Rethinking the ‘smokeproof’ enclosure

Stair pressurization systems in high-rise buildings must be properly designed to avoid creating adverse conditions for egress. Alternate methods to pressurizing the stair bear further evaluation.

02/02/2011


The current design standard for protecting stairwells in high-rise buildings is to provide a “smokeproof” enclosure, using mechanical ventilation systems or other design alternatives for the purpose of prohibiting smoke entry into the stairwell.

The International Building Code (IBC) recognizes three specific means for providing smokeproof enclosures:

  1. Naturally ventilated stair balconies
  2. Mechanical ventilation of a stair
  3. Vestibule or stair pressurization.

Due to cost and space advantages, the provision of a stair pressurization system is the most widely selected design option recognized by the IBC, and has been the predominant method for protecting stairwells in high-rise buildings for more than 20 years. Design difficulties experienced in tall buildings and recent events that challenge the underlying fire scenarios for which the stairwells may be in use may necessitate a rethinking of how stairwells are protected during high-rise fire events.

Design challenges

In tall buildings, particularly those exceeding about 25 stories in height, stair pressurization systems are difficult to design due to the impact of stack effect on maintaining uniform pressures over the building’s height. Stack effect is the vertical air movement through a building caused by the temperature differential between the conditioned building air and the ambient outside air. During cold winter conditions, stack effect causes air to move vertically upward in buildings. Stack effect can have a significant impact on the design of a stair pressurization system and must be considered in its design. The impact of variations in floor-to-floor flow resistance is also a concern. High-rise buildings typically contain a variety of occupancy types, including underground parking, retail spaces, residential spaces, and atriums. The variations in leakage from each type of floor can greatly impact the vertical distribution of pressures within a stairwell.

The effectiveness of a stair pressurization system is dependent on maintaining the doors predominately closed to maintain the required pressure differential to keep smoke from entering the stair. High-rise buildings are often designed using the concept of phased evacuation, where only the fire floor and floor(s) directly above and below are evacuated in the event of a fire. Because only a limited number of building occupants utilize the stairwells, queuing is limited, allowing all of the evacuating occupants to enter the stairwells. This minimizes the amount of time the doors remain open. Once the doors close with the occupants safely within the stairwells, the stair pressurization system maintains a pressure differential to the fire floor, keeping smoke from entering the stair.

Openings in the stair created by doors held open during occupant egress (a particular problem during a full building evacuation) or due to structural damage to the stair would severely compromise the performance of a stair pressurization system. Recent developments such as the World Trade Center bombing in 1993 and terrorist attacks in 2001 have shifted the mind-set toward total building evacuation in the event of a high-rise fire or other emergency event. During a full building evacuation, doors on multiple levels may be fully or partially open due to occupants queuing within the stairwell. Long delays due to occupant queuing can result in occupants being exposed to the environment of the stair for long periods of time during egress from the building. It is therefore imperative for the exit stairs to be free of smoke to the greatest extent possible and to incorporate design features (e.g., path lighting) that improve the speed of occupant egress via the stairs.

Stair pressurization systems

Stair pressurization systems typically utilize a single fan with a ducted shaft to multiple injection points or multiple fans distributed over the height of the stair (see Figure 1). It is desirable to provide supply inlets every three to five floors to evenly distribute air throughout the stair, although stairs serving up to a maximum of 10 stories are capable of being pressurized via a single injection point.

An analysis was performed to illustrate the impact of stack effect on a stair pressurization system designed to provide a minimum pressure differential of 0.15 in. H2O at all stair doors using a single-speed fan and a balanced system using multiple injection points. Stack effect was assessed for a building located in New York City using the ASHRAE recommended winter design temperature of 6 F, and the milder 24 F winter design temperature given by the National Oceanic and Atmospheric Administration (NOAA). The analysis used the CONTAM building airflow model for a typical high-rise residential floor plan.

As shown in Table 1, for even moderately tall buildings, the minimum pressure at any given floor served by the stairway drops off substantially from the design pressure, even for a 15-story building, and depending on the assumed design temperature is lower than the minimum 0.15 in. H2O recommended by NFPA 92A for a 20- to 25-story building. At the 25- to 30-story threshold, air actually enters the stair, as evidenced by the negative pressure differentials, and the maximum door opening force may be exceeded at the upper portion of the stair. Table 1 also shows the impact of opening a single door into the stair, which results in a substantial overall decrease in the pressure in the stair. Even at ambient temperatures (highlighted in Table 1) there is a 50% reduction for the 15-story building and a 30% reduction for the 30-story building.

In cities such as New York, buildings exceeding 30 stories are commonplace. For these buildings, other design considerations must be made to properly design a stair pressurization system. Intermediate doors or transfer corridors are often used in super-tall buildings to break up long stair runs, but in more moderately sized high-rises, architects are often unwilling to sacrifice the space to accommodate these features. The mechanical design must therefore provide a means for mitigating overpressure due to stack effect. To accomplish this, mechanical engineers often turn to modulating systems to account for the impact of stack effect on the stair pressurization system.

Design of modulating systems

To deal with the impact of stack effect on the design of stair pressurization systems in tall buildings, designers often propose to provide modulating stair pressurization systems that adjust the airflow into the stair based on measured pressure differentials between the stair and floors served by the stair. Components of a modulating system are typically a powered or barometric relief damper at the top of the stair, a stair pressurization fan equipped with a variable frequency drive (VFD), and pressure sensors at multiple locations in the stairwell.

Modulating stair systems are difficult to commission, difficult to maintain, and have the potential to create unsafe conditions in the stair if not designed properly. For example, a common mistake is to modulate the stair fan to always maintain above the design minimum pressure differential in the stair, using the lowest measured pressure differential from the sensors. In a stack effect condition, modulating the fan based on a pressure sensor near the bottom of a stair would potentially cause unacceptably high door opening forces high in the stair. The fan may also ramp up due to an open door, creating pressures that would cause the door to slam shut when released, causing the potential for occupant injury. A proper design needs to specify a maximum fan setting to avoid these problems.

For stairwells taller than 20 stories, the data in Table 1 would seem to imply that even if a modulating system were used in a stairwell, modulating the capacity of the stair pressurization fans would still result in unacceptable pressures. If the system sought to maintain a 0.05 to 0.15 in. w.g. positive pressure at every floor in the stairwell, the maximum door opening forces would likely be exceeded. If the system sought to relieve excessive door opening forces, setting a cap at 0.36 in. w.g., minimum pressure differentials for smoke control would not be maintained throughout the stair. More often than not, designers overlook one of these two probabilities when designing modulating systems. These problems are further exacerbated when considering the additional problems that may occur during transient states due to doors opening and closing. When doors open and close, a modulating system is constantly ramping up and ramping down the stair pressurization fan via its variable frequency drive. Slow response times to the changing conditions in the stair often create temporary periods of overpressure that result in unacceptably high door opening forces.

Potential alternatives

The performance of a stair pressurization system may be significantly impacted by stack effect and the presence of openings into the stair due to structural damage during an event or due to open doors. During a full-building evacuation, the opening of multiple doors into the stair for prolonged periods of time may seriously diminish the effectiveness of a stair pressurization system. In tall buildings, it may be very difficult to design a stair pressurization system to provide the minimum pressure differential required to contain smoke while not exceeding the maximum door opening forces on doors entering the stairs.

Recently, alternate approaches have been examined to protect stairwells in high-rise buildings, where both supply and exhaust are provided to ventilate the stair. Use of such a high throughput ventilation system has the potential to maintain a tenable environment in the stair, even when there is minor damage to the stair or doors remain open for prolonged periods of time, while being pressure neutral with respect to the building. Two independent studies were examined that utilized a combination of the computational fluid dynamics (CFD) model fire dynamics simulator (FDS) and the building airflow model, CONTAM, to evaluate the performance of alternate systems without requiring full scale testing. Both models were developed by the National Institute of Standards and Technology (NIST).

The first study (Ferreira and Cutonilli, 2008) used FDS to examine protection alternatives for a 20-story stairwell. Although a 20-story stairwell is a relatively short stair run for high-rise buildings, this resulted in a model domain that allowed reasonable computational times. The relative performance of the scenarios evaluated would also be expected to apply directly to stairwells in taller buildings.

A range of model scenarios was simulated in FDS using the 20-story stairwell model domain, varying the openings present in the stair. For all of the simulations, the exterior temperature was set to 15 F, the ASHRAE winter design temperature for New York City. The internal stair temperature was set to 68 F. A 2,000 kW fire was placed in a small room adjacent to the stairwell on one of the lower levels. Fuel properties consistent with a mix of plastics and wood were used. Three basic stairwell ventilation configurations were evaluated:

  • A baseline case where no ventilation is present to evaluate the smoke conditions that would develop in the stair in the absence of stairwell protection
  • A supply-only stair pressurization system where 10,000 cfm sufficient to pressurize the stair to 0.15 in. H2O with doors closed, is provided via four distributed injection points
  • A supply/exhaust dilution case where 10,000 cfm supply and an equal amount of exhaust are provided to and from the stair.

Simulations varied whether the door to the fire compartment was open or closed to the stairwell and varied the extent to which doors on other floors of the stairwell were held open, simulating occupants queuing to enter the stair in a full-building evacuation scenario. When doors were assumed to be closed, the total leakage of the door was approximated by an area of 0.25 sq-ft. The open fire compartment door had an area of 21 sq-ft (Klote and Milke, 2002). When doors on the upper floors of the stair were assumed held partially open due to a full building evacuation condition, doors on alternate floors were modeled with an opening of 4 sq-ft, which was an assumption made assuming a 7-ft-tall standard stair door remained roughly 6 to 8 in. open due to evacuating occupants blocking the exit. For the purposes of comparing relative performance, visibility distance through smoke was used as the primary criterion, with the target visibility distance being 30 ft through smoke in the stairwell (Ferreira, 2008).

Figure 2 shows the comparative model results for the condition where all doors to the stairs are closed, with the stair subject to the stack effect created by the 15 F outside air temperature. As expected, the pressurization system keeps smoke out of the stair via the pressure differential created across the closed door. While the dilution system allows some smoke to be drawn into the stair due to the stack effect induced negative pressure into the stair enclosure, the dilution system is effective in dramatically improving the stair environment with respect to the baseline “no ventilation” case, and visibility distances are maintained above 30 ft throughout the stair (10 m in the figure), except for the two floors nearest the fire, for which visibility is maintained at roughly 10 ft (3 m in the figure).

The first study hypothesized that during a full building evacuation, conditions would change substantially, as the presence of partially open doors into the stair enclosure would increase the draw of smoke into the stair due to stack effect, and decrease the overpressure in the stair created by the stair pressurization system substantially. As shown in Figure 3, the dilution system actually performs better than the pressurization system in this case, although both active ventilation systems greatly improve the stair environment with respect to the baseline case.

A second study (Klote, 2011) looked at a similar dilution approach for a four-story section of stairwell. The study assumed a fully developed fire in an adjacent space, and the most stringent is a fully developed fire near a stair door. For a specific application, an engineering analysis should be conducted to determine an appropriate design fire, where every object that can burn in a room is burning. The burning material was upholstered furniture filled with polyurethane foam, which is relatively common and produces large quantities of dense black smoke. The ventilation airflow can be upward or downward in the stairs. An upward airflow results in nearly smoke-free conditions in the stair below the fire, and this upward flow was used for the CFD analysis. Door warping from the fire was considered to be the source of leakage into the stairwell. The opening area consisted of two isosceles triangles which have bases of 1 in. and sides of 3 ft and 7 ft.

The visibility criterion for the second study assumed that a person on the landing directly above the fire floor is able to see down to the fire floor landing. This is equivalent to saying that a person on the fire floor landing is able to see the landing directly above. The idea behind this criterion is that if it is met, visibility would also be maintained from landing to landing throughout the stairs. The resulting critical visibility distance was therefore less conservative than the 30 ft critical distance used in the first study.

Figure 4 shows typical results from the second study. Figure 4(a) shows simulated smoke flow in the stairwell without forced ventilation. The stairs were filled with dense smoke resulting in visibility of only 2 to 4 ft. Such smoke would make it impossible for a people to see down to their feet. This visibility was definitely unsatisfactory. Simulations were made with ventilation rates of 10,000, 15,000, and 20,000 cfm. It was determined that 20,000 cfm maintained a tenable environment in the stairs. Figures 4(b) and 4(c) show smoke at this ventilation rate with and without people. In Figure 4(c) a “pocket” of smoke forms in front of the door on the fire floor. The CFD simulations were made without people in this “pocket” of smoke, as it was assumed that it is human nature to avoid such a smoky space.

Figure 5 shows the visibility on a path from the landing above the fire floor to the stairwell door on the fire floor. The ends of the path were 5 ft above floor. The path length was 14.8 ft, and this length is shown on Figure 5 as a dashed line. For visibilities greater than the dashed line, a person can see the door on the fire floor. For all of the simulations without people, visibility is above the dashed line. This means that without people, the visibility criterion is met.

With the stairs full of people, it can be seen from Figure 5 that most of the time the visibility is above the dashed line. Visibility is below this line only for short periods of time. The study shows that in full building evacuations, the presence of people in the stair can impact the performance of the dilution system, and therefore this factor should be considered in future assessments.

Path forward

The predominant means for improving the environment in high-rise stair enclosures is to protect the stairs using stair pressurization systems. Stair pressurization systems must be properly designed to avoid creating adverse conditions to exiting, such as unacceptably high door opening forces due to stack effect. The effectiveness of these systems may also be impacted by openings created in the stairs. Design alternatives such as the “dilution” venting system may be viable solutions for protecting stairs in tall buildings. These alternatives may be less impacted by stack effect, damage to stairwells caused by a localized event in the building, or prolonged opening of doors in a full building evacuation.

Simulations showed that a high throughput dilution venting system has the potential of providing equivalent protection to a typical stair pressurization system, without the associated problems due to door overpressure exacerbated by stack effect. In addition, while the amount of pressurization air that can be delivered to the stair is limited by the potential overpressures created, a balanced dilution system does not have this limitation. Therefore, the performance of a dilution system may be enhanced by increasing the ventilation rates.

The optimal rate for ventilating stairs bears further evaluation, and is likely only limited by ventilation shaft space constraints and system cost issues. Actual use of a dilution venting system would require optimization of the airflow rate based on postulated fire scenarios for the building and the desired performance with respect to tenability conditions within the stair. While the studies reviewed in this article were limited in the scope of their evaluations, the potential for developing alternate designs to protect stair enclosures was demonstrated. Additional work would need to be performed prior to adopting these systems for widespread use, but the design benefits may make this effort worthwhile.

- Ferreira is a senior engineer at Hughes Associates Inc., Baltimore. He is a member of the NFPA Committee on Smoke Management Systems. Klote is a consultant in Leesburg, Va. He has more than 30 years of experience in all aspects of smoke control engineering.


Measuring pressure differential

The design requirements for stair pressurization systems included in the various codes and standards, including the IBC and NFPA 92A, Standard for Smoke Control Systems Utilizing Barriers and Pressure Differences, specify a minimum and maximum pressure differential. The minimum pressure differential ranges between 0.05 and 0.15 in. H2O, and is meant to counteract the anticipated buoyancy force resulting from a compartment fire adjacent to the stair, incorporating appropriate safety factors.

The maximum pressure differential specified by IBC is 0.36 in. H2O and is derived from the maximum allowable door opening force allowed for doors entering the stairs, which is typically specified to be 30 lbf (force to set the door in motion), although Americans with Disabilities Act (ADA) considerations may reduce this maximum force to 15 lbf in some jurisdictions (CBC, 2007).

The maximum door opening force is an often overlooked consideration in the design of stairwell pressurization systems, but it is extremely important. An improperly designed pressurization system has the potential to do more harm than good because if excessive force is exerted on stair doors (usually due to a combination of over-design and stack effect forces), occupants can be prohibited from entering the stairwell due to a door being “pinned” shut. The worst case would be for an occupant who would otherwise be able to safely exit via the stairwell to be overcome by smoke from a small to moderate fire due to overpressure conditions in the stairwell.


References

California Building Code (CBC), “Chapter 11: Accessibility,” California Building Standards Commission, Sacramento, CA, 2007.

Ferreira, M.J., “Fire Dynamics Simulator: Ensure Your Software Provides the Safest Atrium Design for Real World Enforcement,” NFPA Journal, National Fire Protection Association, Quincy, MA, 102 (1), January/February 2008.

Ferreira, M.J., and Cutonilli, J., “Protecting the Stair Enclosure in Tall Buildings Impacted by Stack Effect,” Proceedings of the CTBUH 8th World Congress, Dubai, March 3-5, 2008.

Klote, J.H., “Stairwell Smoke Control by Ventilation,” accepted for publication in ASHRAE Transactions, Vol. 117, Part 1, 2011.

Klote, J.H., and Milke, J.A., Principles of Smoke Management, American Society for Heating Refrigeration and Air Conditioning Engineers, Atlanta, GA, 2002.