysis can be done using the algebraic equation method, and the spreadsheet application, AtriumCalc, can be used for these calculations. The algebraic equation method is appropriate for conventional atrium smoke control systems, but some atrium smoke control systems are tenability systems. A tenability system is one where the occupants are exposed to some amount of diluted smoke that is not life threatening, and these tenability systems are usually analyzed with computational fluid dynamics (CFD).
Atrium smoke venting eliminates the need for exhaust fans, but it is not common in the U.S. Atrium smoke venting is recognized in building codes in many other countries. Wind effects are of special concern with atrium venting. In an air-conditioned atrium during summer, the outdoor air can be hotter that the smoke layer, and this condition needs to accounted for during design of smoke venting systems. When allowed as exception to local building codes in the U.S., atrium smoke venting usually is a tenability system analyzed with CFD.
Smoke filling is an apparent contradiction in that it is both simple and complex. A smoke filling system is simple in that it provides smoke protection in a very large space without any smoke control equipment, but it is complex in that it requires complex analysis to provide a high level of assurance the smoke filling system will provide the smoke protection intended. Smoke filling system is only applicable to very large and tall atria, and smoke filling systems have no smoke exhaust or any other smoke removal. Smoke filling systems need smoke detection to let occupants know to evacuation the atrium and to alert the fire service. Smoke filling is not currently recognized in most building codes in the U.S., but it can be requested as an alternative to code requirements. Most smoke filling systems are tenability systems analyzed with CFD modeling.
Smoke filling works as follows. When a smoke plume reaches the ceiling, the flow turns, forming a jet under the ceiling, as shown in part (a) of the figure below. The arrows indicate air moving into the plume which dilutes the products of combustion. If the ceiling is sufficiently high relative to the properties and size of the fire, the environment in the ceiling jet is tenable. Part (b) of the figure shows an atrium partially filled with smoke. Below the smoke layer, air is entrained in the plume. In the smoke layer, smoke is entrained in the plume. Part (c) of the figure shows the atrium filled with smoke, and only smoke is entrained in the plume. Contaminant concentration will increase until the atrium becomes untenable. This provides time for occupants to evacuate the atrium.
A building with pressurized elevators and
two pressurized stairwells.
Favorable smoke conditions in a unidirectional traffic tunnel.
Atrium smoke exhaust system.
An atrium smoke venting system eliminates the need for the smoke exhaust fan.
A transport tunnel is an enclosed facility that carries traffic including automobiles, trucks, buses, and trains. Fire and smoke control in tunnels is a specialized field. While fires in tunnels are relatively rare events, fires in tunnels pose major safety issues and challenges to the designer, especially with the increase in the number of tunnels, their length, and number of people using them.
Because of the nature of tunnels, ventilation (airflow) is commonly used to control smoke during fires in tunnels, and there is a wide variety of tunnel ventilation approaches that can be used to control smoke. For example, the figure on the right shows smoke being controlled in a unidirectional road tunnel by ventilation. For information and fire and smoke control in transport tunnels, see Chapter 17 of the Handbook of Smoke Control Engineering.
Smoke filling in a large and high atrium can maintain a tenable environment for some time.
Contact John Klote
See about Klote’s Smoke Control Courses and Workshops.
See about the Handbook of Smoke Control Engineering.
Shielded fires are common design fires in spaces open to an atrium, and the figure above shows one of many kinds of shielded fires.
|Scale Model Fire|
|Full Scale Fires|