Common smoke control approaches in high-rise buildings
A fire protection engineer’s approaches to smoke control in high-rise buildings may focus on the rationale behind such approaches — with consideration of the benefits, costs and potential pitfalls.
- Gain an understanding of common smoke control approaches in high-rise buildings.
- Understand origins of the pressurization method as used to protect stairways and floors.
- Learn about active approaches for addressing smoke movement within the smoke zone of origin.
Although the rationale for providing redundant automatic systems to manage smoke movement in tall buildings where manual fire suppression activities cannot be easily undertaken is clear, it is less clear why there is so much variation in how such systems are implemented — if they are implemented at all.
In connection with the current practice of providing smoke control in tall buildings, two fundamental questions have arisen: First, are the methods of smoke control being implemented appropriate to the hazards and use conditions associated with modern high-rise structures? Second, should greater consideration be given to the maintenance of tenable environments within the smoke zone of origin in consideration of the potentially large number of occupants that may be impacted?
Implementing pressurization approaches
During the past 20 to 30 years, high-rise buildings have almost exclusively made use of one form of active smoke control: the pressurization method. Positive pressurization approaches intended to keep smoke out of stairwells have been combined with depressurization or “pressure sandwich” approaches intended to control or restrict smoke movement beyond the zone of fire origin.
Within this seemingly limited application, however, there are numerous variations to the approaches taken by the engineers designing the systems and a multitude of disparate code requirements from jurisdiction to jurisdiction. Adding to this, there has been an increasing number of design professionals who have expressed concern over the industry’s reliance on traditional pressurization systems.
Historically, pressurization systems were used to protect against the spread of biological and chemical contaminants. The application of pressurization systems to stairwells started to gain momentum following a series of studies including the Atlanta City Building Department full-scale fire tests in the 14-story Henry Grady Hotel. This project built on the findings of earlier studies conducted by entities such as the New York City Fire Department in the early 1970s and provided further evidence that pressurization of stair shafts could provide smoke-free exits for the fire scenarios and systems tested.
Subsequent work documented by John H. Klote and James A. Milke at the Church Street Office Building tests exposed the engineering community to such phenomena as stationary vortexes at open doors. This work added to general knowledge about flow through openings in buildings, including a key finding that flow coefficients for open doors were about half of what would otherwise be expected.
In the years since these seminal studies, the use of the pressurization approach in stairwells to create “smokeproof enclosures” was widely adopted and usually implemented with relative ease for buildings under 420 feet in height. However, as the number of high-rise buildings exceeding 420 feet in height being developed has grown, various challenges associated with this approach have been identified.
For example, as documented by Edward K. Budnick and Klote in buildings over 420 feet in height, satisfactory pressurization of a stairwell may be impossible in such tall buildings when all doors are closed. In referring to satisfactory pressurization, the authors were referring to the condition wherein nowhere over the stairwell height is the pressure difference greater than the maximum allowable pressure gradient (such that doors can actually be opened) or less than the minimum allowable pressure gradient (such that smoke intrusion from the fire floor is effectively countered).
The reality highlighted by Budnick and Klote led to the practice of subdividing pressurized stairwells such that pressure gradients caused by stack effect in any single continuous vertical enclosure can be reduced to a more manageable level.
Further adding to the inherent challenges of maintaining the required pressure gradients within stairwells is the fact that fixed air systems, i.e., pressurization systems that cannot dynamically respond to building and environment conditions, will not remain balanced because of inevitable changes in door and wall leakage, automatic door closer settings and as a result of alterations to other active building systems.
To this end, dynamically controlled stair pressurization systems have demonstrated value by providing a means to measure and control air pressures through such approaches as modulating dampers or variable speed drive fans designed to respond to sudden or more gradual pressure changes, respectively.
Ensuring that maximum pressure gradients are not exceeded is of equal importance. Accessibility regulations in many jurisdictions limit the door opening force to 15 pound-force to accommodate occupants who lack the physical strength to push or pull open doors where forces are higher. In practice, it can be challenging to maintain pressures below the threshold value when stair pressurization is combined with floor depressurization and added to fluctuating pressures associated with stack effect.
This reality has forced designers to implement various creative approaches such as pressurization air modulation and automatic door assistors (to facilitate door opening or closing depending on location). However, such approaches cannot easily be implemented after the fact when the final air balance is being conducted.
The subject of how many, if any, doors should be assumed to be open during the process of pressurizing the stair has been sidelined in some jurisdictions due to the inherent challenges of trying to address the multitude of potential demands. This, in turn, has effectively allowed designers to ignore a fundamental reality of building evacuation: pressurized stair shafts will experience numerous and significant fluctuations as a result of people entering and exiting during a fire incident.
Thus, apart from tests that have allowed for one, two or three doors being open under relatively controlled fire conditions, there is not a significant body of knowledge concerning how smokeproof enclosures will actually perform during a major incident. From the standpoint of implementation, even when the rational analysis has accounted for the possibility of some number of doors being opened simultaneously, pressure sensors and dynamic airflow systems capable of responding to sudden variations in leakage are frequently omitted due to questions about the reliability, concerns about cost, doubts of their efficacy or the common belief that it is better to keep systems “simple,” is somewhat limited.
Table 1 provides a summary of minimum and maximum stair pressure gradients allowed by different codes, as well as details about how such codes address number of open doors in the design of the system. The variability in how different codes approach the issue of pressurization illustrates the lack of consensus within the industry as to how much of a pressure difference is sufficient to control smoke movement, as well as how occupant movement through the stairs should be addressed in terms of the effect of opening doors on the quantity of air used.
On-floor pressurization smoke control
Despite the numerous challenges with how stairwell pressurization systems are implemented, there are few professionals that argue that such systems —when properly implemented — do not provide value to building occupants, given the importance of keeping the stairs clear of smoke for the duration of the time required for evacuation.
Conversely, it may be argued that smoke control systems that either pressurize or depressurize part or all of a smoke zone (referred to as on-floor pressurization systems) are of more dubious value in the context of sprinklered fires in high-rise buildings. In fact, many jurisdictions do not require any form of on-floor pressurization in high-rise buildings in their codes.
Nonetheless, there are numerous jurisdictions and design professionals that employ on-floor pressurization with the intent of enhancing occupant safety where reliance on passive barriers alone is not seen as being adequate due to the propensity of such passive systems to become compromised over time as changes are made to the building.
Two of the most common subsets of on-floor pressurization smoke control are:
- Selective depressurization or pressurization of corridors.
- Depressurization or pressurization of full floors (minus core elements and some small rooms).
While the intent of these systems is clear, i.e., to provide a means to counter the expected forces generated by a sprinklered fire due to buoyancy and expansion as documented by Klote and Milke, the rationale behind the implementation of these systems is often less clear. The lack of clarity likely stems from several sources, including a dearth of empirical evidence concerning how such systems perform in actual fires and a lack of consensus on what is trying to be achieved.
The typical questions that arise in connection with on-floor smoke control systems include:
- Why are corridors provided with active depressurization or pressurization to limit smoke movement, while the areas where the fire is more likely to start, e.g., dwelling units or guestrooms, are not provided with active smoke control?
- Can depressurizing or pressurizing corridors potentially cause more harm than good in some circumstances?
- What benefit exists by pressurizing corridors above and below the fire floor when so little leakage often exists in the corridor?
- Does pressurization or depressurization of entire floors (such as in an office building) provide a significant benefit in the context of a sprinklered building?
- Is there a role for the air change method in modern smoke control?
Corridor depressurization approaches
A significant number of active on-floor systems employ some form of selective depressurization. The most prevalent example of selective depressurization is corridor depressurization, which is commonly used in residential occupancies where multiple dwellings or guestrooms are accessed from a single corridor. Upon detection of smoke in the corridor or activation of a sprinkler waterflow alarm anywhere in the building, the system that depressurizes the corridor initiates.
The primary benefit of exhausting the corridor on the fire floor is that the efficacy of the pressure barrier provided at the stairs and other shafts located off the corridor is enhanced and a greater degree of robustness may be achieved due to existence of two systems working in conjunction with one another: stair pressurization and corridor depressurization. Thus, the natural consequence and primary benefit of negatively pressurizing the corridor is that the pressure differential across the stairwell (and vestibule door) will invariably increase — or at least not be subject to a single point of failure as a result of the collaborative relationship between the exhaust and supply fans — and the likelihood of smoke transmission from fire floor to the stair shafts will decrease.
From the standpoint of potential risks associated with exhausting a corridor, there is a likelihood that some smoke from the fire originating in a room served by the corridor will be drawn into the corridor. Thus, some measure of protection against smoke spread is provided to the floors above and below at the expense of the corridor tenability on the fire floor.
However, consideration must be given to the fact that the smoke introduced into the corridor also is being diluted by uncontaminated air that is simultaneously being drawn out of other spaces off the corridor that are not directly connected to the room or area of fire origin.
Corridor pressurization approaches
An alternative to depressurizing the corridor on the fire floor is to pressurize the corridors above and below the corridor above or below the fire. Theoretically, pressurization of the corridors above or below the fire floor provides resistance to the flow of smoke and heat from the fire floor in the same manner as a system where the corridor in the zone of origin is held at a negative pressure.
However, this method is characterized by a number of challenges that can have an impact of the performance of the system. Unlike depressurization of corridors within the smoke zone of origin where there is a direct relationship between the fire source and the corridor, the pressure sandwich method using corridors often is characterized by conditions wherein the corridors above and below are not aligned with the corridor on the fire floor and thus the sandwich of pressure is not aligned with the corridor that is on the fire floor that is most likely to be exposed to the greatest quantity of smoke and heat.
Additionally, because corridors are often located centrally within the building floorplate where there are limited openings through the floor/ceiling slabs, many practitioners believe that pressurizing said spaces offers little practical benefit.
To the contrary, because the most prevalent fire origin is within the dwellings or guestrooms themselves, the implementation of a pressure sandwich above and below would largely only serve to protect the extremely limited openings through the slab directly below the corridor being pressurized, ignoring the fact that the majority of openings that allow for smoke to move through buildings exists within the rooms and areas off the corridor.
Another potential concern with using the corridor pressure sandwich method is that there is effectively no reduction in the overall pressures being generated on the floor of fire origin. To differ, the approach of pressurizing the corridor can create conditions wherein any smoke that contaminates the corridor above the fire floor through such means as the stair or other shaft will be actively pushed into the dwelling units or guestrooms that are by definition at a negative pressure relative to the corridor.
Full-floor depressurization and pressure sandwiches
The scientific basis for providing full-floor depressurization or full-floor pressure sandwiches is well established. Whereas the pressure created on a typical floor in the context of a sprinklered fire would be expected to be 0.05 to 0.10 inches water gauge, maintaining the fire floor negative in the same amount as a fire would be expected to increase the pressure relative to the adjacent zones is common sense. Coupled with the argument that such systems compensate for the inevitable failure of passive systems that occur over time as holes are drilled and new pathways are created between smoke zones at construction joints, there is understandable support for these types of smoke control systems.
The question that many owner and design professionals have asked is whether the potential for the heat– and smoke–induced damage that can arise from the inevitable degradation of the walls, floors and other elements that comprise the building’s passive systems warrants the complexity, controls, equipment and associated lifetime of maintenance that is associated with smoke control systems.
This is an especially valid question in light of the fact that automatic fire sprinkler systems that are required in modern high-rise buildings have consistently delivered an excellent record in controlling fires and minimizing loss of life in high-rise and nonhigh-rise buildings alike. For this reason, there has been willingness in some jurisdictions to forgo the complexities of pressurization shafts, dampers, emergency power systems for fans and controls, as well as automatic initiating devices in favor of “simpler” passive approaches irrespective of the potential for a decrease in performance of passive systems over time.
Corridor tenability and use of air changes
Whereas the broader goal of smoke control systems is to provide a tenable environment for the evacuation or relocation of occupants, it is notable that the pressurization method does not actually require maintenance of a tenable environment in the smoke control zone of fire origin. Alternatively, the goal of such systems is to limit or prevent the movement of smoke and hot gases beyond the zone of origin to facilitate building evacuation and firefighting operations.
In high-rise buildings where floor plates frequently exceed 20,000 square feet, it follows that there is the potential for dozens if not hundreds of building occupants in the zone of fire origin to be exposed to conditions that are essentially identical to those that would exist if there was no smoke control system at all.
For residential buildings where occupants may be nonambulatory or are sleeping, this presents a number of challenges that can better be addressed if a method is devised to improve tenability within the common corridor of the zone of fire origin.
The air change method (providing replacement of the volume of air in a space within a specified time period) has all but been abandoned since the introduction of updated requirements in the Uniform Building Code Section 905, now International Building Code Section 909 and its reference standards including NFPA 92: Standard for Smoke Control Systems.
The method was put aside in part because it was not based on building physics or empirically derived relationships and in part because compliance testing was widely viewed as subjective. The use of smoke bombs created additional challenges due to the fact that such testing tools did not accurately reflect the system’s capabilities, and so when more objective approaches were introduced into the codes, this approach was quickly put aside.
In consideration of the potential benefits that may be conferred upon occupants within the smoke zone of origin using the common egress corridors, abandonment of the concept of changing out air to achieve tenability within corridors may have been premature. It can be argued that in a sprinkler–controlled fire, the provision for air changes within corridors using the same equipment that is already required by the smoke control system will supplement with the protection afforded by on-floor pressurization systems, thereby addressing the goal of limiting smoke movement outside of the zone of fire origin while providing tenability improvements within selected egress components within the zone of fire origin.
When considered in the context of self-closing smoke doors that limit smoke movement from the dwellings or guestrooms into the corridors, the use of air changes (i.e., smoke exhaust with corresponding makeup air) will likely improve tenability. It should be noted, however that while this approach is similar to the exhaust method provided in Section 909.8 of the International Building Code, proper analysis and implementation would likely involve different considerations related to the confined nature of corridors and the typically remote locations of smoke and heat sources. Given the nature of improvements that could be realized without significant additional cost or complexity, however, further study of this smoke control approach is warranted.
Coming to a consensus
With respect to implementation of smoke control approaches in high-rise buildings, there are significant variations in how smoke control systems are implemented and a number of questions about the efficacy of specific applications of the pressurization method.
There is agreement that properly implemented stairwell pressurization systems can provide tenability benefits for evacuating occupants in tall buildings that is supported by a body of empirical data and building physics. However, there is no consensus in the value decisions that can have major impacts on the design of these systems, such as how many doors should be assumed to be simultaneously open and the magnitude of the pressure gradient that is required at those openings serving the stairwell. There does not appear to be a strong rationale for the lack of consistency, which points to the need for additional research to harmonize code approaches for this important life safety system.
In contrast to the reasonably strong consensus that exists relative need for stairwell pressurization, there is far less commonality in whether active smoke control systems are needed to address smoke movement between floors beyond the stair shafts. A variety of methods are currently in use ranging from depressurization of discrete elements such as corridors to depressurization of entire smoke zones. Alternative methodologies also exist wherein pressure sandwiches are implemented above and below the smoke zone of origin.
Perhaps because the real-world efficacy of these systems has not been measured and documented in the same manner as have sprinkler systems or building energy systems, there is very little agreement as to which methods are to be used for specific building types or whether they are needed at all.
Finally, while the vast majority of smoke control used in high-rise buildings focuses on smoke control that is designed to limit smoke movement to the zone of fire origin, renewed attention to approaches that can provide tenability within the smoke zone of origin such as air changes is warranted. Given the significant commitment to resources and coordination that is required to implement active approaches in the first place, the industry should take a close look at the inherent value that is provided when only those occupants outside the zone of origin in such massive buildings are offered protection by active approaches.
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