What every engineer designing smoke control systems should know
This outlines the design requirements and their basis for smoke control systems
- Identify the basis for minimum pressure difference requirements in stairwell pressurization systems.
- Identify the basis for exhaust capacity requirements in atrium smoke control systems.
- Identify the approaches for providing the needed smoke exhaust requirements in atrium smoke control systems.
Smoke control systems are an important component of fire safety systems in high-rise buildings and in buildings with large open areas. With smoke inhalation being the principal cause of death in fires and being capable of inducing damage in contents far removed from the position of a fire, smoke control systems can limit smoke spread or exhaust smoke to limit the hazard posed by smoke.
One challenge in understanding smoke control systems is that there can be appreciable variety in the designs of these systems from building to building, especially for those buildings with large open areas such as atria, arenas, covered malls or airport terminals. This variation in designs occurs because designs of smoke control systems are related to the design goals for the system.
In most high-rise buildings, the principal design goal of a smoke control is life safety, though limiting fire spread beyond the floor of fire origin (either for life safety or property protection) may also be a design goal. For buildings with large open spaces, designs are often based on providing a minimum clear height between the highest walking surface in the large open space and the smoke layer.
The design goals for a particular smoke control system in a building are reflected in the requirements for the systems included in codes, such as the International Building Code and may also be set by stakeholders. In the following sections the design requirements and their technical bases will be discussed.
Stairwell pressurization systems
In high-rise buildings, the IBC requires any stairway serving floor levels more than 75 feet above the lowest level of fire department access to be a “smokeproof enclosure” (Section 403.5.4, IBC 2021). The principal purpose for that requirement is to maintain tenability in these important egress components in high-rise buildings.
Stairwell pressurization systems are one method identified in section 909.20 of the IBC that can be used to meet the requirement for smokeproof enclosures. The label “smokeproof enclosure” is a poor title, as the intent of a stairwell pressurization system (or any of the other design options included in section 909.20 for a smokeproof enclosure), is to limit the amount of smoke which can enter a stairwell so as to maintain tenability in that space and not to keep the stairwell completely free of smoke.
Stairwell pressurization systems seek to make the stairwell a high-pressure zone thereby preventing smoke from spreading into the stairwell (see Figure 1). According to section 909.20.5, the stairwell should be pressurized to a minimum level of 0.10 inches w.g. above the pressure in adjacent building spaces. This pressure difference is adequate to prevent smoke from migrating into a stairwell from an adjacent space by exceeding the positive pressure difference generated by a fire in a sprinklered building.
In section 909.20.5 of the IBC, a maximum pressure difference between the stairwell and adjacent building spaces is stipulated as 0.35 inches w.g.. The maximum allowable pressure difference is based on the 30-pound limit for the maximum force permitted to open a stairwell door that is specified in section 1010.1.3.
According to section 909.20.7.1 of the IBC, any fans used to pressurize a stairwell need to be dedicated solely to that purpose, meaning that they cannot also used as part of a heating, ventilating or air conditioning system for the building. The pressurization may be done with one or more fans. Where one fan is used, it is generally placed at the top of the stairwell or at the bottom.
For systems using multiple fans, these fans may be placed along an exterior wall (if one of the stairwell walls is an exterior wall of the building). If the stairwell is completely in the interior of the building, the airflow may be provided via a shaft that is adjacent to the stairwell. In that case, the fire resistance rating of the shaft enclosure would need to be the same as that for the stairwell.
Pressurization of a tall, continuous stairwell in very tall buildings may be challenging, especially where winter design temperatures are relatively low. The challenge is that it may be difficult to satisfy the minimum pressure difference requirement without exceeding the maximum allowable pressure difference. The specific height limit depends principally on the exterior design temperature and leakage characteristics of the enclosure of the stairwell walls and the exterior building walls. As an example, in the mid-Atlantic area of the U.S., the height limit tends to be about 20 stories.
Determining the capacity of the fans needed to pressurize a stairwell can be done by either hand calculations or application of a computer model. For buildings that have the same floor area for all stories of the building and all doors are considered closed, fans can be sized through a set of algebraic equations. Otherwise, where either the floor area of the level changes from story to story or if open doors need to be considered, then fans need to be sized via the use of software, such as CONTAM.
The default design condition noted in the IBS and NFPA 92: Standard for Smoke Control Systems for sizing fans assumes all doors are closed, unless doors are opened via an automatic device in the event of a fire. Should other doors be opened intermittently during a fire, e.g., by evacuating occupants, the pressure in the stairwell will temporarily decrease, perhaps below the minimum pressure specified by the IBC. Even though some smoke may enter the stairwell, this short-term opening and closing of the door is not sufficient to compromise the tenability of the stairwell, hence the reason why “smokeproof enclosure” is not a good term for a pressurized stairwell.
Even so, in some cases, a local code may require that one or more open stairwell doors be considered in the design. Satisfying this requirement can be challenging. First, consideration of an open door will require an appreciable increase in the fan capacity needed to pressure a stairwell if the minimum pressure difference of 0.10 inches of water is required on the floor level with the open door. Increasing the fan capacity substantially may cause an excess of pressure in the stairwell if all doors are closed at some point during the operation of the system. While there are methods to relieve excess pressure if stairwell doors are closed, the response time of these methods may be much longer than the time needed for a door to open and then return to a closed position.
Being stairwell pressurization fans are dedicated and only used for that purpose, they only need to operate during fire incidents. Consequently, they can be actuated via any fire alarm initiating device in the building, including components such as automatic fire detectors, manual pull stations or sprinkler waterflow switches.
Smoke control in atria
In buildings with atria (or other large open spaces), the IBC requires smoke exhaust to maintain the underside of the smoke layer (i.e., the smoke layer interface) to be at least 6 feet above the highest walking surface in the smoke zone. Again, the implied goal is to maintain tenability of egress paths in the large open area associated with an atrium. Design variations in smoke control systems for atria may be observed from building to building because there are several approaches that can be used to maintain the smoke layer interface at the prescribed elevation. In addition, local codes or building owners may require additional measures to achieve more ambitious design goals leading to even more design variations.
According to Chapter 2 of the IBC, an atrium is a “vertical space that is closed at the top, connecting two or more stories in Group I-2 and I-3 occupancies or three or more stories in all other occupancies.” For spaces classified as “atria,” an engineered smoke control system is required in section 404.5 of the IBC. With the design goal of maintaining the height of the smoke layer to be at least 6 feet above the highest walking surface, the descent of a smoke layer can be stopped if the rate of smoke exhaust is equal to the amount being supplied by the fire.
Estimating the amount of smoke production depends on the heat release rate of the design fire. Annex B of NFPA 92 includes data on a wide variety of potential design fires. Data is also available online in websites such as those created by NIST and the University of Maryland.
The amount of air entrained in a plume that rises unobstructed by balconies or other architectural features can be estimated by applying one of two equations included in NFPA 92. The choice of which equation to apply depends on whether the flame height extends into the smoke layer. Hence, the first step is to estimate the flame height using equation 220.127.116.11.a from NFPA 92:
Given that the flame height for the prescribed design fire is less than the height to the smoke layer interface in most applications involving tall, open spaces, the results of applying equation (18.104.22.168.b) are presented in Figure 2 for a range of fire sizes (in terms of heat release rate) and clear heights.
Some designers may be tempted to underestimate the heat release rate for design fires to reduce the capacity (and hence cost) of atrium exhaust fans. Referring to equation 22.214.171.124.b, the amount of smoke production (and hence the amount of smoke exhaust) is dependent only on the cube root of the heat release rate. Hence, doubling the heat release rate only increases the exhaust rate by about 20%. Providing overly modest design fire sizes may result in constraining how the space is used by restricting the type of combustible items (either permanent or transient) are present in the space.
In specifying a peak heat release rate of a design fire, designers should question why the fire would necessarily stop growing at that heat release rate. Should a fire spread to another fuel package, the potential for a larger fire exists. Fire growth can be limited to a particular maximum size either by separation distances to nearby combustibles or automatic suppression.
In many tall atria, sprinklers may not be present at the ceiling. Hence, the reliance on separation distances becomes important. Administrative controls may need to be enacted to stipulate that design assumptions for the type, distribution and quantity of fuel be maintained throughout the life of the building.
If architectural features of the atrium are present to serve as obstructions to a rising plume, the movement of plumes around such obstructions has the potential to greatly increase the amount of smoke generation. Estimating the amount of smoke production in such cases requires the use of a fire model, such as Fire Dynamics Simulator.
In most cases in North America, the preferred method providing exhaust is via fans, though natural venting is also a possible solution. The design basis for natural vents is the same as that for mechanical venting, i.e., to size the exhaust capacity so that the smoke exhaust rate is equal to the smoke production rate at the design height. The calculation method for sizing natural vents is included in NFPA 204: Standard for Smoke and Heat Venting [2021b].
Natural vents have a benefit of not requiring electrical power to operate and hence do not require secondary power. However, one limitation of natural vents is that smoke only flows through a vent if the smoke is buoyant relative to the outside air. Cooling of a smoke layer by a sprinkler spray may reduce the buoyancy of the smoke and hence the rate of smoke exhaust through a vent, which may be especially problematic on a hot, summer day.
Actuation of smoke control systems
Actuation of smoke control systems is done via fire alarm initiating devices. For stairwell pressurization systems, any fire alarm initiating device may be used, such as area smoke detectors, manual pull stations and water flow switches. Smoke control systems for atria may be actuated by initiating devices in or near the atrium, though in some buildings, all initiating devices might be used.
For an atrium system, if the goal of the system is to arrest the smoke layer descent to a particular elevation, the system needs to be actuated soon enough so that the exhaust fans are operating at capacity before the smoke layer descending to the design elevation (see section 4.5.3 of NFPA 92). Large exhaust fans are likely to require some time to reach capacity. Hence, the proposal of particular fire alarm initiating devices needs to consider the response time of those devices to the prescribed design fire.